Structural Modelling of the Bell-Shaped Size Selection in a Bottom Trawl: A Case Study for a Nephrops Directed Fishery Aiming at Reducing Simultaneously Catches of Cod

In some fisheries the demands for species and size selectivity have forced gear technologists and fisheries managers to adapt selective systems in their trawl fisheries which utilize more than one selective device simultaneously. In the Swedish bottom trawl fishery targeting Nephrops while aiming at avoiding catches of Cod, the selective system consists of a sorting grid followed by a size selective codend. The size selection curve for this system may undertake a characteristic bell-shaped curvature. An approach that can handle a bell shaped curvature is to use a structural-based model which is directly modelling the contribution of the individual part to the overall performance of the system. We demonstrate that this approach can appropriately describe the experimental size selection in a system composed of a sorting grid followed by a size selective codend. Further this approach provides a direct quantification and deeper understanding of the catch processes of the individual parts of the system to the overall size selection in the fishing gear. The structural model approach also enables predictions of the overall performance to be obtained if an individual part of the gear is varied, exemplified by differences in grid bar spacing in this study.