Batch-like arrival waves of glass eels of Anguilla japonica in offshore waters of Taiwan

The larval stage of Anguilla japonica includes a long dispersal time over a long distance. In theory, the larvae should be distributed evenly throughout their transportation route when using both the NEC and Kuroshio, but the hypothesized new moon spawning of mature eels should lead to recruited glass eels exhibiting batch-like arrival waves, with a one-month-long cycle. However, environmental disturbances could mask the expected batch-like waves of glass eel recruitment. Thus, this phenomenon is best observed in glass eels collected from offshore waters, which are closer to the spawning site and less disturbed by these environmental factors. The offshore area of Yilan, Taiwan, is a suitable place to observe the arrival dynamics of the A. japonica glass eel. In this area, batch-like waves of glass eel arrival of A. japonica were observed, with peaks occurring between the last quarter and first quarter lunar periods, with a near one-month periodicity. No arrival peaks were found during the full moon period, suggesting that the glass eels exhibit light-avoidance behavior. Furthermore, all of the batches of arrivals were in the early pigmentation stage and similar in age (around 150-160 days), suggesting that they are likely a new arrival cohort. The tracer simulation showed that the mean tracer drift time, from the presumed spawning site to Yilan, was 155 ± 19.8 days. The observed batch-like arrival waves of glass eels in the offshore waters of Taiwan support the “New Moon Hypothesis,” which suggests that there is synchronized spawning behavior of the eels during the new moon period.