A study of how many dolphins are caught in tuna fishing nets estimates the mammals may now be at just 13 per cent of their numbers prior to 1980.
James Cook University’s Dr Putu Mustika was part of an international group that looked at the bycatch from tuna gillnets (including driftnets) in the Indian Ocean. The group was led by Dr Charles Anderson of the Manta Marine organisation in the Maldives.
“We combined results from 10 bycatch sampling programmes between 1981 and 2016 in Australia, Sri Lanka, India and Pakistan to estimate bycatch rates for cetaceans (whales, dolphins and porpoises) across all Indian Ocean tuna gillnet fisheries,” said Dr Mustika.
She said while some of the official data may be unreliable, scientists had been able to come up with a credible picture of the dolphin catch.
Dr Mustika said tuna fishers operating in theIndian Ocean caught about 4.1 million small cetaceans between 1950 and 2018.
“The vast majority of the cetacean bycatch is dolphins. Estimated cetacean bycatch peaked at almost 100,000 a year during 2004−2006, but has declined to 80,000 animals a year, despite an increase in the tuna gillnet fishing effort,” she said.
Dr Mustika said the numbers may be substantially higher as the figures take little or no account of things such as delayed mortality of cetaceans that escape from the nets or mortality associated with ghost nets.
“But the declining cetacean bycatch rates shown by what we can measure suggest current mortality rates are not sustainable. The estimates we have developed show that average small cetacean abundance may currently be 13 percent of the 1980 levels,” she said.
Dr Mustika said a UN ban on gillnets on the high seas is hard to enforce and tuna fishers are allowed to use gillnets within the territorial waters of states bordering the ocean.
“Cetacean bycatch in Indian Ocean tuna gillnet fisheries has been a concern for decades but has been poorly studied, reflecting the political reality that hundreds of thousands of relatively poor fishermen and their families rely on gillnet fisheries,” she said.
The scientists said there is a need for improvements in monitoring, analysis and governance and for changes to fishing practices if dolphin numbers are to recover.
Tuna gillnets deployed in the Indian Ocean vary in length from 100 m to over 30 km in length, and less than 5 m to more than 20 m in depth.
The current cetacean bycatch rate may be in the order of 175 cetaceans per 1000 tonne of tuna, down from an estimate of 600 in the late 1970s.
If tuna abundance is currently in the order of 44% of pre-commercial exploitation (1980) levels, then average cetacean abundance may now be about 13% of pre-fishery levels (175 / 600 × 0.44).
The countries with the largest current gillnet catches of tuna and likely to have the largest cetacean bycatch are (in order): Iran, Indonesia, India, Sri Lanka, Pakistan, Oman, Yemen, UAE and Tanzania.
Iran and Indonesia have no national monitoring of cetacean bycatch. Dr Mustika said some other countries report numbers that may not be accurate.