Two Saildrone unmanned surface vehicles (USVs) were instrumented with echosounders and deployed in the Bering Sea to make acoustic observations of walleye pollock for 103 days. The Saildrones proved to be a suitable platform for measurement of fish backscatter: they produced high-quality measurements at wind speeds of <10 m s−1. Pollock backscatter measured from the Saildrones was compared to backscatter measured by a noise-reduced research vessel during two “follow-the-leader” comparisons. In a location where pollock were shallowly distributed (30–100 m), there was evidence of depth-dependent avoidance reactions to the ship. This behaviour was not evident in a second comparison, where the fish were primarily deeper than 90 m. Opportunistic comparisons indicate that backscatter where the ship and USVs crossed paths was similar. However, the Saildrones observed higher densities of shallow fish, which is consistent with the diving response inferred in the first follow-the-leader comparison. USVs equipped with echosounders, like all platforms, have inherent strengths (endurance) and limitations (species identification) that should be carefully considered for a given application. USVs can complement traditional ship-based surveys by increasing the spatial and temporal extent of acoustic observations, and their use is likely to become more widespread.
In 2020, the developing COVID-19 pandemic disrupted fisheries surveys to an unprecedented extent. Many surveys were cancelled, including those for walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea (EBS), the largest fishery in the United States. To partially mitigate the loss of survey information, we deployed three uncrewed surface vehicles (USVs) equipped with echosounders to extend the ship-based acoustic-trawl time series of pollock abundance. Trawling was not possible from USVs, so an empirical relationship between pollock backscatter and biomass established from previous surveys was developed to convert USV backscatter observations into pollock abundance. The EBS is well suited for this approach since pollock dominate midwater fishes in the survey area. Acoustic data from the USVs were combined with historical surveys to provide a consistent fishery-independent index in 2020. This application demonstrates the unique capabilities of USVs and how they could be rapidly deployed to collect information on pollock abundance and distribution when a ship-based survey was not feasible. We note the limitations of this approach (e.g. higher uncertainty relative to previous ship-based surveys), but found the USV survey to be useful in informing the stock assessment in a situation where ship-based surveys were not possible.
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