Photographer: Eivind Senneset/Institute of Marine Research
Researchers in Norway have mapped how sea lice spread between fish farms, in work that could help the aquaculture industry target control measures more effectively.
The new report, prepared by the Institute of Marine Research in response to questions from the Norwegian aquaculture industry, uses modelling to show which salmon farming sites release large numbers of lice larvae and which sites receive lice from neighbouring facilities.
Research manager Ingrid Askeland Johnsen said the institute was able to build on existing models already used to assess how lice from fish farms may affect wild salmonids.
“Since we already have models that show how salmon lice from fish farms are transported by currents and infect wild fish, much of the work was done. So it was easy to say yes when we were asked to produce such a report,” Johnsen said in a Norwegian language news release published on the Institute's website.
The report looks at sea lice transmission routes between aquaculture sites in 2024 and 2025, and includes data on how many lice larvae were released from each locality and reached another site, how many larvae arrived at individual sites from other farms, and how lice were transported between facilities.
“Sea lice have been a major problem for many years, both for the aquaculture industry and for wild salmon and sea trout,” Johnsen said, suggesting the findings could support more strategic decision-making by fish farmers seeking to reduce lice pressure.
The work uses mathematical models that combine reported sea temperatures, lice counts on farmed fish and the number of farmed fish at each site. Based on that information, researchers calculated the release of lice larvae from farms, how the larvae were carried by currents, and where they were present during the period in which they could infect fish. The same modelling approach has previously been used by the institute to study the impact of farm-origin sea lice on wild fish, the researchers noted.
“We see that the amount of salmon lice larvae produced in an area is strongly dependent on when the fish are seen in the sea. The production cycle for salmon is typically about two years, which is why we have also included two years in the report,” she said.
According to the institute, sea lice development and behaviour are influenced by temperature, light and salinity. The researchers noted that 2025 brought unusually warm seawater, with high temperatures continuing into the autumn.
“This gave the lice extra good conditions for a longer period than usual, and there were a lot of lice and a big job for many farmers when it came to keeping the level of lice in the facilities down,” Johnsen said.
The modelling also shows that lice dispersal can change substantially within a single day, particularly in fjord systems where water movement is affected by currents, wind and tides. The report presents annual average patterns, but the underlying modelling includes hour-by-hour variation, the researchers said.
Johnsen stressed that the report focuses only on transmission between fish farms, rather than infection pressure on wild salmon, sea trout and Arctic char. The Institute recently began a separate annual monitoring exercise of sea lice on wild salmonids, including wild salmon, sea trout and sea char, in selected coastal areas.
The researchers said the findings could help the industry identify where interventions would have the greatest effect. These could include submerged cage systems or other lice-reducing technologies.
However, Johnsen noted that the modelling includes one important limitation. Around 40 Norwegian sites have adopted submerged systems, which can reduce lice problems, but there is no national register of these facilities. As a result, the model assumes all sites use open surface technology, which the institute said remains true for most locations.
Johnsen said the results could also be used to assess whether submerged systems have been deployed at the sites with the greatest need, or whether other locations should adopt submerged cages or alternative lice-reduction measures.
“We have identified the facilities that produce and spread the most lice, now it is up to the industry to direct its efforts where it will have the greatest effect,” Johnsen said.