Klein Breteler, J., Vriese, T., Borcherding, J., Breukelaar, A., Jörgensen, L., Staas, S., de Laak, G., and Ingendahl, D. 2007. Assessment of population size and migration routes of silver eel in the River Rhine based on a 2-year combined mark-recapture and telemetry study. – ICES Journal of Marine Science, 64: 1450–1456. More than 3000 female silver eels >50 cm were marked and released in the River Rhine at Cologne in 2004 and 2005, and more than 4000 and 6000 per year, respectively, were checked for marks in the different Rhine branches close to the sea. Migration pathways of downstream-migrating eels were also tracked by telemetry from the point of release (300–350 km from the sea, depending on the migration route) through the three main branches of the Rhine (Waal, Nederrijn + Lek, IJssel + Lake IJsselmeer) to the sea. Downstream migration to the sea took from <2 d to more than a year, but was generally in October and November of the year of release. Most successful migrators seemed to find their way to the sea via the Nieuwe Waterweg rather than via Lake IJsselmeer or Haringvliet. Some 23% of released eels of the 2004 cohort and 15% of the 2005 cohort made it to the sea in less than 2 years. The telemetry data suggest that the Nederrijn + Lek watercourse, the only location where hydropower stations have been built in the lower Rhine system, might be important for downstream migration of eels only in the years with greater discharges, suggesting that management measures should concentrate on the Waal and downstream sections to improve spawning escapement of the silver eel population of the Rhine system.

Sea trout Salmo trutta migration in the Rhine delta, The Netherlands, was studied using the NEDAP TRAIL System®, consisting of a chain of fixed detection stations and transponders, each transmitting a unique code. From 16 December 1996 to 31 December 2000, 195 (34%) out of 580 tagged sea trout were detected. Inland migration of sea trout was observed through the sluices in the dam between the Wadden Sea and Lake IJsselmeer (Afsluitdijk), the Nieuwe Waterweg and the sluices in the Haringvliet Dam. Seventy‐five per cent were detected for the first time at one of the detection stations within 30 days after tagging. Inland migration through the sluices in the Afsluitdijk, the Nieuwe Waterweg and the sluices in the Haringvliet Dam was observed for 34, 103 and 70 sea trout, respectively. Based on the number of tagged fish released in the adjacent coastal areas, however, migration through the Afsluitdijk was most important. During migration in fresh water, sea trout was predominantly active during daylight. Fish entering the Rhine delta through the Nieuwe Waterweg or the sluices in the Haringvliet Dam chose eight and six different routes, respectively, for upstream migration. Differences in current velocity in the Rhine distributaries seemed to be an important factor in the choice of migration routes.