Low water temperature and increased discharge trigger downstream spawning migration of ayu Plecoglossus altivelis

Abstract

This study examined the combined effects of water temperature and discharge on the downstream spawning migration of ayu Plecoglossus altivelis, with reference to their hierarchical interactions and relative importance in the Nagara River in central Japan. Fish catches from the sebari-ami traditional fishing method, which can catch ayu migrating downstream for spawning, were recorded at seven fishing sites from September to November in 2020 and 2021. Overall, decision trees showed that the daily catch increased when the daily water temperature (the mean of the 24-hour data) fell below approximately 18 °C and when the water discharge increased regardless of intensity, indicating the hierarchical effects of these hydrological factors on the downstream spawning migration of ayu. After mid-October, when the daily water temperature rapidly dropped below approximately 15 °C, daily catches further increased, and large catches occurred on days of increased discharge, indicating active migration occurring below 15 °C. At a spawning site, the proportion of large individuals (> 80 g) was high in the early phase of the spawning seasons. These findings can help accurately predict the timing of ayu spawning migration and support sustainable fishing practices within a context of climate warming.