More than half of people would be more likely to buy a tuna sandwich if they knew the fish filling was sustainably sourced, a poll suggests.
But less than half of those quizzed (46%) trust claims made by retailers on labels such as "pole and line caught" or "dolphin friendly", the survey for the Marine Stewardship Council revealed.
The poll of 2,090 people by YouGov found four out of five (80%) thought it was very or fairly important that sandwich retailers used sustainably caught tuna, while 54% said they were more likely to buy tuna sandwiches when they knew that was the case.
And while there was limited trust in claims made by retailers, more than two-thirds (67%) said they trusted independent labels such as the Marine Stewardship Council (MSC) or the Soil Association.
The MSC said it certifies around 15% of the global tuna catch, requiring fisheries demonstrate they are well managed to maintain healthy population levels of tuna and minimise impacts on other marine species such as dolphins and turtles.
In the UK just two British retailers - Sainsbury's and Waitrose - have the MSC "blue tick" ecolabel on their tuna sandwiches, accounting for only 3% of the estimated 210 million sold each year in the country, the organisation said.
Toby Middleton, programme director for MSC in the UK and Northern Atlantic said: "Maintaining healthy tuna stocks is essential for the marine environment and for fishing communities around the world that rely on tuna for their living.
"We can all play our part in safeguarding stocks by choosing tuna that's sustainable, but when you're buying a sandwich on the high street, you're often in a rush and don't have time to check what type of tuna it contains, or how it was caught, let alone remember whether skipjack supplies are more plentiful than bluefin. It's no wonder shoppers are baffled.
"By using the MSC's blue tick ecolabel on their tuna sandwiches, Sainsbury's and Waitrose are making it easy for their shoppers to see at a glance that the tuna has been independently certified as sustainable, when tested against the world's most credible and robust environmental fishing standard."
He urged other supermarkets, high street sandwich retailers and coffee shops to follow suit.