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Journal of Fish Biology (2009) 74, 2139–2157 doi:10.1111/j.1095-8649.2009.02293.x, available online at www.interscience.wiley.com Route choices, migration speeds and daily migration activity of European silver eels Anguilla anguilla in the River Rhine, north-west Europe A. W. Breukelaar*†, D. Ingendahl‡, F. T. Vriese§, G. de Laak¶, S. Staas** and J. G. P. Klein Breteler†† *Rijkswaterstaat Waterdienst, P. O. Box 17, 8200 AA Lelystad, The Netherlands, ‡Bezirksregierung Arnsberg, Heinsbergerstrasse 53, 57399 Kirchhundem-Albaum, Germany, §Visadvies bv, Twentehaven 5, 3433 PT Nieuwegein, The Netherlands, ¶Sportvisserij Nederland, Postbus 162, 3720 AD Bilthoven, The Netherlands, **Rheinfischereigenossenschaft im Lande Nordrhein-Westfalen, R¨omerhofweg 12, 50374 Erftstadt, Germany and ††Vivion bv, H¨andelstraat 18, 3533 GK Utrecht, The Netherlands 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. © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles Key words: escapement; lunar cycle; maturation status; river discharge; telemetry. INTRODUCTION The European eel Anguilla anguilla (L.) stock has shown a strong decline and recruitment is as low as 1% of historic levels (Dekker, 2004; ICES, 2004). Concerns †Author to whom correspondence should be addressed. Tel.: +31 653 776 397; fax: +31 320 249 218; email: [email protected] 2139 © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles 2140 A. W. BREUKELAAR ET AL. about declines in recruitment and the status of stocks of A. anguilla have led to the European Union (EU) adopting a regulation establishing measures for the recovery of the stock (EU, 2007). The member states have to make eel management plans (EMP) in 2008 for each river basin district defined according to the Water Framework Directive (WFD). They have to specify the measures they will take to comply with a 40% escapement target of adult seaward migrating (‘silver’) A. anguilla biomass under undisturbed conditions and to make a time schedule for the attainment of that target level. Detailed data on a river basin level are usually missing in most European countries, specifically on target and current levels of escapement of silver A. anguilla to the sea and on factors responsible for the losses of emigrants. Known anthropogenic factors directly influencing A. anguilla survival during seaward migration are fisheries, and hydropower and pumping stations (Dekker, 2004; ICES, 2004). Acou et al. (2008) also suggest indirect mortalities can occur in A. anguilla with delayed migration because of barriers such as weirs and sluices. The efficiency of measures against these anthropogenic factors partly depends on the ability to predict downstream migration of the fish, both with regard to timing and migration routes chosen by the migrating A. anguilla. It also depends on migration speeds of the fish, because longer residence times underway increases risks of natural mortality or being captured by fisheries. Timing of downstream migration of A. anguilla depends on both internal and environmental variables. Reported environmental variables in timing of migration include seasonal factors (month, water temperature in current or preceding months and photoperiod), diurnal factors, weather influences (precipitation, rainfall, wind, microseismic changes, atmospheric depressions and sunshine hours), hydrographic and hydrological conditions (water level, flow rate, river discharge and turbidity) and lunar phase (Vøllestad et al., 1986; Cairns & Hooley, 2003; Haro, 2003; Tesch, 2003; ICES, 2005; Durif & Elie, 2008). Internal metamorphic changes towards ‘silvering’ of A. anguilla include ones in endocrine profiles (Van Ginneken et al., 2007a) and morphological and metabolic variables (Van Ginneken et al., 2007b)andare associated with changes in external morphological characteristics such as the silvery colour on the belly, pectoral fin length (LPF), total length (LT), mass (M)andeye diameter (DE)(Durifet al., 2005). Haro (2003) proposes a damped time distribution of downstream migration in the mainstem of large rivers due to superposition of different migrational peak events in the tributaries at different distances from the sea. In rivers with only one mainstem, there is only one route to the sea, but in braided rivers with networks of river branches and multiple discharge points to the sea, A. anguilla may choose between different escapement routes. This is the situation in the lower River Rhine. Recent studies in such river systems (Winter et al., 2006; Klein Breteler et al., 2007) suggest that the fish just ‘go with the flow’ because silver A. anguilla, except from tidal areas where they actively use the ebb current, seem to migrate downstream in the middle depths of rivers and in the main current (Tesch, 2003). Downstream migration of A. anguilla may be inhibited at bifurcation points in rivers due to locally operating sensory cues (Haro, 2003) such as ambient light, visual obstacles, noise or tactile stimuli from racks at hydropower stations (Hadderingh et al., 1992; Adam et al., 1997). The present study formed part of a project with the ultimate goal of determining key factors that could be used to optimize escapement of silver A. anguilla from the © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 2139–2157 MIGRATION OF A. ANGUILLA SILVER EEL IN THE RIVER RHINE 2141 River Rhine to the sea to comply with the 40% escapement target (from the pristine level) set by the EU (2007). Telemetry results of the 2004 and 2005 cohorts of female silver A. anguilla released in Cologne (Germany) showed that most of the fish chose the route via the Waal and Nieuwe Waterweg and that 23 and 15% escaped to the sea, respectively (Klein Breteler et al., 2007). The present study focused on telemetry of these cohorts, and additionally of the 2006 cohort, to determine migration timings, choice of routes and migration speeds of individual fish in relation to river discharges and lunar cycles, factors thought to be relevant for A. anguilla management, including appropriate water and hydropower management. MATERIALS AND METHODS STUDY AREA The study was carried out in the lower River Rhine (Fig. 1). A detailed description of the lower study area is given in Klein Breteler et al. (2007). Names and numbers of the detection stations are given in Fig. 1. In normal years, the yearly mean discharge of the River Rhine at Lobith varied between 1500 and 3000 m3 s−1 during the last century (www.waterstat.nl). Peak discharges mainly occurred in winter and spring during the study period, but some high river discharges also occurred in summer and autumn (Fig. 2). During these periods of high river discharge, the sluices are opened in the Nederrijn and Haringvlietdam (site 7, 7 , Fig. 1). This affects the distribution of the discharged water of the River Rhine over its three main discharge routes the Waal, the Nederrijn-Lek and the IJssel, and is reflected in the ratio of river discharges between the Waal and the Pannerdens Kanaal at Pannerden and between the IJssel and Nederrijn at IJsselkop (Fig. 2). TELEMETRY SYSTEM The telemetry system (NEDAP Trail System; http://www.nedaptrail.com/) used detection stations with antenna cables laid across the river summerbed at 5–22 m depth on average and is described by Bij de Vaate et al. (2003). Transponders implanted in fish ‘fired’ an identification number when activated by the antenna of a detection station and had a battery life of 1·5–2·0 years (Breukelaar et al., 1998). Each individual fish was traced on its migration route to the sea or to the detection station where it was detected for the last time. The data from the detection stations were screened for passing fish from this study up to 1 January 2009, for longer than the possible lifetime of the transponder batteries. Data records obtained from the system and used in this study contained detection station, transponder number and date and time of detection. When the River Rhine discharged >4735 m3 s−1, as measured at Lobith, the ends of the cables at the Rhine-Xanten 1 station were flooded and undetected passages were probably possible. The same applied to the Waal-Vuren station 2 at river discharges >3850–5000 m3 s−1 (depending on tidal movement). Such situations rarely occurred but potentially affected the results.

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