Effects of Habitat Structure and Environmental Factors on Fish Fauna in Temperate River Estuaries

ABSTRACT Riverine estuaries provide several ecosystem services to humankind. Notably, river estuaries are important habitats for both saltwater and freshwater organisms. However, the environment is experiencing degradation due to both direct and indirect anthropogenic impacts across various spatial scales (e.g., dredging, river channelization, land use change and dam construction). This study aims to reveal the relationship between the river estuarine fish fauna and the environmental factors including anthropogenic impacts as the basic knowledge necessary for the conservation and restoration. The Bōsō Peninsula (Japan) was selected as the study area. We identified the environmental factors that affected the fish species diversity and determined the factors that affected the habitat structure in the river, while focusing on anthropogenic impacts and topographical conditions. The fish species that occur in river estuaries are strongly influenced by habitat‐scale factors, such as flow velocity and water depth, along with watershed‐scale factors (such as wave exposure and direct fetch). In addition, the width–depth ratio, which indicates the degree of river channelization, was an important factor that controlled the habitat structure of the fish in the estuary. The results suggest the need for developing restoration plans based on the predictions of the potential physical environment, while considering the predictions for the occurrence of different fish species in the target area. Although this study is limited to the Bōsō Peninsula, our study can be used as a framework for improving future conservation and restoration plans for river estuaries in not just Japan but also other countries.