Effects of salmon lice Lepeophtheirus salmonis on wild sea trout Salmo trutta—a literature review

Abstract

Salmon farming increases the abundance of salmon lice, which are ectoparasites of salmonids in the sea. Here we review the current knowledge on the effects of salmon lice on wild sea trout. Salmon lice feed on host mucus, skin and muscle, and infestation may induce osmoregulatory dysfunction, physiological stress, anaemia, reduced feeding and growth, increased susceptibility to secondary infections, reduced disease resistance and ultimately mortality of individual sea trout. Wild sea trout in farm-free areas generally show low lice levels. In farm-intensive areas, lice levels on wild sea trout are typically higher, and more variable than in farm-free areas. Lice on wild sea trout are found at elevated levels particularly within 30 km of the nearest farms but can also extend to further ranges. Salmon lice in intensively farmed areas have negatively impacted wild sea trout populations by reducing growth and increasing marine mortality. Quantification of these impacts remains a challenge, although population-level effects have been quantified in Atlantic salmon by comparing the survival of chemically protected fish with control groups, which are relevant also for sea trout. Mortality attributable to salmon lice can lead to an average of 12−29% fewer salmon spawners. Reduced growth and increased mortality will reduce the benefits of marine migration for sea trout, and may also result in selection against anadromy in areas with high lice levels. Salmon lice-induced effects on sea trout populations may also extend to altered genetic composition and reduced diversity, and possibly to the local loss of sea trout, and establishment of exclusively freshwater resident populations.

Statistical and ecological challenges of monitoring parasitic salmon lice infestations in wild salmonid fish stocks

“…Although a binary response variable has limited biological information as it does not allow for testing why some individuals have many lice while others have few, the advantage of this measure is that the statistical methods are relatively simple and that the number of observations remain high (N = 4890 individual fish). However, it is of limited interest whether a fish has 1 or only a few lice, since the potential negative effects, such as immune responses, reduced growth and mortality, only occur after the infestation reaches a certain level (Thorstad et al 2015).…”

“…Therefore, the proportion of individuals above a threshold level may be a better measure, especially if this threshold represents a biologically meaningful lice infestation level. A threshold of 0.1 lice g −1 fish weight (lice g −1 ) has been suggested as a critical value for initial physiological disturbances caused by salmon lice on first-time migrating trout in Norwegian fjords (Taranger et al 2015, Thorstad et al 2015 , 100% extra mortality risk is anticipated (Taranger et al 2015). Studies of effects on older and larger trout (i.e.…”

“…Studies of effects on older and larger trout (i.e. maturing individuals or veteran migrants) are lacking, but because studies on char have indicated that the effect of salmon lice is expected to be more severe for larger fish (Tveiten et al 2010), extra mortality is predicted to occur above 0.025 lice g −1 for this group (Taranger et al 2015, Thorstad et al 2015. Based on this, we compared 3 different lice levels in our models with proportional response variables: > 0.1, > 0.05 and > 0.025 lice g −1…”

“…The programme was initiated because the massive increase in biomass of farmed salmonids (of which > 90% are Atlantic salmon; Directorate of Fisheries 2013) during the last decades has raised growing concerns regarding the transmission of lice between farmed and wild salmonids, with subsequent negative population effects for wild salmonids (Heuch & Mo 2001, Taranger et al 2015, Thorstad et al 2015. Since farmed salmon are kept in floating net pens with free water exchange, there is potential for pathogen exchange with the environment.…”

Temporal variation in sea trout Salmo trutta life history traits in the Erriff River, western Ireland

“…In a recent study, Taranger et al (2015) undertook a risk assessment of the effects of salmon lice on wild salmonid populations along the intensively farmed Norwegian coastline and found that sea trout from the majority of sampled sites had salmon lice infections, mainly resulting from salmon farming, which indicated moderate or high mortality risk of sea trout. In an extensive review of the impacts of the salmon louse on sea trout, Thorstad et al (2015) concluded that salmon farming increases the abundance of salmon lice in the marine habitat, and there is extensive published evidence that salmon lice in intensively farmed areas have negatively impacted wild sea trout populations by in creasing marine mortality, changes in migratory be haviour, reduction of marine growth and reduced population sizes. Results of this long-term monitoring programme on the River Erriff demonstrate that significant change in sea trout population life history composition and abundance can occur over a relatively short time period and suggest that the intro duction of salmon farming in the local estuary most likely contributed to the observed changes.…”