The reason these fisheries are throwing away so many fish? It's mainly the types of commercial gear they're using, which are designed to catch as many fish as possible, says Oceana.

And if you've eaten U.S. shrimp, halibut or red snapper, there's a good chance it has come from one of these fisheries. Their fish and shrimp are sold in markets and restaurants across the U.S. and around the world, according to Gib Brogan, the fisheries campaign manager with Oceana.

"It's being sold all over the place," he says.

Brogan says fishing regulations are partly to blame for wasted seafood. That's because fishermen who have permits to catch certain species must throw back other valuable species they accidentally catch — even if the fish are dead.

For example, trawling vessels in Alaska — boats that drag giant nets behind them to catch flounder and sole — threw back two million pounds of Pacific halibut and five million pounds of Pacific cod in a single year, according to Oceana's report. Brogan says a lot, if not most, of bycatch caught with trawl nets, gillnets and longlines is dead or dying by the time it's returned to the water.

Nine fisheries account for more than half of the reported U.S. bycatch. Courtesy of Oceana

The Gulf of Mexico shrimp trawling industry throws back about two thirds, by weight, of its total catch, the report says. So do swordfish gillnetters in offshore California waters and those fishing closer to shore for halibut and white sea bass. And sea turtles, seabirds and various marine mammals are also among the detritus. Longline snapper and grouper fishermen in the Gulf of Mexico, for example, threw back 400,000 sharks in 2011.

Such information is critical for fishery managers and environmentalists who advocate for catching seafood with more sustainable fishing gear. But unless you study the fishing industry for a living, it's tough for a consumer to avoid these fisheries when shopping for seafood. Nonetheless, groups like Oceana, the Natural Resources Defense Council and the Monterey Bay Aquarium have been trying to educate consumers about the bycatch issue through seafood buying guides and other campaigns.

"As a concept, people get what bycatch is and they don't like it," says Sheila Bowman, with the Monterey Bay Aquarium's Seafood Watch. "But the data is often too hard for shoppers to put into context and act on."

According to Whole Foods Markets, the high-end retailer, 59 percent of its shoppers surveyed in 2013 said they believed how a fish is caught is important. The company has pledged to sell seafood that's sustainable as possible.

"But most people don't even know what 'gear type' means," says Margaret Wittenberg, vice-president of quality standards at Whole Foods Markets. "They care about sustainability, and they want to make a good choice, and they know that we've done the homework for them."

Just as consumers and retailers are starting to understand the severity of the bycatch issues, some fishermen also seem to be taking it more seriously. In the Gulf of Mexico, a group of several dozen fishermen catching snapper and grouper have begun using weaker hooks on their longlines that break or bend when large sharks and other potential bycatch species take a bait.

"In general, these [fishermen] don't want to catch fish they can't sell," says Eric Brazer, with the Gulf of Mexico Reef Fish Shareholders' Alliance. He says the fishermen he represents have made efforts to create demand for lower-value species, like small snappers and porgies—fish that often get tossed back dead. "Our goal is to have a zero-discard fishery by creating markets for these species."

Tj Tate, with Gulf Wild, a sustainability program that marks seafood with detailed place-of-origin tags, says that many fishermen are seeing financial incentive to change their practices and reduce their bycatch.

"It's not effective for fishermen to be spending time handling and trying to release sharks or undersized fish that they can't sell," she says. "They're losing fishing time."

Bowman at Seafood Watch says she supports the efforts of groups like Oceana to inform consumers of wasteful fishing industries. But she notes that in the crusade to curb wasteful fishing practices, less information might sometimes be more.

"People don't want to know all this," she says. "In general, they just want to know what [color-coded label] to look for."

Copyright 2014 NPR.

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Why 500 Million U.S. Seafood Meals Get Dumped In The Sea