Periodicities of inshore migration and selective tidal stream transport of glass eels, Anguilla japonica, in Hamana Lake, Japan

During inshore migration, glass eels experience great environmental changes they must adapt to, but there have been few studies on the behavior of juvenile Japanese eels, Anguilla japonica, entering estuaries. To investigate the relative effects of periodic environmental changes on their inshore migration and ascending behavior from the bottom to swim with flood tidal currents, year-round collections were carried out from December 2003 to December 2005 inside Hamana Lake, Japan, where strong tidal flows occur. Glass eel collections (N = 543) near the inlet to the lake indicated that recruitment occurred mainly in winter from November to May. Time-series autocorrelation analysis of average daily catch rates found negative autocorrelation coefficients at 16-20 days periodicities and positive ones at 30-32 days periodicities in the 2004-2005 season, suggesting that the amount of recruiting glass eels fluctuates with a lunar cycle. Abundances of glass eels showed a diel rhythm and a tidal rhythm with eels appearing only at night and catches during flood tide being higher than during ebb tide, indicating 'Selective Tidal Stream Transport' that eels utilize flood tidal-streams selectively at night to migrate inshore. The generalized additive models (GAM) for the abundance suggested that relatively long timescale factors such as 'recruitment season' and 'lunar age' could affect the abundance of glass eels. The GAM for glass eel presence/absence indicated that the timing of glass eel ascending into the water column during flood tide at night was at 150-180 min after low tide, which tended to synchronize with the rise of water temperature caused by the inflow of warm water from the Pacific coast outside the lake. Results suggested that a temperature increase at the beginning of flood tide may be a cue for the ascending behavior, or at least could be a zeitgeber of the circatidal rhythm regulating the ascending behavior.