Gillnets Responsible for 90% Drop in Dolphin Stocks

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Pod of bottlenose dolphins off Reunion Island in the Indian Ocean

The number of dolphins in the Indian Ocean has plummeted by nearly 90 per cent since 1980 and a just-published study reveals that one of the biggest causes is the vast gillnet fishing industry targeting tuna.

It is conservatively estimated that the poorly regulated tuna fishing industry has killed more than 4 million cetaceans between 1950 and 2018 in the Indian Ocean. At one point it was killing 100,000 cetaceans a year - the report by Dr Charles Anderson of the Manta Marine organisation in the Maldives believes that figure has now slightly declined to around 80,000 dying each year as bycatch in tuna nets.

Published in the journal Endangered Species Research, the study looked at the figures in the Indian Ocean for recorded bycatch levels. It looked at ten programmes to reliably count bycatch between 1981 and 2016 carried out in Australia, Sri Lanka, India and Pakistan. It then extrapolated how many cetaceans were likely to have died in the nets of the 24 countries involved in gillnet fishing in the Indian Ocean. 

The report states: '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.'

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Fishing nets drying on a beach in Sri Lanka

The study found that for every 1,000 tonnes of tuna currently being caught, about 175 cetaceans were also being caught.

Iran, which has the largest gillnet fishery in the region, has an annual average catch of 214,262 tonnes of tuna, meaning an annual catch of at least 30,302 cetaceans.

Dr Puti Liza Mustika, a co-author of the study from James Cook University, said solutions to the problem could include working with fishers to switch from the gill nets to other fishing gear, such as pole and line.

There had also been trials using devices attached to the nets that deterred dolphins, including lights or devices that caused a rattling sound. Setting nets at shallower depths reduced the bycatch but also reduced the amount of tuna being caught.

'The solution has to be technology, as well as using fishing gears that are more sustainable,' Mustika said. 'But banning these fishers is not a solution for developing countries.'




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