Size selection of redfish (Sebastes spp.) in a double grid system: Estimating escapement through individual grids and comparison to former grid trials

Abstract

We inserted a new double steel grid system consisting of a lower and an upper grid in a four-panel section to increase the cross-sectional area and improve the size selectivity in the Norwegian bottom trawl fishery. We tested its ability to size select and release typical bycatch species in the cod (Gadus morhua) and haddock (Melanogrammus aeglefinnus) directed fishery, i.e. two species of redfish (Sebastes spp.). The redfish selectivity data were analysed using a new model that included direct quantification of the probability that fish will make contact with the grids. The results showed that the proportion of redfish escaping through the two grids did not differ significantly. The release efficiency of the first (lower) grid was, however predicted to be significantly lower than that of the second grid when accounting for the actual fraction of redfish reaching the zones of the individual grids. An estimated 80% of the redfish made contact with at least one of the two grids. However, the release efficiency and overall size selection for redfish of the new double grid system was not significantly better than obtained in former selectivity trials testing the grid systems used in the fishery today. Moreover, this comparison of results showed that the existing Sort-V single grid system releases significantly more redfish than the new double grid system.