MONTREAL (Canadian Press) — A dramatic shift is taking place in the fish aisle of Canadian supermarkets as a growing number of retailers are opting to only sell sustainably sourced seafood.

A decade after they were first introduced, eco-friendly certified fish are expected to be the only fish on the menu for the country's large food retailers by 2013.

"It's a huge shift," says Bill Fox, vice-president of the World Wildlife Federation.

Eventually, almost all the fish sold by national retailers will be sustainably harvested, up from eight to 10 per cent today, he said in an interview.

Walmart Canada hopped on board Tuesday, announcing that it will within three years sell only "green" varieties of frozen, wild and farmed fish in its supercentres network of 86 stores.

The retailing giant joined Loblaw, Sobey's and B.C.-based Overwaitea, which have committed to sell sustainable supplies.

Metro Inc., the Quebec-based operator of Metro, Food Basics and Super C, said it is weeks away from announcing its own plans to source its fish from sustainable sources.

The shift to sustainable fish got a transformational kick in 2006 when Arkansas-based Walmart said it would only sell Marine Stewardship Council fish by 2011.

On Tuesday, the U.S.-based retailer's Canadian arm went a step further, by requiring all canned tuna to be sourced only from International Seafood Sustainability Foundation members.

While most canned tuna comes from healthy stocks, the move will help to ensure that doesn't reverse itself and that retailers have enough fish to sell, said foundation spokesman Michael Crispino.

"Running out of fish and having fish stocks that are overfished and in a horrible not good for business."

John Lawrence, director of corporate social responsibility, said the policy is part of a broader environmental strategy adopted by the world's largest company.

"This particular policy relates to fish but we certainly have goals around providing a larger selection of environmentally responsible products to our customers," he said in an interview.

Like all Walmart efforts, this one if focused on delivering the products at lower prices because consumers are not interested in paying more for sustainable products, Lawrence added.

The Canadian supermarket business is intensely competitive. Walmart's growing presence in the food aisles has had a major impact on the industry. Retail consultant Wendy Evans said the chain's sustainable seafood policy will force rivals to step up their own environmental plans.

"Walmart being the largest company in the world is obviously a huge impact and they change the way business is done," she said.

Loblaw (TSX:L) announced a year ago that it plans to source all seafood sold in its retail locations from sustainable sources by the end of 2013. It offers the most MSC certified products of any retailer in Canada with the introduction of 16 sustainable fish products in its frozen Blue Menu line.

Nearly 4,900 MSC-labeled products are sold around the world, up from 2,084 in 2008 and just three in 2000.

Fisheries will "realize that in order to sell into key markets they need to ensure that their practices are not depleting the stock and not harming the marine environment," she said from Seattle.