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Salmon do not become immune to salmon lice

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Friday, November 25, 2022, 07:00 (GMT + 9)

It is known that salmon get lice and react to it. Now the researchers have taken a closer look at what happens when the same fish is repeatedly attacked by lice.
"We clearly see that the salmon's immune system reacts when the fish gets lice, but it is not activated enough to prevent the same fish from getting lice several times", says Sussie Dalvin.
She is the project manager for a project where the researchers have specifically examined the salmon's immune response to salmon lice.

"The fish's response is very local, mostly only in the skin directly below the louse".
Important basic research
In the project, the researchers have worked on studies of basic mechanisms in the interaction between salmon and salmon lice. Such basic research does not provide solutions to problems such as how to avoid infection with salmon lice, but builds up the necessary basic knowledge of how the two species react to each other.
"Such research often confirms facts that many people think they know, but this is knowledge that we need to have scientifically confirmed or disproved so that we do not drag consequential errors into other parts of the research", says Dalvin.
Photo of several adult salmon lice, both females and males.  There is a clear size difference between the two sexes, the females being the largest.
This is what adult salmon lice look like. In the pictures there are both females and males.Photo: Sussie Dalvin / HI
A natural parasite
Salmon louse is a parasite that attaches itself to salmon, sea trout and char. Here it eats on the skin. If the fish is heavily attacked, it can get wounded and in the worst case die.
"Salmon lice are found naturally in our sea areas, but the amount has increased sharply as a result of the large number of farmed salmon that act as hosts for the parasite", says Dalvin.
Since farmed salmon are treated against lice, lice in themselves usually do not pose a major problem for the salmon in the cages, but the methods used to remove lice present major challenges with fish welfare. For the wild fish, salmon lice can pose a threat.

"Salmon lice are a problem especially for small salmon migrating from the rivers to the sea in spring/early summer and for sea trout that stay in coastal areas with high levels of salmon lice all summer", she says.
Picture of a salmon where a skin sample is taken from the back. Photographer: Sussie Dalvin / HI
Infected the salmon with lice repeatedly
In the past, experiments have been carried out to show how salmon react when they get salmon lice on them once. In this experiment, the researchers looked at what happens when it is repeatedly infected.
"Both in the wild and in farms, salmon are often repeatedly infected with lice. That is why we have carried out several rounds of lice infestation in different combinations on the same fish to see if previous infestation can have any effect"says Dalvin.
The researchers clearly see that the immune system of the fish reacts:
"We also see that the response changes over time, but not enough to prevent new infections with lice", she says.
To investigate how the salmon react to salmon lice, skin samples were taken where the lice had attached to the fish.Photographer: Sussie Dalvin / HI
The temperature matters
The researchers have also investigated whether different temperatures have an effect on the fish's reaction:
"It is clear that lower temperatures limit the fish's ability to respond to lice attacks, but at the same time the fish are infected with fewer lice at low temperatures", says Dalvin.
The results fit well with observations where areas/seasons with lower temperatures present fewer challenges in dealing with salmon lice.
"Although it looks like the salmon's immune system reacts worse at low temperatures, we see that low temperatures have the greatest negative consequences for the salmon lice", says Dalvin.


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