A comparison of acoustic backscatter between a trawling and a free-running vessel for eastern Bering Sea walleye pollock (Theragra chalcogramma)

High-quality acoustic backscatter is continuously collected with Simrad ES-60 echosounders during the annual eastern Bering Sea (EBS) bottom trawl survey. Scientists at the Alaska Fisheries Science Center are examining the potential of combining the acoustic backscatter data collected between trawl stations with the trawl catch data to improve the precision of the survey index of walleye pollock (Theragra chalcogramma) abundance. A previous study we conducted [von Szalay, P.G., Somerton, D.A., Kotwicki, S., 2007. 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)? Fish. Res. 86, 77–83.] showed a high correlation between walleye pollock trawl catches and simultaneously collected acoustic backscatter; in this study, we compared walleye pollock acoustic backscatter collected while trawling at 3 knots with that of a free-running vessel traveling at 10 knots. Acoustic data were collected by two vessels during the 2006 and 2007 EBS bottom trawl surveys in a pair-wise fashion at 134 trawl stations. The free-running transects were conducted either immediately before or immediately after the trawl hauls and were parallel to the trawl paths but offset by approximately 250 m. The pollock acoustic backscatter was significantly greater for the trawling vessel in both the layer below the headrope and in the layer above the headrope, and was independent of the order in which the trawling and free-running transects were performed. A comparison of the seabed echo between trawling and free-running did not show a significant difference, suggesting that the discrepancy in the pollock backscatter is due to differences in fish behavior and not to differences in vessel attitude between the two operational modes. The higher backscatter values associated with trawling may be due to reactions of fish within a school that produce a coordinated swimming response, which has the potential to increase the average target strength.