Green activists using helicopters, divers and rotten butter yesterday confronted Libyan and Italian fishermen to release hundreds of threatened bluefin tuna which they strongly suspect were illegally caught off the Libyan coast.

In the first action of its kind in north African waters, the international crew of the California-based Sea Shepherd Conservation Society released around 800 tuna from a cage being towed behind the Italian trawler Cesare Rustico.

Stocks of bluefin tuna, one of the most valuable but endangered fish in the Mediterranean, have been decimated by ruthless overfishing in the last 20 years to the point where they are now unlikely to survive more than a few more years. Catches are limited to two weeks of the year and shipowners have been given strict quotas by governments, but with little policing, the industry has been easily able to flaunt the law.

In a statement from the boat, Captain Paul Watson said: "Sea Shepherd's helicopter reconnaissance flight this morning found two fishing vessels. One was engaged in transferring bluefin tuna into one of the two nets being towed by the other vessel.

"The captain of the Cesare Rustico said when questioned that the tuna were caught on the morning of the 14th by the Libyan vessel Tagreft. When we replied that the number of tuna in the cage exceeded the quota for the Tagreft, the captain said the cage also included tuna from seven other Libyan seiners. All the catches were caught on the 14th, the last legal day, according to the captain.

"The problem with this explanation was that we had observed … weather conditions for those two days made fishing virtually impossible.

"The extremely difficult conditions, coupled with the position of the cages only 40 miles off the Libyan coast, when they should have been moving 25 miles a day, suggested to us that the fish were freshly caught within the last three days at the most."

The Sea Shepherd, which annually confronts Japanese whalers in the Antarctic waters, then asked to examine the fish for juveniles. "We were refused. I then put the bow of the Steve Irwin onto the cage so we could look into the cage from the bow to examine it further.

"Suddenly, the Maltese vessel Rosaria Tuna rammed the Steve Irwin on the aft port side and slid alongside the port rail, as a fisherman tried to violently gaff Sea Shepherd crewmembers with a long, sharp-hooked pole."

In the ensuing fracas, the Steve Irwin crew crew retaliated throwing rotten butter at the fishermen, and then sent divers into one of the cages to identify the size, age, and quantity of the bluefin tuna caught.

"Once it was clearly established that the cage was overstocked and that a high percentage were juveniles, Sea Shepherd divers freed the 700-800 tuna," said Watson.

"It is our position that the bluefin tuna we freed from that cage held a large number of juveniles and that the fish were caught after the official closure of the season. It is also our position that the fish that we freed exceeded the quota," said Watson.

"They shot out of that net like racehorses," said Canadian cameraman Simon Ager.