Context-dependent reduction in somatic condition of wild Atlantic salmon infested with sea lice

Roman Susdorf* (Corresponding Author), Nabeil K. G. Salama, Christopher D. Todd, Robert J. Hillman, Paul Elsmere, David Lusseau

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

10 Citations (Scopus)


Wild Atlantic salmon stocks have globally declined over recent decades. On their migratory return to coastal waters individuals typically are infested by ectoparasitic caligid sea lice (Lepeophtheirus salmonis Krøyer, Caligus elongatus Nordmann). Infestation in laboratory trials can exert non-lethal impacts on the host fish, including increased stress levels and reduced growth, physical condition and swimming performance. However, to date no evidence exists for non-lethal effects of sea lice on wild adult Atlantic salmon. Using observations for >6000 return-migrant adult salmon captured from the coastline at Strathy Point (SP), North Scotland, in the estuary of the River North Esk (NE), East Scotland, and the lower reach of the River Tamar (TA), Southwest England, we show that the somatic condition (weight at length) of wild salmon is associated with mobile sea lice density. This putative sea lice-mediated reduction of condition varied with year and seasonal date of freshwater return, and increased with the proportion of adult female parasites on a given fish. Influence of host sex, sea-age and smolt age was negligible. The estimated impact differed between the three sampling sites likely due to underestimation of infestation levels at NE and TA – largely attributable to negative influences of reduced salinity on parasite retention prior sampling. Caligid infestation in the present samples explained a small, but discernible, proportion of the variation in host condition. Reductions in somatic condition of Atlantic salmon are associated with disproportionate declines in accreted lipid reserves, which are critical to up-river migration and ultimately the provisioning of eggs.
Original languageEnglish
Article numberM 12746
Pages (from-to)91-104
Number of pages14
JournalMarine Ecology Progress Series
Publication statusPublished - 15 Nov 2018


  • Atlantic salmon
  • Salmo salar
  • Condition effect
  • Parasite-host interaction
  • Sea lice
  • Lepeophtheirus salmonis
  • Caligus elongatus


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