Authors

J James¹; H Bradley¹; A J Reading¹; R Hillman¹; P Elsmere¹; C F Williams¹;  ¹ Environment Agency

Discussion

The sea lice, Lepeophtheirus salmonis and Caligus elongatus are amongst the most damaging parasites of farmed salmonid fish globally. Infections of sea lice may also impact wild fish populations following spill-over from aquaculture production. However, little is known about the interactions between wild and farmed populations, with a paucity of data on 'natural' sea louse levels in wild fish. Here, we screened wild adult sea trout (Salmo trutta) and Atlantic salmon (Salmo salar) returning to the River Tamar, England, for the presence of sea lice. From 2013 to 2015, the average prevalence of sea liceon these fish was 55% (range: 51-58%) and the mean intensity of 6.19 (range: 1-66). All 2520 lice collected were identified as L. salmonis. Mean infection intensity did not differ between fish species or across years. Epidermal erosion and localised haemorrhage were observed more frequently in infected than uninfected fish (28% and 11%, respectively). In addition, within infected fish, infection intensity was positively correlated with the amount of external damage present on both S. trutta and S. salar. This is the first study to establish infections of sea lice in wild migratory salmonids in England. Whilst the prevalence of L. salmonis in salmonid fish returning to the river Tamar was low compared to other regions elsewhere in the UK, there is the potential that future development of marine aquaculture could increase disease risk to wild fish populations, already under pressure from declining marine survival, climate change, habitat loss and barriers to migration.