Organisms are frequently infected with multiple parasite species. Such coinfecting parasites interact with each other, and these interactions could affect impacts on host fitness and parasite geographic distribution, but field evidence is still limited. Here, we examined a wide-range geographic distribution pattern of three species of ectoparasites (gill infecting freshwater pearl mussel FPM, gill infecting copepod S. edwardsii, mouth infecting copepod Salmincola cf. markewitschi) infecting congeneric stream salmonids (white-spotted charr WSC and Dolly Varden DV). We specifically hypothesized that (1) induced host immunity by FPM infections reduces infection levels of Salmincola spp., (2) host body condition would be strongly reduced when coinfection occur. We captured fish by electrofishing at 75 sites in eastern Hokkaido. Contrary to our initial predictions, FPM were likely to co-occur with Salmincola spp., resulting in higher frequencies of coinfections. Host body condition decreased with S. cf. markewitschi infections, but the effects became larger when coinfected with FPM, especially under higher water temperature. Our results indicate that the costs of coinfections were larger than the infections by a single parasite, and its impacts depend on ambient environments.