Abstract
The salmon louse is a highly abundant ectoparasitic copepod of salmonids in the North Pacific and Atlantic. Widespread and rapid development of resistance to chemical agents used to delouse salmonids on marine farms is now threatening the continued development of the aquaculture industry and have served as a potent catalyst for the development of alternative pest management strategies. These include freshwater and warm‐water treatments to which the louse is sensitive. However, given the well‐documented evolutionary capacity of this species, the risk of developing tolerance towards these environmental treatments cannot be dismissed. Two common‐garden experiments were performed using full‐sibling families of lice identified by DNA parentage testing to investigate whether one of the fundamental premises for evolution, in this context genetic variation in the capacity of coping with fresh or warm water, exists within this species. Significant differences in survival were observed among families in both experiments, although for the salinity experiment, it was not possible to unequivocally disentangle background mortality from treatment‐induced mortality. Thus, our data demonstrate genetic variation in tolerance of warm water and are suggestive of genetic variation in salinity tolerance. We conclude that extensive use of these environmental‐based treatments to delouse salmonids on commercial farms may drive lice towards increased tolerance.
Highlights
Human ecological impact has massive evolutionary consequences and can greatly accelerate evolutionary change in many species, namely aqua- and agricultural pests, disease organisms or species hunted commercially
A low salinity and a heat challenge, were conducted. Both experiments follow the overall experimental design detailed in Ljungfeldt et al (2014), which includes the following steps (Figure 1) (i) Acquisition of two strains of salmon lice, L. salmonis salmonis, from fish farms situated in two different salinity/ thermal environments, respectively. (ii) Synchronized production of single-s train parental populations. (iii) Synchronized creation of full-sibling families to be mixed in a common pool. (iv) Common- garden infection in replicate salmon tanks with an exact number of copepodids from each of the families. (v) Experimental treatment of lice. (vi) Sampling of lice sorted by trial response. (vii) Individual genotyping of parents and sampled offspring for family identification and subsequent quantification of family performance
It was primarily designed to investigate the potential for genetic variation in tolerance to low salinity and a heat challenge in the salmon louse, an economically and ecologically highly significant parasite of farmed and wild salmonids in the North Atlantic and Pacific oceans
Summary
Human ecological impact has massive evolutionary consequences and can greatly accelerate evolutionary change in many species, namely aqua- and agricultural pests, disease organisms or species hunted commercially. Given the high reproductive output, short generation time and very high abundance of this species, the potential for rapid evolution, including human-induced selection regimes, is foreseeable In this context, the emerging use of unfavourable environmental conditions as a nonchemical alternative strategy to treat lice infestations on farmed salmonids We used the protocol and infrastructure established by Ljungfeldt et al (2014) to quantify family differences (as a proxy for genetic variation) in tolerance to a low salinity and a heat challenge This was to evaluate whether the emerging practice within the commercial aquaculture industry of delousing farmed salmonids with fresh- and/or warm-water treatments may elicit an evolutionary response in this parasite and lead to reduced treatment effectiveness
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