Quantifying bycatch of cold-water gorgonians to propose a dynamic management strategy for sustainable artisanal fishing on the mediterranean continental shelf

Abstract

Cold-water coral (CWC) gardens, mainly composed of gorgonians, soft corals, sponges and a highly diversified associated mobile fauna, are among the most complex and structurally significant benthic communities on the Mediterranean continental shelf. These communities, hosting long lifespan species, have limited recovery capacity from disturbances such as bottom-contact fishing activities, widespread on continental shelf environments. These fishing activities encompass both bottom trawling and small-scale fishing using trammel nets or longlines. Bottom trawling impacts on benthic communities have been widely addressed; however, the extent to which small-scale fisheries affect CWCs is less known. In this context, the present study aims to explore and quantify the impact of trammel net fishing on cold-water gorgonian populations dwelling on the continental shelf of Cap de Creus (Northwestern Mediterranean Sea) during two consecutive fishing seasons, and to propose effective management measures to reduce impacts. A high gorgonian bycatch (i.e., the accidently caught of non-target species in a fishery) was observed in more than half of the assessed fishing nets, with a mean rate of 2.4 ± 0.8 (mean ± standard deviation) gorgonian colonies or fragments per 200 m of trammel net. The higher impact was detected in specific areas at 80–90 m depth and when the wave height exceeded 1 m. Trammel nets mostly captured large colonies with a mean height of 20.2 ± 6.6 cm, being clearly higher than the mean height observed in the study area (12.9 ± 5.5 cm), explored with an Autonomous Underwater Vehicle (AUV). Thus, bycatch effects could lead to changes in population structure and overall ecosystem functioning. Although our results demonstrated that impact of small-scale trammel net fishing on CWC gardens in the area is much smaller compared to bottom trawling, it should not be underestimated. Moreover, this impact can be significantly reduced by implementing simple measures, such as avoiding fishing at 80–90 m depth during adverse weather conditions. The success of these measures requires close collaboration with local fishers as part of a dynamic management approach.