The indicator is an important tool for detecting additional mortality by incidentally occurring by-catch in key populations of the highly mobile mammal and waterbird species. The populations of marine mammals (cetaceans and seals) and diving waterbirds evaluated in the indicator represent highly mobile animals in the Baltic Sea that are sensitive to additive mortality caused by various métiers of fishing gear due to their characteristic slow reproduction rate.

The distribution and abundance of marine mammal populations is closely linked to abundant fish stocks and is impacted by many human activities. For harbour porpoises, by-catch has been identified as the main known cause of human-related mortality and it is likely to inhibit population recovery towards conservation targets. For seals, by-catch adds to directed takes by hunters, both having a direct effect on the populations. Eurasian otters could not be assessed. They often use coastal areas and are mainly territorial whereas juveniles disperse over wider areas. Due to their coastal distribution otters may be especially vulnerable to specific gear such as static nets, fyke nets and traps, both commercial and recreational, and may need more attention in future evaluations.

Drowning due to by-catch in fishing gear is a significant pressure on waterbirds. It has a strong potential to affect their population trends and demography. In vulnerable species, the numbers of drowned birds may represent a relatively large proportion of the total population size. In some Baltic Sea countries, selected waterbird species are hunted. Also oiling of birds can have an additional substantial impact on waterbird populations. This implies that the loss of individuals due to all human-induced mortality can impact the populations and needs to be taken into account.

2.1 Ecological relevance

Mammals and waterbirds are prone to become entangled in various types of fishing gear and to die by drowning. They belong to species with a high longevity and low reproductive rates. Their populations are therefore vulnerable to the loss, especially of adult individuals, as it takes a relatively long time to compensate for such losses (Bernotat & Dierschke 2021).

For harbour porpoises, by-catch is a significant threat (ASCOBANS 2012, 2016) and may be the main cause of human-related mortality in the Baltic Sea and likely inhibits population recovery towards conservation targets. For seals, by-catch in static nets or traps, especially for those without mitigation devices, is a significant anthropogenic cause of death (Vanhatalo et al. 2014, Oksanen et al. 2015). For seals, by-catch adds to the number of animals killed by hunters, both having a direct effect on the populations.

Harbour porpoise and seal species are top predators in the Baltic Sea marine food web and thus have an important functional role in the ecosystem. Due to their population dynamics, they are especially vulnerable to additive mortality (Bernotat & Dierschke 2021). Additional anthropogenic mortality that exceeds the potential rate of increase in a population will eventually drive a population to extinction. It is thus necessary to keep the sum of all anthropogenic mortality, including by-catch, below a critical value. From the conservation perspective, immediate management consequences are needed if this threshold is exceeded. In order to set such reference points, the Scientific Committee of the International Whaling Commission recommended that incidental mortality should not exceed half of the potential rate of increase (IWC 1991). Furthermore, incidental mortality greater than one fourth of the potential rate of increase should be considered cause for concern (IWC 1996).

Harbour porpoise

The figure for the potential rate of population increase for harbour porpoises used in simple population models by ASCOBANS and the IWC as well as in the frame of the US Marine Mammal Protection Act (MMPA) is 4% per annum based on their known life history parameters. Advanced technical abilities in computing large amounts of data allow for Management Strategy Evaluation (MSE) frameworks using more sophisticated population models, such as Removal Limit Algorithm (RLA) or modified Potential Biological Removal (mPBR) in which the development of population size can be simulated based on stochasticity of input data and underlying conservation objectives. This has been done for the harbour porpoise populations of the North Sea (RLA) and the Belt Sea (mPBR) (Genu et al. 2021, Owen et al. 2022).

Given the high levels of environmental contaminants, including heavy metals and PCBs, of harbour porpoises in the Baltic Sea and impaired immune function (e.g. Siebert et al. 1999, Beineke et al. 2005, 2007a,b, Ciesielski et al. 2006) and the correlation between e.g., PCB burdens and reproductive failure (Murphy et al. 2015), a precautionary setting of the maximum reproductive rate, an important input value in population models used in the RLA and mPBR methods, is required from a conservation point of view.

The mean longevity of harbour porpoises is severely impacted by anthropogenic activities such as fishing. The average age at death in animals stranded along the German Baltic Sea coast is only 3.67 (±0.30) years, significantly less than in North Sea animals. With a mean age at sexual maturity of 4.95 years, porpoise populations are especially vulnerable to factors that shorten the reproductive lifespan such as additional direct mortality (Kesselring et al. 2017) or pollution. For harbour porpoises, the by-catch risk is highest in various types of static nets, including gill nets and semi-driftnets (gear type: GNS) and entangling nets (trammel nets, GTR) (ICES 2016, MASTS 2016). Driftnets are banned in the Baltic Sea, but some hybrid nets such as ‘semi-driftnets’ which are fixed on one end of the net with the other end drifting around this anchor which are locally used in Poland are of special concern (Skora & Kuklik 2003).

Seals

Seals in general have a higher maximum reproductive rate compared to cetaceans (Wade 1998). In contrast to harbour porpoises, they are still hunted in the Baltic Sea. Thus, there is an additional source of direct takes from the populations which needs to be considered in predictions of a threshold value which still would allow reaching conservation objectives. By-catch numbers of seals in static nets, traps and fyke nets are in the thousands (Vanhatalo et al. 2014) although reported numbers are orders of magnitude lower.

Otters

During the 1970s, European otters had disappeared along the coasts of the Baltic Sea. Environmental contaminants such as PCBs, DDT, dieldrin and mercury have shown to be among the leading causes of the decrease in the population. In the 1980s, otters were only found in small scattered areas in Sweden and they were absent from the Baltic coast. Since then the population started to recover and otters also re-established in coastal habitats (Norrgren & Levengood 2012). Eurasian otters are known to be frequently by-caught in static nets and traps (Hauer et al. 2020, ICES 2021). However, the otter abundance in the Baltic Sea is not monitored and also by-catch is rarely reported. Hence, no evaluation can be made for HOLAS 3 due to lack of data. In Norway it has been shown that by-catch in local fisheries disrupts the natural re-establishment in otter habitats (Landa & Guidos 2020).

Waterbirds

Waterbirds diving during foraging in order to catch demersal or pelagic fish (divers, grebes, cormorants, mergansers, alcids) and benthic invertebrates (ducks), respectively, are prone to become entangled in various types of static nets and to die by drowning. In addition to hunting (Mooij 2005) and oiling (Larsson & Tydén 2005, Žydelis et al. 2006), drowning in fishing gear is a quantitatively important source of mortality for waterbirds living in the Baltic. Scientific studies show that the number of waterbirds by-caught is very high and differs significantly from the much lower numbers reported in official reports (Morkūnas et al. 2022). Due to their population dynamics, waterbirds are especially vulnerable to additive mortality (Bernotat & Dierschke 2021). Additional anthropogenic mortality that exceeds the potential rate of increase will eventually drive a population to extinction. It is thus necessary to keep the sum of all anthropogenic mortality, including by-catch, below a critical value.

High longevity is typical for the waterbirds found in the Baltic Sea. The mismatch between the loss of individuals and the effort to replace them is most pronounced in alcids which have a late sexual maturity and only low numbers of offspring, whereas ducks may compensate more easily owing to higher reproductive rates and lower ages of first breeding. However, other factors promoting or impeding population growth rates may override or possibly add to this pattern. For example, fluctuations in population sizes are at least partly caused by favourable supply of prey fish (increase of alcids; Österblom et al. 2006), reduced mussel stocks (common eider; Laursen & Møller 2014) or low reproductive success (long-tailed duck; Hario et al. 2009).

By-catch of waterbirds is typically occurring also in longline-fishing (Anderson et al. 2011) and the risk varying between species groups, but due to the very low overall effort of long-line fisheries in the Baltic Sea, and in the quasi-absence of data for these gears in the region, it is not considered further for HOLAS 3.

Also recreational fisheries using static nets, traps and long-lines contribute to by-catch of mammals and waterbirds. Their effort and spatiotemporal distribution as well as by-catch rates are largely unknown.

2.2 Policy relevance

The core indicator Number of drowned mammals and waterbirds in fishing gear addresses the Baltic Sea Action Plan’s Biodiversity and nature conservation segment’s ecological objectives ‘Viable populations of all native species’, ‘Natural distribution, occurrence and quality of habitats and associated communities’ and ‘Functional, healthy and resilient food webs’ as well as the management objectives ‘Human induced mortality, including hunting, fishing, and incidental by-catch, does not threaten the viability of marine life’ and ‘Reduce or prevent human pressures that lead to imbalance in the food web’ (Table 1).

Table 1. Policy relevance of the HELCOM core indicator Number of drowned mammals and waterbirds in fishing gear.

HELCOM Contracting Parties have agreed the following specific actions on by-catch: B8, B21, S43 to S49. These actions aim at i.a. implementing operational conservation measures and promoting effective mitigation measures to achieve the close to zero target for by-catch rates of relevant waterbird and mammal species by 2024, especially the Baltic proper population of harbour porpoise by 2022 and setting up conservation schemes for key seabird areas. Further aims are testing, promoting and introducing new technical and operational by-catch mitigation measures (with specific reference to alternative gear) and finally developing and implementing effective data collection for more reliable data on incidentally by-caught birds and mammals and fishing effort for which there has long been a legal obligation (specifically under the EU Birds and Habitats Directive, Common Fisheries Policy and the Data Collection Multiannual Programmes).

For the three seal species occurring in the Baltic Sea, the HELCOM Recommendation (27-28/2) adopted in 2006 relating to seals recommends:

• to take effective measures for all populations in order to prevent illegal killing, and to reduce incidental by-catches to a minimum level and if possible, to a level close to zero;
• to develop and to apply where possible non-lethal mitigation measures for seals to reduce incidental by-catch and damage to fishing gear, as well as to support and coordinate the development of efficient mitigation measures.

For harbour porpoise the HELCOM Recommendation 17/2, adopted in 1996 and updated in 2020, recommends:

• give highest priority to avoiding by-catches of harbour porpoises, particularly following the recommendations of ASCOBANS and the Jastarnia Plan, in order to achieve the ecological objective of the Baltic Sea Action Plan. By-catch of harbour porpoise, shall be significantly reduced with the aim to reach by-catch rates close to zero, recognizing that the Baltic Proper population of harbour porpoise is more threatened than the WBBK population;
• take action for collection and analysis of data on pressures such as by-catch, disturbance, including underwater noise, pollutants, changes in food base and prey quality, habitat deterioration, climate change, and human activities associated with the listed pressures;
• implementing effective and adequate protection measures for the species both inside and outside HELCOM MPAs.

The core indicator also directly or indirectly addresses the following qualitative descriptors of the MSFD for determining Good Environmental Status (European Commission 2008a; see also Table 1):

Descriptor 1: ‘Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions’ and

Descriptor 4: ‘All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity’,

and the following criteria of the Commission Decision 2017/848 (European Commission 2017a):

• Criterion D1C1 (mortality rate from by-catch)
• Criterion D1C2 (population abundance)
• Criterion D1C3 (population demographic characteristics)
• Criterion D1C4 (species distribution)
• Criterion D4C1 (diversity of trophic guild)
• Criterion D4C2 (balance of total abundance between trophic guilds)
• Criterion D4C4 (productivity)

While broad commitments have been made to achieve Good Environmental Status (GES) under the EU Marine Strategy Framework Directive (MSFD), and to Favourable Conservation Status (FCS) under the Habitats Directive, translating these goals into specific targets on by-catch limits under these legislations is as yet unspecified by the EU. However, the EU Regulation 2019/1241 on Technical Measures in Art. 3, 2.(b) formulates the aim to ensure that incidental catches of sensitive marine species, including those listed under Directives 92/43/EEC and 2009/147/EC, that are a result of fishing, are minimised and where possible eliminated so that they do not represent a threat to the conservation status of these species. The threshold setting for waterbirds (Evaluation Method 2) uses a legal interpretation of this in which ‘small numbers’ are defined as an approximation of ‘zero by-catch’, which acknowledges that small numbers of seabirds will probably still be caught even when the most effective mitigation measures are in place (see chapter 3).

The EU Habitats Directive lists the harbour porpoise as a strictly protected species (Annex IV) which requires Member States to establish a system of strict protection in their natural range. The harbour porpoise and the three seal species are further listed in Annex II, meaning that they are also to be protected by the means of the Natura 2000 network.

The EU Birds Directive aims to protect, inter alia, habitats of endangered and migratory birds to ensure their conservation in Europe (European Commission 2009). This not only refers to birds needing specific conservation measures (Article 4 (1)) and listed in Annex I (black-throated diver, red-throated diver, Slavonian grebe, Steller’s eider, smew), but also to all migratory species (Article 4 (2)). Therefore, all waterbird species breeding, wintering and staging during migration in the Baltic Sea are covered by this Directive.

EU legislation clearly requires Member States to take measures prohibiting deliberate killing or capture by any method (Article 5 Birds Directive; Article 12 Habitats Directive) which also includes the mere acceptance of the possibility of killing or capture (Case C‑221/04 Commission v Spain [2006] ECR I‑4515, paragraph 71).

Article 12, paragraph 4 of the Habitats Directive requires that Member States shall establish a system to monitor the incidental capture and killing of the animal species listed in Annex IV (a) (European Commission 1992). In the light of the information gathered, Member States shall take further research or conservation measures as required to ensure that incidental capture and killing does not have a significant negative impact on the species concerned. Member States of the EU are further obliged to develop national programmes for monitoring fisheries, including on board monitoring, under the EU Regulation 2017/1004 (European Commission 2017b). These programmes include detailed data on fleet capacity and fishing effort by metier and fishing area. The Commission Delegated Decision (EU) 2021/1167 (European Commission 2021) requires that by-catch is to be monitored for all marine mammal species protected under Annex II, IV and V of the Habitats Directive. Besides cetacean and seal species this also includes the Eurasian otter. Due to lack of data, a by-catch evaluation for the Eurasian otter needs to be taken forward to HOLAS 4. Further, with reference to the Birds Directive the Delegated Decision requires by-catch monitoring of all waterbird and seabird species, including migratory species. A proposed action in the Action Plan for reducing incidental by-catches of seabirds in fishing gears includes the monitoring of seabird incidental by-catch with a minimum coverage of 10% of the fisheries (European Commission 2012) which is far from being reached in relevant gears (ICES 2021).

As a voluntary instrument within the framework of EU and international environmental and fishery legislation and conventions, the EU Commission has adopted an Action Plan for reducing incidental by-catches of seabirds in fishing gears (European Commission 2012). It aspires to provide a management framework to minimise incidental by-catch by implementing effective mitigation measures as much as possible in line with the objectives of the EU Common Fisheries Policy (CFP), i.e. to cover all components of the ecosystem.

The Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) aims to achieve and maintain a favourable conservation status of small cetaceans. Six of the nine Baltic Sea countries are Parties to the Convention (Denmark, Germany, Sweden, Poland, Lithuania and Finland).

All waterbird species occurring in the Baltic Sea are subject of the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA), for which Denmark, Germany, Sweden, Lithuania, Latvia, Estonia and Finland are Contracting Parties.

The indicator supports the UN Sustainable Development Goal 14: ‘Conserve and sustainably use the oceans, sea and marine resources for sustainable development.’

2.3 Relevance for other assessments

The level of pressures affecting the status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of a complex issue. This indicator provides an evaluation of the numbers drowned mammals and waterbirds in fishing gear, and this information should be considered together with other biodiversity core indicator evaluations in order to achieve an overall assessment of the status of biodiversity, particularly once further developed.

The results of this indicator can be used for HELCOM integrated assessments (i.e. the BEAT integrated assessment tool).

Further, the results can be used for integrated assessments conducted by EU Member States for their reporting under Article 8 MSFD. According to the relevant guidance for waterbirds (European Commission 2022), the by-catch indicator is weighted equally to the criteria abundance (two indicators for the Baltic Sea) and demography (one indicator). For mammals, the same guidance gives the by-catch criterion the same weight as the other four criteria combined, using the “One-Out All-Out” principle. In this case, the criteria other than by-catch are integrated as in the Habitats Directive, i.e. out of the four parameters population, range, habitat, and future prospects three need to be favourable (and the fourth either unknown or favourable) to achieve favourable conservation status (equalling good status under MSFD).

The joint OSPAR-HELCOM workshop to examine possibilities for developing indicators for incidental by-catch of birds and marine mammals (September 2019, Copenhagen, OSPAR & HELCOM 2019) proposed the conservation objective ‘Minimise and where possible eliminate incidental catches of all marine mammal and bird species such that they do not represent a threat to the conservation status of these species’ to be further considered by HELCOM in work on the Baltic Sea Action Plan. An interim management objective could be ‘The mortality rate from incidental catches should be below levels which threaten any protected species, such that their long-term viability is ensured’. A quite similar wording is provided by the EU Commission Decision 2017/848 which says “The mortality rate per species from incidental by-catch is below levels which threaten the species, such that its long-term viability is ensured.”

The threshold proposals outlined here are based on the outcome and recommendations of the workshop, i.e. they are built on the expertise of 52 experts representing 20 countries and several organizations. They have been further developed by the HELCOM BLUES project, taking discussions between HELCOM Contracting Parties and on expert level into account. Limited availability of by-catch and effort data as well as knowledge on species demography parameters was hereby considered. In the development process leading towards HOLAS 3 it was noted that from a policy/management perspective there is a strong wish to have some sort of evaluation in place as this would promote future work.

Available methods for threshold setting are e.g., Removal Limit Algorithm (RLA), modified Potential Biological Removal (mPBR), and Population Viability Analysis (PVA). All three model-based threshold setting procedures require a quantitative objective. This quantitative objective aspires to maintain the assessment units at or above their Maximum Net Productivity Level (MNPL). In marine mammals, early analytical work places MNPL between 50% and 80% of the carrying capacity (K) (Wade 1998). In absence of an agreed conservation objective, for some seal populations, the international widely adopted conservation objective from the US MMPA was used, which assumes MNPL to be at least 50% of K (Wade 1998) and aims to ensure that the bycatch will not deplete the assessment unit and maintain it at or above MNPL after 100 years of exploitation with a probability of 0.95. For the Belt Sea harbour porpoise population, a control rule for deriving the threshold which had been adopted by OSPAR for the North Sea harbour porpoise population was used. The conservation objective used, which aims at restoring, with a probability of 0.8, the population to 80% of carrying capacity after 100 years, the assessment unit is to be maintained well above MNPL. Therefore, for seals we computed thresholds using the PBR approach (Wade 1998) and for harbour porpoise using mPBR (Genu et al. 2021). For waterbirds, the conservation objective is that by-catch mortality shall be below levels which threaten any protected species such that their long-term viability is ensured. Long-term viability is often interpreted to mean that the population size does not decrease more than 30% over three generations (Oliveira 2021, see also Indicator Breeding success of waterbirds).

Thresholds represent the upper limit to the sum of anthropogenic mortality beyond which conservation objectives will not be met (Figure 2). The threshold values derived are thus entirely dependent on the conservation objective to be achieved. Where threshold values were calculated, these are given in Tables 2 and 3.

Figure 2. Schematic representation of the threshold values applied in the ‘Number of drowned mammals and waterbirds in fishing gear’ core indicator (calculated threshold values are presented in Tables 2 and 3).

Table 2. Assessment unit specific threshold values applied to marine mammal populations in this indicator. The indicator is evaluated on the level of populations, which are allocated to scale 2 HELCOM assessment units. Thresholds derived from PBR or mPBR include all anthropogenic mortality such as hunted seals whereas the zero threshold is for by-catch only.

Table 3. Assessment unit specific threshold values applied to waterbirds in this indicator when Assessment Method 2 is applied. Annual adult mortality according to Bird et al. (2020). For data on bird numbers see Chapter 9.2

3.1 Setting the threshold value(s)

Threshold values for marine mammals

For the Belt Sea population of harbour porpoises it was agreed that the threshold should be derived using the mPBR method which is modified with respect to the conservation objective to allow recovery to and maintain the population at 80% or more of the carrying capacity in the long-term (100 years), with a probability of 80%. This objective is different from the PBR approach used in the frame of the US MMPA. In an MSE using the mPBR method, a threshold of 29 animals per year was derived (Owen et al. 2022). This value is based on the most recent abundance estimate of 17,301 harbour porpoises (95% CI = 11,695-25,688; CV = 0.20) from miniSCANS II in 2020 (Unger et al. 2021). Due to subsequent discussions about the input values for the mPBR approach after the threshold value has been proposed, further simulation trials were conducted and three scenarios for the accuracy of the by-catch estimate developed: ‘accurate’, ‘moderate’ (factor 2/3) underestimation and ‘severe’ (factor 2) underestimation. Threshold values for the whole population derived are 117, 73 and 58, respectively (Authier et al. 2022). A source of bias of available by-catch data is that an REM dataset is from fishing vessels voluntarily participating in the REM study (Glemarec et al. 2022). In the areas not covered by the REM, there is an issue with the quality of data consisting of reported by-catch from logbooks and strandings only (HELCOM EG MAMA 2022). These are orders of magnitude lower and hence can be considered minimum numbers at best. Thus, by-catch estimates can be considered systematically and severely underestimated in those countries which do not have a systematic by-catch monitoring whereas the REM provides more accurate estimates. Based on intersessional discussions contracting parties agreed to use a threshold value of 73 by-caught animals per year, the value for ‘moderate’ underestimation, for HOLAS 3 purposes.

The threshold for the harbour porpoise population of the Baltic Proper is set to zero by-catch due to the severe depletion of the population and its conservation status as critically endangered. The size of the population is estimated at only 491 individuals (95% CI: 71–1105) (Amundin et al. 2022).

The threshold for the ringed seal population of the Southwestern Archipelago Sea, Gulf of Finland and Gulf of Riga is also set to zero by-catch due to its conservation status as vulnerable on species level and the fact that the population is at high risk due to climate change as well as the small population size and range. The population is estimated at 1,800 animals (200 in the Archipelago Sea, 100 in the Gulf of Finland and 1,500 in Western Estonia; M. Ahola, pers.comm.).

For the population of the ringed seal in the Gulf of Bothnia the evaluation is based on the PBR approach adopted from the US MMPA. It must be noted that the US MMPA conservation objective differs from the one suggested during the OSPAR-HELCOM workshop to examine possibilities for developing indicators for incidental by-catch of birds and marine mammals. Assumptions were made on maximum reproductive potential of the population (R_max =0.10) and recovery factor (F_R =0.5) as well as a conservative measure of abundance for the management unit (N_min =17,744). The resulting PBR results in a threshold of 443 animals per year for total anthropogenic removal (i.e., hunting and by-catch).

The threshold for the population of the harbour seal in Kalmarsund is set to zero by-catch. This is due to its conservation status as vulnerable on species level and the small population size and isolated spatial distribution range. The abundance is estimated at 2,900 animals (M. Ahola, pers.comm.).

For the population of the harbour seal in the South-western Baltic and Kattegat the evaluation is based on the PBR approach adopted from the US MMPA. It must be noted that the US MMPA conservation objective differs from the one suggested during the OSPAR-HELCOM workshop to examine possibilities for developing indicators for incidental by-catch of birds and marine mammals. Assumptions were made on maximum reproductive potential of the population (R_max =0.12) and recovery factor (F_R =0.5) as well as a conservative measure of abundance for the management unit (N_min =13,917). The resulting PBR threshold is 417 animals per year for total anthropogenic removal (i.e., hunting and by-catch).

For the grey seal population the evaluation is based on the PBR approach adopted from the US MMPA. It must be noted that the US MMPA conservation objective differs from the one suggested during the OSPAR-HELCOM workshop to examine possibilities for developing indicators for incidental by-catch of birds and marine mammals. Assumptions were made on maximum reproductive potential of the population (R_max =0.10) and recovery factor (F_R =0.5) as well as a conservative measure of abundance for the management unit (N_min =53,232). The resulting PBR threshold is 1,330 animals per year for total anthropogenic removal (i.e., hunting and by-catch).

For all seal populations evaluated under the PBR method, hunted numbers must also be accounted for when assessing the impact of by-catch related takes on the population. Hunted numbers were taken from the HELCOM database (unpublished data). Reliable by-catch data from the assessment period 2016-2021 is not available. Thus, an evaluation using this method can only be made for grey seals for which a by-catch number for the year 2012 is available from interviews with fishers (Vanhatalo et al. 2014) and an assumption for the actual by-catch numbers has been made. Whereas the number of legally hunted seals is considered accurate, the by-catch number appears to be severely underestimated. The underestimation factor is likely much larger than 2 (i.e. true mortality twice estimated mortality) which is the maximum factor considered in the PBR approach (Wade 1998). There may be other sources of anthropogenic mortality not considered here.

Threshold values for waterbirds

The joint OSPAR-HELCOM workshop to examine possibilities for developing indicators for incidental by-catch of birds and marine mammals proposed a threshold derived from the conservation objective to ‘minimise and eliminate by-catch where possible’ (OSPAR & HELCOM 2019). This objective aligns with the prohibition of deliberate killing or capture of birds according to Article 5 of EU Directive 2009/147/EC (Birds Directive). It is also aligned with the conservation target of the EU Action Plan for reducing incidental catches of seabirds in fishing gears (COM(2012) 665), which requests Member States to ’minimize and, where possible, eliminate the incidental catches of seabirds’.

Following BirdLife International (2019), the OSPAR-HELCOM workshop proposed a value of 1% of natural annual adult mortality as an approximation of ‘zero by-catch’, which acknowledges that small numbers of seabirds will probably still be caught even when the most effective mitigation measures are deployed. The 1% value is derived from legal interpretations in European Court of Justice of ‘small numbers’^[1] and EU Commission stemming from the EU Birds Directive and EU guide to sustainable hunting (European Commission 2008b). Since for most species it is extremely difficult to identify natural annual adult mortality in the presence of anthropogenic mortality, it is more feasible to use total annual adult mortality as an approximation. Annual adult mortality m was calculated from survival rates s as

m = 1 – s

The survival values of adult individuals from which the mortality calculated for all bird species can be found in the literature, e.g. Bird et al. (2020).

Based on this, the indicator evaluates by-catch against the conservation objective ‘The mortality rate from incidental catches should be below levels which threaten any protected species, such that their long-term viability is ensured’. As by-catch mortality for most species is one out of several pressures acting cumulatively and directly decreases the population size, it can have negative impact on the population development especially for threatened species. Therefore, a precautionary approach is applied to species identified as vulnerable, endangered or critically endangered on the HELCOM Red List (HELCOM 2013). The evaluation of by-catch mortality includes the following three steps (see also Figure 3):

1. Wherever sufficient data are available population modelling will be used to determine if the fishing-induced mortality threatens the long-term viability of seabird populations (Evaluation Method 1 in Figure 3). In accordance with IUCN Red List criteria, ‘long-term’ is defined as a three-generation timespan (Oliveira 2021). A percentage of maximum acceptable decline for each species/population under scrutiny during this period is yet to be determined. If this threshold value is exceeded for the specific species/population, the indicator is considered failing. This method has not been used here due to lack of sufficient data.

2. If population modelling is not possible in species/populations classified as vulnerable, endangered, or critically endangered on the HELCOM Red List, an alternative threshold will be used, corresponding to a reference value of 1% of the total annual adult mortality of the considered species/population (Evaluation Method 2 in Figure 3). The species-specific threshold value of Evaluation Method 2 (TV(2)) is estimated from multiplying the point estimate of the number of birds in the evaluation area N with the species-specific annual adult mortality rate m and 1%:

TV(2) = N * m * 0.01

where N is the estimated population size in the HELCOM subdivision, and m is the annual mortality of adults of the species/population. Subdivisions of the Baltic Sea used for waterbird indicator evaluations are described in Chapter 9.1. Threshold values used for Evaluation Method 2 in this indicator are presented in Table 2.

In case by-catch data for a species/population listed on the HELCOM Red List are insufficient to assess against the reference value TV(2), but this species/population is known to be susceptible to by-catch in fisheries and there is a spatio-temporal overlap between species/population occurrence and the respective fishing method(s) causing by-catch, then the considered species/population fails the indicator (Evaluation Method 3 in Figure 3). In this case, by-catch monitoring would have to be intensified to provide evidence that incidental captures for that species/population are below TV(2). This procedure implements the precautionary approach. For data-poor species which are not classified as vulnerable, endangered, or critically endangered on the HELCOM Red List, no status evaluation for by-catch mortality enters the indicator.

If specific model-based threshold values cannot be evaluated due to a shortage in demographic and/or by-catch data, contracting parties must strive to improve by-catch monitoring and by-catch mortality evaluations, and to reduce by-catch rates aiming to reach values close to zero, as committed to in the Baltic Sea Action Plan.

The threshold setting is identical to the OSPAR candidate indicator B5 Marine Bird By-catch. It should be noted that for birds other evaluation methods have either not yet been explored (RLA) or have at most limited applicability (PBR: O’Brien et al. 2017, Marchowski et al. 2020).

Figure 3. Schematic illustration of the evaluation of waterbird by-catch in fishing gear in the Baltic Sea. Numbers denote the sequence of applicable Evaluation Methods 1, 2 and 3, depending on data availability.

4.1 Status evaluation

With reference to the period 2016-2021 it was possible for the first time to evaluate the number of by-caught marine mammals and waterbirds in the Baltic Sea. Evaluations were possible for at least one population in each of four species of marine mammals (harbour porpoise, ringed seal, harbour seal, grey seal). Waterbirds were evaluated on the geographical scale of subdivisions (aggregated sub-basins), with evaluations available for a total of 11 species in four subdivisions. In all marine mammal populations and those waterbird species considered, the status was sub-GES because the thresholds for good status were exceeded.

Marine mammal by-catch in the Baltic Sea

In cases where scientific by-catch evaluations report an annual by-catch number, this was taken to compare against the threshold. If reported by-catch numbers based on log-books and strandings (HELCOM EG MAMA 2022) were used for the evaluations, these can be assumed underestimating the real by-catch numbers, and thus the highest annual value in the assessment period (2016-2021) was taken. Hunted seal numbers fluctuated during the assessment period and thus the highest annual numbers were taken for a worst case for those evaluations where the sum of by-catch and hunted numbers are compared against a PBR value. Hunted numbers could not be assigned to a population because genetic data are lacking. However, it was assumed that for populations with abundance below the Limit Reference Level no hunting permits were issued by authorities in accordance with HELCOM Recommendation (27-28/2).

For the harbour porpoise population of the Kattegat, Belt Sea and Western Baltic, two available datasets are from fishing vessels voluntarily participating in an REM study in the Danish static net fishery and by-catch reported by fishermen or found during autopsies of stranded animals (as reported to HELCOM EG MAMA). Based on 2010-2019 data the yearly average by-catch in the Danish fishery including ICES areas IIIa21 (Kattegat), IIIb23 (Øresund) and IIIc22 (Belt Sea) is 776 (95% CI: 539-1,044) animals per year. As fishermen were participating voluntarily, the sampling scheme is not random and by-catch numbers may be under- or overestimated (Glemarec et al. 2022). Reported numbers of 29 animals were taken from the year 2016 which had the highest reported values. The underestimation factor here is likely much higher as not all harbour porpoise by-catches may be reported. The sum of both was compared against the threshold of 73 (derived using mPBR method). The sum is 805 which means that the status is sub-GES.

NAMMCO & IMR (2019) estimated by-catch numbers for the harbour porpoise population of the Baltic Proper in a precautionary way from the upper limit of the 95% confidence interval of a by-catch rate for the Belt Sea population, adjusted for the lower density in the Baltic Proper, and multiplied with reported static net fishing effort (métiers GNS and GTR) within ICES sub-areas 25-29 during the years 2009-2017. Data on minimum by-catch was also compiled from records of strandings and voluntary by-catch reports for the years from 1984 to 2012. By-caught numbers derived from strandings are most likely an underestimation of the total number. The estimated by-catch number for 2017 is 7 animals, and the minimum by-catch numbers for the years 2000-2012 is on average approximately 3 animals per year. Thus, the status of this population is sub-GES.

For the ringed seal population of the Southwestern Archipelago Sea, Gulf of Finland and Gulf of Riga the reported by-catch number for 2017 was 3 animals which can be considered a minimum estimate only. This exceeds the threshold of zero by-catch. Thus, the status of this population is sub-GES.

For the ringed seal population of the Gulf of Bothnia the number of annually hunted animals in the assessment period is between 176 and 597. The highest reported by-catch number is 8 animals (2017) which can be considered a minimum estimate only. In the years 2019, 2020 and 2021 the hunted numbers alone were higher (538, 597 and 568, respectively) than the PBR threshold of 443 animals. From a worst-case evaluation the status is sub-GES. By-catch data need to be improved for future evaluations.

For the harbour seal population in Kalmarsund the reported by-catch number for the period 2016 to 2020 is 2 or 3 animals each year which is compared against the threshold of zero by-catch. Thus, the status of this population is likely sub-GES. Sweden reports by-catch in terms of West coast and Baltic Proper. However, even Baltic Proper by-catch cannot be assigned to this population with a high degree of certainty as by-caught animals often are juveniles and sometimes by-caught outside the normal range of the population. However, sporadic data from the 1990s suggest that approximately 20 pups were caught annually in fyke nets set for eel, but modified gear and changed structure of the coastal fisheries were suggested to have reduced by-catches (Härkönen & Isakson, 2010). Since then, the number of seals has grown (HELCOM 2018). From this and the by-catches reported to HELCOM EG MAMA it appears realistic that the by-caught number is above zero. Subsequently the status is sub-GES.

For the harbour seal population of the South-western Baltic and Kattegat the number of hunted animals is between 88 and 380 with a maximum in 2018. The reported by-catch number for 2020 is 2 animals. Based on 2010-2019 Remote Electronic Monitoring data the yearly average by-catch in the Danish fishery including ICES areas IIIa21 (Kattegat), IIIb23 (Øresund) and IIIc22 (Belt Sea) is 370 (95% CI: 106-731) seals per year (Glemarec et al. 2022). However, REM could not be utilised to its full potential as there were challenges with species identification of juveniles in the video footage and thus it is not distinguished between species. However, due to low confidence in reported seal numbers and missing species identification in REM data the status is not evaluated.

For the grey seal population of the Baltic Sea the annual number of hunted animals is between 465 (2016) and 1,717 (2021), with an average number of 1,065. The highest annual reported by-catch number is 35 animals (2017). However, due to low confidence this data does not allow to assess the true status with any of these options. Thus, an estimation based on earlier and more realistic data was made. Vanhatalo et al. (2014) used 2012 data based on interviews with fishermen in Estonia, Finland and Sweden (east coast N of Kalmar) for a more realistic by-catch evaluation. Taking the possible underreporting into account the posterior mean of the total by-catch in that year is between 2,180 and 2,380. The by-catch in the study area was likely to represent at least 90% of the total yearly grey seal by-catch in the Baltic Sea. Reduced fishing effort since 2012 combined with an increased population size suggests a by-catch number in the same order of magnitude for the assessment period. Given the average number of annually hunted grey seals in the assessment period, this is only 265 animals below the PBR threshold. The likely magnitude of by-catch inferred from Vanhatalo et al. (2014) is much higher. In the period 2019 to 2021 the hunted numbers alone exceeded the PBR threshold. Thus, the status is sub-GES.

The by-catch evaluations for marine mammal populations are summarised in Table 4. In Table 5 these evaluations are allocated to the HELCOM sub-basins in which the respective populations occur. Using a One-Out All-Out approach all sub-basins are in sub-GES with respect to by-catch of marine mammals.

Table 4. Overview of marine mammal by-catch evaluations per species and population. Note that the PBR-based evaluation method includes annual numbers of seals hunted.

Table 5. Marine mammal by-catch evaluations allocated to sub-basins and integrated assessment per sub-basin.

Waterbird by-catch in the Baltic Sea

Numbers of waterbirds by-caught in fishing gear during the assessment period 2016-2021 are available from Denmark (Larsen et al. 2021, Glemarec et al. 2022), Poland (see below) and Lithuania (Morkūnas et al. 2022). Nevertheless, it was not possible to apply Evaluation Method 1 because by-catch data were not available across the entire range of the respective bird populations. Using the example of the greater scaup, Marchowski et al. (2020) have demonstrated how Evaluation Method 1 including a PVA can be applied to waterbird populations in the Baltic Sea. However, part of the data used in that study are from before the assessment period. Therefore, it was not considered for an evaluation here.

Evaluation Method 2 could also not be used in most cases because no data on the number of birds present in the area with the by-catch data were available. However, where by-catch occurred it was possible to apply Evaluation Method 3 because evidence for by-catch events is in place. Observed waterbirds and static nets during ship-based surveys allow assessing the threshold of overlapping occurrence of waterbirds and fishing gear in German waters. Though these assessments are allocated to Baltic Sea subdivisions it has to be noted that they usually cover only part of the respective subdivision.

Waterbird by-catch in the assessment unit Kattegat

Evaluation Method 3 – Denmark

The risk-mapping approach conducted by Glemarec et al. (2022) revealed elevated by-catch risk in static nets in the Kattegat, namely north of the coast of Sjælland (Figure 4). This elevated by-catch risk is based on observed by-catch of common eiders and thus constitutes evidence of spatio-temporal overlap of a fishing method causing by-catch and the occurrence of a HELCOM red-listed species according to Evaluation Method 3. In this indicator the status for common eider is at sub-GES.

The average yearly by-catch estimate for common eider in the westernmost Danish section of the Baltic Sea, including the Kattegat, the Sound, the Belts and part of Bay of Mecklenburg is 2623 birds (95% C.I. 1847-3567 birds) (Glemarec et al. 2022).

Figure 4: Quarterly by-catch risk (no unit) for common eider in the Danish commercial gillnet fleet, from model predictions using electronic monitoring data (2010-2019). Taken from Glemarec et al. (2022).

Waterbird by-catch in the assessment unit Belt Group

Evaluation Method 3 – Denmark

The risk-mapping approach applied by Glemarec et al. (2022) revealed elevated by-catch risk in static nets in the Belt Group, namely in the Great Belt (Figure 4). This elevated by-catch risk is based on observed by-catch of common eiders and thus constitutes spatio-temporal overlap of a fishing method causing by-catch and the occurrence of a HELCOM red-listed species according to Evaluation Method 3. In addition, Larsen et al. (2021) report by-caught waterbirds from The Sound for the period 2017 to 2019, which include the following red-listed species: common eider, common scoter, velvet scoter and black-throated diver. Again, these by-caught events are treated as evidence for spatio-temporal overlap of a fishing. Thus, the four species are considered to be at sub-GES. Beyond that the two studies cited list species not red-listed and therefore not evaluated by this method (great cormorant, great crested grebe, razorbill and common guillemot as well as undetermined mergansers and gulls).

The average yearly by-catch estimate for common eider in the westernmost Danish section of the Baltic Sea, including the Kattegat, the Sound, the Belts and part of Bay of Mecklenburg is 2623 birds (95% C.I. 1847-3567 birds) (Glemarec et al. 2022).

Waterbird by-catch in the assessment unit Bornholm Group

Evaluation Method 3 – Denmark

The risk-mapping approach applied by Glemarec et al. (2022) revealed elevated by-catch risk in static nets in the northwestern area of the Bornholm Group, namely south off the coast of Lolland and south of the Sound (Figure 4). This elevated by-catch risk is based on observed by-catch of common eiders and thus constitutes evidence for spatio-temporal overlap of a fishing method causing by-catch and the occurrence of a HELCOM red-listed species according to Evaluation Method 3. In this indicator the status for common eider is at sub-GES.

The average yearly by-catch estimate for common eider in the westernmost Danish section of the Baltic Sea, including the Kattegat, the Sound, the Belts and part of Bay of Mecklenburg is 2623 birds (95% C.I. 1847-3567 birds) (Glemarec et al. 2022).

Evaluation Method 3 – Germany

During the wintering season from November to April (2016-2021), the spatial extent of static net fishing overlapped with occurrence of the HELCOM red-listed species red-throated diver, black-throated diver, red-necked grebe, Slavonian grebe, greater scaup, common eider, long-tailed duck, common scoter, velvet scoter, red-breasted merganser and black guillemot in 5×5 km grid squares (Figures 5 to 15). It is well known from earlier studies that these species are by-caught in static nets in the German section of the Baltic Sea (Schirmeister 2003, Erdmann et al. 2005, Bellebaum & Schirmeister 2012). Therefore, these species are evaluated as being in sub-GES.

Figure 5: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of greater scaup, November to April (2016-2021).

Figure 6: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of common eider, November to April (2016-2021).

Figure 7: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of velvet scoter, November to April (2016-2021).

Figure 8: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of common scoter, November to April (2016-2021).

Figure 9: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of long-tailed duck, November to April (2016-2021).

Figure 10: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of red-breasted merganser, November to April (2016-2021).

Figure 11: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of red-necked grebe, November to April (2016-2021).

Figure 12: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of Slavonian grebe, November to April (2016-2021).

Figure 13: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of black guillemot, November to April (2016-2021).

Figure 14: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of red-throated diver, November to April (2016-2021).

Figure 15: Distribution of static net fishing according to recorded marking flags during bird surveys and distribution of black-throated diver, November to April (2016-2021).

Evaluation Method 2 – Poland

Summing up the average numbers per waterbird species in the five winter seasons 2015/16 – 2019/20, the mean estimated total abundance of all diving waterbirds in the Polish section of the Bornholm Group is 600,845 individuals, of which 94.2% are benthivorous ducks. The mean by-catch estimate for this period is 5,056 birds for October to April. Evaluation Method 2 of the indicator was applied to four benthivorous duck species (greater scaup, long-tailed duck, common scoter, velvet scoter). For each species, the estimated by-catch was higher than the number of birds corresponding to 1% of annual adult mortality. Therefore, these species do not achieve the threshold for good status and represent sub-GES (Table 6).

Table 6. Evaluation of by-catch mortality of waterbirds in the Polish EEZ (Bornholm Group) based on the estimated by-catch (Evaluation Method 2). For the derivation of the threshold value see Table 3.

Waterbird by-catch in the assessment unit Gotland Group

Evaluation Method 2 – Poland

The estimated total abundance of all diving waterbirds in the Polish section of the Gotland Group is 207,114 individuals (mean of the five winter seasons 2015/16 to 2019/20, min.: 186,363, max.: 237,536), of which 82.2% are benthivorous ducks. The average by-catch estimate for these seasons is 7,921 birds from October to April. Evaluation Method 2 of the indicator was applied to four benthivorous ducks (greater scaup, long-tailed duck, common scoter, velvet scoter). In all species, the estimated by-catch was exceeding the number of birds corresponding to 1% of annual adult mortality. Therefore, these species failed to achieve the threshold for good status and represent sub-GES (Table 7).

Table 7. Evaluation of by-catch mortality of waterbirds in the Polish EEZ (Gotland Group) based on the estimated by-catch (Evaluation Method 2). For the derivation of the threshold value see Table 3.

Evaluation Method 3 – Lithuania

An extensive study of waterbird by-catch in static nets was conducted in Lithuania from October 2015 to May 2020, i.e. nearly completely falling into the HOLAS 3 assessment period (Morkūnas et al. 2022). A total of 905 by-caught birds provides evidence of spatio-temporal overlap between waterbird occurrence and static net fishing effort distribution. This includes the HELCOM red-listed species greater scaup, long-tailed duck, common scoter, velvet scoter, red-breasted merganser, red-necked grebe, red-throated diver and black-throated diver, for which this overlap constitutes sub-GES. In addition, some species not red-listed were by-caught (common goldeneye, common merganser, great cormorant, great crested grebe, herring gull, razorbill and common guillemot).

Waterbird by-catch in the assessment units of the Baltic Sea – overview

The results obtained by the help of Evaluation Methods 2 and 3 in four subdivisions of the Baltic Sea are summarised in Table 8. All evaluations are indicating sub-GES for the red-listed waterbird species examined. In all subdivisions evaluated at least one species evaluation is showing sub-GES. Therefore, regarding by-catch GES is not achieved in any subdivision.

Table 8. Overview of waterbird by-catch evaluations per species and subdivision and integrated evaluation per subdivision. AM/C: Evaluation method / country.

* Red-list categories: CR critically endangered, EN endangered, VU vulnerable.

4.2 Trends

Due to reduced fishing opportunities of cod and herring since 2018 and the prohibition of all targeted fishing for Western Baltic cod implemented since 2019, there was likely a decreased effort in commercial static net fisheries in parts of the region. The unresolved conflict between certain fisheries and bird and mammal species remains difficult to tackle. Scarcity of by-catch data coupled with incomplete knowledge on fishing effort as well as unavailable conservation objectives call for a consequent application of the precautionary principle. In this evaluation, with respect to by-catch and fishing effort some assumptions had to be made as the current inadequate data collection of by-catches and reporting of effort does not allow nearly precise estimates (see Chapter 5, confidence).

By-catch of harbour porpoises in the Baltic Proper was reportedly high before the 1970’s. Ropelewski (1957) reported for the Polish fishery annual by-catches between 16 and 250 porpoises (period 1922-1924) and between 23 and 114 porpoises (period 1928-1932). Lindroth (1962) reported 49 by-catches in Swedish salmon driftnet fisheries during a single year. Current lower by-catch numbers reflect the steep population decline since then (Koschinski 2002). This shows that a trend based by-catch evaluation would not reflect the status well. For the Belt Sea harbour population and the evaluated seal populations no reliable baseline data on by-catch exists.

By-catch of waterbirds in fishing gear, especially in static nets, is well known in the Baltic Region since at least the 1920s, when for example numerous black-throated divers were reported to be caught in salmon drift nets (Schüz 1935).

Trends in the amount of mammal and waterbird by-catch are currently not available because

1. there is no earlier evaluation available which could be used for comparison,
2. many studies were running only for a short time,
3. monitoring of waterbird and mammal by-catch is often insufficient, because the métiers responsible for by-catch are not covered adequately,
4. monitoring using modern techniques (e.g., electronic monitoring with camera) is relatively new and cannot provide long data series yet.

No prior evaluation has been applied for this indicator. Therefore it is not possible to directly compare status between assessment periods. Based on information from literature about the distribution of marine mammals and waterbirds as well as on by-catch in fishing gear it would be expected that no change in status category has occurred between HOLAS 2 (2011-2016) and HOLAS 3 (2016-2021), i.e. both periods would not achieve the threshold values and thus not be in GES.

4.3 Discussion text

Evaluations in this indicator have shown that marine mammals and waterbirds are generally not achieving good environmental status regarding additive mortality from by-catch in fishing gear, given the existing hunting mortality for some assessment units. This in turn applies to all HELCOM sub-basins (mammals) and subdivisions (birds) evaluated. Therefore, by-catch mortality is an ongoing and widespread threat for these populations. Moreover, most of the mammal populations and all bird populations dealt with here are of conservation concern. In these species/populations by-catch is one threat continuing to contribute to further decline and/or inhibiting recovery towards favourable conservation status. PBR- and mPBR-derived thresholds for marine mammals show that already small numbers of by-caught animals are problematic for marine mammal populations, and these low thresholds are exceeded. All waterbird species evaluated are already classified as vulnerable, endangered or even critically endangered by HELCOM (2013), therefore it is possible that the by-catch mortality prevents from improving status or continues to deteriorate the state of the populations.

Unfortunately, a severe lack of data on by-catch and fishing effort prevents undertaking a more exact examination of the true extent and the impact of by-catch on the populations. For waterbirds, Evaluation Method 1 intends to apply population modelling in order to quantify the impact of by-catch mortality on population growth. If sufficient by-catch and fishing effort data are available, such an approach is feasible on the level of bird populations, as has been shown for a benthivorous duck species, the greater scaup (Marchowski et al. 2020).

In all thresholds based on population modelling, a conservation objective needs to be defined and all anthropogenic mortality be taken into account. In waterbirds, the main other causes for direct anthropogenic mortality are hunting and oiling (Mooij 2005, Larsson & Tydén 2005, Žydelis et al. 2006). In seals, hunting needs to be included in the evaluation, and in all marine mammals direct mortality by impulsive noise e. g., from underwater explosions (Siebert et al. 2022). The latter might also be relevant for waterbirds (see Danil & St. Leger 2011), but this has been investigated to a lesser extent than in mammals. Further, compromised fitness (e.g. reduced reproductive potential and survival rate associated with disturbance, habitat alteration, induced for example by overfishing or coastal development, and accumulation of pollutants) can further add to the causes of anthropogenic mortality in waterbirds and marine mammals.

The HELCOM Roadmap on fisheries data in order to assess incidental by-catch and fisheries impact on benthic biotopes in the Baltic Sea describes the data needs with respect to by-catch monitoring and reporting of fishing effort, as is also outlined in chapter 8.

The overall confidence is low. Table 9 presents an evaluation of the confidence in four categories.

1. Accuracy of estimate: A compliance check would allow showing a clear signal whether GES has been achieved or not (‘high’), show general GES achievement but with some outliers and variation in the data (‘intermediate’) or only show GES achievement with only a probability <70% (‘low’). This scoring based on expert opinion was used for the HOLAS3 BEAT Tool in case data does not allow calculation of a standard error.
2. Temporal coverage: This is a measure of the temporal coverage of the assessment period. By-catch is subject to year-to-year variation. If monitoring data covers all six years the confidence is ‘high’, for three or four years of data ‘intermediate’ is chosen and otherwise ‘low’.
3. Spatial representability: This is a measure of the spatial coverage with respect to HELCOM sub-basins. If monitoring data is considered to cover the full spatial variation of the indicator parameter in the assessment area (covering at least 90% of the variation) the confidence is ‘high’. For 70 to 89% of the variation ‘intermediate’ is chosen and otherwise ‘low’. The choice was made on the basis of expert knowledge.
4. Methodological confidence: This relates to quality of the monitoring and whether it is according to existing HELCOM or other internationally accepted guidelines (‘high’), whether the data is from mixed sources partly quality assured (‘intermediate’) or data not collected according to guidelines or not quality assured (‘low’).

Future work will require to address these uncertainties specifically when better data are available.

Table 9. Overview of confidence for the evaluation carried out.

In the Baltic Sea, marine mammals and waterbirds are exposed to a number of pressures from various human activities, both directly and indirectly (Table 10). The pressures act variably with regards to seasons, but the effects are cumulative and include carry-over effects from one season to another. The most relevant in this indicator is the “extraction of, or mortality/injury to, wild species (by commercial and recreational fishing and other activities)”, which is directly linked to fishing by static nets and traps, but to a lesser degree also to longline fishing and trawling.

Marine mammal populations suffer from by-catch, most often in combination with threats from other activities. It is difficult to assign which activity adds to what extent to population effects. In particular, many pressures (such as contaminants, disturbance, prey depletion, habitat degradation or habitat loss) are indirect as they affect the viability but do not result in direct mortality. By-catch, hunting of seals or underwater explosions cause direct mortality and the effect on the population is evident in terms of a reduction in the numbers of individuals. Since marine mammals have a late sexual maturity and produce only a low number of offspring (at maximum one per year), they are extremely vulnerable to anthropogenic pressures.

Waterbird populations suffer from the extraction of individuals due to by-catch, but some species are still under pressure from hunting. Loss of individuals can also occur from collisions with offshore wind turbines. At many breeding sites waterbirds suffer from predation by non-indigenous mammalian predators, but also by disturbance from leisure activities. Foraging habitats are downsized due to avoidance of offshore wind farms, but are also physically disturbed by bottom-trawling fishery and aggregate extraction and lost where wind turbines are placed, especially for benthic feeding seaducks. The food supply is influenced by manipulating fish communities by fishing and the input of nutrients. Harmful substances from various sources impair health and body condition of waterbirds, and oil released during shipping leads to plumage contamination, often followed by the death of the affected individuals. It is unknown yet whether impulsive underwater noise as released during the pile driving for offshore wind turbines is threatening the health of diving waterbirds.

Table 10. Brief summary of relevant pressures and activities with relevance to the indicator.

There are two important aspects of possible impact of climate change related to this indicator. The first involves a likely spatiotemporal shift of fisheries (maybe also combined with the use of other gears) and of mammal or waterbird distribution, both related to availability of fish and/or prey and ice-free water, which would in turn affect the by-catch risk. The other is related to a possible reduced fitness of species/populations due to e.g., reduced availability of prey of a suitable quality and quantity. This in turn would negatively affect the population. Then greater efforts would be needed to preserve the population, also with respect to reducing by-catch.

The ringed seal population of the Southwestern Archipelago Sea, Gulf of Finland and Gulf of Riga is already suffering serious impact of climate change. Availability of suitable breeding ice is known to affect pup survival. Reduced ice cover severely limits the population’s growth rate (Sundqvist et al. 2012). At the same time reduced ice cover opens new fishing opportunities in winter which may increase the by-catch risk. All anthropogenic pressures will need to be consequently reduced in order to compensate for the reduced or even negative population growth.

Distribution shifts of fish populations (Heath et al. 2012) and reduced recruitment of fish species (Polte et al. 2021) caused by climate change are already being reported leaving stocks with a lesser resilience to climate-driven changes. Distribution shifts of prey may be partly compensated for by mammals and waterbirds by shifting their distribution range as well which might have implications for the risk of being by-caught. A reduced availability of suitable quantities and quality of important prey species for mammals and waterbirds by climate change and/or overfishing likely will affect their overall fitness. In the North Sea it has been shown that feeding on prey of lesser quality reduces the fitness of harbour porpoises and leaves them starving even with filled stomachs (Leopold et al. 2015). Prey energy density has been shown to govern harbour porpoise reproductive success (Ijsseldijk et al. 2021).

Due to higher winter air temperatures and consequently less ice cover of the Baltic Sea in winter (HELCOM & Baltic Earth 2021, Meier et al. 2022), many waterbird species have been shifting their winter distribution northeastwards – including also diving species such as common goldeneye, greater scaup and smew (Pavón-Jordán et al. 2015, 2019, Marchowski et al. 2017). This not only leads to longer presence of a larger number of waterbirds prone to by-catch in the Baltic Sea, but also fisheries are less restricted by sea ice, so that the exposure of waterbirds to mortality is likely to have increased. Further, due to distributional shifts waterbirds overwinter in increasing numbers in unprotected areas (Pavón-Jordán et al. 2020). Thus, a mismatch between winter distribution and protected areas may have arisen, with possible consequences for measures to prevent by-catch, which need to be adapted spatially and temporally. A higher variability in winter temperatures and ice covered areas might also lead to a higher variability in the use of wintering areas making it difficult to tailor specific spatiotemporal mitigation measures.

Population-scale impacts of ocean acidification on fish can be assumed for the future which will likely have dramatic effects on the ecosystem and also on fisheries. How this impact will be related to this indicator is, however, even more speculative than the effects highlighted above.

This indicator provides evaluations for marine mammals and waterbirds regarding the link between their conservation status and the loss of individuals from populations due to by-catch in fishing gear. In all cases examined, the threshold for good status was not met, meaning that too many individuals are lost. This has implications for efficient measures to be taken in order to achieve a good status of biodiversity in the Baltic Sea.

Poor data has been hampering the evaluation throughout. It has not been possible yet to relate the amount of by-catch to the management objective that by-catch is not threatening the viability of populations directly, because data on both fishing effort and by-catch of mammals and birds was not available due to inadequate recording of fishing effort and insufficient by-catch monitoring. However, in the case of threatened cetacean and seal populations with a very low number of individuals left, a strict threshold value of zero by-catch was exceeded, indicating that intolerable loss of individuals occurred. This situation is certainly also relevant to other (larger) populations of marine mammals and waterbirds, as was shown by the example of the greater scaup (Marchowski et al. 2020). Establishing effective monitoring of fishing effort and by-catch is needed in order to allow more precise evaluations as in this example.

With respect to by-catch monitoring there are large differences between countries and the data quality achieved. Dedicated by-catch surveys and Remote Electronic Monitoring using cameras produce a high data quality if they are conducted in a representative manner including all relevant fishing métiers. Onboard observers in the frame of the EU Data Collection Framework (DCF) can also produce high quality data. However, this requires a protocol which takes specific needs of by-catch monitoring guidelines into account as observers normally focus on the commercial fish catch. Monitoring effort in general needs to be increased to allow robust evaluations. ICES (2018b) showed that métiers relevant for waterbird and mammal by-catch are relatively under-sampled whereas other métiers which have less or no by-catch are over-sampled.

In order to assess by-catch numbers from by-catch rates (derived from by-catch monitoring), it is extremely important to have reliable effort data in all relevant métiers, which is currently not the case. Whereas large vessels have VMS and report their fishing effort in their (electronic) logbooks, smaller vessels do not report their effort in a comparable way. In some countries, fishers are only required to keep sales notes, other countries require monthly journals and even others coastal logbooks. Effort might be given in different metrics (days at sea, hours fished, gear dimensions x time, etc.). The European Commission and Member States are aware of this, but improving legislation is difficult and coordinating CPs is also difficult because a solution would require additional resources. More detailed explanations are given in the HELCOM Roadmap on fisheries data.

In some cases however, monitoring would need a full coverage of fisheries because populations are so depleted that even very low by-catch numbers which are hard to detect further threaten the population. In these cases implementing effective mitigation measures such as time-/area closures, gear restrictions or technical measures are a matter of urgency.

8.1 Future work or improvements needed.

Monitoring effort in general needs to be increased to allow robust evaluations. Important aspects are a sufficient and representative coverage of all métiers and all fleet segments at a relevant temporal scale. REM has been shown to be a cost-effective method for by-catch monitoring (Kindt-Larsen et al. 2012) which can deliver robust by-catch estimates based on high-quality data (Larsen et al. 2021, Glemarec et al. 2022). Onboard observers in the frame of the EU Data Collection Framework (DCF) can also produce high quality data. However, this requires a protocol which takes specific needs of by-catch monitoring guidelines into account as observers normally focus on the commercial fish catch. This is a major drawback as fisheries producing highest by-catches in the Baltic Sea are less in the focus of observer programmes. Observer coverage needs to be corrected if observers are engaged with other duties (e.g., measuring fish under deck) (ICES 2018a). Reporting of by-catches in log-books (self-reporting) or port controls are the least reliable method and they do not account for fishing effort, meaning that they do not allow extrapolating results to the effort of the whole fleet. Previously, logbooks did not even have a field to report by-catches of mammals and seabirds. Thus, self-reporting and port controls do not allow indicator evaluations. A detailed analysis of improvements regarding data availability and quality can be found in the HELCOM Roadmap on fisheries data in order to assess incidental by-catch and fisheries impact on benthic biotopes in the Baltic Sea.

All uncertainties identified show that sufficient monitoring of by-catch, fishing effort, population size, trend analyses and other sources of anthropogenic mortality are a prerequisite for getting a more reliable evaluation. The European Commission has included by-catch monitoring of protected bird and mammal species in the Commission Delegated Decision (EU) 2021/1167. Further participation of HELCOM Contracting Parties on a regional scale is necessary for the implementation process in order to ensure suitable monitoring methods and sufficient coordinated coverage, as well as effort monitoring, are developed into meaningful parameters (static net fishing effort must be measured in length of nets * soak time, see Monitoring Requirements, Description of optimal monitoring). But also the effort must be given as Days at Sea in order to enable comparisons with earlier years. So far, only fishing effort from logbooks and VMS data can be used for by-catch extrapolations from observer or Remote Electronic Monitoring data (ICES 2021). The additional effort by small commercial vessels for which only monthly journals, landing declarations or sales notes are available without precise information about the spatial distribution of fishing effort and their temporal extent as well as effort by recreational fishermen must be estimated and taken into account. Then the uncertainty in the fishing effort estimates which underlie the by-catch estimate needs to be specified by also adding a 95 % confidence interval.

Nevertheless, in the absence of high-resolution data on effort and bycatch rates, the by-catch figures reported by the scientific community (e.g., ICES WGBYC) are likely underevaluating mortality from bycatch in some cases and, consequently, may not reflect the true extent of the impact of by-catch on populations (Peltier et al. 2016, Morkūnas et al. 2022).

The shortcomings in relation to population estimates, trend analyses and the level of anthropogenic impacts on these populations in common give a low confidence in this indicator. High priority should be given to improvement of these shortcomings.

As specific points to be addressed in future by-catch evaluations, also seal by-catch data based on REM must distinguish between species. A model must be developed to allow estimating what proportion of by-caught seals to assign to each species/population. Further, European otters should also be included in future by-catch evaluations as the coastal distribution of parts of the population overlapping with commercial as well as recreational small scale net and trap fisheries suggests that this population may be of conservation concern.

Description of optimal monitoring

Monitoring of by-caught marine mammals and waterbirds should enable the estimation of annual (seasonal) mortality from all métiers and fleet segments to be compared to the population dynamics of the respective species. Besides fishing effort and by-catch data, data on population size and trend and spatio-temporal delineation of sub-populations (and also fishing effort and by-catches) is also required in order to relate by-catch numbers to the adequate population unit. Monitoring results should not only address the problem of by-catch in general, but should allow to quantify impacts in order to propose management measures such as (temporary) closures of specific fisheries or fishing areas. Optimal monitoring would therefore also provide reliable population size estimates for all species considered from the by-catch perspective.

Except for Evaluation Method 3 in birds, the indicator requires estimates of population sizes for those species suffering from by-catch, either on the level of entire populations (marine mammals, Evaluation Method 1 for birds) or on subdivision level (Evaluation Method 2 for birds). While such estimates are available for a number of marine mammals (especially seals) due to target-oriented surveys, they are quite crude for most waterbird species, especially those wintering in offshore areas. Further, uncertainties in population estimates and incomplete knowledge on spatial and temporal distribution patterns have to be addressed. Thus, internationally coordinated offshore surveys need to be organized and should be established in the respective HELCOM Core Indicator “Abundance of waterbirds in the wintering season”.

The species covered by the indicator are highly mobile and fishing methods differ between sub-regions or even on a local level. Due to the resulting variability in by-catch risk, a regionally and fishing method differentiated métier monitoring approach that considers fishing activity per spatial unit (e.g., Statistical Baltic Squares) is recommended. A By-catch Risk Approach (BRA) can be used to identify areas and fisheries that are likely to pose the greatest conservation threat to incidentally caught species, taking into account the uncertainty of their population structure. A BRA was initially developed for cetaceans at an ICES Workshop (ICES 2010). It can also help optimising different methods of monitoring and tailoring mitigation measures. Using REM, Kindt-Larsen et al. (2016) were able to identify a number of high-risk areas in the North Sea. The BRA highlights areas where the greatest problems occur and enables educated fisheries management decisions such as proactive mitigation measures before by-catches occur. This is especially important for threatened species/populations such as the critically endangered Baltic Proper harbour porpoise population. Risk-mapping for harbour porpoise conducted in the HELCOM ACTION project provided additional information about spatial distribution of by-catch risk (HELCOM 2021a). Risk-mapping has been extended to seals and waterbirds in the HELCOM BLUES project.

Effort monitoring, as well as by-catch monitoring, has to be carried out on a fine spatial scale in order to relate by-catch to both fishing effort and abundance of mammals and birds. Fishing effort must be monitored in a meaningful parameter (length of nets * soak time instead of simply days at sea). The documentation of net length in the logbook (i.e. for vessels 10 m and longer) used is only optional in EU fisheries (EU Commission Implementing Regulations 404/2011 and 2015/1962). Some national peculiarities apply. E. g. in Sweden, the coastal static net fishermen (vessels <8m in the Baltic marine region and <10m in the Atlantic marine region) are obliged to report their effort in meters*days for each static net type, mesh size and fishing location. Larger vessels are obliged to report number of nets, net length, and time for set and haul for each static net type, mesh size and fishing location. Since not all effort is recorded (small vessels, recreational net fisheries) and thus effort has to be estimated, the uncertainty in the fishing effort estimates which underlie the by-catch estimate needs then to be specified by adding a coefficient of variation or 95 % confidence interval.

Appropriate monitoring is needed, so as not to put more burden than necessary on fisheries from management measures to fulfil legal conservation obligations. Monitoring must be able to cover all métiers and fleet segments. A comprehensive monitoring would include on-board and in-situ off-board observers, REM using onboard CCTV cameras (Kindt-Larsen et al. 2012, Glemarec et al. 2020), and possibly additional methods such as interviews where the abovementioned are not possible, e.g., in recreational fisheries (ICES 2013a). In exceptional cases, such as in fisheries with small open boats, self-sampling may be a component of the monitoring programme, but data quality must be verified independently.

Human observers are an important component to sample by-catch and collect information on composition and number of by-catch and to deliver specimen to the relevant authorities in order to conduct further examinations regarding age, sex, nutritional state, and injuries. In addition, stomach contents may help to identify in more detail the conflict between marine areas selected by fisheries and habitat demands of mammals and birds. Stranding networks can provide further by-catch information if collected specimen are examined for net marks and previous injury which could have caused by-catch. However, limitations in data quality have to be accounted for (e.g. beached bird surveys may indicate by-catch but never give any information on the type of gear or nationality of the fishing vessel which caused the fatality).

ICES (2013b) has addressed the question of whether it is possible to combine monitoring of protected and endangered species and discard sampling (which will be the main focus of fishery monitoring due to the landing obligation) in the same sampling scheme. If by-caught animals cannot be landed, as a minimum requirement provision must be taken that detailed, meaningful photographs of by-caught mammals and birds can be taken.

The knowledge on by-catch of waterbirds and marine mammals can greatly be improved once a suitable monitoring scheme is implemented on regional and national levels within the DCF, now termed EU Data Collection Multi-Annual Programme EU-MAP: The EU-MAP will guide future fishery monitoring and data collection within the EU, covering a broad range of objectives including the landing obligation. It is crucial that in the regional implementation process an adequate sampling coverage plan is developed including mammal and waterbird by-catch in all relevant métiers and fleet segments (also including part-time and recreational fisheries) in the Baltic Sea.

Further actions for optimizing electronic monitoring

Pilot studies using cameras for monitoring harbour porpoise and waterbird by-catch have shown that these have the potential to be a practical and economic tool for obtaining reliable by-catch data (Glemarec et al. 2020). Further work is required to demonstrate the potential of the technique to perform consistently with regard to species identification and that all incidents are being detected (ICES 2013a). However, fishermen may reject these systems for personal reasons, hence research and international collaboration is needed on how to create a trustful attitude and to overcome personal reservations against onboard CCTV camera systems.

A main drawback of the onboard camera monitoring of bird and mammal by-catch is that a large footage has to be viewed to verify the data from fishermen’s protocols. In order to further reduce costs of a monitoring programme based on video observation, it may be helpful to computerize the work and view only preselected footage. Thus, the development and validation of reliable automated recognition systems for onboard camera systems is desirable.

9.1 Scale of assessment

Marine mammals are evaluated on the basis of populations, and the assessment units reflect the range these populations inhabit (Table 11). With the exception of the Kalmarsund population of the harbour seal, all populations live in more than one Baltic Sea subbasin (HELCOM assessment unit scale 2). Therefore, the outcome of the by-catch evaluation (GES or sub-GES) is applied to all subbasins in which the respective population occurs.

Table 11. Assessments units used for marine mammal populations in terms of inhabited subbasins (HELCOM assessment unit scale 2), which are marked with X for occurrence.

Waterbirds are evaluated in seven subdivisions, which are defined by the merging of up to four of the 17 sub-basins of the Baltic Sea (i.e. HELCOM assessment unit scale 2), the latter following a recommendation by the Joint OSPAR/HELCOM/ICES Working Group on Marine Birds for the waterbird abundance indicators (Figure 16). The seven subdivisions are named as follows:

• A: Kattegat (Kattegat),
• B: Belt Group (Great Belt, The Sound),
• C: Bornholm Group (Kiel Bay, Bay of Mecklenburg, Arkona Basin, Bornholm Basin),
• D: Gotland Group (Gdansk Basin, Eastern Gotland Basin, Western Gotland Basin, Gulf of Riga),
• E: Aland Group (Northern Baltic Proper, Aland Sea),
• F: Gulf of Finland (Gulf of Finland),
• G: Bothnian Group (Bothnian Sea, The Quark, Bothnian Bay).

Figure 16. Grouping of 17 sub-basins (HELCOM assessment unit scale 2) to seven subdivisions as spatial units for waterbird indicators as recommended by JWGBIRD (ICES 2018b).

9.2 Methodology applied

Marine mammal by-catch

The evaluation method relies on comparing available by-catch data of various quality against thresholds which are based on population demographics modelling and simulation trials using mPBR (Belt Sea harbour porpoises) or PBR method as developed for the US MMPA. Both methods have different underlying conservation objectives (Wade 1998, Owen et al. 2022, Authier et al. 2022, see also chapter 3.1).

The models allow population-specific demographic parameters to be utilised to simulate population growth towards carrying capacity (K). In each of the simulations over 100 years, trajectories for the population development are being calculated taking stochasticity of basic input parameters into account. Required parameters are the minimum population estimate (ideally as the 20^th percentile of the abundance estimate), maximum net productivity rate (Rmax), and a recovery factor (F_R) between 0.1 and 1.0. Using a recovery factor of less than 1.0 provides a safety factor to account for levels of unknown bias or estimation problems and would also account for co-occurring biases, such as overestimating RMAX while underestimating mortality. For depleted or threatened populations, or where the confidence of the abundance and/or by-catch estimate is low, F_R must be lower than in populations which already have a favourable conservation status and uncertainty is low. Robustness trials with different population scenarios were completed to determine the value for F_R (Genu et al. 2021, Owen et al. 2022). The scenarios to be further pursued in order to determine the threshold were then selected based on intersessional discussions of HELCOM State and Conservation Working Group.

Waterbird by-catch (Evaluation Method 1)

Evaluation Method 1 is envisaged to be applied to all waterbird species known to be by-caught in fishing gear. Only if the required data is not available, a switch to Evaluation Methods 2 or 3 is foreseen. Evaluation Method 1 focusses on using a population model to quantify the impact on population dynamics by the sum of estimated levels of by-catch mortality and other sources of anthropogenic mortality.

The metric of this approach is the trajectory of the population size over a longer period (three generations time) in relation to elevated levels of mortality due to by-catch. This method needs to be applied on the level of an entire population, and Population Viability Analysis (PVA) appears to be a well-suited method to do this. Age-structure matrix models can be used, they require data for various demographic data such as survival and reproduction of age classes, but also the size of the population.

Using the example of the greater scaup, Marchowski et al. (2020) have demonstrated how a PVA can be applied to a by-catch vulnerable species wintering in the Baltic Sea.

Waterbird by-catch (Evaluation Method 2)

For waterbird species listed on the HELCOM Red List of Baltic Sea species in danger of becoming extinct (HELCOM 2013), which lack sufficient information about demographic parameters and/or by-catch rates at the population level, Evaluation Method 2 can be used. For each of those species, the minimal data requirements for Evaluation Method 2 include the number of individuals present in a given evaluation area and the number of individuals of that species by-caught in fishing gears (which can be estimated from by-catch rates and total fishing effort).

In this evaluation, Evaluation Method 2 was applied to four species of benthivorous ducks (all of them red-listed by HELCOM 2013) in the Polish Exclusive Economic Zone (PEEZ) including coastal waters and coastal lagoons (c. 30,500 km²), separated according to the evaluation units (aggregated sub-basins) used in waterbird indicators, here the subdivisions Bornholm Group and the Gotland Group (Figure 17). The analysis lacks data on most of the area of Lake Dąbie and the lower sections of the Odra and Vistula rivers, which are areas of a significant concentration of diving birds (Meissner & Rydzkowski 2007, Marchowski et al. 2018), at the same time being exploited for fishing, and thus likely generating bird by-catch. Fishing effort and waterbird data were analysed for the five winter seasons 2015/2016 – 2019/2020. The number of waterbirds was obtained from the annual January counts and was considered constant during the winter period (from October 1 to April 30), while the fishing effort was obtained from fishermen’s declarations during this period submitted to the Polish Fisheries Monitoring Centre.

Figure 17. Evaluation area for waterbird by-catch using Evaluation Method 2 in the Polish Exclusive Economic Zone in the southern part of the Baltic Sea, divided into HELCOM subdivisions (Bornholm Group and Gotland Group) and Baltic Squares (20×20 km). Red lines are the transects used for ship-based monitoring of waterbirds offshore.

Waterbird numbers: Ship-based waterbird surveys at sea were carried out along 56 transects ranging from 3.9 km to 28.7 km in length (Chodkiewicz et al. 2012) (Figure 17), following a standard study protocol (Komdeur et al. 1992; Wetlands International 2010) and a standard distance sampling protocol (Buckland et al. 2001), widely used in seabird studies (Ronconi & Burger 2007, Spurr et al. 2012). Key parametric functions were evaluated with cosines and simple polynomials for adjustment terms: uniform, half-normal and hazard rate, and the best fitting function was chosen based on the smallest Akaike Information Criterion (AIC) values (Burnham & Anderson 2002). The analyses were performed in the R environment (R Core Team 2021), using the Distance package (Distance Sampling Detection Function and Abundance Estimation, version 1.0.4, Miller et al. 2019). Bird numbers obtained from the shore during the standard January waterbirds count and under the International Waterbird Census (Wetlands International 2010) were added to the estimated offshore counts.

Fishing effort: To determine total fishing effort, an analysis of the data obtained from the Fisheries Monitoring Centre (CMR) was performed. Only data from the winter period was used – the time when the most seabirds occur in this part of the Baltic Sea (Skov et al. 2011), specifically from 1 October to 30 April. In this analysis, only static nets – the most problematic fishing gear in terms of bird by-catch in this area (Žydelis et al. 2009; 2013; Marchowski 2021) – were considered. Other fishing gears susceptible to capturing waterbirds incidentally and considered less problematic than static nets in terms of bird by-catch in the study area (e.g., long-lines and fyke traps) were not taken into account in this analysis, owing to a paucity in fishing effort and/or by-catch data for these gears (Marchowski 2021). Additionally, the impact of Polish vessels operating in non-Polish waters or in areas of Exclusive Economic Zones of other countries, and of non-Polish fleets operating in the Polish Exclusive Economic Zone were not included. For each record, the standard unit of fishing effort in net*metre*days (NMD) (Bellebaum et al. 2012; Psuty et al. 2017) was calculated. The NMD unit determines how many meters of nets were left in the water for how many days, i.e., the time during which they posed a potential threat to birds.

Waterbird by-catch: Based on the by-catch rates determining the number of caught birds per 1000 NMD in the main static net fisheries operated in Polish sea waters (Table 12), the total number of by-caught birds of all species together was estimated by multiplying these rates with the total fishing effort for each year at the level of Statistical Baltic Squares (SKB), each covering a total area of about 400 km² offshore, while the area of the squares adjacent to the coast and extending beyond the borders of the PEEZ is smaller (Figure 17). Subsequently, individual species total by-catch mortality was estimated as the ratio of the entire bird by-catch mortality to the corresponding share in the waterbird population. By-catch rates were calculated based on surveys carried out with the participation of observers on board fishing vessels in the winter season in the 2013/2014 on few water bodies in Polish EEZ, Kamień Lagoon, Szczecin Lagoon, Pomeranian Bay and Puck Bay. The method of fishing for these water bodies is considered representative of the entire Polish fishery, the study sites were selected in such a way that they included areas with high by-catch, medium by-catch, and areas with no by-catch (Psuty et al. 2017).

Table 12. By-catch rates based on studies carried out in the Polish waters of the Baltic Sea in the winter seasons of 2013/2014 and 2014/2015 (according to Psuty et al. 2017). By-caught birds/1000 NMD: number of by-caught birds per 1000 nets * metres * days.

Setting the threshold values: Species-specific threshold setting for Evaluation Method 2 using bird numbers from the assessment area and annual adult mortality from literature data (Bird et al. 2020) is explained in section 3.1.

Waterbird by-catch (Evaluation Method 3)

Evaluation Method 3 compares spatial distributions of waterbirds and the exercise of fishing methods by-catching those waterbirds for the same part of the year, but shall be applied only to red-listed species (HELCOM 2013) if Evaluation Methods 1 and 2 are not possible due to lacking by-catch or bird population data. Evaluation Method 3 was used for waterbirds in Germany and Denmark.

Germany: Waterbird distribution in German waters (falling into the subdivision Bornholm Group) was recorded during standardised ship-based surveys (HELCOM 2021b) during the wintering season from November to April and entirely falling into the assessment period 2016-2021. Survey data were aggregated to species maps showing mean densities (birds per km²) in a 5×5 km grid across the six years. During the ship-based bird surveys also marking flags of static nets were recorded and aggregated in a 5×5 km grid map (flags per km travelled).

Bird density maps and static net effort maps were combined in order to check for existing of spatio-temporal overlap. Bird occurrence was defined as being relevant for densities above 1 bird/km² in less numerous species and for densities above 5 birds/km² in abundant species in order to avoid overvaluation of insignificant occurrences. Since the fishing effort was only recorded during the bird surveys, published maps of static net fishing (von Dorrien 2019) were checked in order to identify any undetected fishing effort in areas with relevant bird occurrence (which actually was not the case).

Denmark: For three subdivisions (Kattegat, Belt Group, Bornholm Group) covering Danish waters in the Baltic Sea, a risk assessment for common eiders could be used for this by-catch evaluation. Using data from electronic by-catch monitoring, Glemarec et al. (2022) produced risk maps and modelled total annual and quarterly by-catch totals for common eiders among other species and species groups. Recorded by-catch events give evidence for the spatio-temporal overlap of fishing and bird occurrence and thus can feed into Evaluation Method 3 as long as a spatial allocation is possible.

Lithuania: Species-specific by-catch data from an extensive study in Lithuania from October 2015 to May 2020 (Morkūnas et al. 2022) gave evidence for by-catch and thus could be used for Evaluation Method 3.

9.3 Monitoring and reporting requirements

Monitoring methodology

Monitoring relevant to the indicator is described on a general level in the HELCOM Monitoring Manual in the sub-programme: Fisheries by-catch.

Current monitoring

Commission Delegated Decision (EU) 2021/1167 (European Commission 2021) requires by-catch monitoring of protected mammal and waterbird species. Current national discard/by-catch monitoring programmes carried out under the EU data collection framework (DCF) only to very limited extent target marine mammal and bird by-catches. Monitoring of by-catch of cetaceans under Annex XIII of the EU regulation 2019/1241 lays measures concerning by-catches of cetaceans in fisheries using onboard observers but is limited to vessels >15 m and hence results in the lowest observer coverage of fisheries posing greatest threat to porpoises in the Baltic Sea (ICES 2013b).

Thus, monitoring activities relevant to the indicator are only partially carried out by HELCOM Contracting Parties (see HELCOM Monitoring Manual). These consist generally of DCF at-sea monitoring with a low on-board observer coverage in métiers and fleet segments relevant to marine mammal and waterbird by-catch, with the exception of Denmark and since recently Sweden, for which electronic monitoring in static net fisheries can provide data with a level of high confidence. In other areas, self-reported data from logbooks are being reported which are likely incomplete and do not allow extrapolations on fleet effort. These can at best be considered as absolute minimum estimates.

Sub-programme: Fisheries by-catch

Monitoring Concept Table

All HELCOM Contracting Parties which are also EU Member States are obliged to carry out monitoring to provide estimates of population sizes in accordance with the requirements of the Habitats Directive and the Birds Directive.

Contracting Parties currently do not comply with Article 12 Habitats Directive as there is no monitoring in place that gives information that serves the target that incidental capture and killing does not have a significant negative impact on the species. Even more, current monitoring practice led to the unsatisfactory situation that the extent of the by-catch problem is still not known precisely and as a consequence only limited conservation measures regarding by-catch (such as defined in the EU Regulation 2019/1241) are implemented. Some countries like Denmark have been engaged in developing monitoring based on on-board video cameras recently. In Denmark, this programme is now fully integrated to the regular national monitoring programme of Danish fisheries (i.e., DCF or EU-MAP), and a similar programme is on tracks in Sweden.

Monitoring programmes are carried out under the EU Data Collection Framework (DCF). However, DCF monitoring effort has focused primarily on the problem of discard. Available resources have thus been allocated to large vessels operating active gears for which bycatch of protected, endangered and threatened species is a minor issue, rather than on the more problematic small vessels using static nets which are responsible for most of the by-catch in the Baltic Sea. Thus by-catch of marine mammals and waterbirds is not adequately addressed but rather recorded opportunistically at best not providing the needed data to enhance the confidence of the indicator.

EU Regulation 2019/1241 obliges Member States to monitor cetacean by-catch in static nets. Further, monitoring under Regulation 2019/1241 is not suited to the data needs for this indicator because only vessels >15 m are covered by the observer programme and the majority of Baltic static net fisheries is carried out by small vessels which use the same gear. Under Annex VIII of EU Regulation 2019/1241 vessels are allowed to set 9 km (vessel length <12 m) or even 21 km (vessel length >12 m) of static net, respectively, illustrating the high risk of by-catch even by small vessels (European Commission 2019).

Only very limited data are collected for protected waterbird taxa under DCF, and it is not possible to estimate effort or coverage. Besides national differences there are large differences in coverage between fishing métiers favouring larger vessels and mainly trawlers. As a result, from these programmes there are no robust estimates of by-caught waterbirds and marine mammals for various types of fishing gear (mainly gillnets and entangling nets) in the Baltic Sea, because so far no adequate observer coverage has been achieved with existing monitoring programmes such as DCF and EU Regulation 2019/1241. On the other hand, the results of pilot studies such as interviews are frequently questioned by fishermen and fisheries authorities. Especially in métiers which have been identified by pilot studies as fisheries with a high risk for mammal or bird by-catch, monitoring is inadequate and a revision of existing monitoring programmes is urgently needed.

By-catch estimates harbour porpoises from Kattegat, Belt Sea and the Sound were taken from Larsen et al. (2021) and Glemarec et al. (2022) as results from a Danish REM study. Further marine mammal by-catch data was added from a compilation of reported by-catches and strandings data compiled by HELCOM EG MAMA, from NAMMCO & IMR (2019) and Vanhatalo et al. (2014).

By-catch data for waterbird Evaluation Method 2 in Polish waters were supplied by Dominik Marchowski (unpublished data based on Polish bird surveys, by-catch rates published by Psuty et al. (2017) and effort data from fishermen’s declarations submitted to the Polish Fisheries Monitoring Centre). Estimates of annual adult mortality used for Evaluation Method 2 were taken from Bird et al. (2020)

By-catch data for waterbird Evaluation Method 3 in Danish waters was taken from Larsen et al. (2021) and Glemarec et al. (2022). Data for waterbird Evaluation Method 3 in Lithuanian waters was taken from Morkūnas et al. (2022). Data for waterbird Evaluation Method 3 in German waters was taken from German seabird surveys from November to April (2016-2021) which also record the distribution of static net flags, and further from scientific case studies in German waters (Schirmeister 2003, Erdmann et al. 2005, Bellebaum & Schirmeister 2012).

The indicator “Abundance of waterbirds in the breeding season” is led by Germany (responsible experts: Sven Koschinski, Volker Dierschke and Axel Kreutle) and co-led by Poland (responsible expert: Katarzyna Kaminska).

For the waterbird evaluations, analyses were supplied by Poland (Dominik Marchowksi) and Germany (Kai Borkenhagen, Volker Dierschke, Jana Kotzerka, Nele Markones, Henriette Schwemmer) based on the waterbird monitoring in the respective countries and Polish fishing effort data (Fisheries Monitoring Centre). Literature data were used for additional evaluations in Denmark and Lithuania.

The indicator was developed following recommendations from 50 experts at the OSPAR-HELCOM workshop to examine possibilities for developing indicators for incidental by-catch of birds and marine mammals. Regarding waterbirds, the indicator concept was evaluated by experts of the OSPAR/HELCOM/ICES Joint Working Group on Marine Birds (JWGBIRD): Gildas Glemarec, Dominik Marchowski.

For the marine mammal evaluation, Markus Ahola, Mathieu Authier, Julia Carlström, Anita Gilles, David Lusseau, Kylie Owen contributed to the development and testing of threshold setting methods and evaluation scenarios.

HELCOM Secretariat: Jannica Haldin, Owen Rowe, Jana Wolf

This version of the HELCOM core indicator report was published in April 2023:

The current version of this indicator (including as a PDF) can be found on the HELCOM indicator web page.

Earlier versions of this indicator can be found below:

Number of drowned mammals and waterbirds in fishing gear – 2018 (HOLAS 2) (pdf)

Number of drowned mammals and waterbirds in fishing gear – 2013 (Korpinen & Bräger)

Amundin, M., Carlström, J., Thomas, L., Carlén, I., Teilmann, J., Tougaard, J., Loisa, O., Kyhn, L.A., Sveegaard, S., Burt, M.L., Pawliczka, I., Koza, R., Arciszewski, B., Galatius, A., Laaksonlaita, J., MacAuley, J., Wright, A.J., Gallus, A., Dähne, M., Acevedo-Gutiérrez, A., Benke, H., Koblitz, J., Tregenza, N., Wennerberg, D., Brundiers, K., Kosecka, M., Ljungqvist, C.T., Jussi, I., Jabbusch, M., Lyytinen, S., Šaškov, A., Blankett, P., 2022. Estimating the abundance of the critically endangered Baltic Proper harbour porpoise (Phocoena phocoena) population using passive acoustic monitoring. Ecology and Evolution 12: e8554. https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.8554

Anderson, O.R.J., Small, C.J., Croxall, J.P., Dunn, E.K., Sullivan, B.J., Yates, O., & Black, A. 2011. Global seabird bycatch in longline fisheries. Endangered Species Research 14: 91-106. https://www.int-res.com/articles/esr_oa/n014p091.pdf

ASCOBANS 2012. ASCOBANS conservation plan for the harbour porpoise population in the Western Baltic, the Belt Sea and the Kattegat, 7th Meeting of the Parties to ASCOBANS, Brighton, United Kingdom, 22-24 October 2012, Bonn, Germany, 40 pp. https://www.ascobans.org/sites/default/files/document/HarbourPorpoise_ConservationPlan_WesternBaltic_MOP7_2012.pdf

ASCOBANS 2016. Resolution No. 3: Revision of the Recovery Plan for Baltic Harbour Porpoises (Jastarnia Plan), 8th Meeting of the Parties to ASCOBANS, Helsinki, Finland, 30 August – 1 September 2016, Bonn, 94 pp. https://www.ascobans.org/sites/default/files/document/MOP8_2016-3_JastarniaPlan_inclAnnex.pdf

Authier, M., Carlström, J., Genu,M., Gilles, A., Kindt-Larsen, L., Larsen, F., Lusseau, D., Owen, K., Sköld,M., Sveegaard, S. 2022. Personal communication with experts on further scenarios to increase the robustness of mPBR. August 2022

Beineke, A., Siebert, U., McLachlan, M., Bruhn, R., Thron, K., Failing, K., Müller, G. & Baumgärtner, W. (2005) Investigations of the potential influence of environmental contaminants on the thymus and spleen of harbor porpoises (Phocoena phocoena). Environmental Science & Technology 39: 3933–3938. https://pubs.acs.org/doi/abs/10.1021/es048709j

Beineke, A., Siebert, U., Müller, G. & Baumgärtner, W. (2007a) Increased blood interleukin-10 mRNA levels in diseased free-ranging harbor porpoises (Phocoena phocoena). Veterinary immunology and immunopathology 115: 100–106. https://www.sciencedirect.com/science/article/abs/pii/S0165242706002686

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1. See the following judgements: judgment of 9 December 2004, Commission/Spain, case C-79/03, ECR 2004, p.11619, paragraphs 36 and 41; judgment of 15 December 2005, Commission/Finland, case C-344/03, ECR 2005, p.11033, paragraphs 53-54; judgment of 8 June 2006, WWF Italia and others, case C-60/05, ECR 2006, p.5083, paragraphs 25-27. ↑