Context-dependent impact of an ectoparasite on early marine growth in Atlantic salmon

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The impact of sea lice on post-smolt growth (PSG) was studied by comparing growth patterns in the scales of recaptured adult salmon originating from hatchery-reared smolts that were untreated or were previously treated for sea lice infestation during the years 2003–2014. Growth in the returning hatchery-reared fish was also compared to the growth of wild fish from the same smolt year class. PSG was greater in treated versus untreated fish in the early part of the time series, which was a period of low marine survival. However, the differentiation in PSG disappeared in the later portion of the time series, which coincided with a shift from low to higher marine survival. This transition was demarcated by the year 2009. During this later period, PSG correlated with the marine survival of salmon, whereas in the early period, this correlation was not evident. Wild fish had faster PSG than untreated hatchery fish throughout the study period, but had slower PSG than treated hatchery fish during the early part of the time series. The effects of sea lice alone cannot explain the observed shift in marine survival occurring in 2009. However, it seems evident that the effect of antiparasitic treatment was contingent on poor marine survival, showing that the impacts of parasites on the PSG of Atlantic salmon are context dependent.