Quantifying and understanding cetacean bycatch

Abstract

Fishery bycatch mortality is generally considered to be the main threat to cetaceans in European Atlantic waters. The marine waters in the Northwest of the Iberian Peninsula are among the richest in fishery resources in Europe and, consequently, they are also very important for fisheries. Analysis of cause of death and life history data from strandings in Galicia suggested that the bycatch is a major threat in common dolphins (Delphinus delphis) and harbour porpoises (Phocoena phocoena). The mortality rate in the Iberian population of the harbour porpoise could be well above safe limits. MERMACIFRA (“Monitoring, Assessment and Reduction of Accidental Mortality of Cetaceans due to Interactions with the Spanish Fleet - Review and Action”) is a national Project funded by the Fishery Ministry of Spain, the main objective of which is to monitor, assess and reduce bycatch of cetacean species in Spanish Atlantic waters. This includes acquisition of information on bycaught animals from strandings and on-board monitoring, to quantify bycatch mortality and to build a “profile” of bycaught animals in terms of their sex, size, age, reproductive status and health status. The project collaborates with the Galician stranding network, which attends stranded cetaceans on the Galician coast. Between 1990 and 2020, 3,404 strandings of common dolphins (on average 114 per year) and 344 strandings of harbour porpoises (on average 11 per year) were reported. It was observed that, over the last decade, the number of strandings with bycatch evidence has tended to increase. For 2021, considering only those carcasses examined which preservation state were from fresh to slight (n = 52 for common dolphin, and n = 13 for harbour porpoise), it was obtained that 54% of common dolphins (n = 28) and 23% of harbour porpoises (n = 3) had evidence of fishery interaction. During 2022-2023, we received two notifications from fishers regarding bycaught dolphins. In the first, two common dolphins were collected and studied in the north of Galicia. In the second case, two bycaught common dolphins were thrown overboard by the crew. These data are useful to better understand the reasons for bycatch as well as the resulting mortality. MERMACIFRA also aim to assess methods to reduce bycatch in nets and purse seine fisheries (e.g., using net modifications and observers on-board). Pilots using three types of acoustic deterrent devices indicates different efficacy depending on the species and fishery