Evaluating the potential for sea lice to evolve freshwater tolerance as a consequence of freshwater treatments in salmon aquaculture

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.

Trends in de-lousing of Norwegian farmed salmon from 2000–2019—Consumption of medicines, salmon louse resistance and non-medicinal control methods

“…However, no reports exist to date regarding development of increased salmon lice tolerance towards these treatment options. The risk of salmon lice developing increased tolerance towards freshwater was recently reviewed [ 42 ]. For proactive reasons, objective bioassay tests are being developed [ 17 ], and a freshwater tolerance test was in 2019 included in the Norwegian resistance monitoring program [ 10 ].…”

“…Since 2017, the number of treatments using non-medicinal methods of lowering infestation levels has overtaken the number of treatments using medicinal compounds [ 12 ]. Several non-medicinal approaches to de-lousing are available in Norway and other salmonid producing countries.…”

“…Preventive strategies such as synchronized fallow periods within production zones, synchronization of treatments, use of snorkel cages, functional feed, deep water feeding and plankton nets will not be addressed further in this study. In later years, concerns have been raised not only about fish wellbeing during some of these practices [ 13 – 16 ], but also that the lice may in fact adapt to these challenges as well [ 12 , 17 ] as there may be genetic variation in susceptibility, and hence survival and onward input into the gene pool [ 18 ].…”

Prevention not cure: a review of methods to avoid sea lice infestations in salmon aquaculture

“…2018; Groner et al . 2019). Alternatively, around 50 million cleaner fish (lumpfish Cyclopterus lumpus and several wrasse species) are deployed annually at Norwegian salmon farms to eat lice directly off salmon (Norwegian Directorate of Fisheries 2020), with >1.5 million cleaner fish also used in Scotland (Marine Scotland Directorate, 2017).…”

Evolution of salmon lice in response to management strategies: a review

“…Nevertheless, pests have the potential to adapt to non-chemical methods as well, including biological control (Kerr 2012;Tomasetto et al 2017) and crop rotation (Krysan et al 1986;Levine et al 2002). There are concerns that lice may evolve resistance to nonchemical management strategies as well (Ljungfeldt et al 2017;Groner et al 2019;Hamre et al 2020;Coates et al 2020). If suitable genetic variation exists, adaptation to non-chemical pressures would be facilitated by the same factors that permitted chemical resistance to rapidly emerge: widespread use of a limited number of strategies, which imposes strong, homogeneous selection; high gene flow, which disseminates advantageous mutations; and a high relative abundance of farmed salmon, which minimises the opportunity for wild hosts to act as refugia for susceptible lice.…”