Methods for monitoring abalone Young-of-the-Year in the Japan Sea: Habitat utilization and depth distribution of Haliotis species

The four abalone species Haliotis discus discus, H. madaka, H. gigantea, and H. diversicolor are important fisheries resources in Japan. However, the stock abundance of these species has declined. Fluctuations in abalone young-of-the-year (YOY) stocks may significantly influence adult stock abundance. Monitoring methods for YOY are proposed which clarify habitat shifts associated with growth, particularly in H. discus discus and H. diversicolor. While only partial trends could be inferred for H. madaka and H. gigantea. For all four species, YOY were frequently observed in gaps between rocks or beneath sea urchins in these gaps. In H. discus discus and H. diversicolor, a habitat shift was observed with growth, transitioning from cryptic habitats (e.g., rock gaps and areas under sea urchins) to more exposed habitats. The depths at which YOY were found varied each year. However, H. discus discus and H. diversicolor were found in shallower waters than H. madaka and H. gigantea; the former two species appeared to migrate from deeper to shallower areas as they grew. These findings provide essential insights for improving abalone YOY survey methods. Additionally, it was recognized that, since catch-per-unit-effort trends varied among the four species, precise YOY species identification is crucial for effective monitoring.