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Impacts of set net fisheries


Cod (Gadus morhua) caught in a gillnet. Photo: BfN
Cod (Gadus morhua) caught in a gillnet. Photo: BfN

Bycatch in Baltic Sea set net fisheries

Set nets are one of commonest methods of fishing for cod, herring and flatfish in the Baltic Sea. Set nets (primarily gillnets and trammel nets) are also one of the main threats to seabirds and harbour porpoises. Diving seabirds and harbour porpoises are unable to see the fine, monofilament nets under water, swim into them in pursuit of prey, get caught up and drown. The risk of such animals drowning in set nets depends on the mesh size, filament diameter, setting time, time of day and year, and notably the location of the fishing ground and how far it overlaps with bird and porpoise feeding grounds.

Baltic Sea harbour porpoise at risk

Harbour porpoises are frequently caught as bycatch in set net fisheries. Unfortunately, only a small fraction of this bycatch is reported or surrendered. One means of documenting harbour porpoise bycatch is the monitoring of stranded individuals found dead. Between 2000 and 2014, the number of harbour porpoises found dead on the German Baltic Sea coast more than quadrupled from 25 to 129.

Some harbour porpoises caught as bycatch are handed in by fishing crews. These are added to the number of carcase finds. A number of the stranded harbour porpoises found dead show clear indications of having been caught as bycatch (such as death by drowning, scarring from nets, missing body parts, etc.), although it is not possible to say whether trawls or set nets were the cause. The cause of death can only be established beyond doubt where animals are not in an advanced state of decay. The precise bycatch rate is therefore very hard to determine. In relation to all carcases found on the Mecklenburg-Western Pomerania coast between 2003 and 2012, 7.9% were bycatch and 3.6% were suspected bycatch. In relation to carcase finds with known cause of death, 50% were bycatch and 19% were suspected bycatch (data compiled by the German Maritime Museum Stralsund on behalf of the Mecklenburg-Western Pomerania state nature conservation, environment and geology agency).

The condition of the harbour porpoise population in the Baltic Sea remains a cause for concern overall. According to SCANS (Small Cetacean Abundance in the North Sea and adjacent waters) data for the western Baltic Sea, the population shrank in the space of 11 years from approximately 27,920 (1994) to 10,600 (2005). While the MiniSCANS programme once again arrived at some 18,500 individuals for the western Baltic Sea in 2012, such a large population increase cannot be explained biologically. Presumably there had been a shift in the main distribution range.

Most of all, however, the eastern population is under extreme threat, with SAMBAH data indicating only some 450 individuals left (confidence interval: 100 to 1,000 individuals.

Major risks for harbour porpoise

Harbour porpoises are mostly caught as bycatch in the set nets used to harvest cod and flatfish. Juvenile and inexperienced individuals are most likely to become entangled. The longer the setting time (the length of time a net is in the water), the greater the risk of a harbour porpoise getting caught up in it.

There is greatest potential for conflict with set net fisheries where porpoise density is high and set net fishing is widespread. This is the case in the western Baltic Sea.

There is also major potential for conflict in the eastern German Baltic Sea, however, where the harbour porpoise population is critically at risk.


Average harbour porpoise density and gillnet flag markers sighted in the Baltic Sea. A: Winter; B: Spring; C: Summer. Based on aerial surveys 2002-2006. Densities computed for a 10x10 km grid. Symbol size corresponds with frequency. Conflict potential calculated as the product of porpoise density and average set net density in surrounding cells. (Figure: ICES 2008b)

Average harbour porpoise density and gillnet flag markers sighted in the Baltic Sea.

Notable risk to diving seabirds

Set net fisheries are a hazard to all seabird species that dive for food. The species at greatest risk are fish-eating species such as loons, grebes and auks that also pursue their prey underwater. These do not notice nets until it is too late and get caught up in them.

Set nets are also a hazard, however, to sea ducks such as common and velvet scoter, long-tailed duck and common eider that feed on mussels and other demersal organisms in shallow waters. These feeding grounds are simultaneously important fishing grounds for set net fisheries.

 

Seabirds can be caught as bycatch in all types of gill and trammel net, from small-mesh herring nets to large-mesh flatfish nets. Depth, turbidity, time of day and net setting time also affect the amount of bycatch.


Carcase of eider duck (Somateria mollissima) caught in a set net. Photo: M. Vetemaa
Carcase of eider duck (Somateria mollissima) caught in a set net. Photo: M. Vetemaa

Especially high risk in resting and wintering areas

Bycatch of seabirds is a problem most of all where the fishing grounds of set net fisheries overlap with seabird resting and feeding grounds. Special protection areas (SPAs) under the EU Birds Directive along the coast and in the EEZ are especially important as wintering areas for seabirds. Set net fishing nonetheless so far goes on unabated in such areas. Examples include Greifswald Bodden and the Pomeranian Bay, both of which are important wintering areas for sea ducks and loons.

Further research by Bellebaum (2011) in German Baltic Sea waters showed the following:

  • Seasonally speaking, bycatch rates in bottom set net fisheries for cod, flounder, salmonids, zander, pike and perch are at their highest in the winter months from December to April. Bycatch rates in the pelagic set net herring and needlefish fishery are high between January and May. The winter and spring months are a time of year when particularly large numbers of seabirds are present in their wintering areas.
  • Extrapolations showed the herring fishery in the Greifswald Bodden (Feb-ruary to May) alone to have between 918 and 2,259 bycatch victims a year.
  • Most bycatch occurs at depths of up to 10 m.
  • Regardless of fishing gear and fishery target species, benthos-eating div-ing ducks and sea ducks consistently account for over 50 percent of sea-bird bycatch. Diving ducks (such as greater scaup, tufted duck and com-mon pochard) predominate with 65 percent of individuals in bodden (brackish lagoon) waters, while sea ducks (such as common eider, long-tailed duck and common scoter) predominate with 47 percent of individu-als in outer coastal waters and the EEZ (source: Bellebaum, 2011; see PDF).

Seasonal sensitivity of seabirds in relation to the mortality from bycatch in gillnets (southern Baltic Sea, 2000-2008) based on the weighted frequency 17 diving seabirds. (Sonntag et al. 2012)

Seasonal sensitivity of seabirds in relation to the mortality from bycatch in gillnets (southern Baltic Sea, 2000-2008) based on the weighted frequency 17 diving seabirds. (Sonntag et al. 2012)

 
 

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