Local aggregations of unionoids (mussel beds) represent subpopulations constituting a riverine metapopulation and are presumed to be linked by dispersal of parasitic mussel larvae (glochidia) on host fishes. We examined movement patterns of Masu Salmon (Oncorhynchus masou masou), the obligate host fish for the freshwater pearl mussel Margaritifera laevis, during the mussel's parasitic period to determine the dispersal potential of glochidia via host fish in the Shubuto River system, Hokkaido, Japan. We conducted a mark—recapture study in a 650-m river section to describe the distance and directionality of host fish movement, and we used fyke net sampling to quantify the number of host fish moving from mainstem rivers to tributaries. Most individuals of Masu Salmon captured in the study were infected with glochidia of M. laevis. The displacement distance of infected fishes ranged from 37.5 m downstream to 512.5 m upstream (mean = 36.5 m upstream from marking location). The dispersal kernel based on a diffusion—advection model represented a leptokurtic distribution with a fat-tailed upstream bias, but most recaptured fish remained near the initial capture location. Movement of fish from mainstem rivers to tributaries was strongly affected by differences in water temperature among tributaries. Most individuals moved to cooler tributaries, but tributaries did not support populations of M. laevis. These results suggest that glochidial dispersal via host fish is important for local recruitment and upstream dispersal, but dispersal to tributaries may be a source of mortality for M. laevis.
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