Thresholds and Status evaluation

This pre-core indicator and its threshold values are yet to be commonly agreed in HELCOM.
The indictor is included as a test indicator for the purposes of the 'State of the Baltic Sea' report, and the results are to be considered as intermediate.

Due to the lack of sufficient monitoring data, it has not been possible to set quantitative threshold values for this core indicator on the number of drowned mammals and waterbirds in fishing gear for every species concerned. Some tentative threshold values are proposed to allow for a descriptive evaluation, however they should not be considered as finally agreed and are open for revision as more knowledge and monitoring data is accumulated.

The concepts for threshold value setting based on determining removal- and conservation targets are described below. Based on this, initial threshold values for two populations of harbour porpoises and three species of waterbirds can be derived. These have to be refined as further knowledge is gained. Future threshold value setting activities (see sub-chapter 'Alternative threshold setting approaches') are proposed to obtain the basis of a fully operational core indicator.


Initial threshold values used to develop descriptive evaluation

The assessment values applied as initial threshold values for the species and populations assessed in this indicator are shown in Good environmental status table 1.


Thresholds table 1. Initial threshold values based on removal targets for the assessment units of the species to which this tentative assessment applies. 

Thresholds Table 1.png

The term "initial threshold value" was used as an acknowledgement of the shortcomings of the assessment. Given the uncertainty of the available population estimates, the trend analyses as well as the level of anthropogenically induced mortality, great caution should be given to the current threshold values.


Threshold value development concepts

The concept to apply threshold values supported by species specific removal and conservation targets has been developed in other contexts, including ongoing work carried out under the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS), concluded under the auspices of the Convention on Migratory Species (ASCOBANS 2015a). This approach requires setting species specific conservation targets and defining reference points (removal targets) for the annual incidental by-catch rate.

Removal targets are based on 'unacceptable mortality levels' for the indicator species. 'Unacceptable interactions' have been defined for harbour porpoises (ASCOBANS 2000, 2006, 2016a, for details see also species specific targets below). Levels of 'unacceptable interactions' are related to the total human induced mortality of which incidental by-catch is an unknown fraction that may differ regionally. These levels of 'unacceptable interactions' should not be misinterpreted as 'acceptable levels' if the values are below the reference points.

Conservation targets are focused on the state of biological management units (i.e. stocks or populations). A target for a safe human-induced mortality limit (as a consequence of the removal target) is usually the outcome of a simulation over a certain time period using a suitable population dynamic model. During the time period, the conservation target for the stock size is to be reached with a given certainty in a predefined fraction of the simulation time (e.g. at least 95 % likelihood of reaching at least 80 % of carrying capacity within 100 years). In order to set a safe human-induced mortality limit, the time scale of the simulations have to be agreed upon (ICES 2014a, ASCOBANS 2015a). ICES concluded that such human induced mortality limits (or threshold reference points), should account for uncertainty in existing estimates of incidental by-catch and allow for current conservation goals to be met in order to enable managers to identify fisheries that require further monitoring and those where mitigation measures are most urgently required (ICES 2013a).

In the long-term, mortality in a healthy population must not exceed the birth rate (natality) in order to sustain the population. In seriously depleted populations, the human-related mortality must be close to zero to allow for recovery. All the highly mobile indicator species have a slow reproductive rate (K-strategists), and thus the 'unacceptable' mortality due to drowning in fishing gear has to be set at a low level, in order to avoid serious long-term implications for the populations. Due to the fact that the indicator species are affected by several pressures from various human activities, the general aim must be to minimize incidental by-catch of marine mammals and waterbirds as much as possible.

The use of trend-based thresholds of the number of incidentally by-caught animals is not considered appropriate due to the risk of falsely indicating a good status when the threshold value is reached. A slight downward trend may falsely indicate an improvement, as incidental by-catch is less likely to occur in depleted populations close to regional extinction due to the simple fact that fewer animals occur in the area.


Alternative threshold setting approaches

For management purposes, interim objectives or short-term and longer-term removal targets have been set for certain species, such as the harbour porpoise. The simplest management approach for setting an interim target is defining a reference point as a fixed percentage of the best population estimate. However, there are uncertainties regarding both values which have to be taken into account. These have been included in more sophisticated approaches (e.g. potential biological removal (PBR) or catch limit algorithm (CLA)) aiming at more conservative targets. Any interim targets (not only for the harbour porpoise) should be applied keeping in mind the general aim of ultimately reducing incidental by-catches to zero (resolution no. 5- ASCOBANS 2006, 2016a, HELCOM Recommendation 27-28/2 on seals).

The potential biological removal (PBR) can be applied for threshold setting, and is used to set removal targets under the US Marine Mammal Protection Act. The conservation goal is the 'optimum sustainable population' defined as being at or above the population level that will result in maximum productivity (ICES 2014a). For harbour porpoises in the Baltic Sea, Berggren et al. (2002) calculated anthropogenic mortality limits based on minimal demographic information using this approach. For birds, the ICES Workshop to Review and Advise on Seabird Bycatch (WKBYCS) recognises PBR as an initial and rapid assessment tool, which can indicate possible unsustainable mortality levels that would have to be followed by more sophisticated methods for reliable analyses (ICES 2013b). In addition, the workshop pointed out that basic assumptions of the PBR concept need testing and validation before applying to birds. Especially in rapidly declining populations such as long-tailed duck, velvet scoter, red-throated diver and black-throated diver (Skov et al. 2011), this approach has to be treated with great caution as any additional anthropogenic mortality speeds up the ongoing decline.

Population viability analysis (PVA) is another tool often used in similar contexts to forecast the consequences of changes in additional anthropogenic mortality for the population size. Several different types of PVA are being used. A demographic PVA is based on multiple simulated time-series of population growth or decline using extensive demographic data or demographic models of a population. The reliability of a PVA increases with the knowledge of specific demographic parameters such as the distribution of vital rates between individuals of different life history stages and between years.

Within a similar framework, a catch limit algorithm (CLA) has been developed. It is based on the principles of the International Whaling Commission's (IWC) revised management procedure (RMP) for commercial whaling and has been used to calculate anthropogenic mortality limits for harbour porpoises in the North Sea (Winship 2009). The next step should be to expand the capability of the model by incorporating multiple areas in the model. Further, a CLA for the Baltic Sea populations still needs to be developed. In the calculations by Winship (2009), the underlying conservation objective has been assumed to be the ASCOBANS interim conservation objective 'to allow populations to recover to and/or maintain 80% of carrying capacity in the long term' (see below).

Since 2009, ICES has advised the European Commission that CLA is the most appropriate method to set anthropogenic mortality limits on harbour porpoise, but this advice still has not been acted upon (ICES 2014a). CLA also is a suitable method for depleted populations such as the harbour porpoise population of the Baltic Proper. It is to be noted that all approaches rely on suitable programmes monitoring population sizes and incidental by-catches as prerequisites.


Threshold values for harbour porpoise

Within the frame of ASCOBANS, conservation targets have been agreed for the harbour porpoise and can be applied for the two harbour porpoise management units within the HELCOM area: (1) the Baltic Proper population and (2) the Western Baltic, Belt Sea and Kattegat population. ASCOBANS (2002, 2009, 2012) has adopted an interim goal of restoring (and maintaining) the populations of harbour porpoises to at least 80% of their carrying capacity. ASCOBANS has advised that, to be sustainable, 'the maximum annual anthropogenic induced mortality (including incidental by-catch, but also less conspicuous causes of death such as stress caused by pollutants or noise) for harbour porpoises should not exceed 1.7% of the best estimate of the population size' (Resolution No. 3, Incidental Take of Small Cetaceans, Bristol 2000). It has been reaffirmed in Resolution No. 5, Monitoring and Mitigation of Small Cetacean Bycatch (ASCOBANS 2016a) that "a total anthropogenic removal (e.g. mortality from by-catch and vessel strikes) above 1.7 per cent of the best available estimate of abundance is to be considered unacceptable in the case of the harbour porpoise". Also, the intermediate precautionary aim "to reduce by-catch to less than 1 per cent of the best available population estimate" has been reaffirmed. This aim relates to incidental by-catch explicitly and considers an (unknown) proportion of other causes of anthropogenic mortality. The resolution further states that "where there is significant uncertainty in parameters such as population size or by-catch levels, then 'unacceptable interaction' may involve an anthropogenic removal of much less than 1.7%". To date, there is significant uncertainty in central parameters such as estimations of incidental by-catch, population size and population growth for both harbour porpoise management units in the Baltic Sea.

PBR analyses based on data from a survey of the southern and western part of the Baltic Proper indicate that for the critically endangered Baltic Proper population, recovery towards this goal could only be achieved if the incidental by-catch was reduced to two or fewer porpoises per year (Berggren et al. 2002). This resulted in the objective (i.e. a removal target) of the ASCOBANS Recovery Plan for Baltic Harbour Porpoises (Jastarnia Plan) to 'reduce the number of by-caught porpoises in the Baltic towards zero' (ASCOBANS 2002, 2009, 2016b). The later SAMBAH survey found the distribution range of the Baltic Proper population to only partially overlap with the survey area of Berggren et al. (2002) (ASCOBANS 2016b). However the very low abundance estimate of the Baltic Proper population from the SAMBAH survey confirms the need for reducing the number of incidental by-catches towards zero. In such a severely reduced population "unacceptable interaction" involves a much lower anthropogenic mortality compared to healthy populations. Thus, the threshold chosen for the Baltic Proper population is zero. ASCOBANS (2009, 2016b) state that 'as a matter of urgency, every effort should be made to reduce the porpoise incidental by-catch towards zero as quickly as possible'.

For the population of the Western Baltic, Belt Sea and Kattegat the threshold value is tentatively proposed to be the removal target chosen as threshold for this indicator, which is less than 1% of the best population estimate.

As this limit (as all other target setting options such as PBR and CLA) is applied to the 'best' population estimate, there is a need to better define population boundaries of the population of the Western Baltic, Belt Sea and Kattegat (see Sveegaard et al. 2015 and ASCOBANS 2016b) and estimate the abundance (as well as incidental by-catch numbers) within these boundaries.

For improved management of the harbour porpoise populations in the Baltic Sea, removal targets in the form of 'safe' human-induced mortality limits (including incidental by-catch) should be modelled for the distribution range of each population. It would be appropriate to determine targets primarily using the CLA or possibly the PBR approach as these take the uncertainty of data into account. As soon as the results of such simulations are available, the 1% target should be re-evaluated for the population of the Western Baltic, Belt Sea and Kattegat. In order to obtain a more reliable assessment against threshold values in the future, the extent of the uncertainty in underlying data (which currently is greatest for the abundance estimate, Result table2) should be taken into account. As a priority, a CLA analysis should be developed for this population because the method uses time series of data (both population and incidental by-catch) and thus decreases the overall uncertainty. A consequence of significant uncertainty in parameters, such as in the population estimate of the harbour porpoise population of the Western Baltic, Belt Sea and Kattegat, is that a much lower removal target may be needed to reach the conservation objectives. As ASCOBANS resolution no. 5 (ASCOBANS 2006 and 2016) states "... if available evidence suggests that a population is severely reduced, or in the case of species other than the harbour porpoise, or where there is significant uncertainty in parameters such as population size or by-catch levels, then "unacceptable interaction" may involve an anthropogenic removal of much less than 1.7 %". Incidental by-catch again is an unknown fraction of the total anthropogenic removal target. A CLA analysis could produce more reliable targets than this relatively general resolution statement. Thresholds table 2 shows what data may already be available in order to derive CLA values for an assessment.


Thresholds table 2. Available data for further development of CLA values for harbour porpoise populations in the Baltic Sea (x = some data available to fill the knowledge gaps, - = no data available). 

Thresholds Table 2.png

Threshold values for seals

No specific removal targets for seal incidental by-catch have been formulated to date that could directly be applied as a threshold value for this core indicator. The HELCOM Recommendation 27-28/2 recommends reducing incidental by-catches of seals to a minimum level and if possible to a level close to zero and to develop efficient mitigation measures.

The conservation target for seals within the HELCOM area is that the populations grows until limited by the environmental carrying capacity of their Baltic Sea habitat. Recovery towards this target will be allowed as a long-term objective. A lower reference limit below which the survival of the population is at risk and a middle reference limit are used for anthropogenic removal licenses. The overall target is to continually improve the situation of the seal species, but no timescale for its achievement is given (Lonergan 2011).

Information about the distribution of Baltic seal species is provided in more detail in the core indicator on distribution of Baltic seals.

No threshold value can yet be given for seals. As a consequence, the three seal species have to be added in the indicator assessment as soon as threshold values are available. Incidental by-catch estimations for grey seals are available from Vanhatalo et al. (2014, see above).

So far, there are no threshold values for incidental by-catches of seals. However, existing data would allow the development of PBR or CLA for some seal species to begin with. Demographic and abundance data as well as population boundaries have been quite well examined. For grey seals, an incidental by-catch estimate is available (Vanhatalo et al. 2014). Thresholds table 3 shows what data may already be available in order to derive assessment values and add further species to the descriptive evaluation.


Thresholds table 3. Available data for further development threshold values based on PBR and/or CLA values (x = some data available to fill the knowledge gaps, - = no data available).

Thresholds Table 3.png

Threshold values for otters

HELCOM (2013) lists incidental by-catch in fishing gear, among others pressures, as a major threat to Eurasian otters. However, the extent of the problem is not known. Fykenets might pose the greatest threat to Eurasian otters (Raby et al. 2011). No goals or threshold value for incidental by-catch reduction have been formulated yet. As a consequence, otters have to be added in the indicator assessment as soon as better data are available.

So far, there are no threshold values for incidental by-catches of otters. Thresholds table 4 shows that no data are available in order to derive assessment values.


Thresholds table 4. Available data for further development threshold values based on PBR and/or CLA values (x = some data available to fill the knowledge gaps, - = no data available).

Thresholds Table 4.png

Threshold values for waterbirds

A reduction in the number of incidentally by-caught waterbirds is needed to reach their specific conservation goals. For the species concerned, analyses of thresholds for unacceptable losses of individuals are lacking, but are urgently desired as soon as data from incidental by-catch monitoring become available. It has to be stressed that many of the waterbirds species concerned have high longevity, low reproductive rates and late maturity. These characteristics make them vulnerable to the loss of adult individuals in particular (Bernotat & Dierschke 2016). However, knowledge about demographic parameters, such as survival rates, reproductive performance, and delineation of population segments is sparse or unavailable for many of the species affected by incidental by-catch (Žydelis et al. 2009). One option using limited demographic information is the PBR approach. The main advantage of the PBR approach is that it relies on those demographic parameters which are easiest to obtain for many bird species. Further, it is ready for use whereas specific demographic models still have to be developed for species concerned.

Removal targets considered as provisional thresholds for this indicator have been derived using the PBR concept in some initial studies. PBR has been calculated for three waterbird species which are known to be incidentally by-caught in high numbers: long-tailed duck, greater scaup and common guillemot (Žydelis et al. 2009). The use of PBR as provisional targets and thresholds in this indicator will have to be refined at a later stage because this requires further testing and validation before they can be used as a robust basis for threshold value setting (see also Richard & Abraham 2013). For an improved analysis more sophisticated methods may be required in the future of which PVA and CLA are possible options. Ultimately, individual-based modelling has a potential to provide the best assessment, in particular when evaluating multiple anthropogenic pressures in combination.

The thresholds for the waterbird species used in this indicator must not be confused with a 'maximum allowable catch'. The concept of 'maximum allowable catch' of seabirds appears not to be consistent with the EU Plan of Action (European Commission 2012) overall objective to 'minimise and where possible eliminate' incidental by-catch and with Article 5 of the EU Birds Directive, which requires Member States to take measures prohibiting the 'deliberate killing or capture [of birds] by any method'. According to Article 7 of the Birds Directive, exceptions from the prohibition of deliberate killing are allowed in the context of hunting, and some of the species listed in Annexes II/1 and II/2 include species prone to drowning in fishing gear in the Baltic Sea. Also, uncertainties impede the application of PBR in a management context so far to set trigger levels for incidental by-catch in a population (ICES 2013b).

In northern Europe, the impact of incidental by-catch on population dynamics has so far only been estimated for three species by applying the PBR approach. CLAs have not been applied to waterbird populations and would require information on population trends currently unavailable for the majority of Baltic waterbirds. Application of PBR and CLA approaches appears to allow for formulation of species-specific removal targets for waterbirds, as soon as reliable estimates of the species specific mortality levels can be obtained through incidental by-catch monitoring. A prerequisite for the application of PBR and CLA is knowledge about the species specific mortality and population sizes as input parameters, but data are not yet sufficiently available for all species.

An overview of recent estimates for the numbers of waterbirds wintering in the Baltic Sea is given by Skov et al. (2011). Accordingly, the incidental by-catch problem concerns 8,575 red-throated and black-throated divers, 8,300 great crested grebes, 770 red-necked grebes, 2,890 Slavonian grebes, 54,000 great cormorants, 30,450 common pochards, 476,000 tufted ducks, 127,000 greater scaups, 515,000 common eiders, 2,300 Steller's eiders, 1,486,000 long-tailed ducks, 412,000 common scoters, 373,000 velvet scoters, 174,000 common goldeneyes, 12,600 smew, 25,700 red-breasted mergansers and 66,000 goosanders, but also considerable numbers of common guillemot, razorbill and black guillemot (the latter alcid species not quantified by Skov et al. 2011).

For the threshold values in this indicator, so far no calculations have been made. Instead PBR values were derived from Žydelis et al. (2009). It is intended to calculate more PBR values or, in order to account for the large variation between upper and lower confidence levels, to introduce CLA method. For these calculations, preferably recent data on incidental by-catches and population size is needed as well as some demographic data. Good environmental status table 5 shows what data may already be available in order to derive assessment values and add further waterbird species to this initial assessment in the near future.

It is important to consider other relevant anthropogenic mortality than incidental by-catch such as from hunting and oiling when applying PBR or CLA values. Hunting bag data is collected in many - but not all - countries on the migration routes of the waterbirds to be considered. National data must be collected either in form of national reports to HELCOM or in scientific projects. Further, waterbirds are facing the danger of plumage oiling. High numbers of waterbirds (especially long-tailed ducks) were killed due to plumage oiling in the early 21st century (Larsson & Tydén 2005). Since then the number of oil spills and the volume of oil detected in the Baltic Sea have decreased (HELCOM 2017). Scientific studies are required to derive more recent estimates of oil victims per species in areas significantly affected from oil pollution. Recent studies on oiling of waterbirds is available for some coasts of the North Sea which is another overwintering area for some of the waterbirds relevant for this indicator (e.g., Camphuysen et al. 2009).

PBR approach is used in provisional assessments in this indicator because there is data available for three waterbird species of concern. It is however acknowledged, that the CLA approach may produce a more reliable assessment because it uses time series of incidental by-catch and abundance data and thus reduces uncertainties. As in harbour porpoises, priority should be given to develop CLA for the most vulnerable waterbird species. Additional demographic data such as survival rates may be needed for the relevant bird species to improve simulations.

Further demographic modelling and testing of PBR is needed for all species in the future. Whatever approach is used in the assessment, more recent incidental by-catch and population estimates are needed in order to calculate PBR or CLA values (see Monitoring Requirements). Since these indicate a limit of anthropogenic mortality, also estimates for other causes of mortality (especially oiling and hunting) are needed.


Thresholds table 5. Available data for further development threshold values based on PBR and/or CLA values. Hunting bag data from Mooij 2005. Maximum population growth rate (Rmax) was calculated from data in Bernotat & Dierschke (2016) preliminarily.

Thresholds Table 5.png

a European wintering population (BirdLife International 2015), b Baltic breeding population (BirdLife International 2015), c Baltic wintering population (Skov et al. 2011).