Sea lice (Lepeophtheirus salmonis) are ectoparasites of salmon and trout. Heavy infestations can lead to secondary infections, weaker condition and mortality, particularly in young fish. In the wild, infested coastal trout and juvenile salmon can migrate into freshwater to manage infestations. Sea lice do not tolerate freshwater well, particularly after extended exposures and this behavioral adaptation allows trout to ‘delouse’. In areas with co-occurring salmon aquaculture and wild salmonid populations, sea louse infestations can be more severe on wild fish and this ‘delousing’ behavior may be more important. Salmon farmers have historically managed sea lice infestations using a number of methods including treatment of farmed fish with chemicals that kill the sea lice. Overuse of several chemicals has caused sea lice to evolve resistance to chemical treatments and many salmon farmers are using alternative methods for control. For example, in Norway, salmon farmers use freshwater exposures to control sea lice. While this method is more environmentally friendly, there are developing concerns that overuse could cause sea lice to evolve tolerance of freshwater. This would be both detrimental for wild fish, which rely on freshwater to control infestations and for salmon farmers, whom already have limited options for effectively controlling sea lice.
As a first step towards understanding this issue, researchers are conducting a literature review to quantify what is known about how sea lice and how related species respond physiologically to freshwater. From there researchers will assess the potential for sea lice to evolve a tolerance of freshwater and identify knowledge gaps. This work will be used to inform a risk assessment of this topic.
This work will be used to inform salmon aquaculturists and agencies that manage wild salmon about the risks of and potential for freshwater tolerance evolution in sea lice.