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NOAA Technical Memorandum NMFS-AFSC-272

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Development of acoustic-trawl survey methods to estimate the abundance of age-0 walleye pollock in the eastern Bering Sea shelf during the Bering Arctic Subarctic Integrated Survey (BASIS)

Abstract

Here we assess the feasibility of applying acoustic-trawl (AT) methods to quantify the abundance of age-0 walleye pollock (Gadus chalcogrammus) on the eastern Bering Sea (EBS) shelf during the 2011 and 2012 Bering Arctic Subarctic Integrated Salmon International Survey (BASIS).

The abundance of age-0 walleye pollock in the water column was estimated by combining acoustic measurements, the acoustic scattering properties of the dominant organisms, and the relative proportions of animals from trawl samples. Age-0 pollock were expected to dominate much of the backscatter in the survey area to the east of the Pribilof Islands in 2011, and a wider area covering much of the offshore part of the survey area in 2012. The overall numerical abundance estimate for 2011 was 7.0 × 1010 age-0 walleye pollock, and 1.8 × 1011 in 2012. This suggests that the abundance of walleye pollock in the survey area was ~2.6 times higher in 2012 than in 2011. The vertical distribution of age-0 walleye pollock was similar in both years, with ~ 50% of fish observed in the upper 50 m, and ~95% of fish in the upper 100 m of the water column.

A sensitivity analysis indicates that an abundance index of age-0 walleye is relatively robust to the assumptions made in the analysis (e.g., target strengths used, association of trawl catches and acoustic backscatter, net selectivity) and suggests that an AT estimate of age-0 walleye pollock on the EBS shelf is possible in the context of this survey. A series of short-term and long-term recommendations for implementation of an AT-based age-0 pollock abundance index are provided.


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