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 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.