Cetacean bycatch in Indian Ocean tuna gillnet fisheries

Pelagic gillnet (driftnet) fisheries account for some 34% of Indian Ocean tuna
catches. We combined published results from 10 bycatch sampling programmes (1981−2016) in
Australia, Sri Lanka, India and Pakistan to estimate bycatch rates for cetaceans across all Indian
Ocean tuna gillnet fisheries. Estimated cetacean bycatch peaked at almost 100 000 ind. yr−1 during
2004−2006, but has declined by over 15% since then, despite an increase in tuna gillnet fishing
effort. These fisheries caught an estimated cumulative total of 4.1 million small cetaceans between
1950 and 2018. These bycatch estimates take little or no account of cetaceans caught by gillnet but
not landed, of delayed mortality or sub-lethal impacts on cetaceans (especially whales) that
escape from gillnets, of mortality associated with ghost nets, of harpoon catches made from gillnetters,
or of mortality from other tuna fisheries. Total cetacean mortality from Indian Ocean tuna
fisheries may therefore be substantially higher than estimated here. Declining cetacean bycatch
rates suggest that such levels of mortality are not sustainable. Indeed, mean small cetacean abundance
may currently be 13% of pre-fishery levels. None of these estimates are precise, but they
do demonstrate the likely order of magnitude of the issue. Countries with the largest current gillnet
catches of tuna, and thus the ones likely to have the largest cetacean bycatch are (in order):
Iran, Indonesia, India, Sri Lanka, Pakistan, Oman, Yemen, UAE and Tanzania. These 9 countries
together may account for roughly 96% of all cetacean bycatch from tuna gillnet fisheries across
the Indian Ocean.