Just released research is showing that salmon farms may be killing off wild salmon. As this ScienCentral News video explains, the problem is parasites being spread from the farmed salmon to the wild salmon.
Sea Lice
Inside nets just off shore from British Columbia swim large numbers of salmon. These farm-raised fish will eventually wind up on dinner tables around the world, mostly in the United States. It's an example of aquaculture, raising fish in pens to offset declining stocks of wild fish and to help feed a hungry world. But, these pens can be plagued by many diseases, including parasites called sea lice. Evidence is growing that these pens are spreading the lice to wild juvenile salmon, often with lethal results.
Marine Ecologist John Volpe of the University of Victoria in British Columbia, Canada says part of the problem is that the farms are often near the routes of juvenile salmon as they migrate out into the open ocean from the rivers where they hatched.
A typical salmon farm. image: Twyla Roscovich
"What we found," says Volpe, was that during the approach of the salmon farm, the juvenile fish have almost no salmon lice on them." But, he explains, as the fish swim by, "there is a major spike in terms of lice ... over 70 times that of what would occur naturally."
He notes, "A lice for an adult fish isn't really a big problem because of the size of the fish." But juvenile fish are not much larger than the lice and that can be fatal. He describes it somewhat graphically, saying, "You can imagine having a parasite hanging off you that's half your body size literally chewing away at your skin."
Volpe was part of a team that wanted to know just how deadly these "close encounters" between farm-raised adult salmon with lice and juvenile salmon were.
Lead researcher Martin Krkosek of the University of Alberta directed a team that caught, studied, and released 14,225 juvenile salmon every mile or so over three different migration routes that ranged between 25 to 50 miles.
Writing in Proceedings of the National Academy of Sciences, Krkosek wrote that between nine and 95 percent of the juvenile salmon died. He explained that the salmon survived better early in the season because the sea lice are younger and relatively small, resulting in a mortality rate of nine percent. However, as the season progressed, and the lice grew, mortality rates reached 95 percent.
Volpe says researchers became concerned the farms might be having an impact on the numbers of wild salmon after people reported a major drop in the number of salmon returning to some rivers located near the pens in order to spawn. He says, "Those rivers actually collapsed, where more than 95 percent of the fish just did not show up. And yet, rivers to the south and to the north of this area (near the salmon farms) the fish returned as per expected."
A researcher inspects a fingerling (young salmon) for lice. image: Twyla Roscovich
Volpe says the logical solution to the problem would be to remove the pens from the ocean and instead raise salmon in large tanks on land. However, he adds he understands this is not economically practical.
Krkosek says he'd like to next try to work with the industry to see if there is a way to mitigate the problem so that researchers could find a way for the farmed fish and wild salmon to exist side-by-side.
This research was published in the Proceedings of the National Academy of Sciences online early edition for the week of October 2-6, 2006. The research was funded by the National Science & Engineering Research Council of Canada, D. Allen Birdsall Memorial Fund, Tides Canada, NRC MITACS program, Canadian Sablefish Association, British Columbia Wilderness Tourism Association, and the David Suzuki Foundation.
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