Application of conservation genomics to investigate the role of pathogens on the migration of sea-run brown trout (Salmo trutta)

Pathogens play a key role in individual function and the dynamics of wild populations, but the link between pathogens and individual performance has rarely been investigated in the wild. Migrating salmonids offer an ideal study system to investigate how infection with pathogens affects performance given that climate change and fish farming portend increasing prevalence of pathogens in wild populations. To test for effects of pathogen burden on the performance of a migrating salmonid, we paired data from individual brown trout tagged with acoustic accelerometer transmitters and gill biopsies to investigate how pathogen infection affected whole animal activity during the spawning migration. Generalised additive models fitted to the acceleration data revealed individual and temporal variation in acceleration as expected, but also provided a significant effect of relative infection burden on acceleration. However, when linking this pathogen-specific effect to a relevant bioenergetic change, it was evident that the effect had little impact on the exercise-related oxygen consumption at the individual level, especially in cases where fish were not exerting high exercise activity. The results are a powerful example of how pairing non-lethal biopsies with individual tracking technologies can be used to assess how pathogens impact fish in situ.