Eco‐evolutionary responses of a cold‐water fish (Salmo trutta) to the combined effects of an emerging pathogen and temperature

Abstract How wildlife populations respond to the different components of global change and their interactions is still poorly understood. For instance, emerging pathogens can directly impact the genetic and phenotypic attributes of their hosts through directional selection or plasticity, whereas climatic stress can either mitigate or exacerbate the infection outcomes. Studying the adaptive response of wild populations to both pathogens and environmental stressors is thus critical for predicting their short‐ and long‐term persistence. We investigated the combined impact of water temperature and infection by Tetracapsuloides bryosalmonae , a myxozoan pathogen causing the salmonid proliferative kidney disease (PKD), on wild brown trout Salmo trutta populations. We compared the density, the genetic and phenotypic characteristics of trout populations sampled along gradients of infection prevalence and water temperature, before and after the period of disease outbreaks (summer). Population density decreased after the disease outbreak with increasing infection prevalence, regardless of the temperature, except in populations that were already heavily infected the previous year. High infection prevalence reduced genetic diversity at an immune‐related gene, regardless of the temperature, but not at neutral loci, in line with pathogen‐mediated directional selection. We also showed that both temperature and infection negatively affected trout body condition and had an interactive effect on carotenoid‐based coloration, likely reflecting short‐term plastic responses of phenotypic traits to both stressors. This highlights that different global change stressors (warming and pathogens) can have both negative and interactive effects on fish mortality, genetic diversity and fitness‐related traits, and hence on the rapid evolution of wild populations. Read the free Plain Language Summary for this article on the Journal blog.