The meeting between a parasite (salmon louse) and host (salmon) begins when the louse is in its free-swimming larval stage. At this stage, it is called a copepodid. But as a tiny organism in the vast ocean, how does it find the salmon it will live on?
"Chemical signals are believed to play a key role in the communication between host and parasite, and researchers have documented that this is true," says Nick Robinson, a senior scientist at Norwegian research organisation Nofima.
Want to increase resistance
Robinson coordinates the CrispResist project, which brings together a leading team of researchers from Norway, the United Kingdom, the United States, Canada, Sweden, and Australia. They are trying to discover which mechanisms cause different salmon species to have varying degrees of resistance to salmon lice. This knowledge can be used to strengthen the farmed salmon's resistance to salmon lice.
One goal is to identify and document which genes and mechanisms are responsible for the difference in resistance to salmon lice among different salmon species. Nofima states that it is well known that certain species of Pacific salmon are resistant to salmon lice and can kill lice in the early stages of the parasite attack. Atlantic salmon are, however, very susceptible.
Researchers working with chemical signals in salmon, gathered for experiments at Austevoll. Aleksei Krasnov from Nofima is furthest to the right.Photo: David Fields
Aleksei Krasnov, a senior researcher in fish health at Nofima, is researching the chemical communication between salmon lice and salmon. He and his international colleagues have identified potential semiochemicals; compounds of biological origin that affect the behaviour of animals of the same or other species. The type of semiochemicals called kairomones helps the lice find the salmon using scent.
Tested lice behaviour
Behind this discovery was a wide range of chemical analyses and tests of lice behaviour. Water "conditioned" with Atlantic salmon and Pacific salmon, as well as other fish species, was analysed. During conditioning, the fish swim around in the water for a while, releasing chemicals into the water. Twenty-one semiochemicals were selected for tests of lice behaviour.
In addition, mucus from families of Atlantic salmon with high and low resistance to lice was studied to see if the resistance could be linked to the chemical composition of the mucus. The behavioral tests were conducted using different methods in Norway and Sweden.
Nofima writes that the research results showed that water conditioned with only salmon stimulated the activity of copepodids. This confirms that kairomones are present. Conditioned water also contained compounds that repel lice. This suggests that Atlantic salmon also have mechanisms to push away lice.
Furthermore, the tests suggested that semiochemicals can be produced in various tissues of Atlantic salmon, especially in the skin.
Salmon from families that are susceptible to salmon lice were found to produce mucus that had a higher stimulating effect on lice than salmon from families with high resistance to lice.
The way forward
One of the most important conclusions from the study was that the researchers identified compounds for further research.
"Overall, the findings suggest that host-parasite communication is very complex and likely involves multiple signals," says Krasnov.
He sees the development of molecular tests as the most promising approach for further research on semiochemicals.
The research is part of the project CrispResist, and is funded by the Norwegian Seafood Research Fund - FHF. The project is a collaboration between 12 partners from research and industry. Notably, Rothamsted Research (UK), the University of Gothenburg (Sweden), and Bigelow Laboratory of Ocean Science (US) and Nofima, have contributed to the research mentioned here.
