Route choices, migration speeds and daily migration activity of

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 migrationactivity 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, Romerhofweg 12, 50374 Erftstadt, Germany and ††Vivion bv, Handelstraat18, 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 withimplanted transponders were tracked by remote telemetry at 12 fixed detection locations distributedalong the different possible migration routes to the North Sea. Relatively more A. anguilla migratedvia the Waal than the Nederrijn, as would be expected from the ratio of river discharges at thebifurcation point at Pannerden. Downstream migration from the release site to Rhine-Xanten, closeto the German–Dutch border, generally occurred in the autumn of the year of release but migrationspeeds 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 wasa minor detection peak 1–6 h after sunset. Between 2004 and 2009, 43% of the 457 A. anguillareleased were never detected and of the 260 detected entering the Netherlands, 83 (32%) weredetected escaping to the sea, 78 (94%) via the Nieuwe Waterweg and three (4%) and two (2%) viathe sluices in the Haringvlietdam and Afsluitdijk, respectively. Possible causes of non-detections arediscussed and it is suggested that many A. anguilla temporarily ceased migration, but that fishingmortality could have been important during passage through the Dutch parts of the Rhine. Practicalimplications of the results for predicting emigration routes, timings and magnitudes and use inmanagement initiatives to promote escapement of A. anguilla silver eels to the sea are criticallydiscussed. © 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 andrecruitment 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 AuthorsJournal compilation © 2009 The Fisheries Society of the British Isles

2140 A . W. B R E U K E L A A R E T A L .

about declines in recruitment and the status of stocks of A. anguilla have led to theEuropean Union (EU) adopting a regulation establishing measures for the recoveryof 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 FrameworkDirective (WFD). They have to specify the measures they will take to comply witha 40% escapement target of adult seaward migrating (‘silver’) A. anguilla biomassunder undisturbed conditions and to make a time schedule for the attainment ofthat target level. Detailed data on a river basin level are usually missing in mostEuropean countries, specifically on target and current levels of escapement of silverA. anguilla to the sea and on factors responsible for the losses of emigrants. Knownanthropogenic factors directly influencing A. anguilla survival during seawardmigration are fisheries, and hydropower and pumping stations (Dekker, 2004; ICES,2004). Acou et al. (2008) also suggest indirect mortalities can occur in A. anguillawith delayed migration because of barriers such as weirs and sluices.

The efficiency of measures against these anthropogenic factors partly depends onthe ability to predict downstream migration of the fish, both with regard to timing andmigration routes chosen by the migrating A. anguilla. It also depends on migrationspeeds of the fish, because longer residence times underway increases risks of naturalmortality or being captured by fisheries.

Timing of downstream migration of A. anguilla depends on both internal andenvironmental variables. Reported environmental variables in timing of migrationinclude seasonal factors (month, water temperature in current or preceding monthsand photoperiod), diurnal factors, weather influences (precipitation, rainfall, wind,microseismic changes, atmospheric depressions and sunshine hours), hydrographicand hydrological conditions (water level, flow rate, river discharge and turbidity) andlunar 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) and areassociated with changes in external morphological characteristics such as the silverycolour on the belly, pectoral fin length (LPF), total length (LT), mass (M) and eyediameter (DE) (Durif et al., 2005). Haro (2003) proposes a damped time distributionof downstream migration in the mainstem of large rivers due to superposition ofdifferent migrational peak events in the tributaries at different distances from thesea.

In rivers with only one mainstem, there is only one route to the sea, but in braidedrivers with networks of river branches and multiple discharge points to the sea, A.anguilla may choose between different escapement routes. This is the situation inthe 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’ becausesilver 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 bifurcationpoints in rivers due to locally operating sensory cues (Haro, 2003) such as ambientlight, 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 determiningkey factors that could be used to optimize escapement of silver A. anguilla from the

© 2009 The AuthorsJournal compilation © 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 2139–2157

M I G R AT I O N O F A . A N G U I L L A S I LV E R E E L I N T H E R I V E R R H I N E 2141

River Rhine to the sea to comply with the 40% escapement target (from the pristinelevel) set by the EU (2007).

Telemetry results of the 2004 and 2005 cohorts of female silver A. anguillareleased in Cologne (Germany) showed that most of the fish chose the route via theWaal 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 routesand migration speeds of individual fish in relation to river discharges and lunar cycles,factors thought to be relevant for A. anguilla management, including appropriatewater and hydropower management.

MATERIALS AND METHODS

S T U DY A R E AThe 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 detectionstations are given in Fig. 1.

In normal years, the yearly mean discharge of the River Rhine at Lobith varied between1500 and 3000 m3 s−1 during the last century (www.waterstat.nl). Peak discharges mainlyoccurred in winter and spring during the study period, but some high river discharges alsooccurred in summer and autumn (Fig. 2). During these periods of high river discharge, thesluices are opened in the Nederrijn and Haringvlietdam (site 7, 7 , Fig. 1). This affects thedistribution of the discharged water of the River Rhine over its three main discharge routes theWaal, the Nederrijn-Lek and the IJssel, and is reflected in the ratio of river discharges betweenthe Waal and the Pannerdens Kanaal at Pannerden and between the IJssel and Nederrijn atIJsselkop (Fig. 2).

T E L E M E T RY S Y S T E MThe 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 averageand is described by Bij de Vaate et al. (2003). Transponders implanted in fish ‘fired’ anidentification number when activated by the antenna of a detection station and had a batterylife of 1·5–2·0 years (Breukelaar et al., 1998). Each individual fish was traced on its migrationroute to the sea or to the detection station where it was detected for the last time. The data fromthe detection stations were screened for passing fish from this study up to 1 January 2009, forlonger than the possible lifetime of the transponder batteries. Data records obtained from thesystem and used in this study contained detection station, transponder number and date andtime 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 passageswere 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 butpotentially affected the results.

S U R G I C A L P RO C E D U R E S A N D I M P L A N TAT I O N SAnguilla anguilla were caught in the River Moselle and in the River Sieg (Fig. 1),

henceforth denoted as ‘Moselle’ and ‘Sieg’ A. anguilla, respectively. Records of fishingmethods are incomplete, but most fish were fyke-netted in the Moselle or electrofished inthe Sieg. The fish were delivered in batches well spread through August to December inthe years 2004 to 2006. Individuals were selected as being apparently silver from externalcharacteristics. Anguilla anguilla of 640–1100 mm LT were chosen, as surgical incisions



Detection stations

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FIG. 1. Study area with release locations (1, Rhine-Xanten; 2, Waal-Vuren; 3, Beneden Merwede; 4, OudeMaas; 5, Dordtsche Kil; 6, Spui; 7, Haringvlietdam; 8, Noord; 9, Nederrijn; 10, Lek; 11, IJssel-Kampen;12, Afsluitdijk) of Anguilla anguilla ( ) ‘Cologne’ and ‘Sieg’ detection stations ( 1 , etc.) and sluices( ).

could not be closed completely in smaller fish. Transponders were surgically implanted ineach fish (457 in total, 150–157 per year), according to Klein Breteler et al. (2007). Aftersurgery, each fish was weighed (M , g), LT measured (mm) and LPF and DE in horizontaland vertical directions were measured by callipers (0·1 mm).

After recovery of normal swimming behaviour, fish were released, this occurred in sevenbatches in the Rhine at Cologne in 2004, four batches at Cologne and three in the Sieg nearits confluence with the Rhine in 2005 and four at Cologne and two in the Sieg in 2006. Thedistance from Cologne to the sea depends on the migration routes chosen in the Rhine Delta



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and ranges from 360 km to the Haringvlietdam 7 to 400 km to the Afsluitdijk 12 (Fig. 1).The distance from the Sieg to the sea is c. 30 km longer than from Cologne to the sea.

DATA A NA LY S I S

Values of M,LT,DE and LPF were used for calculation of the maturation stages SF-II,SF-III, SF-IV or SF-V of the female fish according to the criteria of Durif et al. (2005).These maturation classes also represent the physiological status of the fish and SF-IV and



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FIG. 3. Relative frequency of maturation stage of Anguilla anguilla used for implantation of transponders inthe years 2004 ( ), 2005 ( ) and 2006 ( ) (n = 427). Classes are according to Durif et al. (2005).

SF-V are considered to be real migrants (Durif et al., 2005). Most fish belonged to the SF-IIIand SF-IV class and in 2004 relatively more fish fell into the SF-III class (Fig. 3), few fishwere classified as stage SF-V.

The number of escapees to the sea at the Afsluitdijk (12) and the Haringvlietdam 7 wasdetermined at the sluices where they entered the sea (Fig. 1). It is physically impossible tolocate a detection station near the mouth of the Nieuwe Waterweg, so the combined detectionsof the three most downstream stations were used instead [Noord 8 , Oude Maas 4 and Lek(10)] to estimate escapement to the sea via the Nieuwe Waterweg (Fig. 1). No correctionswere made to estimates of escapement for fishery mortalities downstream from these detectionstations because catches and effort of these fisheries are unknown.

Detection rates during migration were calculated from detections and from obvious missingdetections, e.g. each A. anguilla that passed Waal-Vuren 2 must have passed Rhine-Xanten1 , even if not detected at Rhine-Xanten 1 . The percentage missing detections at Rhine-

Xanten 1 (Fig. 1) was determined from the number of fish detected in more downstreamdetection stations compared with those missed at Rhine-Xanten 1 . The percentage missingdetections of the whole telemetry infrastructure in the Netherlands (system performance) wasdetermined from numbers of fish detected at the combined stations furthest downstream beforeentering the sea [Haringvlietdam 7 , Oude Maas 4 , Noord 8 , Lek 10 , and Afsluitdijk 12 ]compared with the number not detected upstream in the Netherlands.

Migration routes of individual fish were determined from the sequence of detections and,in cases of no further progress to the sea but repeated detections at some stations without aclear route choice (nine fish), by the last detection station encountered.

River discharge and temperature (daily mean) data were obtained from the DutchMinistry of Transport, Public Works and Water Management (www.waterstat.nl). Daily meandischarges of the River Rhine at Lobith and the Waal at Pannerden were used to calculatethe daily discharge ratio ‘Pannerden’, i.e. the ratio between the discharges of the Waal andthe Pannerdens Kanaal. Daily mean discharges of the IJssel at IJsselkop and of the Nederrijnat Driel were used for the calculation of the ratios between these discharges (discharge ratio‘IJsselkop’). In a similar way, A. anguilla ratios were calculated at Pannerden and IJsselkop,representing the ratios of numbers of fish migrating via these bifurcation points.



For each individual fish that passed between two detection stations, migration speeds werecalculated by subtraction of year, day and time at both stations and division by the distancebetween the stations. For the same periods, the average river discharges of the Rhine at Lobithwere calculated from the daily mean river discharges.

Migration activity after sunset (Tesch, 2003) was analysed by subtracting the individualdetection time of each fish from the time of last sunset (on the same day or the day before). Forthe analysis of the effect of lunar cycle on migration speeds, the days were sine transformedwith full moon = −1 and new moon = +1. Means of these transformed data were calculatedfor the periods for which the migration speeds were calculated. Data were statistically analysedwith non-parametric tests using SPSS (www.spss.com) with P ≤ 0·05.

RESULTS

D E T E C T I O N R AT E S

From all A. anguilla released with a transponder in 2004–2006, 43% werenever detected up to 1 January 2009 and 38 ± 4%, 43 ± 9% and 48 ± 19%(mean ± s.d. of different batches of fish released) in 2004, 2005 and 2006,respectively, were not detected (Table I). The batches released [missing data for batchSeptember 2004 (Sep-04) in Table I] differed significantly in average maturationstage (Kruskal–Wallis test, χ2, d.f. = 18, P < 0·001) and so did the years of release(Kruskal–Wallis test, χ2, d.f. = 2, P < 0·01). The detected and non-detected fishdid not, however, differ in their maturation stage when released (Mann–WhitneyU -test, n = 427, P > 0·05).

Escapement from Germany, as measured from detections at Rhine-Xanten 1 ,ranged from 46 to 62% (Table II). The escapement to the sea via the Netherlandsvaried from 13 to 23% in 2004–2006 (Table II) and detection rates ranged from 28to 38% of the fish passing Rhine-Xanten 1 . The non-detection rates per river stretchwere relatively stable through the years but increased slightly in 2006 (Table II).

M I G R AT I O N RO U T E S

During the study, 83 ± 2% (yearly mean ± s.d.) of the detected A. anguilla (49–57per year) migrated via the Waal. The route via the IJssel was chosen by only fourto ten fish per year (11 ± 5%) and zero to seven fish per year (6 ± 4%) migratedvia the Nederrijn–Lek (Fig. 4 and Table I). The majority of fish migrating via theWaal chose the Beneden Merwede 3 and proceeded via the Noord 8 to the NieuweWaterweg. All fish, except one in 2005, passing Dordtsche Kil 5 or Spui 6 , initiallymigrated to the Haringvliet but did not proceed to the sluices in the Haringvlietdam7 and chose the route to the Nieuwe Waterweg instead. Nine fish temporarily settled

in the Rhine Delta for a time in the year after release and were repeatedly detectedat some stations there, or before the first weir in the Nederrijn 9 .

There were no differences with regard to the route chosen by the fish at thePannerden bifurcation (either the Waal or the Pannerdens Kanaal) between the yearsof release (χ2, d.f. = 2, P > 0·50). The bifurcation ratios of the fish (A. anguillaratios) exceeded the river discharge ratios at Pannerden each year (Table III), andthe fish seemed to show a greater preference to migrate via the Waal (instead of thePannerdens Kanaal) than could simply be explained by the river discharge ratios atPannerden each year. At the bifurcation IJsselkop, more fish chose the IJssel thanthe Nederrijn in 2005 and 2006, but the A. anguilla ratio was lower than the river



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TABLE II. Percentage detection rates of Anguilla anguilla released in the River Rhine nearCologne in the years 2004–2006 and non-detection rates per river stretch (%)

2004 2005 2006 Totals

Detection rates of released A. AnguillaGermany 62 57 46 55The Netherlands 23 17 13 18

Non-detection rates per river stretchFrom release to Rhine-Xanten 1 38 43 54 45From Rhine-Xanten 1 to Waal-Vuren 2 ,

Nederrijn 9 and IJssel-Kampen 1127 33 23 28

From Waal-Vuren 2 and Nederrijn 9 to sea 47 50 60 52From IJssel-Kampen 11 to sea∗ 75 90 100 90

∗Numbers ≤ 10.

discharge ratio in 2004 and 2006. Considering the river discharges, the data withregard to route choice of fish are not consistent and conclusive for the bifurcation atIJsselkop.

M I G R AT I O N S E A S O N S , S P E E D S A N D T I M E S

Most of the fish detected (97, 93 and 90% in 2004, 2005 and 2006, respectively)migrated downstream in the year of release and from the moment of releaseuntil the winter. Detections rates were relatively low between January and August(Fig. 5). The mean ± s.d. of the migration speeds of all fish over all stretcheswas 0·34 ± 0·44 m s−1 (n = 490). The average migration speed to Rhine-Xanten1 was 0·30 ± 0·46 m s−1, and from Rhine-Xanten 1 to Waal-Vuren 2 it was

0·50 ± 0·57 m s−1. The lower mean migration speed up to Rhine-Xanten 1 , ascompared with the more downstream river stretch, was mostly due to 14% morefish in the migration speed class 0·0–0·3 m s−1. The maximum speed in the 154 kmstretch Cologne–Rhine-Xanten 1 was 2·20 m s−1, and the maximum speed in the128 km stretch Rhine-Xanten 1 and Waal-Vuren 2 was 1·45 m s−1. Migrationspeeds tended to be highly skewed (Fig. 6). For those individual fish for whichdata were available from both release locations to Rhine-Xanten 1 and for thestretch Rhine-Xanten 1 to Waal-Vuren 2 , migration speeds were not correlated(r = 0·013). There were only a few fish moving fast on both stretches. On thestretch from release locations to Rhine-Xanten 1 , fish from different maturationclasses showed no significant differences in migration speeds (Kruskal–Wallis test,χ2, d.f. = 2, P > 0·05): fish from the maturation class SF-III migrated on averageat 0·37 ± 0·56 m s−1, SF-IV fish migrated at 0·25 ± 0·38 m s−1 and SF-V fishmigrated at 0·07 ± 0·07 m s−1.

E N V I RO N M E N TA L FAC TO R S

The migration speeds of the fish were not correlated with average river dischargeswhen the fish migrated from the location of release to one of the detection stationsRhine-Xanten 1 , Waal-Vuren 2 , Nederrijn 9 and IJssel-Kampen 11 (Fig. 7,r = 0·08). The individual migration speeds of silver A. anguilla in these riverstretches showed little relation to the mean of the lunar cycle (r = 0·15).



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FIG. 4. Migration routes, survival and escapement to sea deduced from telemetry of silver Anguilla anguillareleased in the Rivers Rhine and Sieg near Cologne in August 2004 to January 2007. No detection stationswere located at Pannerden, IJsselkop and Nieuwe Waterweg. Escapement numbers are minimum estimatesand calculated from the last stations before entering the sea. Ranges of numbers at Beneden Merwede3 , Dordtsche Kil 5 and Spui 6 indicate uncertainties about which of these stations had been passed.

Left-hand panel numbers at Dordtsche Kil 5 and Spui 6 indicate migration from Beneden Merwede3 to Haringvlietdam 7 ; right-hand panel numbers migrating from Waal-Vuren 2 via Haringvliet to

Oude Maas 4 .

The migrating fish were detected throughout the day (Fig. 8) and only showeda slightly increased detection peak, indicating higher activity, in the first 6 h aftersunset. The total daily number of fish detections at all detection stations in the monthsAugust to December, as an indication of activity, did not seem to show a clearrelationship with river discharges at Lobith during the seasons. Only in 2004 were



TABLE III. Ratios of river discharges and of migrating silver Anguilla anguilla at thebifurcation points Pannerden (Waal–Pannerdens Kanaal) and IJsselkop (IJssel–Nederrijn)in the years 2004–2006. River discharge ratios are based on daily mean discharges of thewatercourses involved from 1 August to 31 December. Anguilla anguilla ratios are based on

total numbers of detected fish in the watercourses in the year of release

River discharge ratio A. anguilla ratio

Pannerden2004 3·0 5·22005 3·3 4·52006 3·0 4·9

IJsselkop2004 5·1 0·62005 6·5 10·02006 4·6 2·0

there indications for coinciding peaks (Fig. 9), but the datasets were significantlycorrelated (r = 0·184, P < 0·001). The daily number of detections were not relatedto the lunar cycle (Fig. 9, r = 0·07, P > 0·05).

DISCUSSION

N O N - D E T E C T I O N

Thirty-two, 23 (19%) of the downstream migrating A. anguilla were not detectedat Rhine-Xanten 1 , Waal-Vuren 2 and Beneden Merwede 3 , respectively. Thefailure rates at Rhine-Xanten 1 amounted to 30, 32 and 35% in the successive years2004–2006. Detection failures of the whole detection system between Rhine-Xanten1 and the last stations before entering the sea in the Netherlands (Haringvlietdam7 , Oude Maas 4 , Noord 8 , Lek 10 and Afsluitdijk 12 ) were 21, 8 and 0%

in successive years. Missing detections made determination of the exact migrationroute in the delta part of the River Rhine impossible for six fish. All of these fish

Aug-040

10

20

30

Num

ber

of r

elea

ses

and

dete

ctio

ns

40

50

60

70

80

90

Feb-05 Aug-05 Feb-06 Aug-06

FIG. 5. Number of releases ( ) of silver Anguilla anguilla in the River Rhine at Cologne or Sieg in the years2004–2006 and of detections of these fish at all detection stations in the River Rhine ( ). Repeateddetections of one fish at the same station were counted as one detection.



00·0– 0·3

0·3– 0·6

0·6– 0·9

0·9– 1·2

1·2– 1·5

Migration speed (m s–1)

1·5– 1·8

1·8– 2·1

2·1– 2·4

10

20

30

Freq

uenc

y (%

)

40

50

60

70

80

FIG. 6. Relative frequency distributions of migration speeds of silver Anguilla anguilla in the River Rhineover the river reaches between release location (Cologne or Sieg) and Rhine-Xanten (1) [Release-Xanten

( ); n = 171] and over the reach between Rhine-Xanten 1 and Waal-Vuren 2 [Xanten-Vuren ( );n = 71] in 2004–2006. n is the number of measurements of migration speeds of different individualfish.

00·00

0·20

0·40

0·60

0·80

Mig

ratio

n sp

eed

(m s

–1)

1·00

1·20

1·40

1·60

1·80

1000 2000

River discharge (m3 s–1)

3000 4000

FIG. 7. Migration speeds of individual silver Anguilla anguilla (n = 180) in the transects between location ofrelease and detection stations where they had been detected for the first time, in relation to the averageriver discharge of the River Rhine at Lobith (here the Rhine enters The Netherlands) when the fish passedthese transects.

had chosen the Waal, however, and were used for the analysis of route choice atPannerden.

Why some fish were not detected after release is unknown, but technicalproblems, behavioural factors and natural or anthropogenic mortality could be



00 4 8 12

Hours after sunset

16 20

5

10

15

20

Num

ber

of d

etec

tions 25

30

35

FIG. 8. Number of detections of silver Anguilla anguilla per hour (all detection stations and years combined).The timescale is scaled to hours after sunset. Note that sunrise is undefined in this figure because itchanges in the study area between 8 and 17 h after sunset during the seasons.

involved. Transponder failures could have occurred or some fish may have losttheir transponders, but this did not occur in a 19 week controlled experiment byWinter et al. (2005). There were some technical failures at the Beneden Merwede3 detector station but flooding of antenna cables at some other stations did not

appear to have had significant effects on detection rates.Some fish may have died or moved upstream or settled in the neighbourhood of the

release location, naturally or because of stress due to handling, surgical procedures orthe effects of implanted transponders. In a similar tracking study in the River Meuse,one fish from 150 released moved upstream (Winter et al., 2006) and preliminary dataof telemetered fish in the River Rhine and in the River Berwijn, a small tributaryof the River Meuse, showed that some tagged silver A. anguilla did not migratedownstream (unpub. data). It is possible that some fish may not have been at asufficiently advanced stage of silvering to show full migratory activity. Detected andundetected fish, however, did not differ greatly in estimated maturation stage whenreleased. Moreover, fish released in Cologne in August, September and October 2005(Table I) showed a progression in maturation status during autumn with increasinglevels of haematocrit, oestradiol, oocyte diameters, fat in oocytes and gonado-somaticindex (A. Palstra, pers. comm.), suggesting that maturation continued during theseason.

It is possible, however, that reversal of the silvering process occurred in some fish,as it is known that silver A. anguilla are capable of resuming feeding and delayingmigration for several years (Vøllestad et al., 1994; Feunteun et al., 2000; Durif et al.,2005). The maximum delay in first detection of the 2004 cohort was 10 months, twofish from the 2005 releases were found to delay their migration for up to 14 monthsand one fish of the 2006 cohort did so for 15 months. Longer delays might haveoccurred but this cannot be confirmed by the detection date because of the limitedtransponder battery life (1·5–2·0 years).

Another reason why the released fish might differ is because of their capturelocations and methods of capture. The migration routes chosen and the percentage



20044000

3000

2000

1000

0

18

14

10

6

2

18

14

10

Det

ectio

n fr

eque

ncy

6

2

18

14

10

6

2

4000

(a)

(b)

(c)

3000

2000

Dis

char

ge (

m s

–1)

1000

0

4000

3000

2000

1000

0Aug Oct Dec

FIG. 9. Detection frequency ( ) of silver Anguilla anguilla in relation to river discharges at Lobith ( ) andto the lunar cycle (sine transformed and on an arbitrary scale, maximum = new moon, minimum = fullmoon) ( ), during downstream migration in Autumn (a) 2004, (b) 2005 and (c) 2006.

of undetected ‘Sieg’ and ‘Moselle’ A. anguilla, however, did not seem to differ ingeneral terms and where they did on specific routes (Table I), the numbers weretoo low to reach firm conclusions. Anthropogenic mortality may be another causeof non-detections before silver A. anguilla escape to the sea, as discussed furtherbelow.



M I G R AT I O N RO U T E S A N D M I G R AT I O N O B S TAC L E S

At lower river discharges, most of the water from the Rhine flows via the Waal andenters the North Sea via the Nieuwe Waterweg. Results show that 84% of the fishmigrated via the River Waal towards the sea. The Rivers IJssel and Nederrijn–Lekwere less preferred (11 and 6%, respectively). The choice for the Waal as the primarymigration route was not fully explained by the difference in river discharges in theWaal and the Pannerdens Kanaal. Assuming that the fish passively drift with the flow,more fish (26–42% per year) migrated via the Waal than could be expected fromthe river discharge ratio at Pannerden. At the IJsselkop bifurcation, the choice offish was unclear because of low detection rates. The yearly differences in numbersof A. anguilla migrating via the Nederrijn (Fig. 4), however, may be explainedby differences in the timing of the operation of the sluices in the peak period ofmigration. The sluices in the Nederrijn were simultaneously opened during relativelylong time periods in 2004 (22 October to 30 November and 23 to 31 December) and2006 (16 August to 9 September, 21 September to 18 October, 27 to 30 October,24 November to 1 December and 8 to 21 December). These sluices were onlyopened from 25 August to 20 September (and on 9 October) in 2005 and onlyone fish was detected at Nederrijn 9 in that time period, but this fish evidentlyreturned upstream and was detected 20 days later at IJssel-Kampen 11 . Two otherfish detected at Nederrijn 9 later in that year also returned and were detected atIJssel-Kampen 11 after 8 days or at Waal-Vuren 2 after 45 days (the latter first musthave taken Pannerdens Kanaal upstream). Such searching or hesitating behaviour ofsilver A. anguilla upstream of a weir (and power station) was also observed in theRiver Moselle (Behrmann-Godel & Eckmann, 2003) and under controlled laboratoryconditions (Adam et al., 1997). Evidently, the first weir in the Nederrijn at Drielfunctions as an obstruction to the downstream migration of silver A. anguilla.

The sluices in the Haringvlietdam 7 must also form a major barrier, throughwhich only three fish escaped. There is a tidal influence and saltwater intrusion inthis delta region up to Waal-Vuren 2 and Lek 10 , but in the Haringvliet the tidalrange is reduced to a few decimetres and saltwater intrusion is prevented by themanagement of the sluices in the Haringvlietdam 7 . Most of the fish passing Waal-Vuren 2 migrated via Beneden Merwede 3 (66–73 in total, Fig. 4) and Noord8 towards the Nieuwe Waterweg, the remainder (20–27 in total) migrated via the

Haringvliet but returned and migrated via Dordtsche Kil 5 and Spui 6 towardsthe Nieuwe Waterweg.

The data obtained from this telemetric study suggest that the current watermanagement in the main course of the River Rhine in the Netherlands does notpreclude the silver A. anguilla to find their way to the sea. There are indications,however, that the existing sluices in the Nederrijn and the sluices at the Haringvlietinduce a prolonged stay of some of the fish upstream of these obstacles, enhancingthe risk for being caught by fishermen in the delta region of the River Rhine orreducing the risk for being entrained by hydropower stations in the Nederrijn.

FAC TO R S A F F E C T I N G M I G R AT I O N

Higher commercial catches of silver A. anguilla generally occur at night, atlow lunar light conditions or in deeper water with lower light conditions (Tesch,2003). Locally operating sensory cues are considered to be important therefore for



downstream migrating anguillid eels (Haro, 2003). Silver A. anguilla tend to avoidartificial light at hydropower stations (Hadderingh et al., 1992). Local ambient lightconditions might affect, therefore, both depth and lateral position of the fish in thecross section of the watercourse at a bifurcation point. Subsurface light penetration,however, will be low in turbid and deep rivers such as the River Rhine (yearly averageSecchi-depth range 50–60 cm with s.d. = 12–21 cm in the time period 1987–2005).Thus, light would be expected to have little effect on A. anguilla migrations in theRiver Rhine, both with regard to route choice and timing, because fish can easilyfind lower light levels in deeper water. This is possibly why, contrary to manyother studies (Cairns & Hooley, 2003; Haro, 2003; Tesch, 2003), silver A. anguillamigration in the River Rhine occurred frequently during the day and showed no clearrelationships with the lunar cycle. Downstream migration speeds of anguillid eelsrange from 0·05 to 1·10 m s−1 in telemetric studies (Haro, 2003) and depend on flowrates as fishes make both active and passive use of the currents (Richkus & Dixon,2003). Most individual A. anguilla migrated downstream with highly varying speedsin the River Rhine, and the frequency distribution was shown to be highly skewedtowards <0·30 m s−1 (Fig. 6). These results match with those from a comparablestudy in the River Meuse, where several fish also showed stepwise migrations withlarge intervals up to 20 months (Winter et al., 2006). In the present study, fewfish consistently migrated at high speeds on two successive routes towards the sea(location of release to Rhine-Xanten 1 and Rhine-Xanten 1 to Waal-Vuren 2 ),but many fish did so on one of these routes. Most of the fish migrated more slowlydownstream than can be explained by passive drift in the main current (Fig. 6) andthus they must have shown intermittent migrations. Such behaviours could explainwhy river discharges and lunar cycles did not show a clear effect on detectionfrequency of the A. anguilla in this study. Reasons for the delays in the migrationsbetween detection stations, however, are unknown.

E S C A P E M E N T TO S E A

Of the 2004, 2005 and 2006 cohorts of silver A. anguilla that passed Rhine-Xanten1 and immigrated into The Netherlands, 38, 29 and 29%, respectively, passed the

last detection stations before escaping to the sea. All others probably ceased migrationfor a prolonged period or died. Silver A. anguilla fisheries and passage throughhydropower plants may have been anthropogenic causes of mortality. The only riverstretch where hydropower stations might have affected survival, however, was in theNederrijn and here, only one to seven eels per year from the 2004–2006 releaseswere detected (Fig. 4). The total losses of detections in this river stretch duringthis study were low, and the effect of hydropower appeared to be insignificant.The fish in this study seemed to show a relatively continuous migration activityfrom August to December, with a series of successive smaller peaks spread overthe whole period. Migration also occurred during daytime nearly as intensively asduring night hours, thus the use of discharge data, lunar cycles and night-times inforecasting possible peaks of silver A. anguilla migration to aid escapement willtherefore be limited. For example, temporary closures of hydropower plants in theNederrijn reaches to minimize turbine mortalities cannot be planned in advance withthe current knowledge.



Fishing mortality might have been a cause of non-detections and affected esca-pement rates. The intensive A. anguilla fishery in Lake IJsselmeer (Dekker, 2000)may have been a principal cause of the 75–100% decrease in detection rates over2004–2006 between IJssel-Kampen 11 and the sea, but the numbers of fish involved(four to 10 per year) were small (Table II). On all other Dutch river stretches, the Lekand Nieuwe Waterweg included, the only known anthropogenic causes of mortalityare A. anguilla fisheries. These are estimated (in orders of magnitude) by the Ministryof Agriculture, Nature Management and Food Quality (MANMFQ, 2008) in TheNetherlands Eel Management Plan to cause a loss of 100 t (25%) of the currentannual escapement to the sea of 400 t. Winter et al. (2006) estimated fishing mortalityof silver A. anguilla released with transponders in the River Meuse to be 22–26%.Silver A. anguilla migration from the German part of the River Rhine to the Dutchpart is estimated in The Netherlands Eel Management Plan to be in the order of 300t year−1, of which 100 t are landed, i.e. a fishing mortality of c. 33% per year. Theseestimates contrast with an overall non-detection rate between Rhine-Xanten 1 andthe sea of 71% (Fig. 4), suggesting that perhaps half of the fish not detected mayhave been lost to the fisheries.

The transponder and telemetry systems deployed in the present study proveduseful in tracking large female silver A. anguilla in the River Rhine. Many fishwith implanted transponders, however, were not detected after release, possibly dueto technical problems but also due to fish not migrating downstream within the studyperiod and within the 1·5–2·0 year lifespan of the transponder batteries. Natural andanthropogenic mortality factors could also be important but more data are neededon causes of non-detections to reach reliable estimates of anthropogenic mortalitiesdue to hydropower plants and fisheries. For example, the escapement rate to the seaof silver A. anguilla detected at Rhine-Xanten 1 in this study appears to be c. 29%(Fig. 4), but it remains to be determined to what extent this apparent failure to reachthe 40% EU target is due to mortalities rather than technical or behavioural factors.

This study has shown the telemetry system has potential in large rivers like theRiver Rhine for studying routes used and the effects of environmental factors onmigration speeds and timings and escapement to the sea. More detection stationsand tracking in local environments of individual fish, however, are needed for afuller understanding of the migration behaviour of silver A. anguilla. The lifetime ofthe transponder batteries should also be doubled at least in order to detect all delayedmigrants.

Despite these provisos, this study has shown that most of the silver A. anguilladetected chose the River Waal and the Nieuwe Waterweg on their downstreammigration to the sea, and consequently relatively few were exposed to the hydropowerstations in the Nederrijn, the fishery in Lake IJsselmeer or the Haringvliet sluices.The current management of the sluices in the Haringvlietdam 7 and Afsluitdijk 12

does not seem to prevent fish from escaping to the sea.This study also showed that silver A. anguilla in the Rhine showed more or

less continuous migration activity from August to December, with a series ofsuccessive smaller peaks spread over the whole period. Fish did not seem to migrateat constant speeds or follow the main flow patterns at bifurcation points and relationsof migration speed to river discharge and lunar cycles were weak and absent,respectively. There was a small migration peak during the first 6 h after sunsetbut fish migrated all day in the turbid River Rhine. It, therefore, appears that the use



of discharge data, lunar cycles and night-times in forecasting possible peaks in silverA. anguilla migration are limited. This has practical implications, because if suchpeaks could be accurately predicted in advance in rivers like the Rhine, temporaryprotection measures could be imposed, such as temporary closures of hydropowerturbine intakes and water diversions.

In conclusion, although there are some limitations that need addressing, this studyhas shown it is feasible to use the NEDAP Trail telemetry system for trackingfemale silver A. anguilla in large and highly modified rivers like the Rhine todevelop understanding of their emigration and assess management options to achieveescapement targets.

The project was made possible by using the eels caught in the yearly trap-and-releaseactions by the Struktur und Genehmigungsbehorde Nord (Rheinland-Pfalz). LOBF-NRW,OVB, Sportvisserij Nederland and Visadvies contributed to the field work. The Universitatzu Koln made a field and laboratory facility available in Cologne. RWS Waterdienst providedthe telemetry infrastructure and the labour for its maintenance. OVB and LOBF co-ordinatedthe project and are acknowledged for funding the project. Another part of the project wasfunded by EU through the FIFG to Rheinfischereigenossenschaft. The authors acknowledgethe participation of E. Winter and A. Palstra for personal comments on maturation of the eels,B. Knights and C. Belpaire for useful comments on a draft paper and the staff of all partnersto the field work, especially G. Feldhaus, A. Hehenkamp and J. Merkx.

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Behrmann-Godel, J. & Eckmann, R. (2003). A preliminary telemetry study of the migrationof silver European eel (Anguilla anguilla L.) in the River Mosel, Germany. Ecologyof Freshwater Fish 12, 196–202.

Bij de Vaate, A., Breukelaar, A. W., Vriese, T., de Laak, G. & Dijkers, C. (2003). Sea troutmigration in the Rhine delta. Journal of Fish Biology 63, 892–908.

Breukelaar, A. W., Bij de Vaate, A. & Fockens, K. T. W. (1998). Inland migration studyof sea trout (Salmo trutta) into the Rivers Rhine and Meuse (Netherlands), based oninductive coupling radio telemetry. Hydrobiologia 371/372, 29–33.

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Feunteun, E., Acou, A., Laffaille, P. & Legault, A. (2000). European eel (Anguilla anguilla):prediction of spawner escapement from continental population parameters. CanadianJournal of Fisheries and Aquatic Sciences 57, 1627–1635.

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Electronic References

ICES (2004) Report of the ICES/EIFAC Working Group on Eels. International Councilfor the Exploration of the Sea, Sukarrieta, Spain, 7–11 October 2003. ICES CM2004/ACFM:09. Available at http://www.eaa-europe.org/fileadmin/templates/uploads/Eels/ICES_WGEEL_2004.pdf

ICES (2005) Report of the ICES/EIFAC Working Group on Eels. International Coun-cil for the Exploration of the Sea,Galway, Ireland, 22–26 November 2004. ICESCM 2005/ACFM:01 Available at http://www.eaa-anglingineurope.com/fileadmin/ tem-plates/uploads/Eels/ICES_WGEEL_2003.pdf


Documents

Route choices, migration speeds and daily migration activity of