Ecocentric

A Virus Threatens Farmed Salmon

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The future of fish—at least the sort that end up on our dinner plates—is not in the ocean. As we’ve steadily overfished the seas, the stock of wild fish have been declining fast. Only around 25% of commercial stocks are in a reasonably healthy state, and some 30% of fish stocks are already considered collapsed. A famous study that appeared a few years ago in the journal Science predicted that if current trends continued, commercial fish stocks might utterly collapse by 2048—meaning there would be virtually no wild seafood left.

Instead, we may have farmed fish—and indeed, chances are the salmon in your sushi roll was raised in a pen, not caught by fishermen. The global fish farming harvest is now at 110 million tons a year and is growing at an 8% clip. Farmed fish are an important source of protein and oils, but the practice can have serious environmental consequences, with farms pouring liquid waste into waterways. One of the biggest concerns, however, is disease, which can spread quickly among growing fish penned in a farm—and then to their wild cousins in the ocean, placing additional burdens on a population that is already declining. Farmed salmon in particular have proved vulnerable to a condition called heart and skeletal muscle inflammation (HSMI), a disease that can kill up to 20% of infected fish. Since it was first detected in a single farm in Norway in 1999, the disease has spread to hundreds of farms there and now in Britain as well—but scientists didn’t know what caused it.

An epidemiologist at Columbia University, however, may know. In a study published in the open-access journal PLoS ONE, a team led by W. Ian Lipkin—the director of the Center for Infection and Immunity at Columbia’s Mailman School of Public Health—found that the disease may be caused by a previously unknown reovirus, a form of double-stranded RNA viruses that infect a wide range of animals. And they found the virus using the tools of the geneticist.

Using more than 450 high throughput DNA sequencing and bioinformatics, the team—which included investigators from Norway—looked for viral sequences in heart and kidney samples taken from fish that were suffering from HSMI and healthy farmed salmon. They found evidence of the virus in 96.5% of the HSMI samples and none of the healthy fish. “Our data provide compelling evidence that HSMI is associated with infection with a new reovirus,” said Gustavo Palacios, the first author of the study and an epidemiologist at the Center.

Further research is still needed to definitively prove that the reovirus is causing HSMI—Koch’s postulate, the gold standard for proving that a pathogen causes a disease, requires scientists to be able to culture the new germ from a sick subject and then infect a healthy one with it. The Columbia researchers haven’t done that yet, but scientists in Norway are already developing a vaccine that would be able to protect farmed salmon—and if it works, that would also be conclusive evidence that the reovirus is behind the disease. Given how important farmed fish is becoming as the oceans are emptied—and given the risk that a disease in penned fish could decimate the waning stocks of wild salmon—there’s no time to wait.

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