​​​​​Salmonid aquaculture is an important industry on both sides of the North Atlantic, contributing to the local societies and producing highly demanded marine food. However, the industry has its environmental challenges, such as the ectoparasitic salmon louse (Lepeophtheirus salmonis). The salmon louse occurs naturally in the North Atlantic wherever there are wild salmonids. With the onset of salmonid farming in open net pens, the parasite obtained access to hosts held at high densities throughout the year. Fish farmers around the North Atlantic have since the 1970s used chemicals to control the number of lice in the fish pens. However, the salmon louse has developed resistance to four out of five chemical groups that have been available, forcing the industry to look for alternative non-chemical methods.

A fascinating aspect with the salmon louse is that there appears to be only one, pan-Atlantic, population. Because of the migratory life history of one of the main hosts, the wild Atlantic salmon, salmon lice travel long distances and can spread their offspring to salmon that return to both sides of the North Atlantic. Coupled with large reproductive potential and short generation time, the salmon louse is well prepared to meet whatever the fish farmers throw at it. In the latest Editor's Choice paper Aquaculture-driven evolution: distribution of pyrethroid resistance in the salmon louse throughout the North Atlantic in the years 2000-2017​, the authors have used a genetic marker associated with resistance to the delousing chemicals based on pyrethroids to document how this trait has dispersed and developed in time and space in the North Atlantic from the turn of the century.

Close to 15 000 salmon lice were genotyped and rated as either resistant or sensitive to pyrethroids. The authors did not detect the resistance marker in the samples prior to 2009, but from this year on the marker is found in most regions on the European side of the Atlantic, even on hosts from regions that have not been treated with pyrethroids. The authors have looked closely at two of the most important aquaculture regions on the European side, namely Norway and Scotland. The high-resolution screening of pyrethroid resistance in Norway for the period 2012–2015 revealed very high frequencies (>90 %) in the most intensely farmed counties. The similar study from Scotland in the period 2015–2017 found resistance frequencies >75 % in the most intensely farmed regions.

The paper confirms previous findings on the high connectivity of salmon lice between different parts of the North Atlantic and the role of aquaculture in salmon louse evolution. On the European side, where use of pyrethroids has been widespread, lice sensitive to pyrethroids have been almost eradicated in the most aquaculture affected regions. On the Canadian side no pyrethroids have been used and no resistant lice were detected in Canadian samples. However, resistance was detected in 20% of the lice sampled from wild Atlantic salmon caught off Greenland, where both North American and European salmon are present. The authors suggest that the resistant genotype is present also on the western shores of the North Atlantic, and that an introduction of the chemical in Canadian fish farms would rapidly lead to high frequencies of resistant lice also there.

The findings of this paper should have implications for the future management of delousing methods. The louse's ability to adapt and disperse beneficial traits over large distances within relatively short time spans implies that the aquaculture industry from all parts of the North Atlantic should come together and make a common strategy in order to prolong the efficacy of the available methods.

Read the full paper in ICES Journal of Marine Science.