Bycatch in Cetaceans from the North-Western Mediterranean Sea: Retrospective Study of Lesions and Utility of Bycatch Criteria

Bycatch is the most common cause of death of small delphinids worldwide, including the Mediterranean Sea. The diagnosis of bycatch as cause of death in stranded cetaceans depends on the cumulative presence of multiple findings, termed bycatch criteria. In this study, we retrospectively evaluated the presence of bycatch criteria in 138 necropsied cetaceans, 136 stranded and 2 confirmed bycaught, in the Catalan Mediterranean Sea across a 13-year period. With the aim of identifying the most specific and reliable bycatch criteria, the animals&rsquo: cause of death was classified as either bycaught or other causes. Animals were necropsied according to standard procedures with complete histopathological examination and ancillary diagnostic techniques. We reviewed the necropsy reports and photographs of 138 cetaceans of seven species. Bycatch had been determined as the cause of death/stranding in 40 (29%) necropsied cetaceans. Both sexes were equally represented in the bycatch group. Bycatch was diagnosed in the Mediterranean common bottlenose dolphin (10/14: 71.4%), striped dolphin (29/108: 26.9%), and Risso&rsquo:s dolphin (1/11: 9.1%). Sixty-seven out of 98 (68.3%) cetaceans that had been classified as non-bycatch had one or two bycatch criteria. Cetaceans with two and three major criteria had an overlap of causes of death, as some animals were diagnosed with bycatch and others with other causes of mortality. Animals with four criteria were invariably diagnosed as being bycaught. Recent feeding, absence of disease, good nutritional status, marks of fishing gear, multiorgan intravascular gas bubbles, hyphema and amputations or sharp incisions presumably inflicted by humans were significantly more likely to result in a diagnosis of bycatch, while loss of teeth and cranial fractures were not. None of the dolphins diagnosed as bycatch had ingested fishing gear. Our results highlight the relevance of bycatch as the cause of death of dolphins in the Mediterranean and suggest that some criteria traditionally linked to bycatch are not specific for bycatch in our region.