Gear modifications for bycatch reduction in the Bay of Biscay demersal trawl fishery

<p>Bottom trawl fisheries have great social and economic importance for coastal communities in the Basque Country. However, the activity of the demersal trawl fishery in the Bay of Biscay, which includes the coastline of the Basque country, can be compromised due to its multispecies nature, high proportion of unwanted species in the catch, and the increasingly strict legislation implemented aiming to ensure sustainable fisheries. In this scenario, developing gear modifications to reduce unwanted bycatch is increasingly important. This thesis presents results from six selectivity research papers that address unwanted catch issues in the Bay of Biscay bottom trawl fishery. <p>Hake (<i>Merluccius merluccius</i>), horse mackerel (<i>Trachurus trachurus</i>), blue whiting (<i>Micromesistius poutassou</i>), and mackerel (<i>Scomber scombrus</i>) are some of the most relevant species for the bottom trawl fishery in the Bay of Biscay and constitute the main choke species in the fishery. Papers I and II present selectivity results for different square mesh panel (SMP) designs aimed to improve fish-SMP contact probability and release efficiency for these species. Among the designs tested, the results demonstrate that modifying SMP size and position can increase fish contact probability with the SMP. Paper III investigates the size selection process through SMP and codend meshes for blue whiting based on fish morphology and behavior. The results demonstrate that SMP size selection can be explained by different fish contact angles with SMP meshes, which allows making accurate predictions for fish size selectivity. Paper IV explores the effect of alternative SMP and codend mesh combinations on the size selectivity of hake and blue whiting and on the fishery exploitation pattern for a variety of fish population scenarios. The results demonstrate that changes both in SMP and, especially, codend designs can have a significant effect on the size selectivity and exploitation patterns of hake and blue whiting. This paper also outlines new ways for investigating and illustrating the effect of multiple gear changes on the size selectivity and exploitation pattern indicators by means of diagrams named treatment trees. These may aid in the identification of promising gear designs and help the industry in the pursuit of specific catch goals. In Paper V a trawl configuration for species separation is tested. This new configuration intends to guide those species that hold themselves close to the lower panel of the trawl through a horizontal grid into a lower codend, while the rest of the species are directed to an upper codend. The findings in Paper V demonstrate that, under the conditions in which this fishery operates, the trawl configuration tested is not able to efficiently separate species based on their behavior. Finally, in Paper VI the effect of shortening codend lastridge ropes on codend size selectivity compared to a standard codend is tested, and fish escape chances estimated based on fish morphology. The results show that a codend with shortened lastridge ropes can improve the size selectivity of horse mackerel and blue whiting, while the selectivity of hake was not affected. The results indicate species-dependent variability in the ability to utilize open meshes located at different places. <p>In general, the work presented in this thesis provides technological advances and knowledge that contributes with guidance on how to reduce unwanted bycatch and generate alternative exploitation patterns in the Bay of Biscay demersal trawl fishery.