Abstract
Increasing usage of non-medicinal methods (NMMs) to control sea louse infestations on salmon farms has raised questions about whether sea lice may be able to evolve tolerance of NMMs. Of particular concern is the potential for sea lice to evolve freshwater tolerance as a result of freshwater treatments. Wild trout and some juvenile salmonids swim into freshwater to control infestations and regain ionic balance after disruption by sea lice; freshwater tolerance would compromise this potentially adaptive behavior. Here we evaluated the potential for freshwater tolerance to evolve in the sea louse Lepeophtheirus salmonis. When exposed to low-salinity water, parasitic stages of sea lice are able to osmoregulate through the host, while larval planktonic stages are not. Transcriptomic work suggests that sea lice mount a costly polygenic stress response when exposed to brackish water. The population structure of sea lice is panmictic in both the Pacific and Atlantic, making it conducive to rapid evolutionary responses. It is unknown how much heritable genetic variation these panmictic populations have for freshwater treatments. While usage of freshwater treatments on wellboats is increasing, it is unclear whether the freshwater itself is a strong selective force; during the freshwater exposure, sea lice can die from physical disruption during pumping and filtration on the wellboat. Future studies are advised to quantify the heritable variation in freshwater tolerance in sea louse populations, characterize mechanisms for freshwater tolerance in planktonic and attached sea lice, and assess the risk of freshwater tolerance evolution under different management strategies.
Highlights
Pest control is a major challenge in high-intensity farming
We describe the various methods for applying freshwater treatments and consider the potential influence of differing management strategies on salmon farms to contribute to resistance evolution
We identify knowledge gaps related to these topics and provide parameters which could potentially inform a risk analysis for freshwater tolerance evolution in sea lice, given the continued use of freshwater treatments
Summary
Pest control is a major challenge in high-intensity farming. High densities of single species are susceptible to density-dependent infestations (Lafferty et al 2015). As the use of NMMs (freshwater, warm water, brushes, flushing, etc.) for sea louse infestations on farmed salmonids increases, so do concerns about the potential for evolving tolerance This is concerning for freshwater treatments because freshwater exposure is a method that wild trout use to self-control infestations. This behavior comes at a cost of reduced growth and/or mortality (Birkeland & Jakobsen 1997), this is presumably less costly than maintaining an infection (Wells et al 2007) If this behavior becomes less effective due to artificial selection for freshwater tolerance in sea lice, or if control of sea lice on farms is compromised because sea lice have evolved tolerance to treatments, these wild salmonid species would be even more susceptible to the deleterious effects of infestations on behavior and health. We identify knowledge gaps related to these topics and provide parameters which could potentially inform a risk analysis for freshwater tolerance evolution in sea lice, given the continued use of freshwater treatments
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