Combining bottom trawls and acoustics in a diverse semipelagic environment: What is the contribution of walleye pollock (Gadus chalcogrammus) to near-bottom acoustic backscatter in the eastern Bering Sea?

Abstract

The abundance of walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea is estimated in part through fisheries-independent acoustic trawl (AT) surveys, which currently use acoustic backscatter data down to 3 m above the bottom. A large portion of adult pollock are demersal, and these estimates will become more accurate if the survey is extended closer to bottom. The purpose of this project was to assess the feasibility of extending the AT survey closer to the bottom by estimating the contributions of each demersal fish species to observed acoustic backscatter in the highly diverse near-bottom region. This was accomplished by fitting a regression model to simultaneously collected acoustic backscatter and bottom trawl catch data. Pollock were the dominant source of acoustic backscatter among demersal species, accounting for 85.9% ± 4.8% of acoustic backscatter (mean ± standard deviation). A method was developed to extend the AT survey to within 0.5 m of the bottom, and when applied to the 1994–2014 surveys, pollock biomass increased by a mean of 28% ± 9%.