Reduction of seal-induced catch and gear damage by modification of trap-net design: Design principles for a seal-safe trap-net

Abstract

During the last decade, seal-induced catch and gear damage have increased dramatically in the coastal trap-net fishery in the northern Baltic Sea. Our trials show that it is possible to markedly reduce seal damage by appropriate gear modifications and by careful choice of netting material. Five trap-net modifications and two traditional traps (four replicates of each) were compared under commercial fishing conditions. Modified traps were equipped with various types of fish bags made of strong seal-safe netting and a wire-grid in the funnel to prevent seals from entering into the bag. Four of the five modified models caught as much or more salmon as the traditional traps. In traditional traps, 30–50% of the total observed salmon catch was damaged. Trap modifications that were equipped with a fish bag made of double-layer netting held under tension offered the best protection; only 1–2% of the catch was damaged using these modifications. The proportion of seal-damaged catch varied between 16 and 27% for other modified trap designs. The use of thick and stiff polyethylene netting in the wings and middle chambers effectively prevented entangling of fish and thereby reduced their vulnerability to seal predation. Moreover, the seal-induced damage in the thick net was negligible compared with that of the thinner and more elastic nylon net of traditional traps.