The specialized market for sustainable seafood has gone mainstream. Even Wal-Mart is promoting its environmental credentials in the seafood aisle.

More supermarket shoppers are demanding to know they are buying from operations that don't threaten fragile fish and seafood populations, or damage habitats with nets and other commercial gear. And supermarkets are meeting the demand.

While brands such as Whole Foods Market are known for being environmentally minded, traditional grocers are increasingly promoting seafood that's labeled sustainable. Safeway says it's nearing its goal of selling only "responsibly caught or farmed" fresh and frozen seafood by next year. At Wal-Mart and Sam's Club, more than 90 percent of fresh, frozen, farmed and wild seafood is certified as sustainable.

"Consumers are more and more aware of the food that they eat and how it's grown and how it's raised and how it's caught, and we hear from customers it's important to them," said Gregory A. Ten Eyck, a spokesman for Safeway's Eastern division, adding that sourcing sustainable seafood has become a bigger focus for the company.

"We don't want consumers who care deeply about the environment or organics to feel they have to go to another retailer to get those products," he said. "We want to be on the forefront of this."

Safeway, Target, Harris Teeter and Ahold USA, owner of Giant Food, ranked in the top 10 of Greenpeace's annual seafood sustainability score card for 2014, joining the likes of Whole Foods and Trader Joe's. Wal-Mart ranked 12th on the list.

Supermarket chains have increasingly embraced the sustainability movement, prompted not only by consumer demand but by the growing realization that supply could be in jeopardy, experts said. All but three of the top 20 grocery retailers in North America now work with nongovernmental organizations that advocate for sustainable seafood, according to the Conservation Alliance for Seafood Solutions.

"Some are leaders and others were pulled along, but the sentiment within the environmental community is that these companies are working hard to reduce their environmental impact in seafood. Most of the major retailers are engaged in the issue," said Tobias Aguirre, executive director of FishWise, a nonprofit sustainable-seafood consultant that helps retailers and suppliers put sustainability programs in place.

"For those in the industry the longest, they've seen changes where they're not able to get certain products or the prices are going up," Aguirre said. "They know these changes are occurring and are starting to think about the long-term viability of their business."

Public awareness of overfishing and other practices threatening species such as swordfish and Chilean sea bass has stoked the sustainability movement. Even as Greenpeace noted progress among grocers, the group warned that problems persist.

"Our oceans continue to suffer from overfishing, destructive fishing and illegal fishing," the group said in its report ranking seafood sustainability in U.S. supermarkets.

Some estimates indicate that a quarter of the seafood sold to U.S. consumers comes from illegal sources, Aguirre said. It's a challenge for supermarkets to ensure all their sustainable seafood comes from legal sources.

Safeway began overhauling its seafood sourcing in 2009. Now the chain, the second-largest Baltimore-area grocer, no longer sells grouper, red snapper, orange roughy or shark, and two years ago it launched a store brand of responsibly caught canned tuna.

Meanwhile, Ten Eyck said, the retailer is seeing strong growth in sales of organic and natural product lines, including packaged and fresh food made mostly without synthetic pesticides, fertilizers, antibiotics or added growth hormones, and meat from animals raised without antibiotics or added hormones.

While retailers such as Whole Foods helped set the all-natural trend, "it didn't catch on until the prices started dropping," said retail consultant Jeremy Diamond, a director of the Diamond Marketing Group.

"A lot of the mainstream grocery chains have expanded their natural food offerings as shoppers have asked for more natural and organic products," Diamond said. "The larger mainstream grocers have been doing a better job of lowering prices on natural products."

A number of different groups set standards for seafood to be labeled sustainable, assessing fishing methods of suppliers, the health of fish populations, and whether fishing alters a habitat or harms nontargeted fish species.

The Monterey Bay Aquarium's Seafood Watch is considered by environmental groups the leading rating system in North America. It rates seafood products as "Best Choice" or "Good Alternative," and flags items consumers should avoid.

Giant has been working since 2000 with the New England Aquarium to ensure the sustainable seafood it sells meets "strict" criteria. "Our customers and the public are very sensitive to overfishing of the ocean, so we do our best to act in a way that minimizes the impact on the environment," said Jamie Miller, a spokesman for the chain.

The grocer sells the "Sustainable Choice" seafood identified by seafood buyers working with the aquarium, including farm-raised tilapia and wild-caught Alaska pollock as well as Alaska sockeye salmon and Pacific cod caught using fishing gear that doesn't damage the ocean floor.