Immigration timing and activity rhythms of the eel, Anguilla japonica, elvers in the estuary of northern Taiwan, with emphasis on environmental influences

The biological characteristics, upstream behavior and migration period of the elvers of Anguilla japonica in the estuary of northern Taiwan were studied. Seasonal decline in size was found for the elvers both from rivers and coastal waters. The maximal catch of the elvers in the coastal waters seems to be correlated with the temperature. The maximum catch often occurred on the data or several days after the daily seawater temperature reached the lowest. Instantaneous maximal catch occurred when the salinity increased to plateau and the instantaneous rate of seawater influx reached maximum. Upstream elvers also became most active during this period. The biological rhythm of elver-activity was found to follow the lunar cycle in the coastal waters, but a semilunar rhythm corresponding to spring tide was observed in the rivers. The moon-light seemed to inhibit the elver activity in the coastal waters during the full moon period.