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Migration is a common behaviour among salmonids, but not all individuals within a population migrate, instead becoming residents and remaining in their natal streams. This phenomenon, known as “partial migration,” is well studied among sea‐run and lake‐run populations; however, the lower migratory benefits and costs for mainstem‐run individuals question whether the same mechanisms can be applied for stream‐dwelling salmonids. In this study, we investigated main stem‐run timing and body condition of partially migratory stream‐dwelling Dolly Varden charr (Salvelinus malma) in the Shiisorapuchi River in central Hokkaido, Japan. Based on commonly observed patterns in sea‐run and lake‐run salmonids, we predicted that migration would occur during spring and consist predominantly of age 1 +  females with some small subordinate males. Traps were placed at the mouths of two small tributaries along the Shiisorapuchi River periodically from May to December 2015. Adipose fin samples were taken for DNA to sex the individuals. Consistent to initial predictions, downstream movement occurs only once in the spring, dominated by age‐1 +  females. Trapped fishes (presumed migrants) were slightly longer but slimmer compared to the fish caught in the tributaries (presumed resident), which may be equivalent to smolts in anadromous populations. Regardless of migratory habitat, mechanisms driving partial migration in salmonids may be the same as long as production between natal stream and feeding habitats is significantly different.

Large-bodied ectoparasites are often observed only in low numbers in the field. How such rarely found parasites persist and maintain viable populations has been an intriguing question inadequately addressed. The simplest hypothesis is the existence of distribution hot spots, and another, but not mutually exclusive, possibility is a form of metapopulation structure where local populations are effectively connected via dispersal. In this study, we conducted an intensive epidemiological survey of a piscicolid leech Taimenobdella amurensis to elucidate spatial population structure and potential dispersal of this rarely found parasite. Four years of potential-host screening (n = 20 664) from 28 tributaries and 10 main stem reaches in a mountain river system of Hokkaido, northern Japan, revealed that occurrences of T. amurensis (n = 1348) were confined to spring-fed tributaries. Since most spring-fed tributaries were small (<1 km in length), it would seem to be unlikely for the ectoparasite to form a persistent local population in each tributary. The main host fish was Dolly Varden charr, which is known to disperse among neighbouring tributaries. These findings suggest that, along with the host, the ectoparasite displays a potential metapopulation structure, in which host-dependent dispersion may overcome local extinction by keeping the local populations connected.

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