Cetacean bycatch in tuna drift gillnet fisheries off Pakistan (Arabian Sea)

DOCUMENT WITHDRAWN - FOR ADDITIONAL INFORMATION CONTACT THE AUTHOR DIRECTLY

Bycatch is the most significant threat to cetacean populations at the global scale. Therefore, assessing
and identifying potential mitigation measures is critical for conservation and management. Here we
provide the first assessment of cetacean bycatch in tuna drift gillnet fisheries off Pakistan. Using a
network of trained captains (four 15-20m vessels), targeted captures (tunas) and bycatch data were
systematically collected from 2013 to 2017. Over the study period, a total of 3,874drift 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 meters below the surface (net height varied from 10 to 14m). A total of 203 cetacean captures were
recorded (0.04% of all captures, all taxa included). 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). Captures
Per Unit of Effort (CPUEs) were calculated for both targeted species and cetaceans, and were also
compared between the two fishing methods used. Overall, tuna CPUEs in surface and subsurface
gillnets were not significantly different, whereas cetacean bycatch was significantly higher in surface
gillnets. Cetacean bycatch is potentially a major issue in the northern Indian Ocean since drift gillnet
fisheries are rising in the region. Our results most likely reflect minimal bycatch levels. However,
although this study should be expanded spatially and using other methods (electronic monitoring
systems), subsurface gillnet deployments could significantly reduce cetacean bycatch in Indian Ocean
tuna drift gillnet fisheries and should be recommended as a precautionary solution.