Fishing Behavior and Fish Populations

The mortality exerted by mixed fisheries on marine resources crucially depends on the spatial overlap between the fishery and the resources. Here, we focus at using optimality models to understand and predict the spatial and temporal distributions  of fishing effort. For mixed fisheries, the spatio-temporal distribution of fleets can be viewed as the sum of all individual decisions made by skippers. Dynamic state variable models allow modelling the individual decisions under the assumption of utility maximization. The factors taken into account are the different size categories of fish of different species, fish prices, and fuel costs are taken into account. When fisheries management puts constraints on the total landings by species, or the fishing effort of fishing fleets, it effectively alters the optimal choices of the individuals. As a result, fishing effort distribution changes, and the overlap and thus the mortality exerted on the different resource components. Also, discarding practices in the fleet may change . In this study, we analyse effects of landings and effort quota on the fishing behaviour of different fleets targeting size structured demersal fish populations. These stocks are managed by annual TACs, divided in ITQs for individual vessels. In this system, a vessel has to take into account meeting the ITQs for both species, given the realized catch in a sequence of decisions on effort allocation and discarding. We study whether discarding and effort reallocation disrupt the linkage between effort and landings, which may bias our perception of the fish stocks and negatively affect the quality of the management advice.