Migration failure in stocked eels Anguilla anguilla

Silver eels Anguilla anguilla L., stocked as elvers in a freshwater lake on the island of Gotland, were caught in an outlet stream leading to the Baltic Sea. After tagging, they were released either into the lake or into the stream above a fine-meshed trap, and were recaptured after a period of 3 to 120 mo. During the period between tagging and recapture, the majority lost weight and decreased in length and fat content. It is hypothesized that the stocked eels had had no opportunity to imprint the directional cues necessary for migration; i.e. they were unable to recognize the outlet stream as the starting point of migration and lacked the orientation mechanism necessary to locate the outlet to the Baltic Sea. The lake itself comprises an initial trap, and the Baltic Sea a secondary trap. It is concluded that stocked eels lack imprinting, and that consequently their contribution to recruitment is null.