Winter is a challenging time for salmon farmers, as fish can develop skin sores and infections when seawater temperatures fall to 8ºC or lower, leading to fish welfare problems and mortality incidents.
However, researchers at Nofima, Norway's Institute of Food, Fisheries and Aquaculture Research, have made some advances in knowledge about both salmon skin and vaccine efficacy which may be useful in warding off winter ulcers.
While salmon skin infections can result from a wide range of pathogens, "classic" winter ulcers are most often caused by the bacterium Moritella viscosa. Salmon farmers can vaccinate their fish against this, and there are now several vaccines available on the market.
However, there's one major stumbling block: the bacterium has many different strains. That means some vaccines can be less effective if they are developed with the wrong strain of bacteria - in fact, researchers have observed development of ulcers even in vaccinated fish.
In order to gain better insights into the disease, the research team took an in-depth look at the salmon skin itself, to understand how the different strains of the bacteria manage to colonise the skin initially, and to what extent vaccines are effective in preventing infections or mitigating their severity.
The researchers found that the bacteria first gets a foothold on the salmon skin by getting between the outside mucus layer and the scales themselves.
“It sticks to the surface of the scales. It then multiplies and forms a colony,” lead researcher Christian Karlsen explains. “It can then create large wounds that can even reach muscle tissue."
Although vaccines did not provide 100% protection, the researchers found that these preventative measures do have an important part to play in mitigating the severity of skin infections.
"Vaccination resulted in significant protection against both M. viscosa-induced mortality and skin ulceration, although protection varied according to the vaccine formulation," the researchers concluded.
In unvaccinated fish, the researchers found that bacteria invade deeper into the skin at an earlier stage, leading to winter ulcers. Vaccinated salmon appeared to hold the bacterium at bay with more superficial skin infections.
The Nofima research was conducted in collaboration with the vaccine company Pharmaq, and financed by the Research Council of Norway, published in the journal Aquaculture.
