New Delhi, May 7 -- The European fishing fleet has long been a powerhouse at catching tuna, with a fleet of massive vessels known as purse seiners that can hold as much as 4 million pounds (1.8 million kilograms) of fish at a time. Dozens of them roam the Indian Ocean, fishing for skipjack, yellowfin and bigeye tuna destined for cans on grocery store shelves.

So when Jess Rattle began seeing purse seine ships fishing the Indian Ocean under the flags of Mauritius, Tanzania and Oman, she wondered whether European corporations might be involved.

"We wanted to understand who really owned these vessels," said Rattle, head of investigations at the London-based environmental charity Blue Marine Foundation. "Were they owned by the coastal state...