Silent ships sometimes do encounter more fish. 1. Vessel comparisons during winter pollock surveys

De Robertis, A., Wilson, C. D., Williamson, N. J., Guttormsen, M. A., and Stienessen, S. 2010. Silent ships sometimes do encounter more fish. 1. Vessel comparisons during winter pollock surveys. – ICES Journal of Marine Science, 67: 985–995. Avoidance of approaching vessels by fish is a major source of uncertainty in surveys of fish stocks. In an effort to minimize vessel avoidance, international standards for underwater-noise emission by research vessels have been established. Despite widespread investment in noise-reduced vessels, the effectiveness of noise reduction on vessel avoidance remains poorly understood. Here, we report on vessel comparisons of pollock abundance recorded by the NOAA ships “Oscar Dyson” (OD), a noise-reduced vessel, and “Miller Freeman” (MF), a conventionally designed vessel. The comparisons were made during three acoustic surveys of prespawning aggregations of walleye pollock (Theragra chalcogramma) in Alaska. The experiments demonstrate that a noise-reduced vessel will detect significantly more fish backscatter than a conventional vessel in some situations. OD detected 31% more pollock backscatter than MF in the Shumagin Islands, where pollock were distributed between 100 and 200 m deep, and 13% more pollock backscatter in Shelikof Strait, where pollock were primarily distributed 200–300 m deep. However, there was no difference in the Bogoslof Island area where pollock were found at 400–700 m. In the Shumagin and Shelikof areas, the discrepancy between vessels tended to decrease with fish depth, consistent with a decreasing response to a stimulus propagating from the surface. Analysis of the depth distributions of pollock supports the conclusion that the discrepancies in backscatter stem from differential behavioural responses to the two vessels.