Freshwater mussels, Unionoida, are endangered across the globe due to recruitment failures. In the present study, we investigated the relative effects of biotic (host fish density) and abiotic factors (water depth, fine sediment, water temperature, water quality) on the recruitment success of Margaritifera laevis in Hokkaido, northern Japan. We additionally examined factors regulating the density of host fish, Oncorhynchus masou masou. Statistical analysis revealed that the proportion of juvenile mussels had a unimodal relationship with the host fish density, which was the most influential factor among candidate variables. The positive effects of host fish may be attributable to the increased infection rate. One possible reason for the negative correlation at high density is the reduction of larval growth associated with the low host fish's body condition. We also found that the host fish density was negatively affected by nutrient enrichment, which was caused by farmland expansion. Our results suggest that mitigations of water quality degradation should be preferentially conducted to recover the host fish density and improve mussel recruitment. While fish stocking is widely used to enhance salmon production, conservation practitioners should be aware that excessive host densities can potentially disturb mussel recruitment.