Migratory behaviour of displaced homing yellow eels (Anguilla anguilla) in the North Sea

Kurzfassung

Vier Gelbaale (Anguilla anguilla L., standorttreue Entwicklungsform) wurden aus dem Gebiet des Limfjordes in die Deutsche Bucht oder in die Mittlere Nordsee verpflanzt, mit Ultraschallsendern markiert und nach dem Aussetzen 7 bis 35 Stunden verfolgt. Sie schwammen meistens auf einem Kompaßkurs in südöstlicher Richtung (140°). Der mittlere Kurs dieser und vier weiterer, schon früher untersuchter Gelbaale betrug 126°. Das beobachtete Heimfindeverhalten entspricht den Ergebnissen früherer Untersuchungen mit konventionell markierten Aalen. Die errechnete Schwimmgeschwindigkeit betrug 1,5 km/h und lag unter der Geschwindigkeit der früher untersuchten Exemplare. Markierung mit einem druckmessenden Ultraschallsender lieferte vorläufige Daten über die Schwimmtiefe; sie lag zwischen 4 und 18 m, während die Wassertiefe 41 m betrug. Es wird diskutiert, wie die Aale Untiefen der Küstengebiete meiden.

Summary

1. Four yellow (stationary) eels (Anguilla anguilla L.) were transplanted from Northern Denmark to the German Bight and to the central North Sea about 300 km SSW of the home area. They were tagged with ultrasonic transmitters and tracked for 7 to 35 hours.

2. The mean direction adopted by the four individuals was SE (140°), away from the direction of the home area (NNE). One eel discontinued activity after 5 h of active swimming. Combined with earlier tracking results on yellow eels a significant directional choise of 126° is calculated; it is concluded that a South-Eastern or perhaps a North-Western compass course is the first swimming performance of yellow eels towards home, regardless of the direction of their home area.

3. Earlier results with conventionally tagged yellow eels transplanted over long distances have also shown a South-Eastern trend and are now explainable.

4. With a mean migratory speed below 1 kn (1.85 km/h), the experimental specimes moved more slowly than individuals tracked one year previously in similar experiments.

5. One yellow eel tagged with a pressure sensing ultrasonic transmitter got lost twice because of an insufficiently adjusted receiver. During the first minutes of tracking it preferred a depth between 4 and 18 m; 24 h later it swam in a depth of 7 to 9 m. The water depth was 41 m.

6. From the discussion it becomes evident that yellow as well as silver eels prefer trenches if water depth decreases. Upon arrival in inshore waters directional behaviour in yellow eels probably might change; at daylight a minimum water depth of about 4 m is preferred.