The Effects of Wave-Induced Vessel Motion on the Geometry of a Bottom Survey Trawl and the Herding of Yellowfin Sole (limanda Aspera)

Bottom trawl surveys need constant catchability over time to produce estimates of relative abundance that are most informative for stock assessment modeling. However, environmental conditions during surveys can potentially produce both interannual and long-term changes in catchability. Here we consider the effects of one environmental variable, sea state, on vessel motion, trawl performance, and, presumably, catchability. We present the results of 2 field experiments performed on chartered commercial trawlers typical of those used by the National Marine Fisheries Service for conducting its eastern Bering Sea bottom trawl survey. The first experiment shows that wave-induced vessel motion is transmitted down the towing warps, creating vertical oscillation in both trawl bridles and the footrope, which potentially influence both the herding and footrope escapement of fish. The second experiment shows that the herding of yellowfin sole (Limanda aspera) by the survey trawl decreases with an increase in wave height. A proposed mathematical model of flatfish herding is used to provide a hypothetical mechanism describing the effect of surface waves on flatfish herding. In addition, the annual biomass estimates of yellowfin sole determined from the bottom trawl survey data are shown to decrease with increasing wave height. The potential impacts of these relationships on the yellowfin sole assessment model are then considered.