Ecological risk assessment of cetaceans to Indian Ocean tuna fisheries

Bycatch, or the incidental capture in fishing gears, is the most significant threat to marine megafauna in the world’s oceans. It is currently the main driver of the decline and extirpation of cetaceans (whales, dolphins and porpoises) in many regions around the globe, both in coastal and open-ocean ecosystems. However, the magnitude of bycatch remains poorly quantified in many regions and fisheries. Over the past decade, there has been increasing concerns about the extent of cetacean bycatch in the Indian Ocean, particularly in expanding drift gillnet fisheries. Here, an ecological risk assessment including a productivity-susceptibility analysis (PSA) designed for data-poor situations was adapted to investigate the vulnerability of cetaceans to bycatch in tuna fisheries, particularly in drift gillnets, pelagic longlines, and purse seines within the IOTC (Indian Ocean Tuna Commission) Area Of Competence. The PSA revealed that risk varies greatly between gears and species. Overall, risk is higher and for more species in drift gillnets than in pelagic longlines and purse seines. Species at higher risk include oceanic small delphinids, medium-sized delphinids, and, to a much lesser extent, baleen whales. For pelagic longline fisheries, risk was also relatively high for several large oceanic delphinids. Risk for purse seine fisheries was lower than for other gears, but was relatively high for some baleen whales (particularly B. edeni). Most species with high susceptibility to capture also had high vulnerability scores based on their life history traits. Overall, the highest vulnerability scores were for gillnets across all species, but particularly small oceanic dolphins. An assessment of the spatial overlap between cetacean occurrence generated by AquaMaps (https://www.aquamaps.org) and tuna fishing effort also allowed assessment of vulnerability of species groups for each gear. The spatial overlap between gillnet fisheries and baleen whales is limited to the northern portion of the Indian Ocean. Small and large oceanic dolphins exhibit similar patterns of overlap for all three gears, with high overlap in the northern Indian Ocean with gillnets, and with pelagic longlines and purse seines in the western tropical Indian Ocean. Large toothed whale distribution overlaps extensively with the three gears, including gillnets in the northern Indian Ocean and pelagic longlines in the southern and southwestern parts of the IOTC area. Overall, this study highlights the need to better quantify cetacean bycatch in Indian Ocean tuna fisheries, particularly in gillnet fisheries.