Monitoring the impacts and infection levels of parasites are central issues in resource conservation. While many studies have investigated host-parasite relationships within a restricted time window, the impact and infection levels should change throughout the year because resource allocation into immunity is seasonally variable. Here we examined the host-parasite relationship between white-spotted charr and the ectoparasitic copepod Salmincola sp. among four seasons in Hokkaido, Japan. Specifically, we predicted that the negative effects caused by these parasites and their infection levels would increase in autumn (breeding season) and winter (overwintering season) because hosts expend a great deal of energy and cease feeding activities during that period, resulting in low energy allocation to immunity. We found that body condition significantly decreased with increasing numbers of copepods in all seasons, suggesting that Salmincola sp. can strongly affect host fish. Infection levels sharply increased in winter, possibly reflecting the copepod’s recruitment during the hosts' breeding. Contrary to our initial predictions, however, the impact of parasite infections on host condition did not change between seasons or host maturity states. The time-lag between infection and parasite effects on host condition or the low pathogenicity of these parasites during cold water seasons, may have masked the differences.