Downstream migration of Anguilla anguilla silver eels was studied in the Lower Rhine, Germany, and the Rhine Delta, The Netherlands, in 2004-2006. Fish (n = 457) released near Cologne with implanted transponders were tracked by remote telemetry at 12 fixed detection locations distributed along the different possible migration routes to the North Sea. Relatively more A. anguilla migrated via the Waal than the Nederrijn, as would be expected from the ratio of river discharges at the bifurcation point at Pannerden. Downstream migration from the release site to Rhine-Xanten, close to the German-Dutch border, generally occurred in the autumn of the year of release but migration speeds tended to be low and variable and unaffected by maturation status or river discharge rates. Detection frequencies were not significantly related to discharge peaks or lunar cycles, but there was a minor detection peak 1-6 h after sunset. Between 2004 and 2009, 43% of the 457 A. anguilla released were never detected and of the 260 detected entering the Netherlands, 83 (32%) were detected escaping to the sea, 78 (94%) via the Nieuwe Waterweg and three (4%) and two (2%) via the sluices in the Haringvlietdam and Afsluitdijk, respectively. Possible causes of non-detections are discussed and it is suggested that many A. anguilla temporarily ceased migration, but that fishing mortality could have been important during passage through the Dutch parts of the Rhine. Practical implications of the results for predicting emigration routes, timings and magnitudes and use in management initiatives to promote escapement of A. anguilla silver eels to the sea are critically discussed.

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.

No abstract

The onset of downstream migration of European eels is accompanied by a cessation of feeding and the start of sexual maturation which stresses the link between metabolism and sexual maturation, also suggesting an important role for exercise. Exercise has been tested with eels in swim tunnels and was found to stimulate the onset of sexual maturation. In this study, we have investigated the interplay between migration and maturation in the field during the downstream migration of female silver eels. Temporal changes in migratory status and sexual maturation among silver eels of the upstream Rhine River system over 3 months of the migration season (August, September and October) were determined in biometrical parameters, plasma 17β-estradiol and calcium levels, oocyte histology and gonadal fat levels. Furthermore, the ecological relevant parameters age as determined by otolithometry and health aspects indicated by haematocrit, haemoglobin and swim-bladder parasite load were measured. Silver eels were estimated to be 14 years old. A strong temporal progression in migratory stage was shown over the months of downstream migration. Catches probably represented a mix of reproductive migrants and feeding migrants of which the ratio increased over time. Furthermore, this study confirmed our hypothesis linking the migratory stage to early maturation as indicated by enlargement of the eyes, oocyte growth and fat deposition in the oocytes, exactly the same changes as found induced by exercise but not ruling out environmental influences. Migrants show extensive fat uptake by the oocytes, probably stimulated by the swimming exercise. In addition, at least 83% of the silver eels in this spawning run may have suffered from negative effects of swim-bladder parasites on their swimming performance.