Countries involved in bluefin tuna fishing have decided to do more to protect a species of shark against collateral killing, environmental groups said Saturday.

Elizabeth Griffin Wilson of the Oceana group said the 48-state International Commission for the Conservation of Atlantic Tunas (ICCAT) had ruled that tuna fishermen who find a silky shark in their nets must put it back in the sea.

The only exceptions should be coastal communities who hunt the shark for consumption, she said.

Wilson hailed the move, saying studies had shown the silky shark to be highly vulnerable to long-line tuna fishing in the Atlantic.

The US Pew Environment Group also praised the decision, while deploring the fact that another shark, the porbeagle, was not similarly protected.

Marine protection groups claim that three-quarters of migratory shark species that inhabit bluefin fishing areas are threatened with extinction.

Pew wants fishermen to use new materials that allow sharks to escape, such as nylon fishing lines that can be severed by a shark but not a tuna.

ICCAT has been meeting in Turkey to discuss ways to put tighter controls on catches of the endangered fish, savoured by sushi eaters for its firm meat.

One decision was to introduce an electronic tracing scheme for catches to replace the returns on paper which environment groups say is open to widespread fraud.

ICCAT also cut from 20 metres to 12 metres the size of boat which must undergo a mandatory inspection of its bluefin haul on return to port, Pew said.

But the WWF condemned as weak and insufficient a decision to set the minimum size of swordfish allowed to be caught at 90 centimetres (three feet).