I sink therefore I am: 20 years of tagging small cetacean carcasses in the North-East Atlantic for bycatch estimation

International audience

Английский. <div><p>For common dolphin (Delphinus delphis Linnaeus, 1758) and harbour porpoise (Phocoena phocoena Linnaeus, 1758), bycatch levels exceed the thresholds in the North-East Atlantic. Reverse drift modelling was developed to infer bycatch from stranding data. It is then necessary to define three parameters: the proportion of carcasses that float, the probability of a floating carcass drifting until stranding; and finally, the proportion of beached carcasses that are discovered and reported. The proportion of floating carcasses is the most influential corrective factor in the reverse drift modelling process. The objective of this study is to provide a better estimation of this proportion following bycatch in fishing gear. The predicted drift of 455 common dolphins and harbour porpoises that were tagged and discarded by fishing vessels was calculated by a drift prediction model (MOTHY). Of the tags deployed, stranding was predicted for 309 carcasses (278 common dolphins and 31 harbour porpoises), and 84 animals were discovered (78 common dolphins and 6 harbour porpoises). Bycatch in trammel nets and in midwater pair trawls resulted in a higher probability of these carcasses being recovered stranded. The predicted duration of the drift, on the other hand, had a negative effect on the number of strandings observed. The percentage of harbour porpoises that float was 27% (95% CI [12%,43%]), while it was 31% (95% CI [24%, 39%]) for common dolphins. These results contribute to the improvement of estimates of bycatch of small cetaceans inferred from strandings, which are used to inform conservation measures, particularly for the common dolphin in the North-East Atlantic.</p></div>