Greenpeace says it filmed seven ships within 20 square kilometres using driftnets to catch tuna. It detected another eight vessels on radar using navigational patterns that also suggested use of nets.
Greenpeace has uncovered widespread use of illegal driftnets in the northwest Indian Ocean, which it says are decimating marine life in what is one of the world's most ecologically vulnerable fishing grounds.
During two weeks at sea, the environmental organisation says it filmed seven ships within 20 square kilometres using driftnets to catch tuna. It detected another eight vessels on radar using navigational patterns that also suggested use of nets.
"If yellowfin tuna continues to decrease at the current rate then food security in the region, as well as local economies is going to take a huge hit," Greenpeace said.
Nicknamed the "walls of death" for the quantity of other sea life they catch in addition to the fish they are set for, the nets were banned by the United Nations 30 years ago.
Greenpeace shared footage of sharks and manta rays that had been killed in the nets, set some 500 kilometres east of Somalia.
"Because of the issues of bycatch we're concerned about all fish in the Indian Ocean," it said, adding that the same area had also seen a huge increase in unregulated squid fishing.
"We need a global ocean treaty"
"What's the point in a UN ban on driftnets when all the fishing vessels we saw are using driftnets?" asked Will McCallum, head of oceans for Greenpeace UK.
"There is little to no enforcement in international waters... We need a global ocean treaty ... to resolve this enormous governance gap."
Nations are due to meet in August for negotiations over such a pact, designed to attempt to set up safeguards for parts of the ocean similar to reserves established on land.
Last month, representatives of 30 nations met to discuss ways to save fast-depleting tuna stocks in the Indian Ocean. The meeting ended without any new agreement.