Greenpeace says they have uncovered widespread use of illegal driftnets in the northwest Indian Ocean.
They say it is decimating marine life in 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 miles using driftnets to catch tuna.
And detected another eight vessels on radar that also suggested use of nets.
The group shared video footage that showed sharks and manta rays that had been killed in nets, set some 500 miles east of Somalia.
These nets are nicknamed the "wall of death" for the quantity of other sea life they catch in addition to the fish they are set for.
They were banned by the United Nations 30 years ago.
This is Greenpeace UK head of oceans, Will McCallum:
"As we arrived though what we weren't prepared for was just how many we'd actually been able to see at night. The reason is because they have got these football stadium lights so they shine onto the water that attract the squid. So they are fishing at this unbelievable scale and it is really surprising how little we know about just how they are doing this fishing."
He added that enforcement of the U.N. ban is needed in international waters to resolve an enormous governance gap.
Nations are due to meet in August for negotiations over a global ocean treaty, designed to set up safeguards for parts of the ocean.