Effects of multiple stressors on recruitment of long‐lived endangered freshwater mussels

Abstract

Multiple stressors can affect populations interactively. However, the process by which they affect recruitment remains unclear, especially in long‐lived organisms with complex life cycles. This study explored the bottleneck stages of juvenile recruitment in the endangered freshwater pearl mussel Margaritifera togakushiensis and the interactive effects of nutrients and fine sediments on these stages. The proportion of M. togakushiensis juveniles aged ≤10 years was investigated in 24 rivers from north‐eastern Japan. Margaritifera togakushiensis has a complex life cycle with an obligate parasitic larval stage. Gravid female density, glochidia density (the number of glochidial infections per stream area), and juvenile survival rate, used as indices for adult, parasitic, and post‐parasitic juvenile stages, respectively, were obtained from 11 of these rivers. Twenty‐four population status assessments showed that the proportion of juveniles aged ≤10 years ranged from 0.00 to 0.23, and that they were absent in 12 rivers. Parasitic and post‐parasitic juvenile stages were bottlenecks for recruitment; juvenile survival rates had a stronger positive effect on recruitment in rivers with high glochidia density. Juvenile survival rate was negatively associated with the synergistic effects of nutrients and fine sediments, although factors limiting glochidia density were not identified. These findings suggest that the interaction between nutrients and fine sediments can synergistically reduce the recruitment of M. togakushiensis, indicating an underestimated impact of stressors on mussel recruitment if treated individually. Future mitigation measures should consider both nutrient levels and fine sediments to restore mussel recruitment. These results also emphasize the importance of considering both parasitic and post‐parasitic juvenile stages of mussels to maximize the success of future restoration actions.

Does winterkill explain contrasting demographics of winter-breeding freshwater mussel populations in Ishikari River floodplain?

“…Under such conditions, a chain of adult abundance decreases in winter, high mortality of gravid mussels and newly born juveniles occurs in summer, and population size decreases with a lack of juveniles at annual or longer timescales. Recent studies have indicated that multiple stressors can limit the community structure, distribution, and reproduction of freshwater species, including freshwater mussels, in complex ways (Birk et al, 2020;Miura et al, 2023a). Our findings extend the multiple-factor paradigm and underscore the importance of considering survival limitation factors acting at multiple life stages of freshwater mussels at different times of the year.…”

“…One of the most prevalent signs of population decline in freshwater mussels is the cessation of their reproduction, which can often be reflected in disproportionately fewer juveniles in population demography and small population sizes (i.e., abundance) (Österling et al, 2010;Negishi & Kayaba, 2010). Efforts have been made to identify how population decline and reproduction cessation occur by examining single or multiple life-history stages of species in relation to various external environmental factors (Geist et al, 2006;Österling et al, 2010;Strayer & Malcom, 2012;Tremblay et al, 2016;Brian et al, 2021;Miura et al, 2023a). Furthermore, as intrinsic properties of mussels, demographics, including size frequency, age structure, sex ratios, and survival rates, are among the key parameters in inferring impaired populations and projecting how and whether the populations would change in the future (Ferreira-Rodríguez et al, 2019).…”