Pathogen diversity cannot explain variation in fate and behaviour of trout across a latitudinal gradient

Abstract Pathogens play a significant role in the evolution of many species and are increasingly prevalent in salmonid populations, potentially owing to fish-farming activities. To understand the role of pathogens in the migration of sea-run brown trout, we replicated an acoustic telemetry and gill biopsy study across four rivers along a latitudinal gradient in Norway to test for an interaction between infection by pathogens and latitude (as a proxy for climate). We predicted that pathogen infection would affect the migration performance of trout differently in the four rivers. While infection richness was high within individuals (up to 11 agents per individual detected), we found relatively similar infection profiles across the four populations, and no clear latitudinal gradients in metrics of behaviour and fate within a year of sampling. It appeared that fate and behaviour were more closely linked to the river of origin, with some indications of poorer survival in the rivers that have warmer annual temperatures linked to the hydrology of the watershed rather than to the latitude. Pathogen diversity alone did not have a major effect on the behaviour and life history of adult sea trout from the focal populations. For the purposes of marine spatial planning and aquaculture management, impacts of microbes on adult trout do not currently appear to be a priority issue.