Evaluating the role of bycatch reduction device design and fish behavior on Pacific salmon (Oncorhynchus spp.) escapement rates from a pelagic trawl

Abstract

Pacific salmon (Oncorhynchus spp.) bycatch is a significant driver in the management of walleye pollock (Gadus chalcogrammus) pelagic trawl fisheries in the North Pacific. Various bycatch reduction devices that permit salmon to escape from the trawl (‘excluders’) have been developed. High variability in escapement rates from the excluders underscores a lack of understanding regarding mechanisms that promote escapement. We developed an excluder that provided a 360˚ open area for escapement during towing, turns, and haulback. We used computational fluid dynamics simulations and flume tank testing to expedite development by producing quantitative flow and net mensuration data, which reasonably predicted performance at full scale under commercial conditions. During at-sea trials, salmon escapement rates were high (mean 0.58 ± 0.18); however, more comprehensive testing is needed among salmon species and over a breadth of fishing conditions. Video footage revealed that salmon disproportionately escaped by swimming forward from aft of the excluder during haulback and turns. This highlights the importance of providing an open path to the escapement area during these periods. Retention of any salmon despite the expansive, easily accessible open area reflects the important role played by perception of the open area and motivation of salmon to escape at that point in the fishing process.