Understanding and predicting size selectivity of saithe (Pollachius virens) in trawl codends

Abstract

Saithe (Pollachius virens) is common in the Northern Atlantic and is often captured with demersal and pelagic trawls that comprise diamond mesh codends. However, size selectivity data is scarce for saithe. The meshes in these trawl codends have limited openness due to the drag forces acting on the catch accumulation in the aft end, which reduces escape opportunities for roundfish like saithe. Shortening lastridge ropes attached to codend selvedges can increase codend mesh openness and make it more constant though the capture process potentially improving size selectivity. Thus, this study investigated the size selectivity of saithe in diamond mesh codends with two different configurations: lastridges with the same length as the codend selvedges and 15% shortened lastridge ropes. Results from experimental fishing proved that shortened lastridge ropes increased codend size selection of saithe. By combining these results with saithe morphology data relevant for mesh size selectivity, we were able to gain deeper understanding on codend size selectivity for this species and the influence of the shortened lastridge ropes. The analysis conducted enabled us to predict the size selectivity of saithe for codends with a mesh size different to that tested in the sea trials. By comparing our predictions for codend size selection with results obtained based on experimental fishing reported in literature, we demonstrate that our predictions are well-aligned with experimental results obtained for a wide range of codend mesh sizes. Our approach and results can act as guidelines for technical regulations and codend design in saithe fisheries leading to exploitation patterns that comply with local legislations as the minimum legal size defined by the authorities.