Downstream Migration of Female Silver Eel in the River Meuse (2007-2008)

Instituut voor Natuur- en Bosonderzoek - Kliniekstraat 25 - 1070 Brussel - T.: +32 (0)2 558 18 11 - F.: +32 (0)2 558 18 05 - [email protected] - www.inbo.be

Downstream migration of female silver eel in the River Meuse (2007-2008)

Hilde Verbiest, Claude Belpaire en Breukelaar André

INBO.R.2008.59 INBO.R.2008.59

INBO.R.2008.59.indd 1 09-03-2010 11:17:40 Auteurs: Hilde Verbiest(1), Claude Belpaire(1) en Breukelaar André(2) (1) Instituut voor Natuur- en Bosonderzoek (2) Rijkswaterstaat Waterdienst, Nederland

Instituut voor Natuur- en Bosonderzoek Wetenschappelijke instelling van de Vlaamse overheid

Instituut voor Natuur- en Bosonderzoek Het Instituut voor Natuur- en Bosonderzoek (INBO) is het Vlaams onderzoeks- en kenniscentrum voor natuur en het duurzame beheer en gebruik ervan. Het INBO verricht onderzoek en levert kennis aan al wie het beleid voorbereidt, uitvoert of erin geïnteresseerd is.

Rijkswaterstaat Waterdienst, Nederland Rijkswaterstaat (RWS) is de Nederlandse Rijksdienst die werkt aan droge voeten, schoon en voldoende water én aan de vlotte en veilige doorstroming van het verkeer.

Vestiging: INBO Brussel Kliniekstraat 25 - 1070 Brussel www.inbo.be

e-mail: [email protected]

Wijze van citeren: Verbiest H., Belpaire C., Breukelaar A. (2008). Downstream migration of female silver eel in the River Meuse (2007-2008). Rapporten van het Instituut voor Natuur- en Bosonderzoek 2008 (INBO.R.2008.59). Instituut voor Natuur- en Bosonderzoek, Brussel.

D/2008/3241/399 INBO.R.2008.59 ISSN: 1782-9054

Verantwoordelijke uitgever: J. Tack

Druk: Management ondersteunende diensten van de Vlaamse overheid

Foto cover: Y. Adams/Vilda

Dit onderzoek werd uitgevoerd in samenwerking met: Rijkswaterstaat Waterdienst, P.O. Box 17, 8200 AA Lelystad, Nederland

INBO.R.2008.59.indd 2 09-03-2010 11:17:40

Downstream migration of female silver eel in the River Meuse (2007-2008)

Hilde Verbiest, Claude Belpaire, André Breukelaar

INBO.R.2008.59 D/2008/3241/399

Acknowledgements

We would like to express words of thanks to Jean-Claude Philippart and Michael Ovidio of the University of Liège for their support during the present study and to the ‘Service Public de Wallonie’ for their logistic support. We also are very greatful to Alain Vanderkelen whose efficiency and enthusiasm during field work proved infectious.

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Preface

Populations of eel species of the northern hemisphere have declined since 1980. In June 2007, the European Commission adopted a regulation to protect and restore the eel stock (EU, 2007). Within this regulation all countries of the EU have to formulate a national eel management strategy before the end of 2008, aiming to restore a silver eel escapement from the river basin to the sea of about 40% compared to the non-human impact situation (EU, 2007).

This study focuses on the downstream migration of silver eels in the River Meuse. It gives essential information on the silver eel escapement nesesary for the national eel management plan.

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Summary

Populations of eel species of the northern hemisphere have declined since 1980. The stock of the European eel, Anguilla anguilla (L.) is considered to be outside biological safety limits (ICES, 2004). In June 2007, the European Commission adopted a regulation to protect and restore the eel stock (EU, 2007).

Within this EU regulation the elaboration of measures to enhance the escapement of silver eels is an essential element for the recovery of the eel stocks. For this rerason the downstream migration of female silver eels is monitored within the River Meuse, a large transboundary river flowing from France, through Belgium to The Netherlands. For this study the migration of female silver eels was monitored by tagging fish in combination with a fixed detection equipment on the river (NEDAP TRAIL System®). Each detection station incorporates an antenna loop, consisting of three parallel cables (10 m in length) crossing the bottom of the river, an antenna connection box, a receiver and a transmitter. A detection station of INBO is situated on the River Berwijn, a tributary of the River Meuse. This station links up with a network of more than 30 of these detection stations in The Netherlands.

31 eels were tagged after determining the silvering status of each eel. After tagging all eels were released (between June and November) into the River Berwijn 3km upstream the mouth. Once a tagged eel leaves the River Berwijn it can be detected at the different detection stations on the River Meuse. The total distance from the release site in the River Berwijn until the North Sea is 326 km.

Downstream migration was tracked between August 2007 and April 2008. Nine (29%) eels did not start their migration and were located by manual tracking in the Berwijn. These eels were searched for 5 times by manual tracking with a portable Nedap TRAIL station. This tracking reveals that the non-migrants behave as resident after being released in the River Berwijn. This shows clearly that a considerable percentage of silver eels is not ready to migrate to sea and show resident behaviour, yet all tagged eels were silver eels at the time of tagging. Probably these eels have stopped silvering. These observations highlight the need to evaluate the proportion and behaviour of non-migrants when assessing downstream migration of silver eels, especially in the process of implementation of the European Eel Regulation where specific benchmarks for silver eel escapement are defined. In studies evaluating the migration success of released silver eels, success rates could be underevaluated. We therefore recommend to incorporate protocols to evaluate the proportion of silver eels not starting migration within studies assessing migration success of silver eels.

Thirteen eels (42%) left the Berwijn and were detected at the second station downstream. Eight out of these eels were detected at the station before the Meuse delta, but only two eels (15%) passed the most downstream detection station and arrived at the North Sea.

Two detection stations were positioned at hydropower stations (HPS). Migration activity of the eels at these stations is clearly delayed. This delay is much higher at Linne than at Lith. One eel passing through the HPS in Linne presumably died after passing through the turbines. At all other detection stations (without HPS, n= 10) migration was not delayed.

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Recommendations for management and/or policy

Female silver eels released in the River Berwijn at 326 km from the North Sea seem to be able to reach the North Sea via the River Meuse. However the escapement rate seems small. Only 2 of out 13 individuals which effectively started their migration were shown to reach the sea.

Some individuals were lost and it is not clear whether these eels were caught by fishermen, died for other reasons or stopped their migration and settled in the river delta. However the impact of hydropower stations is evident, eels show a delay in their migration, and some may die. Fisheries might be another important cause of mortality. Obviously, both factors need further research within the implementation of the eel management plans for the River Meuse basin.

This study with promising results carried out on small scale with a limited number of individuals has shown its strengths and shortcomings, but clearly justifies and calls for a broader international approach.

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Table of contents

Acknowledgements...... 1

Preface ...... 2

Summary ...... 3

Recommendations for management and/or policy...... 4

1 Introduction ...... 6 1.1 Motivation ...... 6 1.2 Objectives ...... 7 1.3 Participating partners ...... 7 2 Materials and Methods ...... 8 2.1 The River Meuse...... 8 2.2 The River Berwijn ...... 9 2.3 Telemetry...... 10 2.3.1 Nedap Trail detection stations...... 10 2.3.2 Tagging...... 14 2.3.3 Data collection ...... 17 3 Results ...... 18 3.1 Silver eels ...... 18 3.2 Registrations of migration activity of silver eels...... 19 3.2.1 Relation between migration activity and length/weight of the eel ...... 22 3.2.2 Relation between migration activity and water discharge ...... 23 3.2.3 Migration time and speed...... 23 3.2.4 Relation between eel movement and time ...... 25 3.2.4.1 Hour of day ...... 25 3.2.4.2 Delay...... 25 3.2.5 Relation between migration activity and maturation stage of the eel ...... 26 3.2.6 Relation between success and origin of the eel...... 27 3.3 Resident eels ...... 28 4 Discussion and conclusions ...... 29

Appendix 1: Fish inventory on the Grensmaas and the Berwijn (Van Thuyne et al., 1997, 2003, 2005a & 2008; Breine et al., 1998; Vrielinck et al., 2002; Van Gils et al. 2001) (after Buysse et al, 2008) ...... 32

Appendix II: Transponder implantation data of eel...... 34

Bibliography...... 35

List of figures...... 38

List of tables ...... 40

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1 Introduction

1.1 Motivation

In most large rivers, fragmentation of the longitudinal corridors by dams, navigation weirs and hydroelectric power plants have resulted in the drastic range reduction and the extinction of numerous migratory fish species. Also in Belgium, such physical barriers obstruct the main migration routes such as rivers Scheldt, Leie, Demer, Dender and Meuse which inhibits the downstream and upstream migration of anadromous and catadromous fish species to their spawning or growing area.

Populations of eel species of the northern hemisphere have declined since 1980. The stock of the European eel, Anguilla anguilla (L.) is considered to be outside biological safety limits (ICES, 2004). Long term time series on glass eel recruitment are available for the Nieuwpoort station at the mouth of the river Yser. These series show a drastic decline in glass eel catches (Belpaire, 2006).

1000 100

900 Total year catch 90

800 Maximal daycatch 80

700 70

600 60

500 50

400 40 Total year catch (kg) catch year Total Maximal daycatch (kg) daycatch Maximal 300 30

200 20

100 10

0 0 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

In June 2007, the European Commission adopted a regulation to protect and restore the eel stock (EU, 2007). Within this regulation all countries of the EU have to formulate a national eel management strategy before the end of 2008, aiming to restore a silver eel escapement from the river basin to the sea of about 40% compared to the non-human impact situation (EU, 2007). The management plans will be considered on river basin level, consistent with the EU Water Framework Directive (WFD).

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Measures to enhance the escapement of silver eels will be essential for the recovery of the eel stocks. The River Meuse was selected as study area as it is a large transboundary river flowing from France, through Belgium to The Netherlands and draining an important part of Belgium and The Netherlands.

1.2 Objectives

The main objectives of this study are:

- to assess to what extend female silver eels released at 326 km from the sea are able to reach the sea

- to trace the preferred pathway of downstream migrating eels in the River Meuse

- to gain insight in the effects of barriers (dams, fish pass, hydropower station) on migratory activity, especially damage caused by hydropower stations

- to gain insight in the behaviour of tagged non-migrant silver eels

1.3 Participating partners

- Research Institute for Nature and Forest, Kliniekstraat 25, 1070 Brussel, Belgium

- Rijkswaterstaat Waterdienst, P.O.Box 17, 8200 AA Lelystad, The Netherlands

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2 Materials and Methods

2.1 The River Meuse

The River Meuse has a total length of 925 km from its source in France to the North Sea. The river basin covers a catchment area of about 36000 km2, situated for 26% in France, 2% in Luxemburg, 39% in Belgium, 11% in Germany and 22% in The Netherlands. The Meuse is nourished by rainwater, whereby the discharge can fluctuate fairly strong. Mean discharge is 230 m3 s-1.

In France the River Meuse is characterized by a small river basin, without any important tributaries. The Belgian part of the River Meuse has a very broad river basin, with important tributaries such as Semois, Lesse, Samber and Ourthe. The lower reach from the Belgian- Dutch border till the estuary can be divided into an non impounded part (the Grensmaas) and the River Meuse downstream of Maasbracht.

Several migration obstructions are localized on the River Meuse (Table 1). The Walloon part of the River Meuse counts 15 dams, equipped with a ship lock (except for Lixhe). The 6 dams below Namur are equipped with hydropower stations. All dams are equipped with fish passes. In the Upper-Ourthe the dam of Nisramont hinders upstream migration. On the tributaries upstream Liège (Samson, Lesse, Bocq, Molignée, Hermeton, Semois, Houille and Viroin) important obstructions hinder free migration. On the Grensmaas no migration barriers are situated. On the other hand the most important tributaries (Dommel, Bollisserbeek, Bosbeek, Warmbeek, Abeek, Lossing, Itterbeek, Witbeek, Jeker, Voer, Veurs and Berwijn) have got a lot. The lower reach of the River Meuse counts 7 weirs and 2 HPS (at Linne and Lith), each weir equipped with a fish pass.

The fish stock of the river Meuse is very abundant although most migratory fish have disappeared over the years. Still a notable eel concentration is situated in the basin of the River Meuse (Figure 3). Due to its non impounded character, its relatively high fall and coarse substrate the Grensmaas holds a unique fish population with rheophylic species as barbel, common nace, common dace, chub. Especially the populations of barbel and common nace are unique in Flanders and only appear in the River Grensmaas. Fish inventories on the Grensmaas (1997-2005) show 34 fish species, only few migratory fish included (Van Thuyne et al., 1997, 2003, 2005a; Breine et al., 1998; Van Gils et al. 2001; Buysse et al, 2008). Completing this list with historical data 15 species can be added (Vrielinck et al., 2002), mainly migratory fish species (Appendix I).

The river Meuse is an important route for migratory fish to reach growing or spawning areas. In Flanders 13 migratory fish species were present: eel, see trout, atlantic salmon, atlantic sturgeon, twaite shad, elft, houting, river lamprey, sea lamprey, smelt, , flounder, dunlipharder and three spined stickleback (Vrielinck et al., 2002). Most of these species disappeared in the river Meuse. Since the nineties, through the restoration of fish migration in the river Meuse basin in Belgium and The Netherlands, migratory species are growing. Salmon, which disappeared halfway through the 20th centuary, has gradually come back. Since 1994 salmon catches increase. Sea trout and sea lamprey have, in contrast to salmon, always been present in the downstream part of the river Meuse, although in very low numbers. Twaite shad, which enters the river Meuse estuaria to spawn, has disappeared in the Meuse after cutting of the Haringvliet. From 1990 the number of twaite shad seems to grow around the coast and in the tidal rivers of the Netherlands (Jansen et al., 2007).

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On the Meuse, the amount of ascending young eels is monitored in a small pool-type fishpass at the Visé-Lixhe dam (1992 to 2008). The study based on a constant year-to-year sampling effort revealed a regular decrease of the annual catch (Baras et al., 1994, Philippart and Rimbaud, 2005) (Fig. 2).

6000

5000

4000

3000 * 2000 Number of eels 1000

0 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 Year

Figuur 2. Variation in the number of ascending young yellow eels trapped at the fish trap of the Visé-Lixhe dam. Data from University of Liège (J.C. Philippart) in Philippart and Rimbaud (2005), Philippart (2006) and Philippart (pers. comm.) (in Bepaire et al, 2008). * Data incomplete, catches until 31/07/08.

2.2 The River Berwijn

The River Berwijn is a small tributary of the River Meuse in Belgium (Moelingen) near the Dutch-Belgium border. It is situated 326 km from the sea and has a total length of 31,9 km from its source in Aubel to its mouth in the River Meuse. The River Berwijn basin has a drainage area of about 128 km2. Mean discharge is 1,32 m3 s-1.

Different fish inventories show a valuable fish stock in the river Berwijn (Van Gils et al., 2001; Van Thuyne et al, 2005b). In total 24 spieces are seen in the River Berwijn during 5 fishing experiments (Appendix I). Some of the species are considered as quite rare in Flanders, some of the species are Red List species (bullhead, chub, common dace, barbell, minnow, brown trout, nase carp, Schneider, eel and ide). Stone loach is a dominant species in the Berwijn as well as gudgeon and three spined stickleback. Also minnow and chub are seen in large numbers. Barbel and nase carp are present, but in low numbers. Perch and common dace are sporadically seen. Apendix I lists the inventory of the Berwijn and Meusse.

The river Berwijn is important for eel, as well as for salmon and sea trout. It carries suitable and (for Flanders) rare habitats for different life stages. This is why this river deserves necessary attention. Figure 3 reflects the natural character of the River Berwijn.

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Figuur 3. View on the River Berwijn

2.3 Telemetry

2.3.1 Nedap Trail detection stations

For this study the NEDAP TRAIL System® is used (Fig. 4). The system is based on inductive coupling between an antenna loop and the transponder. Each detection station incorporates an antenna loop, consisting of three parallel cables (10 m in length) crossing the bottom of the river or canal, an antenna connection box, a receiver and a transmitter.

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Figuur 4. Schematic view on the Nedap Trail system (Breukelaar et al, 1998)

Each transponder consists of a biocompatible glass tube with a diameter of 15 mm and a length of 65 mm (Fig. 5). Inside the transponder is a ferrite rod with a length of 25 mm, a custom made integrated electronic circuit, and a battery with a diameter of 12 mm and a length of 10 mm. The battery ensures a lifetime of about two years.

Figuur 5. Nedap Trail transponders. The middle transponder (65mm, 25g, life span of 2 years) is used to tag the silver eels. The left transponder is 90mm, 40g (life span of 4 years), right transonder is 35mm, 12g (life span of 6 months) and is used in smolt research.

Every 4 seconds the transmitter generates an interrogating signal which triggers any transponder that might pass the antenna loop. The transponder responds by transmitting its unique signal, which is subsequently demodulated by the receiver, decoded and recorded by a microprocessor unit. This unit is connected to a telephone modem. Transmission of a transponder signal takes two periods of 8 seconds, separated by 8 seconds of silence. In each transmission period of 8 seconds the unique code number of that particular transponder is transmitted 32 times. After the complete transmission period of 24 seconds the

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transponder is mute for two minutes to prevent the batteries from running down in case a tagged fish should stop above an antenna (Breukelaar et al., 1998).

In The Netherlands, a network of more than 30 of these detection stations registers large scale movements of migratory fish in Rhine and Meuse (Fig. 6). Stations are located at each weir and/or HPS and/or fishpass on the rivers.

Figuur 6. Network of detection stations in The Netherlands, belgium and Germany on the River Meuse and the River Rhine (situation April 2007)

More detailed information on the locations of these detection stations is presented in table 1.

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Tabel 1. Locations of the Nedap Trail detection stations in The Netherlands, Belgium and Germany.

Station River Location Number 2 IJssel Kampen 5 Spui Zuidland 6 Dordtsche Kil ‘s-Gravendeel 7 De Noord Alblasserdam 8 Beneden Merwede Boven Hardinxveld 9 Lek Nieuwegein 10 Waal Vuren 11 Bergsche Meuse Capelse Veer 12 Grensmaas Stevensweert 13 Rhine Xanten 14 Oude Meuse Spijkenisse 15 Ruhr St. Odiliënberg 16 Sieg Menden 19 IJsselmeer Den Oever 18 IJsselmeer Kornwerderzand 25 Meuse Sambeek 23 Meuse Balgoij 26 Meuse Afferden 24 Meuse Sambeek below weir 22 Haringvliet Stellendam South 21 Haringvliet Stellendam North 28 Meuse Borgharen 27 Meuse Itteren 36 Meuse Linne HPS 37 Meuse Linne fish pass 33 Meuse Linne 35 Meuse Alphen HPS 34 Meuse Alphen fish pass 32 Maas Lith 31 Neder Rhine Arnhem 30 Neder Rhine Maurik 29 Neder Rhine Hagestein 38 Berwijn Moelingen 39 Wupper Durrig 40 Lippe Wesel

One Nedap TRAIL detection station is located on the River Berwijn in Belgium (Figure 7). Tagged eels leaving the River Berwijn can be detected at the different detection stations on the River Meuse. Different migration routes are possible once the eel reaches the network of tributaries in the western part of the River Meuse delta. River Meuse and Rhine share the Haringvliet as their former estuary.

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Figuur 7. View on the detection station at the River Berwijn. Left: one of the 3 antenna cables crossing the bottom of the river; right: Nedap Trail semi portable detection station where data are collected.

The NEDAP TRAIL System® has proven to be an adequate system to monitor the downstream eel migration. Advantages of this system for monitoring fish migration: - traceable long distance route of tagged fish - information on passage of/through obstacles - information on delay by obstacles - individual detection of fish - international network (France, The Netherlands, Germany, Berlgium) allows information through different countries and lowers the investement to built a network - easy downloading of data through internet - low disturbance by other signals - usable in freshwater as well as seawater - low maintenance cost - low time investement

2.3.2 Tagging

Eels were caught by electro fishing or by fyke netting between June and December 2007 at different locations in Flanders (Belgium): River Berwijn, River Meuse, canal Bocholt- Herentals and canal Brussel-Charleroi (Figure 8). Electrofishing was carried out with a 5 kW generator (DEKA 7000, 3000 and Z) with an adjustable output voltage ranging from 300 to 500 V. The pulse frequency is 480 Hz. Fykes are 20.5 m long, the enterence is 90cm high and 1,05 m wide. Each time they were set up for 48 hours.

After tagging all eels were released into the River Berwijn 3km upstream the mouth.

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Figuur 8. Origin of the tagged silver eels

The silvering status of each eel was determined using external parameters: (1) eye-diameter (vertical and horizontal), (2) body length, (3) body weight and (4) pectoral fin length. The maturation stage was determined after Durif et al. (2005). Figure 9 shows the measurement procedures of eye and pectoral fin. Only eels classified as ‘silver’ were used for implanting the transponders (stages III, IV and V). 11 silver eels were potentials migrants (stage III) and 20 were migrants (stage IV and V).

Figuur 9. Measuring length of pectoral fin and eye diameter to determine the silverig stage of the female eels.

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31 female silver eels were selected with a length ranging from 643 mm to 896 mm , their body weight varied between 491 g and 1487 g. Tags were inserted into the body cavity after eels were anaesthetized with a 0,1 ml.L-1 solution of clove oil. The incision was closed with three sutures (figure 10). Data on transponder implantation are shown in appendix II.

Figuur 10. Tagging procedure of the silver eel.

After surgical procedures eels recovered in a 1000l water tank for 1 to 5 days. All tagged silver eels were released individually into the River Berwijn, 2,5 km upstream the detection station and about 3 km from the river mouth.

Eels were released at different times, depending on the date of electro fishing or by fyke netting and surgery. Tabel 2 gives an overview of the release date of the tagged eels.

Tabel 2. Origin, release date and number per location of released silver eels used for monitoring their downstream migration in the River Meuse N eels date Release Origin of

Berwijn June 5 6

Canal Bocholt-Herentals October 17 12

Canal Brussel-Charleroi Oktober 26 7

Meuse (Lixhe) November 14 2

Meuse (de Spaanjerd) November 23 4

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2.3.3 Data collection

The detection station on the River Berwijn is installed and managed by INBO. Data registered on this station (detections of migrating eels in the River Berwijn) can be downloaded by INBO.

The detection stations on the River Meuse are property of different Dutch agencies and maintained by Rijkswaterstaat Waterdienst. The detection station on the River Berwijn links up to the network of Dutch detection stations in the River Meuse (Fig. 6). Data collected on these Meuse stations give insight in the migration activity and routes of the each individual silver eel migrating towards the North Sea leaving the River Berwijn. Because of the cooperation with The Netherlands in this study the Meuse data (migration through the Meuse) can also be obtained. They are provided through Rijkswaterstaat Waterdienst.

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3 Results

3.1 Silver eels

31 female silver eels were tagged. Their length varied from 643 mm to 896 mm. Their body weight varied between 491 and 1487 g. Figure 11 and 12 show their length and weight distribution.

2 number of tagged eels

0 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 length (cm)

Figuur 11. Length distribution of the tagged silver eels released in the River Berwijn.

2 number of tagged eels

0 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 weight (g)

Figuur 12. Weight distribution of the tagged silver eels released in the River Berwijn.

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The silvering status of each eel was determined according to Durif et al. (2005). All eels classified as ‘silver’ (stages III, IV and V) were tagged. 11 silver eels were potentials migrants (stage III) and 20 were migrants (stage IV and V). Figure 9 shows their distribution.

4 number of tagged eels

0 III IV V silvering stage

Figuur 13. Distribution of the silvering stage of the tagged silver eels.

3.2 Registrations of migration activity of silver eels

From August 2007 till August 2008, 22 of the 31 transpondered silver eels left the River Berwijn. They were detected at least at one detection station. The total number of detections of tagged silver eels at different detection stations in the River Berwijn and the River Meuse in this study is 1079. These are all registrations of eels, including continuing registrations of eels resting on an antenna for some time (registrations every 4 seconds). For further analsys the number of registrations are filtered by omitting continious registrations.

The 22 eels leaving the River Berwijn and reached the River Meuse, 13 of these eels were detected at Station 27 on the River Meuse (at Itteren). These eels migrated 22,5 km from the release site to Itteren. We assume that these eels have started their downstream migration towards the North Sea. Eight eels migrated into the Haringvliet complex of the Meuse delta via the Bergsche Maas (Station 11). Two eels of this group reached the North Sea via the harbour of Rotterdam (Station 14). The other six eels were lost in the River Meuse delta (Fig. 14).

This results in 42% of the eels being detected at least at two detection stations. 16% of the tagged eels migrated between 22,5km and 216,5km. These eels are on the way, but ceased

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there migration and therefore did not reach the North Sea. 26% migrated more then 216,5km. 6% reached the North Sea, about 326km from the release site.

2 eels Oude Maas Spijkenisse NORTH SEA

8 eels Bergsche Maas

8 eels Meuse Lith (Hydropower station)

8 eels Meuse Balgoij

8 eels Meuse Sambeek

9 eels Meuse Linne (Hydropowe station)

9 eels Meuse Stevensweert

13 eels Meuse Itteren

22 eels Berwijn Moelingen

31 tagged silver eels in the Berwijn

Figuur 14. Downstream migration routes of tagged silver eels in the River Meuse. At every detection station the number of detected eels is presented.

Figure 15 shows the downstream migration of one female silver eel reaching the North Sea. This individual is a small one, only 65,7cm and 540,8g. Remarkable is the yellow belly of this female ‘silver’ eel. Although this eel has a yellow belly, it was determined as a silver eel according to Durif et al. (2005). It was released October 17th and first detected on the River Berwijn at November 2nd. It started it’s downstream migration on December 3rd and arrived December 7th. Hence it migrated 303.5 km during a 5 days period (exactly 102 hours and 55 minutes).

Some individuals interrupt their migration for a period of a few days up to a month and then start migrating again, while others carry out their migration in one trip, without stop.

The downstream migration of the silver eels is hindered by the 2 HPS in Linne and in Lith. To bridge these 2 HPS eels can select the route via the turbines of the HPS or the alternative via the weir (Fig. 15). No tagged eel used the fish pass to go downstream as alternative for the weir or turbines. The weir-route is choosen by the majority of the downstream migrating eels. In Linne 3 out of 9 eels approach via the HPS. All three passes through this HPS. One individual was never detected afterwards. This eel is presumed to have died as a consequence of passing through the turbines. The other 2 eels continued their downstream migration. At Lith 3 eels approached the HPS. All three eels passed through the turbines and continued their downstream migration.

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7.12.2007 Oude Maas Spijkenisse (326km) NORTH SEA

6.12.2007 Bergsche Maas (231km)

5.12.2007 Meuse Sambeek (139km),Balgoij (170km), Lith (193km) 4.12.2007 Meuse Stevensweert (62km), Linne (72km)

3.12.2007 Meuse Itteren (22,5km)

2.11.2007 Berwijn Moelingen (3km)

Figuur 15 Downstream migration of a succesfull silver eel reaching the North Sea. Dates and distances are given. Notice the yellow belly of this succesfull female ‘silver’ eel.

Figuur 16. HPS in the River Meuse at Lnne. The HPS is situated in between the fish pass (left) and the weir (right).

21 Downstream migration of female silver eel in the River Meuse (2007-2008) www.inbo.be

3.2.1 Relation between migration activity and length/weight of the eel

Silver eels with a length varying between 643 mm and 896 mm and a body weight varying between 491 and 1487 g were tagged. We see no relation between the success of the downstream migration and the length of the silver eels. Small, as well as large individuals, are migrating.

In figure 17 the length and weight of the migrating eels is shown in relation to the success of their downstream migration. The green spots indicate ‘migrating’ eels (registrated at least at 2 detection stations). These eels are assumed to have started their downstream migration. The red spots indicate the ‘non migrating’ silver eels (registrated at 0 or 1 station).

The length and weight of the 2 eels reaching the North Sea is:

• Eel1: 83,4cm and 1308,4 g

• Eel 2: 65,7cm and 540,8g

3,2

3,1

2,9 Log Weight 2,8

2,7

2,6 1,8 1,82 1,84 1,86 1,88 1,9 1,92 1,94 1,96 Log Length

not migrating migrating

Figuur 17. Log length and log weight distribution of the tagged silver eels in relation to the success of their downstream migration. (migrating=detected at more than 1 detection stations; not migrating=detected at less than 2 detection stations).

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3.2.2 Relation between migration activity and water discharge

Mean discharge during the study period in the river Meuse is 605,32 m3/s, in the River Berwijn the mean discharge during study period is 2,12m3/s. More detections appear at discharges higher than the mean discharge in the River Meuse as well as in the River Berwijn, as shown in Figure 18.

1200 12

1000 10

800 8

600 6 mean discharge Meuse Water discharge (m3/s) Water discharge(m3/s) 400 4

200 2 mean dischargeBerwijn

0 123456789 0

Figuur 18. Detections of silver eels in the River Meuse and Berwijn during downstream migration in relation to mean discharge (left panel: 1= River Berwijn; 2= Meuse at Itteren; 3= Meuse at Stevensweert; 4= Meuse at Linne, 5= Meuse at Sambeek; 6= Meuse at Bolgoij; 7= Meuse at Lith; 8= Meuse Bergsche Maas; 9= Oude Maas Spijkenisse and right panel: detail of River Berwijn)

3.2.3 Migration time and speed

Table 3 shows the duration of the downstream migration of the eels in the River Meuse and their migration speed. We see a great variation in the duration of downstream migration. The fastest eel covers a maximum of 167 km a day, the slowest migrates over a distance of 300m in one day. It is clear that eels alternate periods of migration with periods of inactivity. The duration of an inactive period differs highly between individuals. This explains the great difference in migration time and speed.

23 Downstream migration of female silver eel in the River Meuse (2007-2008) www.inbo.be

Tabel 3. Migration time and speed (min-max-mean) of the tagged eels during downstream migration between two consecutive detection stations on the River Meuse

) -1

River Location Distance covered (km) Migration time (hours) time Migration Number of eels detected Detection Station number Migration speed (km day

Min Min Max Max Mean Mean

1 Berwijn Moelingen 22 3

2 Meuse Itteren 13 19 6 723 214 0,6 73 16

3 Meuse Stevensweert 9 41 9 1274 301 0,8 115 32

4 Meuse Linne HPS 9 8 2 728 124 0,3 115 28

5 Meuse Sambeek 8 76 11 92 49 20 162 60

6 Meuse Balgoij 8 29 5 22 12 25 167 98

7 Meuse Lith HPS 8 24 4 698 101 0,8 130 71

Bergsche 8 Meuse 8 39 8 103 30 9 120 78 Maas

Oude Maas 9 Meuse 2 70 27 29 28 58 63 60 Spijkenisse

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3.2.4 Relation between eel movement and time

3.2.4.1 Hour of day

Figure 19 shows the registrations of the eels in a diurnal cycle. If individuals are continiously registrated at one detection station (with detections every 4 seconds) only one detection is presented in this graph. This should give a beter idea of the activity.

detections

0 1 2 3 4 5 6 7 8 9 101112131415161718192021222324 hour

Figuur 19. Number of detections at all detections stations of downstream migrating eels in the rivers Berwijn and Meuse during the registration period (June 2007 – April 2008) presented in a 24 hour cycle.

3.2.4.2 Delay

During their downstream migration eels experience a loss of time or delay. This delay is determined as ‘the difference in time between the first (arrival at a detection station) and the last detection (leaving at the same detection station)’. Figure 20 shows the delay at the detection stations along the downstream migration route in the River Meuse.

At all detection stations without HPS there is no delay, at the two HPS at Linne and Lith we see a clear delay. This delay is much higher at Linne than at Lith.

25 Downstream migration of female silver eel in the River Meuse (2007-2008) www.inbo.be

140

120

100

60 (hours) Delay

0 Berwijn Itteren Stevensweert Linne Hydropower Sambeek Balgoij Lith Hydropower Maas Bergsche

Detection stations

Migrating via the hydropower stations causes a delay of 18 hours to 1 month. The delay at the HPS is only seen by eels being detected at the detection station in front of the HPS and not by eels approaching via the weir next to the HPS. At all other detection stations at the River Meuse eels reveal no delay.

3.2.5 Relation between migration activity and maturation stage of the eel

As mentioned above the silvering status of each eel was determined before tagging (after Durif et al., 2005). Maturation stages III, IV and V were used. 11 silver eels were potentials migrants (stage III) and 20 were migrants (stage IV and V).

Individuals of all stages seem to start their downstream migration, as shown in figure 21. Stage IV-individuals are the most successful eels in this experiment. When they take off to the North Sea (detected at more than 1 detection station) they don’t stop their migration. They all can enter the River Meuse delta. Individuals of Stage III and IV also can reach the River Meuse delta, but a number of individuals of these stages cease migration before reaching the River Meuse delta. The 2 individuals who reached the North Sea belong to silvering class III and IV.

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6 Number of eels 5

0 III IV V silvering class (Durif) resident 1-7 stations 8 stations

Figuur 21. Maturation stage of all tagged silver eels in relation to the success of their downstream migration

3.2.6 Relation between success and origin of the eel

The batch of 31 transpondered eels consisted of eels from different origins. The sampling of the eels within Rivers Meuse and Berwijn, resulted in an insufficient number of suitable individuals. Hence eels from other locations had to be included. All the individuals from the different locations were handled in the same way, tagged and released in the River Berwijn, all at the same site, as mentioned above.

Table 4 summarizes the migration activity of silver eels from the different origins. Apparently, the translocated eels seem to be able to start their migration, and at least some of them succeed in completing their migration up to the River Meuse delta and/or the North Sea. Conspiciously, out of 6 eels sampled in the River Meuse not any seems to be successful.

Tabel 4. Migration activity of silver eels originating from different locations N % Origin succesful succesful

River Meuse 6 0 0

Canals 19 6 31,6

Berwijn 6 2 33,3

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3.3 Resident eels

The overall result of the downstream migration of the female silver eels shows that 13 tagged silver eels migrate downstream. This means that 18 individuals are not migrating over relative long distances. They are not detected at more than 1 detection station. These individuals have covered distances less then 22km. We suppose that these eels did not start a downstream migration towards the North Sea. 9 not migrating silver eels stayed in the River Berwijn, 9 left the River Berwijn into the River Meuse, but not being detected at any detection stations on the River Meuse.

Between November 2007 and April 2008 all 9 non migrants in the River Berwijn were searched for 5 times by manual tracking with a portable Nedap TRAIL station. During this tracking a 3km river strech was explored. This station has a measurement range of 10m, which ensures detection of the tagged individuals by walking along the river bank.

This tracking reveals that the non-migrants behave as resident after being released in the River Berwijn. From the point of release they move downstream to a site which seems to become their new home site. From this supposed new home site they perform small migrations. No upstream movement was observed, except for return to the home site.

Distances covered between release site and the new home site varied from 0 m to 650 m from their release site. Once the eel found a new home site, they were never registered more than 300 m from this site.

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4 Discussion and conclusions

From August 2007 till April 2008, 22 of the 31 (71%) transpondered silver eels were detected at least at one detection station. 13 of the eels (42%) were detected at two or more Nedap TRAIL detection stations. Eight migrated into the Haringvliet complex of the Meuse delta via the Bergsche Maas. Two individual eels of this group were able to reach the North Sea via the harbour of Rotterdam.

6 out of 8 individuals are lost in the River Meuse delta. It is not known whether these eels were caught by fishermen, died for other reasons (HPS or natural predation) or stopped their migration and settled in the river delta. Most probably fishing pressure has the largest impact, although we lack factual information on the catches. Winter et al. (2006) and Dekker (2000) also suggested fisheries as an important mortality factor. In the River Meuse Winter at al (2006) found a fisheries mortality of of at least 16% (reported catches) and estimated it to be 22-26%.

Eels alternate periods of migration with periods of inactivity. The duration of an inactive period differs highly between individuals. This variation in individual data recorded at the detection stations indicates that migrating silver eels show heterogeneity in migration behaviour and flexibility during downstream migration. This flexibility was also seen by Durif et al (2003) during a behavioural study, where eels stopped their migration for longer periods (up to one month) if environmental conditions were not favourable. Durif et al (2005) stated that the presence of dams could stop or delay the run. Therefor, an eel that has missed favourable environmental windows would probably stop silvering and revert back to the growth phase. Hain (1975) suggested that Anguilla rostrata makes several trial runs before finally leaving for the Sargasso Sea. Migratory characteristics weaken after each false start until the next migratory season. The high variability in age and length of female silver eels could be explained by their physiological and behaviourable flexibility.

There’s a tendency to higher success for largest individuals, although small as well as large individuals are successful. The number of fishes is unfortunately to small to do some reliable statistics.

The diurnal movements shows more detections during evening hours than during morning hours. The largest migration activity occurs between sunset and midnight, during the first dark hours of the night. This is also seen by Hammond et al. (2003) and Acou et al. (2008).

58% (n=18) of the tagged eels are behaving non migrating or resident. This shows that a considerable number of silver eels is not ready to migrate to sea, yet all tagged eels were silver at the time of tagging. Probably these eels have stopped silvering or even reversed their silver condition (Durif et al, 2005). More frequent tracking could reveal more interesting information on the behaviour of resident silver eels. It is advisable to repeat this experiment to find out whether this phenomenon and the proportion of non migrating eels is consistent over the years, and to find out if and when the resident eels start their downstream migration in the following year.

Studies on downstream migrating eels are mainly focused on the success of downstream migration in relation to seasonal, meteorological and hydrological parameters (Cullen &McCarthy, 2003, Jansen et al, 2007; Winter et al, 2006; Klein Breteler et al, 2007), but not taking into account the resident stay-behinds. These non migrating individuals are often classified as ‘lost’ or ‘delayed migrants’. The present study however shows that non-migrants are not lost, but simply not participating in the downstream migration.

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Taking this into account the telemetry data show an overall escapement of female silver eels in the River Meuse of 15% (2 out of 13 migrants) instead of 6,5% (2 out of 31 migrants and non-migrants). This shows the importance of the non-migrants in concluding about the success of downstream migration, especially in the process of implementation of the European Eel Regulation where specific benchmarks for silver eel escapement are defined. In studies evaluating the migration success of released silver eels, success rates could be under evaluated. We therefore recommend to incorporate protocols to evaluate the proportion of silver eels not starting migration within studies assessing migration success of silver eels. It is therefore advisable to repeat this experiment to find out whether this phenomenon and the proportion of non migrating eels is consistent over the years.

There’s also more information needed on why some tagged silver eels cease migration and whether they later resume migration and, for those eels apparently starting their migrations, on the causes of non-detections at downstream sites. Such information is needed to plan, execute and interpret meaningful studies of the escapement of tagged silver eel.

This study also demonstrates the difficulty in determining migrating phases in silver eels by external characteristics. The maturation stage (III, IV or V) does not seem to influence the success of the downstream migration, although the amount of eels is to small for statistical analysis.

Eels of all silvering stages (III, IV and V) can start their downstream migration, but of all stages eels stay behind and do not start their downstream migration. A maturation determination method excluding non migrants is preferable. Therefore additional research is necessary.

Wether the origin has an impact on migration activity and success of downstream migration could not be revealed due to the small numbers within individual batches.

Water discharge seems to influence downstream migration. Eels migrate more at discharges higher than the mean discharge (over the study period). This phenomenon is observed for the Meuse as well as for the Berwijn. Vøllestad et al (1986) also found that an increase in water flow tends to encourage eels to migrate. For the period August till October he could explain 91% of the total variation in the start of the yearly silver eel run by the water discharge. Although the importance of water flow is highlighted, many other environmental factors, such as temperature, wind and lunar periodicity influence the silver eel migration (Cullen & McCarthy, 2003).

At all detection stations without HPS we observe no delay of downstream migration. At the two HPS (Linne and Lith) however we see a clear delay of 18 hours to 1 month. This delay only noticed at HPS indicates a hesitation of eels to pass the turbines.The delay is much higher at Linne than at Lith. No explanation could be found for this large difference. Delay at HPS was also seen by Durif et al (2003), Jansen et al (2007). Also Winter et al (2006) found a clear difference in passage behaviour at hydropower stations, where 40% showed recurrence, indicating a hesitation to pass the turbines. The attraction of the HPS during downstream migration and the delay caused by this are factors which should therefore not be underestimated.

Also passing through a HPS is not without risk. One eel passing through the HPS in Linne presumably died after passing through the turbines. This shows that evaluating the impact of HPS on downstream migration on silver eel is important. Hadderingh and Bakker (1998) studied fish mortality at two hydropower stations on the rivers Meuse and Vecht in the Netherlands, where they found a 13.5% immediate mortality for only one hydroelectric power plant. Winter et al (2006) found a hydropower mortality in the River Meuse of eel of at least 9% and estimated it to be 16-26%.

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For the experiments eels of different origine were used. The (possible) influence of this translocation experiment is not examined. Focusing on the main objectives of this study, (1) assessing to what extent female eels released at 326km from the sea are able to reach the sea and through which pathway and (2) gaining insight in the effects of barriers on migratory activity, the possible effect of translocation in itself is of minor importance.

The (possible) influence of translocation should get more attention concerning the third objective (3) to gain insight in the behaviour of tagged non-migrant silver eels. 18 tagged eels did not migrate over relative long distances. 9 eels have left the Berwijn, but were not detected at the second detection station at less then 22km from the release site. We suppose that these eels did not start a downstream migration towards the North Sea. Possibly translocation could leed to confusion and drove these eels to uncontrolled swimming movements leaving the river Berwijn and entering the river Meuse looking for a new home site. These uncontrolled swimming movements looking for new home sites were also seen in chub after translocation (Buysse et al, 2006). The 9 eels not leaving the Berwijn were manually tracked. From the point of release they move downstream to a site which seems to become their new home site. No uncontrolled swimming movements were registered. The possible influence of translocation on these individuals is apparently low. This difference between individuals in reaction on translocation is also observed in chub, as well as the variation in high and low swimming activity (in native as well as translocated fish) (Coeck et al, 2001; Buysse et al, 2006).

More information is needed on why some tagged silver eels cease migration and whether they later resume migration and, for those eels apparently starting their migrations, on the causes of non-detections at downstream sites. Such information is needed to plan, execute and interpret meaningful studies of the escapement of tagged silver eel.

31 Downstream migration of female silver eel in the River Meuse (2007-2008) www.inbo.be

Appendix I: Fish inventory on the Grensmaas and the Berwijn (Van Thuyne et al., 1997, 2003, 2005a; Breine et al., 1998; Vrielinck et al., 2002; Van Gils et al. 2001; Buysse et al, 2008)

Species data (Grens) Meuse : Hisotical Grens Meuse : 1991-1992-1998 Grens Meuse : 2002 Grens Meuse : 2005 Berwijn : 2001 Species data (Grens) Meuse : Hisotical Grens Meuse : 1991-1992-1998 Grens Meuse : 2002 Grens Meuse 2005 Berwijn : 2001

1 Bleak x x - - X 26 European eel x x x X x

2 Atlantic salmon x - - - - 27 Ruffe x x x X -

3 European pearch x x x X - 28 Rainbow trout x x - - X

4 barbel x x x x X 29 Common rudd x x - X X

5 Brown trout x x - X X 30 European bullhead x x x X X

6 Brook lamprey x - - - - 31 Gudgeon x x x X X

7 Stone loach x - x X X 32 European river lamprey x - - - -

8 Bitterling x - - - - 33 Asp - - x - X

9 Roach x x x X X 34 European plaice x - - - -

10 Flounder x - - - - 35 Common dace x x - X X

11 Bream x x x X X 36 Common nace x x x - X

12 Brown bullhead x - - - - 37 Pike x x x X -

13 Three spined stickleback x x x - X 38 Zander x x x - X

14 Allis shad x - - - - 39 European smelt x x - - -

15 Common minnow x - x - X 40 European sea sturgeon x - - - -

16 Twaite shad x - - - - 41 Ten spined stickleback - x - - -

17 Largemouth bass x - - - - 42 Moderlieschen - x - - -

18 Spirlin x - - - - 43 Grayling x - - - x

19 Prussian carp - x - x - 44 Ide x x - x x

20 Houting x - - - - 45 Sea trout x x - - x

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21 Common carp x x - - - 46 Tench x x - X -

22 Spined loach x - - - - 47 Sea lamprey x x - - -

23 Silver bream x x x X - 48 Pumpkinseed - x - - -

24 Chub x x x X X 49 Smallmouth bass x - - - -

25 Burbot x - - - - Totaal 44 29 9 18 21

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Appendix II: Transponder implantation data of eel

Transponde Lengt Weigh Maturation Origing Release r h t Class Date Code 2400 75,3 941 IV Bernaux (spoorwegbrug) 5/06/2007 2403 78,2 1058,4 IV Bernaux (spoorwegbrug) 5/06/2007 2404 64,3 866,7 IV Bernaux (spoorwegbrug) 5/06/2007 2407 79,8 929,2 III Bernaux (rugby) 5/06/2007 2410 70,3 765,4 V Kanaal Bocholt-Herentals 17/10/2007 2411 71,7 648 V Kanaal Bocholt-Herentals 17/10/2007 2412 78,6 708,8 V Kanaal Bocholt-Herentals 17/10/2007 2414 83,4 1308,4 IV Bernaux (rugby) 5/06/2007 2416 78,9 580,6 V Kanaal Bocholt-Herentals 17/10/2007 2418 83 1215 IV Bernaux (rugby) 5/06/2007 2680 74 491 V Kanaal Bocholt-Herentals 17/10/2007 2682 75,3 818,4 V Kanaal Bocholt-Herentals 17/10/2007 2683 65,7 540,8 III Kanaal Bocholt-Herentals 17/10/2007 2690 89,6 1415,2 IV Kanaal Bocholt-Herentals 17/10/2007 2693 74 860,9 V Kanaal Brussel-Charleroi 26/10/2007 2701 66 667,2 III Kanaal Brussel-Charleroi 26/10/2007 2702 76,8 848,1 III Maas Lixhe 14/11/2007 2703 82,3 1197,5 IV Kanaal Bocholt-Herentals 17/10/2007 2705 80 877,7 III Maas Lixhe 14/11/2007 2710 71,6 705 V Kanaal Brussel-Charleroi 26/10/2007 2712 74,5 734 V Kanaal Brussel-Charleroi 26/10/2007 2713 72,4 933,6 IV Maas De Spaanjerd 23/11/2007 2717 77,2 811,8 III Kanaal Brussel-Charleroi 26/10/2007 2719 72,9 786,4 III Maas Lixhe 14/11/2007 2720 81,7 1111,7 IV Kanaal Brussel-Charleroi 26/10/2007 2721 66,3 643,9 III Kanaal Bocholt-Herentals 17/10/2007 2724 76,6 984,6 IV Kanaal Bocholt-Herentals 17/10/2007 2728 74,4 737,6 III Kanaal Bocholt-Herentals 17/10/2007 2731 87,4 1487,4 IV Kanaal Brussel-Charleroi 26/10/2007 2733 81,4 943,8 III Maas Lixhe 14/11/2007 2736 74 774,6 III Maas De Spaanjerd 23/11/2007

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actuele toestand. Rapporten van het Instituut voor Bosbouw en Wildbeheer – sectie visserij, 2002 (89). 271 p.

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37 Downstream migration of female silver eel in the River Meuse (2007-2008) www.inbo.be

List of figures

Figuur 1. Overview of the glass eel catches (1994-2006) at river Yser using the dipnet catches in the ship lock at Nieuwpoort (total year catches and maximum day catch per season) (in Belpaire et al, 2008)...... 6 Figuur 2. Variation in the number of ascending young yellow eels trapped at the fish trap of the Visé-Lixhe dam. Data from University of Liège (J.C. Philippart) in Philippart and Rimbaud (2005), Philippart (2006) and Philippart (pers. comm.) (in Bepaire et al, 2008). * Data incomplete, catches until 31/07/08...... 9 Figuur 3. View on the River Berwijn ...... 10 Figuur 4. Schematic view on the Nedap Trail system (Breukelaar et al, 1998)...... 11 Figuur 5. Nedap Trail transponders. The middle transponder (65mm, 25g, life span of 2 years) is used to tag the silver eels. The left transponder is 90mm, 40g (life span of 4 years), right transonder is 35mm, 12g (life span of 6 months) and is used in smolt research...... 11 Figuur 6. Network of detection stations in The Netherlands, belgium and Germany on the River Meuse and the River Rhine (situation April 2007) ...... 12 Figuur 7. View on the detection station at the River Berwijn. Left: one of the 3 antenna cables crossing the bottom of the river; right: Nedap Trail semi portable detection station where data are collected...... 14 Figuur 8. Origin of the tagged silver eels ...... 15 Figuur 9. Measuring length of pectoral fin and eye diameter to determine the silverig stage of the female eels...... 15 Figuur 10. Tagging procedure of the silver eel...... 16 Figuur 11. Length distribution of the tagged silver eels released in the River Berwijn...... 18 Figuur 12. Weight distribution of the tagged silver eels released in the River Berwijn...... 18 Figuur 13. Distribution of the silvering stage of the tagged silver eels...... 19 Figuur 14. Downstream migration routes of tagged silver eels in the River Meuse. At every detection station the number of detected eels is presented...... 20 Figuur 15 Downstream migration of a succesfull silver eel reaching the North Sea. Dates and distances are given. Notice the yellow belly of this succesfull female ‘silver’ eel. .. 21 Figuur 16. HPS in the River Meuse at Lnne. The HPS is situated in between the fish pass (left) and the weir (right)...... 21 Figuur 17. Log length and log weight distribution of the tagged silver eels in relation to the success of their downstream migration. (successful=detected at more than 1 detection stations; not successful=detected at less than 2 detection stations). .... 22 Figuur 18. Detections of silver eels in the River Meuse and Berwijn during downstream migration in relation to mean discharge (left panel: 1= River Berwijn; 2= Meuse at Itteren; 3= Meuse at Stevensweert; 4= Meuse at Linne, 5= Meuse at Sambeek; 6= Meuse at Bolgoij; 7= Meuse at Lith; 8= Meuse Bergsche Maas; 9= Oude Maas Spijkenisse and right panel: detail of River Berwijn) ...... 23 Figuur 19. Number of detections at all detections stations of downstream migrating eels in the rivers Berwijn and Meuse during the registration period (June 2007 – April 2008) presented in a 24 hour cycle...... 25

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Figuur 20. Delay (hours) at the detection stations along the downstream migration route in the River Meuse. Right picture shows the two choosen routes by downstream migrating eels approaching HPS site: HPS or weir...... 26 Figuur 21. Maturation stage of all tagged silver eels in relation to the success of their downstream migration...... 27

39 Downstream migration of female silver eel in the River Meuse (2007-2008) www.inbo.be

List of tables

Tabel 1. Locations of the Nedap Trail detection stations in The Netherlands, Belgium and Germany...... 13 Tabel 2. Origin, release date and number per location of released silver eels used for monitoring their downstream migration in the River Meuse ...... 16 Tabel 3. Migration time and speed (min-max-mean) of the tagged eels during downstream migration between two consecutive detection stations on the River Meuse ...... 24 Tabel 4. Migration activity of silver eels originating from different locations ...... 27

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