Recruitment status and host-fish-limitation threats to endangered freshwater pearl mussel (Margaritifera laevis) in eastern Hokkaido, northern Japan

Abstract

Recruitment failure is a major threat to freshwater mussel (Order Unionoida) populations worldwide. Assessments of the recruitment status and determining the bottleneck factors of mussel recruitment are crucial for preventing future declines in mussel populations. In this study, we investigated the recruitment status (i.e., size structure and the proportion of juveniles within a population) of the endangered freshwater pearl mussel Margaritifera laevis in 22 rivers in eastern Hokkaido, northern Japan. We also quantified the density of the host fish Oncorhynchus masou masou and examined the relationship between the proportion of juveniles and host-fish density to assess host-limitation threats to M. laevis recruitment. Our assessments showed that 13 (59.1%) rivers had no signs of recent recruitment within 10 years, with a low mean proportion of juveniles (mean:0.02 [range:0.00-0.09] fraction), indicating that these populations are threatened by sustained recruitment failure. The proportion of juveniles was positively associated with host-fish density, suggesting that host-fish limitation could be a bottleneck factor for M. laevis recruitment. These results highlight the urgent need for prompt conservation measures, including the enhancement of host-fish availability, to sustain M. laevis populations in the study region.