Escaping the net: Assessing midwater gear selectivity for the Joint United States and Canada Integrated Ecosystem and Pacific hake (Merluccius productus) Acoustic-Trawl survey

Sabrina Beyer, Julia Clemons, Alicia Billings, Stephen de Blois, Elizabeth Phillips, John Pohl, Rebecca Thomas, Amanda Vitale, Stéphane Gauthier

Acoustic-trawl surveys use trawl catches to validate the species and size composition of fish aggregations detected acoustically. However, certain sizes of fish may be more likely to escape some trawls, which can bias the size and age distribution of the catch used to estimate biomass. To quantify size-selectivity, we studied 3 midwater trawls used for the United States and Canada joint survey of Pacific hake (Merluccius productus). The survey most recently used an Aleutian Wing Trawl (AWT) with different codend liners until 2023, then switched to a Multi-Function Trawl (MFT) in 2025. To prepare for the switch, we assessed differences in escapement and catch rates using recapture nets, and in paired trawls of both net-types sampling the same aggregations. All nets retained greater than 85% of hake longer than 30-cm (age 2+). In general, the MFT was more efficient than the AWT, with near full retention of all sizes. A substantial fraction of small hake (age 0 to 1) escaped the AWT. A power analysis indicated a low probability of detecting differences in escapement from the AWT with different liners. Gear selectivity information is important to improve the accuracy of fishery survey data and account for changes in sampling gear.