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Abundance and Distribution of Age-0 Walleye Pollock in the Eastern Bering Sea Shelf During the Bering Arctic Subarctic Integrated Survey (BASIS) in 2014
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    We conducted an acoustic-trawl (AT) survey of the eastern Bering Sea during the 2014 Bering Arctic Subarctic Integrated Survey (BASIS). Because of the mixed species assemblages in the study area, we implemented proportional allocation of backscatter to the multiple species found in the trawl catch samples, modeled after the methods used for the 2011-2012 BASIS survey analysis. The abundance of age-0 walleye pollock (Gadus chalcogrammus) was estimated by combining acoustic measurements, the acoustic scattering properties of the dominant organisms, and the relative proportions of animals from trawl samples. Our survey results found that age-0 pollock dominated the near-surface and midwater fishes by number and we estimate their numerical abundance at 6.85 × 1011 fish. This estimate is approximately 9 times higher than what was reported in 2011 and 3.5 times higher than what was reported in 2012. Nearly half of the overall age-0 abundance was observed in the upper 30 m of the water column. Spatially, the highest numerical densities of age-0 pollock were observed where bottom depths were less than 75 m. Sensitivity analyses indicated that both the method used to assign length and species composition from trawl hauls to backscatter (expert assignment versus using the nearest haul) had a relatively modest impact on our estimates of age-0 pollock abundance. However, both the use of 1) a trawl selectivity function to account for escapement of small fish and 2) an alternative pollock target strength-length relationship that included observations of age-0 pollock changed abundance estimates by as much as 22%. These findings suggest that more research is needed in these areas to obtain more accurate abundance estimates.
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