Fish Labeled as Eco-Friendly Chilean Sea Bass May Not Be

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Think you’re buying eco-friendly Chilean sea bass at your local supermarket? Think again, say researchers, who found that 1 in 5 certified sustainable Chilean sea bass may not be as environmentally responsible as the labels claim.

The Marine Stewardship Council (MSC), an international organization that recognizes eco-friendly fishing practices, has certified about 8,000 seafood products. These are confirmed to have been harvested from sustainable fisheries, where strict protocols protect species against overfishing and ensure that the fish can continue to propagate in healthy numbers. (See here for more on overfishing from my colleague Bryan Walsh.)

As far as Chilean sea bass goes, only one fishery, located near South Georgia Island in the sub-Antarctic, is MSC certified. But according to DNA analysis by Clemson University researchers, 15% of Chilean sea bass labeled as sustainable and sold at U.S. grocers did not come from the certified fishery. What’s more, 8% of sustainable Chilean sea bass were of a different species entirely.

Taken together, that means more than 20% of fish that’s sold as sustainable Chilean sea bass isn’t what it claims to be. “I was surprised that so many of the MSC-labeled fish were not what they were supposed to be, but I guess I’m not shocked,” says Peter Marko, a professor of biological sciences at Clemson, who led the study.

Marko notes that many cases of fish mislabeling are driven by economic incentives, in which less desirable (and inexpensive) species are substituted for more desirable and valuable ones. But he and his group decided to track how faithful the supply chain was in bringing sustainable fish to market.

The Chilean sea bass fishery off South Georgia Island provided a perfect setting for the experiment. Because of its remote location, fish stocks in those waters are relatively isolated from other species; the currents provide a geographic barrier to genetic mixing and therefore keep the sea bass relatively DNA-pure. (It’s also why fish from these waters tend to have lower levels of mercury than those harvested farther north.) Adult sea bass don’t tend to migrate much, which also helps to keep the stocks relatively contained.

The fact that so many different genetic signatures appeared in the MSC-certified sea bass that reached retailers suggests that other species are being introduced somewhere in the supply chain, between when the certified fish are harvested and when they are sold. “There are lots of sub-hypotheses about where that happens,” says Marko. “It could happen anywhere from the time the fish is caught to the time the consumer buys the fish at the grocery store. It could be that retailers are swapping uncertified fish and other species for MSC certified Chilean sea bass when they don’t have MSC certified fish on hand. We don’t know where it’s happening.”

It’s also possible that other fish are migrating into the waters where the MSC-certified Chilean sea bass are living and mating, and therefore changing the genetic makeup of the species. Marko’s team took their samples from the fishery in 2008, and compared them to reference samples taken in 2001, an eight-year gap. But such genetic alterations are unlikely to have occurred in such a relatively short period of time, he says, since Chilean sea bass have long larval stages which would mean any introduction of new DNA would take several more years to show up in the fish.

The most likely explanation is that uncertified sea bass or other species of fish are being substituted for MSC-certified Chilean sea bass. Marko hopes that the genetic testing method he used to test these fish will become more widespread, so that MSC-certified fish from specific fisheries can be tracked more accurately all the way to market.

In the meantime, he says, “it’s fairly difficult to promise that every fish that is labeled Chilean sea bass is going to only come from the MSC fishery.” Which, for the consumer, means that when it comes to buying sustainable fish, it’s caveat emptor.

The new study was published in Current Biology.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.