Cetacean bycatch in tuna drift gillnet fisheries off Pakistan (Arabian Sea)
Bycatch is the most significant threat to cetacean populations worldwide. Therefore, assessing and identifying bycatch mitigation measures is critical for cetacean conservation and management. Here we provide the first assessment of cetacean bycatch in tuna drift-gillnet fisheries in the Arabian Sea. Using a network of trained captains (four 15-20 m vessels), targeted-catch (tunas) and bycatch data were collected systematically from 2013 to 2017. Over the study period, a total of 3,874 drift-gillnet sets was monitored. Two fishing methods using multifilament gillnets were used: surface and subsurface gillnets. Surface gillnets were deployed at the surface, whereas subsurface gillnets were deployed at 2 m below the surface; net height varied from 10 to 14 m). A total of 203 cetacean captures were recorded (0.04% of all catch). A total of seven species of cetaceans was recorded as bycatch, including spinner dolphin (Stenella longirostris), common bottlenose dolphin (Tursiops truncatus), common dolphin (Delphinus delphis), striped dolphin (Stenella coeruleoalba), pantropical spotted dolphin (Stenella attenuata), Risso’s dolphin (Grampus griseus), dwarf sperm whale (Kogia sima) and an unidentified baleen whale (Balaenoptera spp., probably Balaenoptera edeni). Catch per unit of effort (CPUE) were calculated for both targeted species and cetaceans, and were also compared between the two fishing methods used. Overall, tuna CPUE in surface and subsurface gillnets were not significantly different, whereas cetacean bycatch was significantly higher in surface gillnets. Cetacean bycatch has become a major issue in the northern Indian Ocean because drift-gillnet fisheries are expanding in the region. Although this study should be improved in its spatial extent and use of other monitoring methods (e.g., electronic monitoring systems), the conclusions reached here are sufficient to recommend subsurface gillnet deployments as a precautionary solution.