Linking by-caught cetacean traits to fishing techniques: Insights from two species of small cetaceans.

By-catch is one of the main threats currently looming over small cetaceans worldwide. Improving knowledge of the interactions between fishing activities and small cetaceans is paramount to design cost-effective mitigation measures. In particular, not all individuals within a population may be exposed to the same by-catch risk: in dolphins, juveniles and males appear to be more sensitive to by-catch. Yet, few studies have investigated how individual-level characteristics (such as age, sex, body size) correlate with fishing practices in these species. Using French by-catch observations, declarations, and stranding databases on two small cetaceans (Delphinus delphis and Phocoena phocoena), we explored phenotypic vulnerability to by-catch by correlating the phenotypes of by-caught individuals to fishing-operation characteristics (including fishing gear, mesh size, the presence of an acoustic deterrent, targeted and fished taxa, and fishing effort). This investigation allowed us to outline by-catch sensitivity and vulnerability profiles. Again, we found that males and young individuals were more sensitive to by-catch, with spatiotemporal sensitivity patterns. Smaller individuals appeared to be caught on the northern French coast and in spring, and more males were caught on the southern French coast. We then found larger body-sized dolphins to be more vulnerable to trawls compared to gillnets. For the latter fisheries, the size and body mass of by-caught harbor porpoises were positively correlated with mesh size. Targeting soles or hakes was also associated with a larger body size of by-caught dolphins compared with targeting Sparidae or sea bass. Finally, we found larger individuals to be by-caught in the presence of an acoustic deterrent device. Our results suggest age-specific by-catch sensitivity and vulnerability to fishing techniques, which may be due to biological factors such as social behavior and diet. Our study therefore advocates for a better consideration of spatiotemporal patterns in individuals' sensitivity to by-catch and age- or sex-specific vulnerabilities to particular fishing activity profiles.