With all the hype made by Jamie Oliver & Gordon Ramsey about buying sustainably sourced fish and seafood for quality and the growth of declining breeds. However it is confusing labels that prevent the average British consumer from doing so.

In a new report and survey, Which? Says the UK's supermarkets – which dominate fish sales – need to do more to help consumers to understand how to shop ethically to protect the world's fast-diminishing stocks. The majority of fish bought in the UK is via supermarkets, this is a staggering 80%. Supermarkets are chosen rather than fish mongers due to cheaper prizes, freezing options and variety of fish types.

Nearly 90% of fish stocks are overfished. According to United Nations estimates, 71-78% of the world's stocks are fully exploited or significantly depleted.

The Which? survey warns that packaging information about catch method and origin is often "far too vague" – and says that "supermarkets aren't making it easy for [consumers] to understand how to shop sustainably". In addition, 80% of shoppers said they thought supermarkets should sell only fish from sustainable sources.

The report singled out Marks & Spencer and Waitrose as "the best of the bunch", with other retailers fast catching up. But it questioned the decision by Sainsbury's to use tuna that is not pole and line caught in its sandwiches and sushi. All its tinned tuna is pole and line caught, which is safer for dolphins and other marine life.

Sainsbury's said in a statement that it was changing this policy: "Sainsbury's is the largest retailer of MSC [Marine Stewardship Council] certified fish in the UK and we offer twice as many MSC labelled products as our nearest competitor. All of the tuna in our sandwiches and sushi will be pole and line caught by the end of the year and we are the UK's largest retailer of RSPCA Freedom Food approved salmon."

The report also called for a more standardised approach to certification from industry bodies, after identifying seven different labels – including the MSC, Fish for Life and RSPCA's Freedom Food. Philip MacMullen, head of environment at industry body Seafish, said: "Some fisheries don't lend themselves to the process and others may not want to pay the often high costs of certification."

Government guidelines recommend that adults eat at least two portions of fish (one of which is oily, such as salmon or mackerel) a week. We spend more than £2.7bn a year on fish in UK shops.

Separately, Ethical Consumer magazine is today urging people to help save threatened sharks and turtles by boycotting tuna sandwiches from Boots, Greggs, Subway and 
Tesco – four of the UK's biggest sandwich retailers. Its research indicates that the fishing methods these companies employ to catch the skipjack tuna used in sandwiches results in the unnecessary death of thousands of sharks, turtles and other marine wildlife every year.

Jane Turner from Ethical Consumer magazine said: "People will be shocked to learn that their favourite tuna sandwich results in the slaughter of thousands of sharks, turtles and other marine wildlife. We want companies to follow the lead of M&S and only use pole and line caught tuna in their sandwiches."

Tesco would not address the issue of tuna in its sandwiches but said in a statement: "We currently source tinned tuna from fisheries that use a net (purse seine), all our tinned tuna is skipjack and by the end of the year 10% of our tinned tuna will be caught by pole and line. We take sustainable sourcing of fish very seriously and are committed to taking fish only from responsibly managed sources."