2022 Volume 24 Issue 2 Pages 217-234
Ayu (Plecoglossus altivelis), the most important fish in Japan as a food item, as an export, etc., is used not only as a food item but also as a target of recreational fishing. Stocks of P. altivelis have been affected by releases from fishing cooperatives and fisheries research institutes, but many rivers have maintained their resources naturally through production. Plecoglossus altivelis feed on algae in the riverbed, which increases the levels of certain organisms such as cyanobacteria and benthos. This species of fish is strongly associated with the algal and benthic fauna in rivers, occupying an important position in the ecosystem. Therefore, the conservation of the spawning ground environment of P. altivelis and maintenance of its resources are important for the preservation of the river ecosystem and environment. It is generally well known that alewives spawn on gravel bottoms in middle and lower river rapids in late fall. However, because ayu is a widely distributed species and its spawning tendencies differ from river to river, greatly increasing the amount of information available, data regarding the spawning behavior of ayu have not been sufficiently organized and comprehensively examined. Therefore, we reviewed the spawning grounds of ayu fishes with the aim of summarizing this information and creating guidelines that contribute to the conservation of the river environment. In this study, we comprehensively collected literature on studies that describe the spawning of alewives published between 1895 and 2019, extracting and organizing the physical conditions of their spawning environments from a total of 339 articles. Most of these references used data from natural river spawning grounds, but examples of experiments in artificial rivers and data from artificially created sites were also included. The physical conditions were extracted and organized as follows: spawning season, spawning ground velocity, water depth, riverbed material, penetration (riverbed softness), and egg burial depth. These data provided an overview of the general physical conditions of P. altivelis spawning grounds. Based on combining these five physical conditions, physiological information of P. altivelis, and physical consideration of riverbed environment, we have clarified that the place used by P. altivelis as a spawning habitat is rapid with soft and unstable gravelbed. In addition, the place is defined as "temporary spawning habitat" that is temporarily formed in the river depending on the flood disturbance.
