SPAWNING MIGRATION OF SEA TROUT
(SALMO TRUTTA TRUTTA) IN THE LOWER
RHINE, THE NETHERLANDS

Abraham bij de Vaate¹ & André W. Breukelaar²
Institute for Inland Water Management & Waste Water Treatment
¹ P.O.Box 17, NL-8200 AA Lelystad, The Netherlands
² P.O.Box 9072, NL-6800 ED Arnhem, The Netherlands

INTRODUCTION An important aim in the ecological rehabilitation of the River Rhine is the reintroduction of anadromous fish species like Atlantic salmon (Salmo salar), which was extincted from the river Rhine in the first part of this century. This species spawns in Germany and France. The Lower Rhine only serves as a migration route. In the last decades spawning migration possibilities of anadromous fish in general have been drastically reduced as the result of river engineering in the Lower Rhine. Unknown is the effect of physical obstacles constructed, specially in the delta area. By the absence of Atlantic salmon, Sea trout (Salmo trutta trutta) was selected as a representative species in this study on spawning migration possibilities for the Atlantic salmon. Sea trout density in the river increased considerably due to water quality improvement from the 1970's.

METHODS Mature sea trout is tagged, on several locations in the Dutch coastal area of the North Sea, with tags of the transponder type. Taged animals are released on the same spot were they were caught. The tags only transmit a signal when activated for a short time by an interrogation signal. This signal is picked up when passing fixed detection stations along possible migration routes. The telemetric system used in this study is the so-called NEDAP TRAIL system. Its technology is based on inductive coupling between an antenna loop on the bottom of a river or canal and a ferrite rod antenna as part of the transponder.

RESULTS Sea trout tagging started in December 1996. At least 200 specimens of mature Sea trout are expected to be tagged before March 15, 1997. Background information and first results of the study will be presented.

DISCUSSION The NEDAP TRAIL system is a new technique in fish migration studies based on telemetry. In general systems being used are based on two principles of signal transmission: acoustically or via radio waves. Both radio and acoustic tags could not be applied. Conductivity in the River Rhine varied between 32-139 mS/m (1987-1994), which was considered to be too high for optimal functioning of radio tags. Intensive shipping (on average every two minutes a vessel passes the German-Dutch border) was the reason that acoustic tags could not be applied, because background noise of shipping interferes the acoustic signal too much.