ABOARD THE STEVE IRWIN - Tuna fishermen battled environmentalists on the Mediterranean, hurling heavy links of chain at them as the environmentalists tried to disrupt illegal tuna fishing under the no-fly zone north of Libya on Saturday.

The fishermen also tried to lay a rope in front of the activists' boat, the Steve Irwin - owned by the U.S. group the Sea Shepherd Conservation Society - hoping to disable it. Environmentalists responded with fire hoses and stink bombs.

Several hundred feet above the fray circled a French fighter jet summoned by the fishermen, who claimed, falsely, that activist divers were trying to cut their net.

The 195-foot Steve Irwin, named after the Australian conservationist who died in 2006, left the Sicilian port of Syracuse early Friday, heading for a rendezvous with a smaller, faster sister ship, the Brigitte Bardot, just north of Libyan waters. The Bardot had traversed the area and reported that more than 20 purse seiners were operating there.

Purse seiners are boats with large nets that draw closed like a purse, ensnaring the tuna. The fish are then sometimes put in floating net-cages and slowly towed to port.

Sea Shepherd is on a mission to disrupt boats that are fishing illegally. The stock of bluefin tuna, which spawn in the Mediterranean and then swim out to the North Atlantic, has been depleted to the point that some experts fear it will soon collapse.

Saturday's confrontation began to take shape at first light. Ten purse seiners were working several miles from the Steve Irwin in one direction, and five were spotted in another direction.

Late in the day, having broken off the earlier confrontation, the Irwin and the Bardot entered Libyan waters in search of illegal fishing boats there.