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

Abstract

Bycatch (accidental capture in fisheries) is the most common cause of death of small delphinids worldwide. Determining bycatch to be the primary cause of death in a free-ranging stranded cetacean relies on the detection of lesions termed “bycatch criteria”, that vary in their specificity. Here, we retrospectively reviewed the bycatch criteria found in 138 necropsied cetaceans from the North-western Mediterranean Sea in a 13-year period to identify the most reliable criteria. Bycatch was determined as the cause of death/stranding in 40 (29%) of 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’s dolphin (1/11; 9.1%). Recent feeding, absence of disease, good nutritional status, marks of fishing gear, 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 cause of death of dolphins in the Mediterranean and show that some criteria traditionally linked to bycatch are not specific for bycatch in our region.