Route choices, migration speeds and daily migration activity of European silver eels Anguilla anguilla in the River Rhine, north‐west Europe

Abstract

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.

Silver eel downstream migration in fragmented rivers: use of a Bayesian model to track movements triggering and duration★

“…Many have focused on the behaviour of silver eels passing downstream through hydroelectric power stations to estimate mortality (Boubée and Williams, 2006;Carr and Whoriskey, 2008;Travade et al, 2010;Pedersen et al, 2012) or to improve mitigation solutions (Gosset et al, 2005;Russon and Kemp, 2011;Calles et al, 2013). Some studies have tracked silver eels along fragmented watercourses to estimate escapement (Haraldstad et al, 1985;Jansen et al, 2007;Acou et al, 2008;Breukelaar et al, 2009;Verbiest et al, 2012;Piper et al, 2013;Marohn et al, 2014;Mccarthy et al, 2014;Reckordt et al, 2014). Other studies have focused on migration triggered by environmental factors, to predict migration activity and especially peaks of migration, and consequently when to shutdown turbines (Vøllestad et al, 1986;Trancart et al, 2013).…”

Escapement, route choice, barrier passage and entrainment of seaward migrating European eel, Anguilla anguilla, within a highly regulated lowland river

“…In heavily impacted rivers, the cumulative effect of multiple structures 21 may reduce overall escapement to low levels. For example, previous studies on the 1 rivers Meuse and Rhine, estimated silver eel escapement at 15% (Verbiest et al, 2 2012), and 15 -32% (Breteler et al, 2007;Breukelaar et al, 2009), respectively, 3 which in both cases was influenced by entrainment at hydropower facilities. The 4 current study focused on the most downstream 10% (9.5 km) of the freshwater river.…”

“…To meet legislative escapement targets it is important to identify key locations of 12 silver eel loss and delay during freshwater migration (Breukelaar et al, 2009). 13…”

Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration

“…Additional argumentation would be that 47% of the eels that were used for the mark-recapture and telemetry studies in this project did not show downstream migration after release in the Rhine (Klein Breteler et al 2007). Other similar studies have also reported on significant numbers of eels that do not migrate downstream (Durif et al 2005; Winter et al 2006; Breukelaar et al 2009). Moreover, these eels may especially be those caught early in the migration season (Fig.…”

“…To reduce the mortality at hydropower stations in the Moselle (Germany), downstream-migrating eels are caught annually with fyke nets upstream from the hydropower stations (3,000–5,000 kg year −1 ) and are then translocated downstream of these obstructions (Klein Breteler et al 2007). Samples of these eels have been used for mark-recapture and telemetry studies (Klein Breteler et al 2007; Breukelaar et al 2009), and the health status of recaptured eels in The Netherlands has been determined (Haenen et al 2010). Eels were caught with fyke nets upstream from the hydropower stations in the River Moselle (Germany) specifically aiming for those eels migrating downstream.…”