The Relative Effects of Biotic and Abiotic Factors on the Recruitment of Freshwater Mussels (Margaritifera laevis)

Keita Kawajiri,Nobuo Ishiyama,Masanao Sueyoshi,Kazuki Miura,Akira Terui,Futoshi Nakamura

doi:10.3390/w13091289

Abstract

Freshwater mussels, Unionoida, are endangered across the globe due to recruitment failure. In the present study, with general linear mixed models, we investigated the relative effects of biotic (host fish density) and abiotic (water depth, fine sediment, water temperature, and water quality) factors on the recruitment of Margaritifera laevis in 10 streams of Hokkaido, northern Japan. We additionally examined the factors regulating the density of the host fish Oncorhynchus masou masou with general linear models. The proportion of juvenile mussels had a unimodal relationship with the host density, which was the most influential factor among the others examined. The positive relationship between mussel recruitment and host density can be attributed to an increased host fish infection rate. The negative correlation between mussel recruitment and host density at high fish densities may be due to reduced larval growth on host fish that are in poor physical condition. We also found that host fish density was negatively affected by nutrient enrichment. Our results suggest that mitigating water quality degradation to recover host fish density should be prioritized to improve mussel recruitment. Although stock enhancement is effective for increasing the salmon population density, excess stocking can further disturb mussel recruitment.

Highlights

Biodiversity has rapidly declined during the sixth mass extinction [1]
Examining the factors that regulate host fish density is critically important for the conservation of unionid mussels. This is because habitat degradation indirectly influences the parasitic stage, which is crucial to the initial growth of juvenile mussels, by reducing the host fish density [24,25]
We examined the relative effects of biotic and abiotic factors on the recruitment of freshwater mussels

Summary

Introduction

Biodiversity has rapidly declined during the sixth mass extinction [1]. Clarifying the causes of species decline is an essential first step for evidence-based conservation. The recruitment of freshwater mussels is influenced by biotic (i.e., host fish) and abiotic factors during their parasitic and benthic life stages. Examining the factors that regulate host fish density is critically important for the conservation of unionid mussels This is because habitat degradation indirectly influences the parasitic stage, which is crucial to the initial growth of juvenile mussels, by reducing the host fish density [24,25]. Margaritifera laevis is a Japanese freshwater mussel whose glochidia are obligate parasites of the gills of juvenile masu salmon, Oncorhynchus masou masou This long-lived species has an estimated lifespan of 80 years and is distributed in cold streams in Sakhalin, Russia, and the Honshu and Hokkaido Islands of Japan [26]. We examined the relative effects of biotic (host fish density) and abiotic (sedimentation and water quality) factors on the recruitment of freshwater mussels. We expected that biotic factors (i.e., host fish density) had the most influence on the recruitment of M. laevis because released glochidia from female mussels cannot begin the juvenile stage if they cannot encounter the proper host fish individuals

Quantitative Sampling and Species Detection

Abiotic Factors

Biotic Factors

Statistical Analysis

Results