The influence of non-native salmonids on circulating hormone concentrations in juvenile Atlantic salmon

Behavioural endocrinologists have shown that stressors including competition for resources can affect an individual’s circulating level of cortisol, whereas agonistic interactions typically affect androgen levels. Conservation biologists have used such data to facilitate the restoration and management of biodiversity by monitoring social interactions and stress among individuals and species. Here, we examined whether competition and agonistic interactions with non-native salmonids is hindering restoration of native Atlantic salmon, Salmo salar, in Lake Ontario. Using semi-natural streams, we examined the effects of competition with non-native brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, on plasma cortisol and 11-ketotestosterone (11-KT) concentrations in juveniles from three Atlantic salmon strains, as well as the relationship between these hormones and social dominance and growth. Basal hormone levels and the hormonal response to the presence of the trouts varied among the strains. Cortisol increased in the presence of the trouts for two strains and was associated with lowered aggression and food consumption. Conversely, the presence of the non-native species had little influence on overall concentrations of 11-KT in the Atlantic salmon, although unexpectedly, there was a negative relationship between 11-KT and initiated aggression when either one of the trouts was present. Interestingly, elevated 11-KT levels were associated with declines in both food consumption and growth. Overall, our results suggest that competition with non-native salmonids negatively impacts Atlantic salmon. We discuss how our results can improve poststocking success and restoration of Atlantic salmon in Lake Ontario.