Using acoustics to estimate the fish-length selectivity of trawl mesh

Estimation of the retention probability of a trawlnet traditionally involves conducting experiments during which the fish escaping through the meshes are recaptured using either small-mesh pocketnets attached to the outside of the net or by enclosing the entire trawlnet in a small-mesh net. A new method of estimating the length selectivity of trawl mesh is demonstrated; it does not require the recapture of escaping fish but instead uses standard acoustic methods to estimate the abundance of fish entering the net before mesh selection. The method was applied to the 83-112 eastern otter trawl used by the Alaska Fisheries Science Center (AFSC) to conduct bottom-trawl surveys in the eastern Bering Sea (EBS), and the Aleutian wing trawl used by the AFSC to collect midwater biological samples of walleye pollock (Theragra chalcogramma) during fishery acoustic surveys of the EBS and Gulf of Alaska. The length selectivities of both trawls were also estimated using standard recapture experiments. For both, the estimated lengths at 50% selection (L-50) from the acoustic method were similar to the estimates from the recapture experiments, but the estimated selection ranges were narrower. The advantages of the acoustic method are that it is simpler to use than traditional fish-recapture methods and it does not alter normal trawl performance.