Salmon louse Lepeophtheirus salmonis predominates over Caligus elongatus on Atlantic salmon post-smolts in central and western Norway

Atlantic salmon Salmo salar are exposed to parasitic sea lice as they enter the marine environment, particularly in areas with high densities of open net-pen salmon farming, where infestations can reach harmful levels. Given the correlation between infestations on farmed and wild fish, the highly specialized salmon louse Lepeophtheirus salmonis , which often causes disease outbreaks in salmon farms, is considered the main cause for sea lice infestations in wild populations. However, this has not been validated, as sessile life stages of L. salmonis cannot easily be distinguished from other caligid sea lice species. Through diagnostic PCR analysis of sessile sea lice (n = 1243) collected from wild Atlantic salmon post-smolts in central and western Norway, we document a large disparity in the occurrence of L. salmonis (98%) compared to Caligus elongatus (2%), the other sea lice species found on salmonids in Norway. The occurrence of L. salmonis was substantially greater among sessile sea lice in all fjords (n = 6) and sampling events (n = 9), and the contribution of C. elongatus only exceeded 8% in the 2 sampling events with the markedly lowest prevalence and mean intensity of sea lice. These findings demonstrate that L. salmonis is the dominant caligid sea louse on wild Atlantic salmon post-smolts migrating through fjords in central and western Norway.