Do silent ships see more fish? Comparisons of walleye polloc kbackscatter recorded by a noise-reduced and a conventional research vessel in Alaska

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Avoidance of approaching vessels by fish is a major source of uncertainty in fisheries resource surveys. Vessel noise is considered to be the primary stimulus for vessel avoidance, and standards for noise emission by research vessels have been established under the auspices of ICES to minimize vessel avoidance. Although several vessels meeting these criteria are now in service, the effectiveness of noise reduction on vessel avoidance remains poorly understood. For example, a recent study indicated that noisereduced vessels can elicit increased avoidance reactions compared to conventional vessels. This raises concern regarding the current understanding of vessel avoidance reactions. We conducted a series of comparisons of fish abundance recorded by the NOAA ships Oscar Dyson, a noise-reduced vessel and Miller Freeman, a conventional research vessel. Experiments were conducted in 2006-2008 during five acoustic-trawl surveys targeting walleye pollock (Theragra chalcogramma) over a broad range of depths, environmental conditions and behavioral situations (e.g. non-spawning vs. spawning). The vessel comparison results demonstrated that survey abundance estimates could be up to 31% higher from the noise-reduced vessel in some situations. The discrepancy between vessels tended to decrease with depth, which is consistent with a decreasing response to a stimulus propagating from the vessel. Observations from a buoy-mounted echosounder confirmed that the discrepancies are due to decreased avoidance of the noise-reduced vessel. These results illustrate that biases may be introduced into a fish stock abundance time series by switching vessels, particularly when initiating use of a noise-reduced vessel designed to minimize vessel avoidance.