Salmon lice are the most common parasite on farmed salmon, and the biggest disease problem in the industry

Salmon lice: Progress and conclusions of the Lakselus project

NORWAY
Thursday, January 14, 2021, 17:00 (GMT + 9)

An FHF project shows that prevention and control of salmon lice are demanding and that various factors can interact both positively and negatively.

FHF has co-funded the Lakselus project: Drug-Free Control by Combined Measures that was started at the Center for Aquaculture Competence (CAC) in 2016, in collaboration with the Marine Research Institute, Skretting ARC, MOWI, University of Melbourne and Quantidoc. The objective has been to describe the effect of different combinations of preventive measures measured through the number of lice and the frequency of treatment in the different experimental groups.

Salmon lices Photo: Havforskningsinstituttet

Four preventative strategies were tested over a 13-month period at a large-scale R&D facility in Rogaland. Various preventive measures were tested alone and together to see if any of the measures would be reinforced if combined. The test regimes were; cleaner fish alone; cleaner fish and use of functional feed; cleaner fish, functional feed and deep feed / deep light; as well as a final group with all the measures mentioned in addition to lice skirts around the cage. The experiments are described in detail in the final report and the research group concludes with a series of clear recommendations:

•         Preventive strategies should be developed and implemented in all cages at each location. This is to maximize the chance of fewer lice and fewer internal infections.
•         Functional diets should not be used only as a preventive measure. If they are to be used, it must be combined with other measures.
•         The corkwing wrasse / Grønngylt (Symphodus melops) should not be used in conjunction with deep feeding, deep light and especially lice skirts. Green seabream was not a particularly effective lice eater in any group, and careful evaluations should be made by the individual breeder on the rationale for using Vello fish for biological control of lice.
•         The results of this study suggest that the environmental conditions at the site are important for the success of a preventive strategy. In particular, the fish's distance to a possible halocline (fresh / brackish water over saltier water) can be decisive, and if the fish swims close to the halocline, this may mean that preventive measures go against their purpose. This should be included in plans for the use of preventive measures, and in the case of long-term, deep brackish water layers on the site, the preventive measures in use should be changed (e.g. avoid the use of skirts during those periods to ensure that the fish maximizes the distance from the halocline).
•         Both operations, such as net change and net washing, affect the barrier fabric of the fish. This supports the above recommendations to avoid or greatly reduce the discharge of overgrown nets into the sea at the site, or change nets operations on nets particularly overgrown.
•         Hydrogen peroxide treatment contributes to the drying of the skin of the fish and thus it becomes less resistant to external stressors and less able to establish a relative immune response to such stressors (for example, addition of lice). The breeder should carefully consider the use of hydrogen peroxide against the production cycle time and possible lice infestations in the time after treatment.

"This has been an extensive experimental setup, with significant efforts from all involved, and clearly shows how complex the interplay between different prevention measures against lice infestation really is. That researchers can make recommendations as clear as what have done in this project is very useful knowledge for the industry and should contribute to a greater focus on all measures, used alone or in combination, they have their prerequisites for whether they work well or not ", says department head Kjell Maroni of FHF who has followed the project.

Source: FHF (Translated fron original in Norwegian)

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