Parasite transmission in aquatic ecosystems under temperature change : effects of host activity and elimination of parasite larvae by filter-feeders

A moderate raise in temperature was suggested to enhance the impact of parasites on aquatic ecosystems. Under higher temperatures, poikilothermic animals (e.g. fish) increase their activity, which can result in a more frequent encounter with parasites. However, temperature increase may also trigger processes counteracting an increased risk of parasitic infections. Thus, the removal of free‐living stages of parasites by filter‐feeding organisms can increase with temperature and potentially mitigate disease risk in ecosystems under climate change. We aimed to study whether an increased infection transmission under higher temperatures can be compensated by the increased removal of parasitic larvae by aquatic predators. In addition, we planned to reveal the behavioral mechanism underlying the more successful transmission of the parasite at higher temperatures. We studied experimentally how temperature, the behavior of fish hosts (rainbow trout), and the presence of filter‐feeding mussels in the environment influence the infection success of trematode larvae Diplostomum pseudospathaceum cercariae. We found that temperature raise increased while the presence of filter‐feeding mussels in the environment decreased infection intensities in fish. However, the effect of mussel's presence was constant within the tested range of water temperatures (15–23°C), which suggests that it cannot compensate for the observed increased transmission of parasites under temperature raise. Fish activity before the exposure to parasites was a substantial factor affecting the host's vulnerability to infection. However, fish activity only weakly correlated with temperature, therefore, it is unlikely to be the only factor responsible for the increased infection success under warmer conditions. After the exposure, fish activity decreased and did not influence the infection's success. This decrease was temperature‐dependent and more pronounced in more active fish. In general, we showed that the elimination of trematode larvae by filter‐feeders is unlikely to deter the potential effects of global warming on host–parasite interactions in temperate freshwater ecosystems.