Alternative host identity and lake morphometry drive trematode transmission in brook charr

Both biotic and abiotic factors have been invoked to explain the large variations observed in the prevalence and abundance of parasites in aquatic ecosystems. However, we have only a poor knowledge of the potential interplay among these factors in natural systems. It is, therefore, important to analyze the effects of multiple potential environmental drivers together to get an integrated view of their influence on the prevalence and abundance of parasites. To this end, we selected two genera of digenean trematode parasites that require at least two hosts to complete their life cycle and use two different transmission strategies. Crepidostomum moves through a trophic pathway via consumption of infected prey by the host, while Apophallus infects its hosts via direct penetration of their skin. This study was conducted in 23 Canadian Shield lakes exhibiting orthogonal gradients of biotic (fish species richness and biomass) and abiotic (morphometry, physico-chemical) variables. We quantified prevalence and abundance of these parasites in the skin and intestine of brook charr (Salvelinus fontinalis). Our results show that biotic factors are key drivers of parasite abundance and prevalence, with Apophallus being negatively associated with the fish species richness-biomass gradient, and Crepidostomum responding more to identity of host than to the diversity gradient. Among the abiotic variables, lake area was found to be positively related to both prevalence and abundance in Apophallus. Our results suggest that taking into account the interplay of both biotic and abiotic factors is crucial for understanding patterns of parasite transmission success in boreal lakes.