Rapid loss rates of farmed Japanese eels stocked into two different rivers

Monitoring the survival rates of stocked fish is important for the success of stocking programs. Stocked fish reared in artificial environments may exhibit reduced performance until they adapt to the natural environment, potentially leading to a high loss rate immediately after stocking. In this study, we conducted high-frequency tracking of farmed Japanese eels Anguilla japonica stocked into the Fukada River (small size, no previous stocking) and the Amikake River (larger size, previous stocking) in Japan, starting from the first day after stocking using a mobile Radio Frequency Identification system. Apparent survival rates of stocked eels including mortality and emigration was estimated using the Cormack–Jolly–Seber (CJS) model with state-space formulations. The CJS model showed that apparent survival rates of stocked eels were lowest during the initial detection survey after stocking (i.e., on the first day in the Fukada River and during the first four days in the Amikake River), and then the rates gradually increased over time. These results suggest that stocked eels experience significant depletion within the initial days following stocking. Emigration and predation were inferred as potential causes of the high loss rate in the initial phase following stocking. Furthermore, the CJS model also showed that smaller stocked eels with high condition factors exhibited higher apparent survival rates. This study highlighted that identifying effective stocking methods to improve post-release survival is essential for enhancing the local catch and population of Japanese eels. • Eels were tagged and stocked into 2 sizes of rivers before being tracked. • The apparent survival rate was estimated from the first day after stocking. • A rapid loss rate of stocked eels was observed, but some eels remained in rivers. • Developing methods for improving survival rate immediately after stocking is essential.