Tracking Migration Routes of Formerly Stocked European Eels Anguilla anguilla in the Baltic Sea

ABSTRACT

Since the early 1980s the European eel (Anguilla anguilla) stock is in steep decline. Management measures for a recovery differ between European Union member states, but are united under the aim of reaching a targeted 40% spawner escapement, relative to the best possible estimate of spawner biomass in a pristine state. The German Eel Management Plan (EMP) relies primarily on an extension of the nationwide stocking program. As artificial reproduction of the European eel is still not feasible, stocking entirely depends on wild-caught juveniles and is mostly accompanied by a significant translocation of juvenile eels. It has been suggested that translocation might prevent imprinting of migration routes and will thus lead to silver eel losses in migration. Up to date, it is still in debate to which extent formerly stocked European eels are actually capable of finding their way out of the Baltic Sea to proceed to their spawning grounds in the Sargasso Sea.

To better understand eel migratory behavior, we tagged 257 silver eels in 2009/2010 of the Schwentine River near Kiel (Germany). As the Schwentine system has been closed for any natural ascend of eels since 1904 by two hydro-power stations, the local eel population entirely consists of stocked eels. To confirm stocking history, individual life history of the tracked eels was reconstructed by otolith microchemistry. Eels were tagged with T-bar anchor tags and released downstream the hydro-power stations to allow unobstructed migration out of the freshwater system into the Baltic Sea. 22 eels (approximately 8%) were recaptured along the eastern coast of Denmark – the Danish Belt Sea indicating target-oriented migration behavior towards the spawning grounds. Only 2 individuals (approximately 0.8%) were reported from more easterly sites.

 Keywords: Anguilla anguilla, spawning migration, stocking, Baltic Sea, anchor tags, Sr:Ca