​​​​​​​​​Key message

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 mid-2017 'State of the Baltic Sea' report, and the results are to be considered as intermediate.


This core indicator provides a descriptive evaluation of whether the number of incidentally by-caught marine mammals and waterbirds are below mortality levels that enable reaching good status. Currently no quantitative threshold values have been defined for the core indicator. Only concepts for determining the threshold values based on removal- and conservation targets have been described and are proposed to form the basis of future core indicator threshold setting activities.

Initial assessment values (i.e. initial threshold value) have been used to develop a first descriptive indicator evaluation. The removal targets are used as tentative threshold values for two populations of harbour porpoises and three species of waterbirds (Key message table 1). Available incidental by-catch estimates (e.g. ICES 2015, 2016a) are evaluated against these threshold values, which also account for other sources of anthropogenic mortality than incidental by-catch to the concerned species. The threshold values have to be refined and further species added as further knowledge is gained. The initial descriptive evaluation shows that the incidental by-catch over all species included so far fails to meet the threshold in all areas where an initial evaluation was possible. Key message table 1 shows in which sub-basins the species assessed occur and where by-catch is proven.​

Key message table 1. Assessment availability of incidental by-catch per species and sub-basin.

Drowned mammals and waterbirds Key message table 1.png

(1) to be assessed by OSPAR indicator M6 (thresholds should be harmonised), X = incidental by-catch proven, ? = incidental by-catch mortality remains to be shown (occurrence of bird species and gillnet fishery in area but spatial/temporal overlap uncertain), n.a. = not assessed (occurrence of this population in the area uncertain)​

Key message table 2. Initial threshold values based on removal targets for the assessment units of the species to which this tentative assessment applies. 

Drowned mammals and waterbirds Key message table 2.png

For harbour porpoises, increased mortality due to drowning (including death by suffocation) in fishing gears is recognised as the most significant threat to the populations in the Baltic Sea (Hammond 2008a,b, HELCOM 2013). The number of drowned animals exceeds the tentative removal target for the Baltic Proper population. For the harbour porpoise population in the Western Baltic, Belt Sea and Kattegat, the preliminary incidental by-catch estimate is in the same range as the removal target. Due to uncertainties in both the population estimates and estimate of fishing pressure, a bycatch rate close to the tentative threshold does not imply a good status (by-catch rate < tentative threshold) or bad status (by-catch rate > tentative threshold).

Recent modelling efforts have shown that incidental by-catch is a relevant source of human induced mortality in grey seals (Vanhatalo et al. 2014). No recent incidental by-catch estimates are available for ringed seals and harbour seals.

For waterbirds, drowning in fishing gear is believed to be a significant pressure on the populations of long-tailed duck, scoters, divers and some other waterbird species in wintering areas with high densities of waterbirds (Larsson & Tydén 2005, Žydelis et al. 2009, 2013, Bellebaum et al. 2012, European Commission 2012). The initial assessment based on case studies reveals that tentative threshold values are exceeded in all three waterbird species included in this evaluation. A declining trend in numbers of incidentally by-caught birds has been detected in the last two decades, however this is generally not believed to be a result of improved fishing practices but due to declining trends detected in the abundance of wintering waterbirds populations (e.g. due to factors such as poor breeding success) which likely contributes to declining incidental by-catch numbers. Also other anthropogenic sources of mortality such as oiling and hunting contribute to declines and must be considered in the indicator assessment. This means also that progress in the reduction of hunting and oiling would also have a positive effect on the assessment. In countries such as Denmark, Poland and Sweden the reported fishing effort has decreased during this time. Thus, a change in fishing pressure may also have contributed to the declining trend in by-caught birds. However as the fishing effort in some cases is measured in days at sea, the effective effort reduction cannot be quantified.

The overall confidence of the indicator is low.​

The indicator is applicable in the waters of all countries bordering the Baltic Sea. Only for harbour porpoise, the assessment in the Kattegat has been agreed to be carried out by OSPAR (within their by-catch indicator M6, which does not cover any other species). Since the harbour porpoise assessment is needed for the overall assessment over all species as proposed in this indicator, the threshold value for the Western Baltic, Belt Sea and Kattegat population should be harmonised between HELCOM and OSPAR.​


Relevance of the core indicator

The populations of marine mammals (cetaceans, seals and otters) and diving waterbirds evaluated in the indicator represent highly mobile animals in the Baltic Sea that are sensitive to additive mortality caused by fishing gear due to their characteristic slow reproduction rate. The indicator is an important tool for detecting intolerable mortality in key populations of the highly mobile species due to fishing activities.

The distribution and abundance of marine mammal populations is closely linked to healthy fish stocks and influenced by many human activities. For harbour porpoises, incidental 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.

Drowning due to incidental by-catch in fishing gear is a significant pressure on population trends and demography of waterbirds as in vulnerable species the numbers of drowned birds represent a relatively large proportion of the total population size.

Policy relevance of the core indicator

​ BSAP segment and objectiveMSFD descriptors and criteria

Primary link


  • Viable populations of species

  • Thriving and balanced communities of plants and animals​

D1 Biodiversity

​​- D1C1 The mortality rate per species from incidental by-catch is below levels which threaten the species, such that its long-term viability is ensured ​

Secondary link


  • Natural distribution and occurrence of plants and animals​


​D1 Biodiversity

- D1C2 (population abundance)

- D1C3 (population demographic characteristics)

- D1C4 (species distribution)

D4 Food web

​- D4C1 (diversity of trophic guild)

​​- D4C2 (balance of total abundance between trophic guilds)

​Other relevant legislation: In some Contracting Parties also EU Birds Directive, EC Action Plan for reducing incidental catches of seabirds in fishing gears, EU Habitats Directive, Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) and Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA)​

The indicator is applicable in the waters of all the countries bordering the Baltic Sea.

Cite this indicator

HELCOM (2017) Number of drowned mammals and waterbirds in fishing gear. HELCOM core indicator report. Online. [Date Viewed], [Web link].​

ISSN 2343-2543

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