Confessions first – I love sushi. But getting hedonistic pleasure from biting into a firm, cold piece of fish paired with salty-sweet vinegared rice doesn’t mean that I also want to be chowing down on an endangered animal. Unfortunately, according to a study published in PloS ONE last week, there’s a good chance that the fish inside my tuna roll came from an endangered or threatened species.
There are eight species of tuna in the genus Thunnus, any of which may be served up as sushi: yellowfin, bigeye, longtail, blackfin, albacore, as well as the threatened Pacific bluefin and northern bluefin, and the critically endangered southern bluefin. Conservation organizations urge consumers to avoid eating any kind of bluefin tuna, but unless your ahi is a slice of the light-fleshed albacore tuna, it’s hard to tell which species you’re getting on your plate. Tuna species look different from one another when they’re whole fish, but slice them into filets and they all look pretty much alike. People may want to avoid eating endangered types of tuna, but they can’t do it effectively unless they can identify the fish in the restaurant or the market. (They could, of course, give up eating all tuna. But few people are willing to push their convictions that far.)
Jacob Lowenstein and his colleagues at the American Museum of Natural History tackled this problem by developing a DNA barcode system for all eight species of tuna. By looking for small, species-specific differences in the DNA sequence for a single protein – an important mitochondrial enzyme called cytochrome c oxidase – they developed a key that could match the DNA from a piece of tuna with the species of fish it came from. They tested the accuracy of their method by collecting samples from the places where most people meet tuna – sushi restaurants.
Over a period of six months, the researchers went to sushi restaurants in Manhattan and Denver and bought tuna: sashimi if they could get it, to reduce contamination with foreign human and rice DNA. They’d ask the waiter what kind of tuna they were getting, then pop a sample into alcohol to preserve it. Back in the lab, they extracted the DNA from the fish and checked its cytochrome c sequence against their key. And their key worked: it matched up their samples with known sequences from tuna species 100% of the time.
But what they found out about sushi restaurants in the course of this research wasn’t encouraging. They’d collected 68 samples of tuna. Twenty-two of them – about a third – came from a species of bluefin tuna, including the critically-endangered southern bluefin. And what’s more, the restaurants didn’t seem to know what they were serving: about a third of the samples were being sold as one species, but the molecular analysis showed that they were a different species altogether.
It wasn’t that restaurants were trying to hide the bluefin – they simply couldn’t tell which species they were serving any more than their customers could. In fact, 9 of the 22 samples that restaurant waitstaff said were bluefin tuna weren’t from one of those species at all. And 5 of the 9 pieces of “albacore” or “white tuna” collected by Lowenstein and his team weren’t even tuna. So this is a problem that goes further down the food chain than the customer or the waiter or even the chef: it goes back to the buyer at the fish market and the fishermen who catch the fish. Unless they change, or someone invents a handheld tricorder that can instantly take the DNA fingerprint of a piece of fish, most of us are going to be stuck in the same ignorant boat.
Source: Lowenstein JH, Amato G, Kolokotronis S-O, 2009. The Real maccoyii: Identifying Tuna Sushi with DNA Barcodes – Contrasting Characteristic Attributes and Genetic Distances. PLoS ONE 4(11): e7866.