Stress makes farmed salmon more susceptible to viruses

Fish with IPN

Fish with IPN. (Credit: Bård Skjelstad)

It’s something salmon farmers know from experience: stress makes fish sick. That’s why they take so many steps to avoid stressing the fish, measures like only feeding when water and temperature conditions are just right; maintaining a low density of fish in the pens; and handling the fish as little as possible.

But the scientific cause-and-effect relationships between stressors and fish health are not well-documented. What is actually happening in the fish? It’s not surprising that we are still learning how this works, given that ocean farming is still only a few decades old. But the great thing about developing a new way of farming in an age of science and technology is that we learn quickly.

Koestan Gadan

Koestan Gadan, doctoral candidate at the Norwegian School of Veterinary Science.

Koestan Gadan did her doctoral thesis on the link between stress in farmed salmon and viruses, specifically the Infectious Pancreatic Necrosis virus (IPN), which is a concern for farmers in Norway. Gadan defended her doctoral research last month at the Norwegian School of Veterinary Science with her thesis, titled “Studies on stress and innate immunity in relation to infectious pancreatic necrosis virus in Atlantic salmon (Salmo salar L.).”

Gadan discovered that these stress factors lead to increased production of the stress hormone cortisol in the fish. This increase in the level of cortisol affects the immune system of the fish and weaker immunity makes them more susceptible to infections. The fact that stress can trigger an outbreak of IPN is corroborated by the experiences of many in the field. Gadan’s research shows that various environmental factors are stressful to the fish, in turn triggering an increase in the level of cortisol in the blood and causing harm to their congenital immune defence system, which is essential to their capacity to ward off viral infections.

Gadan discovered something else that is quite interesting.

Gadan also demonstrated that chronic stress leads to a high level of cortisol over time, which results in increased mortality, a high propagation of viruses and chronic infections. For the first time, she was able to prove that when infected salmon fry were exposed to stress, otherwise benign variants of the IPN virus changed into pathogenic viruses. In other words, stress lowers resistance, increases the “production” of IPN virus in the fish’s internal organs and can lead to benign viruses changing into pernicious variants of the virus.

IPN has not been found in B.C. but it has been observed in Eastern Canada and Europe in different kinds of fish since 1941. But the specific virus isn’t the interesting thing here; it’s how stress makes fish more susceptible to viruses and can even weaken fish to the point that otherwise benign viruses can evolve into harmful variants.

It’s very helpful for farmers to have scientific evidence to explain what they have experienced in the ocean. And hopefully this research will help salmon farmers all around the world learn more about keeping the stress levels of their fish low, and find new ways to prevent viral infections.

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It seems that only poor, agenda-driven science attacking salmon farms gets mass media attention. We will show there is a lot more to the story, and the science.

Posted on 2013/01/03, in News, Primary Sources and tagged , , , , , , , , , , , , , . Bookmark the permalink. 2 Comments.

  1. I would think by now this issue would have made it’s way into the management of the wild-catch fisheries? As a 37 year commercial salmon fisher, distress and it’s affects to salmon is nothing new but disgarded by DFO in it’s feeble efforts to manage the wild-catch BC salmon fisheries. Years ago there was but one commercial fishery per river or species and once the fish got past the fishery, their greatest concern was avoiding upriver sport fisheries and Native food fisheries. Did DFO ever factor in Alaskan interception as a stess factor? In today’s commercial salmon fishery, what once was is nothing like what DFO has created today, especially when it comes to stress overload and disease/parasite resistance.

    Once a salmon begins it’s return migration today, it first encounters acidifying Ocean waters and warming Ocean waters, both of which by default decrease the Oceans ability to balance oxygen levels which in turn causes distress. As they close in on the route along coastal landfall, they next encounter various fisheries including trawl, commercial salmon and sport and each adds to the distress for those that get past. Once they get down to their home river, no longer is their but the one serious concern but today a mutlitude of commecial fisheries all the way right up to and including at the very front of their spawning grounds. Add to that recipe for stress related disaster is the growing snag fishery for sockeye, where even the slightest “touch” triggers the flight or fight response, otherwise known as a host of chemicals shot into the blood-stream to initiate the flight response until a point is reached when the cumulative affects of distress leave the fish wide open to who knows what in the way of pathogens that are more than a little likely to be contribiuting to the overall decline of our salmon.

    Some may and always have, wondered why I am a supporter of our fish farm industry but when the facts are exposed, given we have no non-politicized management address on these issues to protect the canning industry and mass cheap volumes, I don’t need to ask why our salmon are failing, or our halibut, etc, etc, I know we need the BC fish farm industry to provide the only support necessary to give real science the support it needs to stop the crash in our salmon and other species such as halibut, etc!

    (PS- how many know, since 2004 the harvest quota on halibut has been cut by 224%! In the last three years alone it will have been chopped 70% if the scientists concerns are allowed into the management plan! It’s so serious in fact, the scientists are now using words such a “grim” to describe what is happening and they have no idea how to stop it – without shutting the fishery down and what would the stores sell then? Hint- farmed halibut)

    • Thanks for sharing these great points, Fred.

      You are right — there are so many stressors wild salmon face from the moment they hatch to the moment they spawn and die. Salmon farms may be a stressor (although no science done to date shows any impact outside mathematical simulations), but as you point out, salmon farms are also science-based and deeply invested in research that improves fish health. That same research helps everyone, because it can be used to better understand and sustainably manage wild fish stocks.

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