Interim report of the HELCOM CORESET project

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4.2. By-catch of marine mammals and birds
1. Working team: Marine Mammals
Author: Stefasn Braeger
2. Name of candidate indicator
Bycatch of marine mammals and birds
3. Unit of the candidate indicator
Numbers of individuals bycaught in fi shing gear
4. Description of proposed indicator
Bycatch in fi shing gear is known to be the most important threat to biodiversity and potential disruption
of the food web as far as marine mammals in the wider Baltic Sea are concerned. Bycatch is also
regarded as one of the main direct anthropogenic pressures on marine diving bird species. It can be
measured as number of bycaught porpoises or seals either by a near-complete coverage with on-board
observers/ CCTV-recording or by examining beached individuals. All net-setting vessels should be monitored
since monitoring only a subset of vessels would lead to an estimate with considerable variance.
Beached individuals with clear signs of by-catch injuries confi rmed by pathological investigations can give
indications that by-catch occurred in an area, but it is very diffi cult to use such data for obtaining total
numbers of animals being affected and hence impacts on populations.
For healthy mammal populations (with an abundance ≥80% of a population at carrying capacity) a tolerable
bycatch rate may amount to 1.0% (plus another 0.7% anthropogenic take due to other impacts
such as pollution, noise etc.) of the local population. For depleted populations such as the “critically
endangered” (according to Hammond et al. 2008) porpoise population of the Baltic Proper and the
rapidly decreasing (according to Teilmann et al. 2011) porpiose population of the Belt Sea, bycatch was
recommended to be reduced to near zero immedeately (cf. ASCOBANS 2002). For birds it has been
shown that present bycatch rates are close to, or for some species even exceeding, levels that can be sustained
by the populations.
5. Functional group or habitat type
Toothed whales, seals and birds (i.e., the piscivorous top predators)
6. Policy relevance
Habitats Directive, Marine Strategy Framework Directive, and national obligations under a number of
IGO resolutions (e.g., HELCOM, OSPAR, CMS, ASCOBANS, AEWA etc.)
The Baltic Sea Action Plan provides the following targets: “By 2012 spatial/temporal and permanent
closures of fi sheries of suffi cient size/duration are established thorough the Baltic Sea area” and “By
2015 by-catch of harbour porpoise, seals, water birds and non-target fi sh species has been signifi cantly
reduced with the aim to reach by-catch rates close to zero”
7. Use of the indicator in previous assessments
ASCOBANS (2002 & 2009) in the Jastarnia Plan, none for seals and birds.
8. Link to anthropogenic pressures
Directly linked to commercial fi shery and recreational fi shing with gillnets, and to a lesser degree and
indirectly (through prey depletion) with other commercial fi shery such as pelagic and bottom trawling as
well as other coastal stationary gear.
9. Pressure(s) that the indicator refl ect
Selective extraction of species, including incidental non-target catches
10. Spatial considerations
Since harbour porpoises are highly migratory mammals, the spatial considerations would be determined
by the desired species distribution, e.g. Baltic-wide. Bycatch of birds is higher in areas where large bird
concentrations occur. Such areas, however, are distributed throughout the Baltic.