Institute of

Baltic Sea Fisheries

Reduction of unwanted bycatch

Innenansicht eines Schleppnetzes mit Leinen unter dem Fluchtfenster im hintersten Teil eines Schleppnetzes (dem Steert) zur Stimulation einer Fluchtreaktion von Dorschen
Auftriebskörper unter dem Fluchtfenster leiten die Fische nach oben. (© Thünen-Institut/D. Stepputtis)

A current focus is the improvement of the selection of trawls for several species at the same time (multi-species selection). For this we use the behavior of the fish. Previous development approaches have focused on improving the selection for a species, for example for cod in bottom trawling in the Baltic Sea.

As a result, it has hardly been possible to catch other fish such as flatfish, which are almost inevitably caught, more selectively and thus reduce unwanted by-catches of these species. A main problem here is the different selection properties of the individual fish species caused by the very different body shape, so that innovative solutions for species separation are required.

We can take advantage of the different behavior of fish - cod, for example, flee usually up, flatfish down. The development of suitable escape options, such as larger meshes in the upstream network or species-specific escape grids, is therefore of great importance. However, the efficiency of these developments depends crucially on whether the unwanted fish and other creatures also use these escape options. With the help of underwater observations, the behavior of the fish in the net and the properties of the net when fishing can be examined and used to improve the network selection.

An example: It is not possible to install escape windows with large meshes in the codend (rear part of the trawl) in the Norway lobster fishery in the North Sea, in order to enable fish species such as cod and whiting to escape without a large part of the catch of the smaller ones To lose target species. If you install such an escape window in the front part of the network, no Imperial grenade will escape - but the window is hardly used by the fish. The solution could be to stimulate the fish in front of the net so that they use the escape options offered.

We achieved very good results by installing lines with buoyancy bodies near the escape windows that move in the current and thus irritate the fish in the net and guide them upwards to the escape windows. This significantly increases the likelihood of the unwanted fish escaping.

Sometimes it's the little things that can make a big difference. And the smaller the vessels, the more urgently simple, robust, inexpensive and efficient solutions are needed.



Here you can see the rear part of a bottom trawl with an escape window, specially developed for flatfish. The camera looks towards the vessel.