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What is the most effective treatment against sea lice in salmon farming?

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By Milthon Lujan

Sea lice of salmon. Source: Nofima
Sea lice of salmon. Source: Nofima

Sea lice, or salmon lice, are one of the main parasites that affect the salmon industry worldwide.

Traditionally, salmon lice have been controlled using antiparasitic medications, which are added to the feed or used in bath treatments. However, many of these products lead to resistance in sea lice.

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The effectiveness of different treatment methods is evaluated by fish farmers and regulatory authorities because each treatment affects the well-being of the fish and increases costs, which need to be balanced to determine the benefits of the treatments.

A team of researchers from the Norwegian Computing Center and the Norwegian Veterinary Institute estimated the effectiveness of ten types of treatments currently used in the salmon industry by analyzing daily data and complete production cycles in Norway.

They performed calculations based on a stage-structured population model for sea lice and accounted for the data structure.

Control treatments for salmon lice

“We divided the treatment types into three main groups: medicated feeds, medicinal baths, and non-medicinal treatments,” they reported.

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The researchers subdivided non-medicinal treatments into thermal treatments (hot water, up to 34 °C), mechanical treatments (brushing and washing), and freshwater treatments.

They also highlighted that medicated feeds are provided for one or two weeks and have little relative effectiveness on a daily basis, but the effects are seen over a longer period.

Medicinal and non-medicinal treatments are applied in seconds or minutes, with a more or less immediate effect.

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According to the scientists, some treatments are effective against all life stages of sea lice (copepodids, chalimus, pre-adults, adults), while others only have an effect on certain stages.

Most efficient methods for killing sea lice

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“The results suggest that the most commonly used treatment methods in the dataset, namely thermal treatments, mechanical treatments, and freshwater treatments, kill an average of 70% to 80% of the lice, but with considerable variability,” they reported.

According to the study, feed treatments with emamectin benzoate are also commonly used but are estimated to kill only around 35% of lice on average.

“Bath treatments with hydrogen peroxide are estimated to kill around 74%, and pyrethroids around 50% of the lice on average,” they stated.

The researchers estimate that recently approved bath treatment with imidacloprid eliminates over 99% of sessile lice and 98% of pre-adult and adult lice on average.

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In this regard, the scientists emphasize that the most effective treatments were medicated treatments with the active ingredients imidacloprid and the benzoylureas diflubenzuron and teflubenzuron.

“However, imidacloprid cannot be a single treatment option since historically, medication treatment methods have lost effectiveness over time due to resistance,” the study cited.

Furthermore, the researchers note that a cruder estimate of effectiveness based on comparing lice counts before and after treatments in a subset of data confirmed most of the results based on the model.

“An exception was that the crude estimate only showed around an 80% reduction in the number of lice after imidacloprid treatments, which is lower than the model-based estimate of close to 100% effectiveness,” they concluded.

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Conclusion

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“These results allow for a coherent and comprehensive comparison of the effectiveness of different treatment methods, enabling fish farmers and authorities to better balance the expected benefits of treatments in terms of lice control against economic costs, fish welfare, and the risk of resistance development,” the researchers concluded.

The project was funded by the Norwegian Seafood Research Fund and the Norwegian Research Council.

Reference (open access):
M. Aldrin, R.B. Huseby, L.C. Stige, K.O. Helgesen. 2023. Estimated effectiveness of treatments against salmon lice in marine salmonid farming, Aquaculture, Volume 575, 2023, 39749, ISSN 0044-8486, https://doi.org/10.1016/j.aquaculture.2023.739749.

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