Verhalten der Glasaale (Anguilla anguilla) bei ihrer Wanderung in den Ästuarien deutscher Nordseeflüsse

Abstract

Upon observing the tidal section of the Elbe River, it was found that the elvers (Anguilla anguilla L.), gathering in shoals immediately below the surface of the water, moved only in the early part of the morning. The area of such below-the-surface movements probably ranges from approximately the uppermost point reached by the flood stream to the uppermost point of measurable vertical tidal fluctuations. In the Elbe this area spreads over a distance of about 50 kilometers. Subsequent to such early day movements, the elvers bury themselves in the river bottom. They are passively carried upstream and, following a midnight high tide, appear at the upper boundary of the flood stream. Above this boundary the elvers swim against the current and close to the river bank. If the direction of the downstream current occasionally changes, for example, due to the special morphology of the river banks, the elvers swim with it, but, in principle, they do not alter their upstream direction of swimming. The elvers reach the section above the weir near Geesthacht only when the water is extremely high and the surrounding land flooded, or, though more seldom, through the sluice. Behavioural patterns and hydrographical relationships similar to those described for the Elbe were found by other authors to exist in the Weser and Ems Rivers. The periodic emergence of the elvers to the surface in the upper tidal region can be explained by previous experiences and resulting internal rhythms, which do not need reinforcements by external factors over shorter periods of time. The question of how ebb and flood streams are perceived in outer and inner coastal waters by the elvers, as well as possible direct and indirect influences through the moon, are discussed. © 1965 Biologischen Anstalt Helgoland.