Quantifying fish escape under a bottom trawl

Abstract

In this study, we investigated the length-dependent escape of groundfish underneath a commercial bottom trawl. Escaped fish were captured in three collecting bags (center (bosom) and wings (port and starboard)) mounted underneath the fishing line and behind the rockhopper footgear and compared to fish that were captured in the codend. Generally, how fish escaped under the trawl was often species-specific and differed according to size. For roundfish, length-dependent escape was pronounced for Atlantic cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and blue ling (Molva dypterygia) with the proportion of escapes decreasing as fish length increased. However, saithe (Pollachius virens) showed limited a length-based effect and redfish (Sebastes spp.) none. For flatfish, the escape proportion of American plaice (Hippoglossoides platessoides) and dab (Limanda limanda) decreased as fish length increased. However, European plaice (Pleuronectes platessa) had a very small length effect. Monkfish (Lophius piscatorius) escape decreased with size. However, monkfish retention probability remained less than 30% at the largest lengths observed. Differences in escape proportion between day and night were found for Atlantic cod, haddock, and dab, but not for other species. Small Atlantic cod and haddock escaped more often at night, and mid-sized dab (between 18 and 30 cm) escaped more often during the day. Results show that overall trawl selectivity is strongly affected at the trawl mouth, particularly for small fish of specific species.