High-resolution post-release movement of stocked Japanese eels Anguilla japonica tracked by a mobile radio frequency identification system

Abstract The Japanese eel ( Anguilla japonica ) is an important fisheries species in East Asia, with stocking programs implemented to enhance local fisheries. However, the downstream movement of stocked eels, which has been reported in previous studies, may reduce the stock enhancement effect due to emigration from fishery sites. To investigate the immediate movement of stocked eels, passive integrated transponder (PIT)-tagged eels were tracked using intensive surveys of a small stream along the west coast of the Izu Peninsula in Shizuoka Prefecture (Japan). Of the 168 stocked eels that were tagged, 64.9% were detected within 15 days. Stocked eels released immediately below weir or waterfall drop-offs exhibited a higher probability of downstream movement. Furthermore, stocked eels with lower condition factors were more likely to move downstream or remain undetected, indicating a loss from the study area. These findings suggest that drop-offs within the stream and the nutritional condition of stocked eels relate to post-stocking dispersal, indicating that carefully considering stocking sites and selecting eels in good condition can enhance stock enhancement efforts. The results of this study provide insights into optimizing stocking strategies for Japanese eel, highlighting the importance of riverine physical factors and the biological characteristics of stocked eels.