The Phe362Tyr mutation conveying resistance to organophosphates occurs in high frequencies in salmon lice collected from wild salmon and trout

Helene Børretzen Fjørtoft,Vidar Aspehaug,Frank Nilsen,Kevin Alan Glover,Pål Arne Bjørn,Ann-Kristin Tveten,Anne Stene,Bengt Finstad,Francois Besnier

doi:10.1038/s41598-017-14681-6

Abstract

The parasitic salmon louse, and its resistance to chemical delousing agents, represents one of the largest challenges to the salmon aquaculture industry. We genotyped lice sampled from wild salmon and sea trout throughout Norway with the recently identified Phe362Tyr mutation that conveys resistance to organophosphates. These results were compared to data from lice sampled on farmed salmon in the same regions. The resistant (R) allele was observed in salmon lice from wild salmon and sea trout throughout Norway, although its frequency was highest in farming-intense regions. In most regions, the frequency of the R allele was higher in lice collected from wild sea trout than wild Atlantic salmon, and in all regions, the frequency of the R allele was similar in lice collected from wild sea trout and farmed Atlantic salmon. The R allele is only selected for in fish-farms where organophosphates are used for delousing. Therefore, our results suggest extensive exchange of lice between farmed and wild hosts, and indicate that in farming-dense regions in Norway, aquaculture represents a major driver of salmon louse population structure. Finally, these data suggest that the wild hosts within the regions studied will not delay the spread of resistance when organophosphates are used.

Highlights

Background data from farmsThe dataset, based upon the raw data presented in Kaur and colleagues[46] on farmed salmon from 2014, demonstrates a very low prevalence of RR in Finnmark (0%, 2013 data) and Sørlandet(1.7%)
At all locations sampled along the coast, both wild Atlantic salmon and sea trout were infected with salmon lice carrying the mutation Phe362Tyr, which is associated with reduced sensitivity towards organophosphates (i.e., RS or RR genotypes) (Fig. 1, Supplementary Table 1)
The RR genotype was present in all samples of lice from wild salmonids, with the exception of the sample taken from wild sea trout at Sørlandet, southern Norway

Summary

Introduction

The dataset, based upon the raw data presented in Kaur and colleagues[46] on farmed salmon from 2014, demonstrates a very low prevalence of RR in Finnmark (0%, 2013 data) and Sørlandet(1.7%). In order to select the background data that were relevant to our study, we used the border of the municipality where we had sampling stations and set a perimeter of 50 kilometers based on the dispersal distances of salmon lice and sea trout[16,53,57]. If the perimeter crossed the border of a municipality reported in Kaur and colleagues[46], we included their data in our study (Fig. 3). The only farms included in Kaur and colleagues[46] are located in the westernmost part of the region, while the National monitoring program’s wild salmonid sampling location is in the easternmost part of this region, ~190 kilometers away

Methods

Results

Conclusion