Study: Lice-Infested Farmed Salmon Not to Blame for Wild Salmon Die-Off

By Andrew Moseman | December 14, 2010 11:12 am

Pink_salmonThe wild pink salmon of western Canada are in trouble: In the early 2000s, their numbers in some locations swiftly dropped by 90 percent or more. One explanation put forth for this steep population decline is that sea lice, parasites ubiquitous on farmed salmon, jumped to the wild variety of the fish. But this week in the Proceedings of the National Academy of Sciences, a new study casts doubt on that idea and says the sea lice are not to blame.

When Gary Marty of the University of California, Davis, and his colleagues looked at that aspect for the Broughton Archipelago of western Canada, they found that salmon survival was not lower in years when the juveniles passed by louse-infested farms. This, they say, suggests that something other than sea lice must be reducing survival rates. [New Scientist]

Marty’s team checked up on a decade worth of data dating back to before the 2002 crash, and found a few interesting things. First, they say, the predominance of the lice in wild populations appears to predict the number found in farms a little later, suggesting the parasites travel from wild salmon to farmed ones and not the other way around. Second, they argue, it does appear that a high number of lice in the farmed fish predicts higher than normal exposure for the juveniles of the wild variety, but that increased exposure can’t account for the huge population drop in the wild salmon.

“Based on extrapolations from controlled laboratory studies, infestation levels… might have killed 8% if the juvenile salmon [leaving the area].” In other words this was not enough to destabilise the wild population, they added: “Death caused by sea lice exposure replaced death caused by other causes, resulting in no net change in generational survival.” [BBC News]

But if not sea lice, then what killed so many wild salmon in so short a time?

Marty suggests that the 2002 crash was due to some unrelated cause, perhaps a virus or bacterium. He admits that it is possible that sea lice were part of a combination of factors that led to the declines; perhaps lice or viral infection alone aren’t enough to kill that many fish, but if both occur in the same year, many juveniles die. However, he doesn’t think that this is likely. [Nature News]

Marty notes that many of the sick wild salmon showed reddening or bleeding at the base of their fins—an ailment associated with environmental stress or infection, not with the sea louse parasite. Plus, though the sea lice are a salmon parasite, the salmon also like to eat the sea lice, which could help them fight off its parasitic effects.

For his part, Martin Krkošek—the researcher whose earlier study said that wild salmon in rivers that were exposed to farmed fish populations tended to decline—isn’t convinced. Krkošek argues that the new study is too limited by studying just the Broughton Archipelago location, and says that its statistical connections aren’t convincing.

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Image: Wikimedia Commons

  • Matt B.

    How closely related is the sea louse to regular lice like the head louse?

  • Barbara watson

    Natures way of dealing with the sea lice on adult salmon is to have the lice die with the adult when they go up the river to spawn. This leaves a clean environment for the baby fry to grow and begin their journey to the ocean. The salmon feedlots are like the Golden Arches to the young fry, attracted by food. This intense exposure to concentrations of lice infested farmed fish allow the lice to transfer to the young salmon in mortal numbers, instead of growing in the open ocean to a size that is not as vulnerable to the lice. Nature has no Golden Arches. The young fry are not intended by nature to mingle intensely with diseased or infected adults.

    The science of this report is faulty and self serving to the industry. It should be ignored.

  • Vick

    Your article states that the authors findings were that : “the parasites travel from wild salmon to farmed ones and not the other way around”.

    Absolutely NOT true. In fact, later on the author corrects or counterdicts this statement by stating that the authors found: ” high number of lice in the farmed fish predicts higher than normal exposure for the juveniles of the wild variety”.

    The authors looked at prevalences (% population with sea lice), instead of intensities (# sea lice per infected fish). It’s the INTENSITIES (per gram of host) that are used to estimate mortalities of fish, not Abundances. So why didn’t the authors also examine INTENSITIES as well as PREVALENCES? That data would have been right in front of them. Why hasn’t the media questioned them on it?

    Let’s also NOT FORGET that sea lice on farmed salmon is the BEST predictor of sea lice on wild outmigrating juveniles that these authors found. The question is whether or not sea lice prevalences is the best metric to use to determine the impacts of farm-origin lice on wild salmon populations. Seems not. what about intensities?

  • Bronwen

    New study suggesting sea lice not to blame for harming wild salmon is inconclusive and fails to convince when weighed against the full scope of science on this subject.

  • YouRang

    re Vicks’s comment.
    It seems to me that that is exactly their contention: I.E. the prevalence of infestation has been asserted to be killing the fish; but the intensity was only capable of killing an additional 8% of the population. In fact it looks to me like the people claiming the link between farming and lice killing have been looking at prevalence rather than intensity. So thank you Vick for proving the opposite side of the case to your position (or are you just a shill?).
    My position actually is we’d be better off if we farmed the babies, protecting them and then releasing them into the wild.

  • nick

    YR: good point on farming the babies, but you know would end up doing it in toxic conditions anyway, unleashing plagues across our sea. :)

    Did they check for parasites that infect salmon internally, carried by the sea lice? Like the way our pets can get gross intestinal infections from eating fleas? That’s what I’m curious about. Especially since they apparently enjoy eating them. Unfortunately I don’t have a PNAS account so I can’t check for myself.

  • Barbara watson

    Recommended reading. “Listening to Whales” by Alexandra Morton.

    Written from a love of the natural world and a lifelong commitment to the whales and their habitat. Read it and then you decide who’s science is motivated in YOUR best interests. A fascinating wealth of information about the Orca whales that should not be missed.

  • Sara

    There is a whole other story about the well funded anti salmon farming activists intent on demarketing farmed salmon. Dismissing science that is contrary to their campaign is part of the package. Recommended reading:

  • Jim Bruining

    I love the lady above (Barbara Watson) that says “it (study) should be ignored”.

    Wow. I take it Barbara is an activist, ’cause that doesn’t sound very scientific!! Science is to be debated (usually by scientists though, and not weird people that seek cults and leaders).

    Whether you agree with this study or not, you simply can’t “choose” your favorite and ignore the rest. It’s all important.

    In this case, I think salmon farmers have made some very good changes because of what other “less favorable” studies have concluded in the past. These positive changes would suggest that salmon farmers aren’t ignoring any studies, so it would be good to see activists like Barbara paying attention as well.

    I’ll bet Barbara stopped reading this comment after the first sentence…


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