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Vision is important to the sea lice. 
Photo: Karin Kroon Boxaspen
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Production of lice families reveals valuable insights into pesticide resistance in lice

Scientists at the Institute of Marine Research, and the Salmon Louse Research Centre at the University of Bergen, both located in Norway, have for the first time produced families of sea lice in order to investigate resistance development in salmon lice. The ground-breaking work was conducted under the Norwegian Research Council funded project PrevenT, with PhD student Lina Ljungfelt leading the experiment.

Pesticide resistance represents a global challenge to food production. Specifically for the Atlantic salmon aquaculture industry, salmon lice and their developing resistance to delousing chemicals, for example emamectin benzoate, represents a challenge to sustainability. Therefore, it is important to quantify the degree of resistance development within lice, and understand its underlying genetic mechanisms. In the present study, researchers produced full-sibling families of salmon lice for the first time. This involved pairing sexually mature virgin females with males, mixing thousands of their offspring together to infect salmon, then implementing DNA parentage testing to identify the lice surviving chemical treatment with emamectin benzoate. Results from the study demonstrated increased genetic resistance tolerance in families produced from strains suspected to have decreased emamectin benzoate susceptibility. Furthermore, no fitness cost to the lice, measured as reduced survival under the experimental conditions, was observed with the resistance mechanism. The latter strongly suggests that when resistance to this chemical develops, it will be difficult to eradicate within wild lice populations. Finally, while this experiment provided a unique insight into emamectin benzoate sensitivity among lice families, the experimental design represents a novel methodology to experimentally address both resistance development and other evolutionary questions in parasitic copepods.

The article is open online:

A common-garden experiment to quantify evolutionary processes in copepods: the case of emamectin benzoate resistance in the parasitic sea louse Lepeophtheirus salmonis.

Facts about sea lice

Latin name: Lepeophtheirus salmonis
Distribution: occur naturally in Norwegian waters. Their numbers have risen significantly in parallel with the growth of the aquaculture industry.
Biology: sea lice are parasites with eight life stages, three of which are free-swimming, two of which are stationary and three of which are mobile. They attach themselves to salmon in the third life stage.
Size: adult female: 12 mm (approx. 29 mm including egg strings); adult male: 6 mm.
Diet: the skin and blood of salmonids. The lice only start feeding when they have attached themselves to a host fish (stationary and mobile stages).
Reproduction: all year round, but reproduce increasingly quickly as temperatures rise in spring.
Dispersal: free-swimming stages spread on currents in fjords and coastal waters.
Treatment: biological methods (wrasse) or chemicals (medication).

Sea lice