Using species-specific behavior to improve catch efficiency of target species in mixed trawl fisheries

Demersal trawl fisheries are increasingly challenged by new and more ambitious gear regulations alongside rising fuel costs. However, knowledge of behavioral differences between species are yet poorly integrated and exploited in commercially operated trawl designs. In the demersal mixed species trawl fishery for Nephrops ( Nephrops norvegicus ), many fish species are herded by the netting and actively avoid contact with the meshes as opposed to Nephrops , for which most individuals are tumbling along the bottom panel of the gear towards the codend. By reducing the mesh size of the entire lower half of the trawl we reduced the loss of marketable sized Nephrops through the bottom panels significantly by 47.2 % (CI: 33.6–60.2 %) in the North Sea. The unchanged catches of the round fish, cod ( Gadus morhua ) and hake ( Merluccius merluccius ) confirm that these species do not come in physical contact with the bottom panel. In contrast, witch flounder ( Glyptocephalus cynoglossus ) escapes through the bottom panel of the baseline trawl as the treatment gear caught 65.1 % (CI: 39.5–104.9 %) more marketable-sized individuals and 259.7 % (CI: 144.4–459.5 %) more undersized individuals. Our results confirm known species-specific behavior in the forward part of the trawl and demonstrate how this can be exploited with simple design changes to increase the catch efficiency for Nephrops and likely other species without affecting the catches of roundfish. The undersized individuals captured will largely escape through the meshes used in commercial codends.