Development and application of bottom trawl instrumentation systems in fisheries resource assessment

Catch results from bottom trawl surveys conducted by the Resource Assessment and Conservation Engineering (RACE) Division of the Northwest and Alaska Fisheries Center, National Marine Fisheries Service, NOAA, are used to estimate the abundance of demersal fishery resources in the northeast Pacific Ocean and the Bering Sea. The magnitude and composition of bottom trawl catches is influenced, among other considerations, by the physical performance of the trawl. An understanding of the factors that affect trawl performance is essential for correct interpretation of trawl survey catch data, which in turn has an impact on the management of commercial fisheries. In order to study trawl performance, hydroacoustic net mensuration instruments were developed in 1974 and have been used since then by RACE biologists. These observations have revealed, among other things, that the horizontal opening of bottom trawls can vary a great deal and that the trawls can be quite erratic in terms of attaining and maintaining contact with the bottom during a tow. Programs aimed at the development of systems for specifically evaluating these two trawl performance parameters have been initiated, and preliminary results have been promising. Possible avenues for further development of trawl instrumentation are discussed.