The supermarkets were standing their ground over tuna sourcing this week as pressure mounted on them to ensure tuna was 100% pole or line-caught.

Marks & Spencer and Pret a Manger both changed their sourcing policies ahead of the first UK screening of a new documentary, the End of the Line yesterday.

However, although Tesco claimed to stock some pole and line-caught tuna on the fresh counter, it said there was not enough available to satisfy demand for canned own-label product. "Tuna for our canned product is currently caught using 'dolphin-friendly' nets as there simply is not the volume of pole and line-caught tuna," said a spokesman.

Neither Morrisons nor Asda, both of which sell net caught tuna as well as MSC-certified and pole and line-caught tuna, have yet shifted to 100% line and pole-caught either.

Sainsbury's, meanwhile, claimed it had already made the move to 100% pole and line-caught skipjack so didn't need to change its policy. The tuna on its fish counter and its pre-packaged tuna is also line-caught yellowfin.

The stance of its big four rivals mirrored that of John West. "Pole and line fishing can only supply a small percentage of the world market. We use both pole and line and purse-seining," said a spokesman.

End of the Line, based on a book by The Guardian's Charles Clover, claimed fish stocks were in a perilous state. The New Economics Foundation, however, said that sourcing from properly monitored, sustainable stocks was more important than the method used.