Correlating trawl and acoustic data in the eastern Bering Sea: A first step toward improving biomass estimates of walleye pollock (Theragra chalcogramma) and Pacific cod (Gadus macrocephalus)?

The charter vessels used for the annual eastern Bering Sea bottom trawl survey have recently been equipped with echosounders, which are relatively sophisticated compared to the traditional depth sounders, and are capable of collecting acoustic backscatter data of a quality approaching that of scientific echosounders. Because these echosounders provide a large amount of inexpensive and continuous backscatter data between trawl stations, it is of interest to determine whether these acoustic data can be used in conjunction with the trawl catch data to improve the precision of walleye pollock (Theragra chalcogramma) and Pacific cod (Gadus macrocephalus) trawl indices of abundance. Catch and acoustic backscatter data collected from 98 stations executed during the 2005 field season were analyzed to estimate the correlation between trawl catch per unit swept area and acoustic backscatter integrated over various layers above the seafloor. The correlation for walleye pollock was good, with the highest correlation obtained for the layer between the seafloor and the headrope (r2 = 0.64). The correlation for layers above the headrope monotonically decreased with increasing height, indicating a lack of vertical herding by pollock. There was no correlation (r2 = 0.02) between trawl and acoustic data for Pacific cod, the only other important fish source of acoustic backscatter.