International eel symposium, 2003 - Quebec August 11, 2003
Poster Session

Otolith Marking as a Useful Tool in the Evaluation of Eel Stock Enhancement Programmes

Wickström, H.* Swedish National Board of Fisheries, Institute of Freshwater Research, SE-178 93 DROTTNINGHOLM, Sweden

Presenter email address: hakan.wickstrom AT fiskeriverket.se

Abstract Text: Small eels (Anguilla anguilla) are difficult to tag. Passive Integrated Transponders (PIT) have been used to tag eels as small as 10 g, and recently, Coded Wire Tags (CWT) also were successfully employed in a Danish study of eels (>=2 g) before stocking. The stock enhancement programme in Sweden uses mainly imported (from UK) elvers, which after an approved period in quarantine, are stocked in lakes and along the coast of the Baltic Sea. To identify recaptures and evaluate the outcome of stocking schemes, otoliths were marked by bathing elvers of about 1 g in a bath of SrCl₂ or alizarin complexone before stocking. In one lake 5,000 alizarin-marked elvers were stocked in 1997. Another lake was stocked yearly in 1999-2002 with 17800 Strontium-marked elvers in total. Among those about 2900 were also tagged with PIT. Up to and including the first part of 2003 a number of recaptures were made from both lakes. I describe the methods used, during both marking and detection, and provide examples of the appearance of marks, both in reference eels (samples from the marking occasions) and in eels recaptured after some years in the lakes, respectively