Gradients of Variation in the At-Vessel Mortality Rate between Twelve Species of Sharks and Skates Sampled through a Fishery-Independent Trawl Survey in the Asinara Gulf (NW Mediterranean Sea)

SIMPLE SUMMARY: The impact of human activities on marine environments is driving many elasmobranch species toward the brink of extinction. Fishing activities play the most important role in causing the mortality of these animals and the substantial decrease observed in their populations. In this context, we aimed to measure species resistance to catch of poorly selective fishing gear, such as trawl, and investigate the patterns of variation found in the sample. Fishery-independent standardized data indicated that small-sized deepwater sharks are the most affected by stress due to trawl catch. On the contrary, large and coastal species, particularly skates, appeared more resistant to trawl capture. Overall, the at-vessel mortality rate of the studied species results from the intermingled effect of inhabited depth, species type, and fish size. The information provided can help refine best practices to reduce direct and indirect fishing mortality of the studied species in trawling activities. ABSTRACT: Elasmobranchs are priority species for conservation due to their rapid decline determined by the unbalanced struggle between a fragile bio-ecology and strong anthropogenic impacts, such as bycatch from professional fishing. In this context, measuring species resistance to catch of poorly selective gear is of paramount importance. During June–October 2022, five experimental fishing campaigns were carried out in the Asinara Gulf (northern Sardinia) through 35 geographically and bathymetrically representative hauls of an area between 30 and 600 m in depth. Skates prevailed over sharks in the number of species, with seven and five species, respectively. We first evaluated the status of each individual with respect to stress due to the trawl’s catch using a three-graded scale. We also recorded individual biometrics (total and disk length, weight and sex, and maturity for males) on board by implementing the best practices in manipulating individuals for physiological recovery and release at sea. After capture, skates resulted in generally better conditions than sharks, although deepwater species of both groups exhibited a worse state than coastal species. The estimated vitality rates also depended on the size of the individuals. This work provides standardized data on the intermingled effect of size, species type, and inhabited depth on the resistance response of some elasmobranch species against capture by trawl fishery activities.