Sea lice infections on wild Atlantic salmon and sea trout in the River Tamar, UK: a temporal study

Sea lice are amongst the most ecologically and economically damaging parasites of farmed salmonids globally. Spill-over from aquaculture can increase parasite pressure on wild fish populations, but quantifying this effect is challenging due to the relative paucity of data available on 'natural' salmonid louse burdens in the absence of aquaculture, particularly for Atlantic salmon Salmo salar. Here, wild Atlantic salmon and sea trout S. trutta were screened at the tidal limit of the River Tamar (UK) for the presence of sea lice. During 2013 and 2015, the prevalence of sea lice ranged from 41 (n = 361) to 60% (n = 275) and 55 (n = 882) to 58% (n = 800) in Atlantic salmon and sea trout, respectively. All sea lice collected were identified as Lepeophtheirus salmonis. Mean L. salmonis infection intensity across the study period was 5.84 (range: 1-66) in Atlantic salmon and 6.45 (range: 1-37) in sea trout. Infection intensity was positively correlated with the amount of external damage present for both fish species. Given that the fish were examined when returning to freshwater, the lice burdens obtained may represent an underestimate. Nevertheless, these data provide important baseline information on 'natural' sea louse infections in South West England, which has been proposed as a potential region for aquaculture development.