Cetacean bycatch in the western Indian Ocean: a review of available information on coastal gillnet, tuna purse seine and pelagic longline fisheries

Bycatch is the most significant and immediate threat to cetacean populations at the global scale. Here we review available information on cetacean bycatch in industrial and small-scale (mostly artisanal) fisheries in the western Indian Ocean (WIO). In coastal waters of the WIO region, the impact of bycatch has not been quantified in small-scale fisheries, except in bottom-set gillnets and driftnets targeting large pelagic fish (including tuna) off Zanzibar. Based on bycatch data from observer programs and estimated abundance of coastal dolphins, the bycatch off the south coast of Zanzibar was found to be unsustainable. Elsewhere in the region, other species are also bycaught, including coastal, oceanic and migratory species such as humpback whales (Megaptera novaeangliae), mostly in bottom-set and drift gillnets. In open-ocean fisheries, bycatch in pelagic longlines has been recorded but seems to be rare. Species affected are mostly medium-sized delphinids (Globicephala macrorhynchus, Grampus griseus, Pseudorca crassidens) involved in depredation (on either bait or catch). Areas of high co-occurrence of cetaceans and purse seine fisheries have been identified, particularly east of the Seychelles (December to March) and in the Mozambique Channel (April and May). However, few cetacean deaths have been reported. The presence of observers could affect purse seining practices, and therefore affect data reliability. Overall, cetacean bycatch is very poorly documented in the region and more systematic assessment is critical, particularly for those fisheries that use gear known to entangle or entrap cetaceans; gillnets are of greatest concern.