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Do Silent Ships See More Fish? Comparisons of Walleye Pollock Backscatter Recorded by a Conventional and a Noise-Reduced Research Vessel in Alaska

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sea and skyscape
The NOAA ships Miller Freeman (left) and Oscar Dyson (right) in Dutch Harbor, Alaska. Although the ships are of similar length, the Dyson has approximately 30% more displacement and approximately 40% more horsepower. Photo by Alex De Robertis.

It has been widely reported that, under some circumstances, fish detect and avoid approaching vessels. A sizeable body of work has demonstrated that fish respond to vessels by diving towards the seafloor or by moving horizontally out of the vessel’s path. These reactions are often initiated well before the vessel passes over the fish. Such vessel-induced avoidance behavior is potentially a major source of error in surveys of fish populations; this has led to concern that vessel avoidance will bias survey results used in fish stock assessments. Acoustic surveys estimates are thought to be particularly vulnerable to vessel avoidance as only those fish directly below the vessel are enumerated, and the intensity of the acoustic return from a fish is strongly dependent on the orientation of the fish.

NOAA is building four noise-reduced fisheries research ships of a single design that are intended to conform to international recommendations for underwater radiated noise. The first of these, the NOAA ship Oscar Dyson, operates in the North Pacific, where it is scheduled to replace the conventional (i.e., not noise-reduced) NOAA ship Miller Freeman as the primary vessel used to continue a long time-series of acoustic-trawl assessment surveys of walleye pollock in Alaska. Because the Dyson is a noise-reduced vessel whereas the Freeman is not, there is concern that biomass indices derived from the two vessels will differ.

To ensure consistent results as the acoustic surveys transition to the Dyson, the Alaska Fisheries Science Center’s (AFSC) Midwater Assessment and Conservation Engineering (MACE) program has undertaken a series of field experiments designed to establish if walleye pollock differentially avoid the two ships.

 

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