Rhine Silver Eel 2006

VisAdvies BV

Postadres Vondellaan 14 3521 GD Utrecht Nederland

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Rhine Silver Eel 2006

Project Titel: “Population study of female downstream migrating silver eel Anguilla anguilla in the Rhine system in 2006/2007”

Project: VA2006_62

31 January 2007

Author(s): F.T. Vriese, J.C.A. Merkx (VisAdvies) & A.W. Breukelaar (RWS-RIZA) Statuspage

Titel “Population study of female downstream migrating silver eel Anguilla anguilla in the Rhine system in 2006/2007”

Compiled by VisAdvies BV Vondellaan 14 3521 GD Utrecht The Netherlands Telephone 030-285 1066 E-mail [email protected] Homepage http://www.visadvies.nl Author(s) F.T. Vriese, J.C.A. Merkx (VisAdvies) & A.W. Breukelaar (RWS- RIZA) E-mail adres [email protected] ; [email protected] ; [email protected] Number of pages 31 Keywords Transponder, Silver eel, Rhine, population study Version Final Project number VA2006_62 Date 31 January 2007 In commission of Rheinfischereigenossenschaft, Petrusstr. 20, 53639 Königswinter.

Bibliographical reference :

Vriese, F.T , J.C.A. Merkx.(VisAdvies) & A.W. Breukelaar (RWS-RIZA), 2007. Population study of female downstream migrating silver eel Anguilla anguilla in the Rhine system in 2006/2007. VisAdvies BV, Utrecht. Project VA2006_62, 31 pages.

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Table of contents

1 Rhine Silver Eel project...... 1

2 Material and methods ...... 3 2.1 Telemetry (after Breukelaar et al. , 1998) ...... 3 2.2 Mark recapture ...... 7

3 Data and data analysis ...... 9 3.1 Telemetry...... 9 3.2 Mark recapture ...... 9

4 Results...... 11 4.1 Telemetry...... 11 4.1.1 Eel detection and migration activity...... 11 4.1.2 Migration routes of eel in the Netherlands, escapement to sea and losses in the river branches...... 15 4.1.3 Swimming speed during downstream migration ...... 22 4.2 Mark recapture ...... 23

5 Discussion and conclusions...... 27 5.1 Discussion ...... 27 5.2 Conclusions...... 29

6 References ...... 31

7 Annexes...... 1 7.1 Transponder implantation data of eel...... 1 7.2 Registrations of eel on the NEDAP TRAIL system®...... 1 7.3 Migration routes of eel in order of importance...... 1 7.4 Data of floy tagged eel...... 5 7.5 Telemetry data of eels transpondered in 2005 and detected in 2006...... 7

Rhine Silver Eel project

1 Rhine Silver Eel project

Strategic objective: to quantify the population size and mortality of downstream migrating silver eels in the Rhine system, to trace their migration pathways in 2006/2007.

Abstract: Transponders were surgically implanted in female silver eels > 50 cm from the rivers Mosel and Sieg (139 from the Mosel, 11 from the Sieg) in late summer-autumn 2006. Migration pathways at downstream movement and mortality (transponder signal loss) were traced along the downstream sections of the river Rhine in Germany and The Netherlands.

Floy tags were applied to female (> 50 cm) silver eels routinely caught in the Mosel. These marked eels were released in the Rhine in Cologne. Catches of silver eels made by an anchored stow net near Grietherbusch/Rees (DE) and commercial catches of silver eels in the most downstream sections of the Rhine (NL) were checked for this mark. A mark-recapture estimate of the total female silver eel (> 50 cm) run in 2006/2007 is made for the Rhine.

Objectives of the project:

1) Migration pathways and total mortality (TELEMETRY), a).To determine the relative importance of different migration pathways for female silver eels in the lower Rhine system in 2006/2007. b). To quantify the total silver eel mortality in the lower Rhine in 2006/2007 2) Total- and subpopulation size (MARK-RECAPTURE), a). To quantify the total silver eel run in the Lower Rhine in 2006/2007.

Table 1 Participants in the project No. Participant Acronym 1 Universität zu Köln, Zoologisches Institut, Allg. Ökologie und Uni-Köln Limnologie, 50923 Köln, Germany. Coordinator: Jost Borcherding, Aussenstelle des Zoologischen Instituts, Ökologische Forschungsstation, 46459 Rees- Grietherbusch, Germany 2 VisAdvies BV, Vondellaan 14, 3521 GD Utrecht, Netherlands. VisAdvies Coordinator: Tim Vriese (from August 2005) 3 Rijksinstituut voor Integraal Zoetwaterbeheer en RIZA Afvalwaterbehandeling, Postbus 17, 8200 AA Lelystad, Netherlands. Coordinator: André Breukelaar 4 Rheinfischereigenossenschaft im Lande Nordrhein-Westfalen, RFG Römerhofweg 12, 50374 Erftstadt. Coordinator: Stefan Staas 5 Landesanstalt für Ökologie, Bodenordnung und Forsten NRW, Postfach 10 10 52, 45610 Recklinghausen, Germany. LÖBF Coordinator: Dr. Detlev Ingendahl, LÖBF NRW Kernteam Wanderfischprogramm, Heinsbergerstr. 53, 57399 Kirchhundem-Albaum 6 Struktur und Genehmigungsbehörde Nord, Rheinland-Pfalz SGN Coordinator: L. Jörgensen

Rhine Silver Eel project

No. Participant Acronym 7 Rheinischer Fischereiverband Nordrhein e.V., Webersbitze RFV 20, 53804 Much Coordinator: Armin Nemitz, c/o Amt für Agrarordnung Siegburg, Frankfurter Straße 86-88, 53721 Siegburg. 8 Combinatie van Beroepsvissers, Postbus 72, 2280 AB CVB Rijswijk, Netherlands. Coordinator: A. Heinen

Material and methods

2 Material and methods

2.1 Telemetry (after Breukelaar et al. , 1998)

In this project the NEDAP TRAIL System® has been used. This method is based on the inductive coupling between an antenna loop and ferrite rod antenna within transponders. The identification system can be used in rivers, canals and estuaries. For the transmission mode a narrow Low Frequency (LF) band (33.25 kHz), low bit rate, approach was chosen. This combines a high sensitive receiver with a low chance of interference from radio signals and other man made disturbances. The choice of the frequency is based on the consideration that coupling between the tag, (in this study a transponder), and the antenna on the river bottom is a loosely coupled transformer. This means that no radio signals are emitted and thus no interference is caused to radio users. The system consists of a detection station and implantable transponders. The network of detection stations is shown in figure 1 (situation on 1 May 2006) and the station names are given in table 1 (next page).

Figure 1. Network of detection stations (situation 1 May 2006)

Material and methods

Table 2 Names and locations of the detection stations in Rhine and Meuse Station nr. River (branche)/lake Location Start date 2 IJssel Kampen 01-12-96 5 Spui Zuidland 01-12-96 6 Dordtsche Kil ‘s-Gravendeel 01-12-96 7 De Noord Alblasserdam 01-12-96 8 Beneden Merwede Boven Hardinxveld 01-12-96 9 Lek Nieuwegein 01-12-96 10 Waal Vuren 01-12-96 11 Bergsche Maas Capelse Veer (Dussen) 01-12-96 12 Grensmaas Stevensweert 01-12-96 13 Rijn Xanten (BRD) 22-05-97 14 Oude Maas Spijkenisse 03-11-97 15 Roer St. Odiliënberg 19-06-98 16 Sieg Menden (BRD) 01-07-98 19 IJsselmeer Den Oever 24-02-00 18 IJsselmeer Kornwerderzand 24-02-00 25 Maas Sambeek 21-09-00 23 Maas Balgoij 21-09-00 26 Maas Afferden 21-09-00 24 Maas Sambeek below weir 01-10-01 22 Haringvliet Stellendam zuid 01-11-01 21 Haringvliet Stellendam noord 03-12-01 28 Maas Borgharen 10-10-01 27 Maas Itteren 01-05-03 K4 Maas Linne wkc 04-09-02 R2 Maas Linne vistrap 09-09-03 K3 Maas Linne dorp 27-08-02 K2 Maas Alphen wkc 04-09-02 R1 Maas Alphen vistrap 09-09-03 K1 Maas Lith dorp 27-08-02 31 Nederrijn Arnhem 24-02-05 30 Nederrijn Maurik 16-03-05 29 Nederrijn Hagestein 17-03-05

Each detection station incorporates an antenna loop, consisting of three parallel cables (distance 10 m) crossing the river or canal bottom, an antenna connection box, a receiver and a transmitter. In the antenna connection box the antenna loop is tuned.

Every 4 seconds the transmitter generates an interrogating signal. This signal triggers a transponder passing the antenna loop. The transponder responds by transmitting its unique signal which is demodulated by the receiver, decoded, and recorded by a microprocessor unit connected with a telephone modem. Transmission of a transponder signal takes two periods of 8 seconds, separated by 8 seconds of silence. In each 8 seconds transmission period the unique code number is transmitted 32 times. After the whole transmission period (24 seconds) the transponder is mute for two minutes to prevent the batteries from running down in the case where a tagged fish stops above an antenna.

The implantable transponder consist of a biocompatible glass tube with a diameter of 15 mm and a length of 65 mm. Inside is a ferrite rod with a length of 25 mm, a custom made integrated electronic circuit, and one battery with diameter of 12 mm

Material and methods

and a length of 10 mm, ensuring a lifetime of at least one year. Weight of a transponder is approx. 17 g (in water).

Field tests showed that the detection system still functions well with a maximum antenna length of 550 m, a water depth of 15 m and a passing speed of 5-6 m s -1 at maximum. Effects of ship engine noise are negligible. When the transponder is close to the hull of a vessel (less than 10 cm) the signal detection is interrupted. Maximum tolerable conductivity of the river water was not tested. However, calculations showed that a conductivity < 6,000 µS cm -1 does not affect transmission when the distance between the antenna and the transponder is < 15 m.

The transponders are implanted in the abdominal cavity of the animals (Lucas, 1989; Moore et al ., 1990; McKinley et al., 1994). Rainbow trout ( Oncorhynchus mykiss (Walbaum)), kept in captivity, did not show any significant difference in growth and behaviour during a half year period after implantation of 25 fishes with dummy transponders (Vriese, 1995). Histological analyses showed some encapsulation of the transponder in only two of them, most probably the effect of insufficient cleaning. None of the implanted fishes died as result of implantation.

In several other studies transponders have been successfully implanted in eel (Baras & Jeandrain, 1998; Bruijs et al ., 2003). In this project the same surgical implanting procedure was used as in aforementioned studies, with several slight changes. As an anaesthetic Benzocain (80 mg/l) was used. The incision was closed with three sutures (Vicryl 3-0, FS-1 24,0 mm 3/8c). In total 150 silver eels (139 from the Mosel and 11 from the Sieg) were to be implanted with transponders at the boathouse in Cologne or at the Sieg. On the following days eel were implanted: 16 August (Sieg), 17 August, 7 September, 13 September (Sieg), 14 September, 23 November and 24 November.

The external silvering status of each implanted eel was determined by measuring the eye-diameter (vertically and horizontally - C), length (A), bodyweight and pectoral fin length (B) and analysed using Caroline Durif’s algorithm. See photographs for details how the measurements were taken (figures 2-13). The eels were also checked for their silvering status (silver, intermediate or yellow) according to the criteria used at the mark-recapture part of the project. Only eels classified as “silver” and “intermediate” (not “yellow”) were used for implanting transponders. After the surgical procedures were finished and the measurements were taken, the eels were released individually into the river.

Material and methods

Figure 2. Operating table Figure 3. Surgical instruments

Figure 4. Location of the incision; Figure 5. Making the incision ventral, equidistant from head and anus

Figure 6. Opening of the perito- Figure 7. A Gigli probe is used neum to protect the internal organs

Material and methods

Figure 8. The transponder is Figure 9. Incision after insertion inserted of the transponder

Figure 10. A sterile drape is used Figure 11. The first suture is applied

Figure 12. Three sutures close Figure 13. Measurement of the the wound pectoral fin

2.2 Mark recapture

The downstream migrating silver eels in the Mosel are caught by SGN annually (3,000-6,000 kg/year) upstream from the hydropower stations and trucked and released once a week downstream of these obstructions in the migration routes or

Material and methods

directly into the River Rhine nearby the confluence of Rhine and Mosel. The total silver eel (> 50 cm) run in the Lower Rhine is quantified by mark-recapture procedures, using the eels caught at the Mosel.

The silver eels > 50 cm were to be marked at the Mosel in alternating weeks spread through August – November, starting in week 31. The target was 3.000 eels (>50 cm) to be marked and released. The mark is a floy tag, applied with a injector. The location of the batch mark is at the front of the dorsal fin. Eels were classified as silver, intermediate or yellow on external appearance. Only silver eels and intermediate eels were marked. Size was estimated by eye as >70, 50-70 and < 50 cm. Eels < 50 cm were not marked. All handling was done without anaesthetization.

Figuur

The batch marking of eel done by the German partners in the project took place on the following dates: 10, 17 and 31 August, 7, 13 and 27 September, 18 and 26 October and 22 November 2006.

Recapturing of eels was done with fykenets by commercial fishermen in the most downstream parts of the Rhine branches (the Netherlands). Recapture locations in the Netherlands were selected in downstream sections of the river Waal (Nieuwe Merwede and Beneden Merwede) (Fisherman Klop). In total 9 visits were made: 19 September (Nieuwe Merwede), 19 September (Beneden Merwede), 27 September (Beneden Merwede), 27 September (Nieuwe Merwede), 11 Oktober (Beneden Merwede), 12 October (Beneden Merwede), 3 November (Beneden Merwede), 9 November (Beneden Merwede) and 19 December (Beneden Merwede). Eels were checked for Floy tags and a sub sample of eels was measured.

Data and data analysis

3 Data and data analysis

3.1 Telemetry

On the following days eel were implanted (either at the boathouse in Cologne or in case of eel from the Sieg at Buschdorf am Rhein): 16 August (n = 6 Sieg), 17 August (n = 26), 7 September (n = 30), 13 September (n= 5 Sieg), 14 September (n = 27), 23 November (n = 43) and 24 November (n=13). In total, 150 eels were implanted (139 from the Mosel and 11 from the Sieg, the latter due to low catches). In Annex 7.1 the complete transponder implantation data on eel are given. Of the 150 eels tagged 16 individuals were classified as “intermediate” (11%) and 139 were classified as “silver” (89%).

On a regular basis RIZA delivered spreadsheets with information on the detections of eel on the network of detection stations (see figure 1). The spreadsheets were ‘cleaned’ of false detections, detections of other fish species, system checks and from routine checks of transponder signals. Data up to the end of January 2007 were used to write this report. From the collective data each individual eel was followed on its journey to the sea or to the detection station it was detected for the last time. In several cases it became clear that eels were not detected on one or more stations during their migration. The majority of these events were related to stations being out of order.

Transmission time of the unique transponder signal was two times 8 seconds, with a pause of 8 seconds in between, followed by a mute period of two minutes to prevent the batteries from running down in case when a tagged fish stops in the reach of an antenna. In case of a detection series at a single detection station with intervals between the detections of three minutes or less, all detections were joined together and considered to be one registration. To describe migration routes of the fishes tagged, the assumption was made that in a sequence of registrations swimming direction continuously turns. Such assumption was needed because with the NEDAP TRAIL System® swimming direction can not be detected. In an earlier study 91,5% of the animals reached next detection station as was determined from the sequence of registrations on a particular detection station (Bij de Vaate & Breukelaar, 2001).

Based on the detection data several aspects like eel activity, relative importance of different routes, influencing factors on migration direction, swimming speed of migration, mortality and escapement of eel were analysed.

3.2 Mark recapture

The tagging of eel by the German partners in the project took place on the following days: 10 August (n = 295), 17 August (n = 257), 31 August (n = 198), 7 September (n = 619), 13 September (n = 188), 27 September (n = 460), 18 October (n = 138), 26 October (n = 302) and 22 November (n = 170). In total 2.627 individuals were tagged. Table 3 shows the composition of the Floy tagged eels in 2006 (data from Stefan Staas, RFG).

Data and data analysis

Table 3 Floy tagging statistics 2006

total of total of > 70 cm 50 - 70 cm < 50 cm Floy tagged eels counted eels total ( marked ) 2.627 879 1.748 silver 1.560 750 810 15 1.575 intermediate 1.067 129 938 131 1.198 yellow - 0 67 119 186 sum 879 1.815 265 2.959

In table 4 the complete statistics of the recapture samples, in terms of the proportion of silver and intermediate eels in the different size classes, are given. In total 3.091 eels were checked for batch marks of which 275 were intermediate 50-70 cm, 208 were intermediate >70 cm, 1.954 were silver 50-70 cm and 654 were silver > 70 cm. The amount of checked eel is almost the target set in 2006 (3.600 eels), although not as many visits (planned were 12) could be made to fisherman Klop due to planning difficulties (on the arranged day no catch etc.).

Table 4 Composition of recapture samples taken in 2006 in the Waal Total weight Sample nr Intermediate 50-70 Intermediate >70 Silver eel 50-70 Silver eel >70 (kg) 1 74 15 377 5 195 2 12 48 315 24 183 3 23 51 239 101 246 4 32 17 142 55 147 5 44 0 219 132 246 6 30 20 193 105 216 7 14 7 117 63 124 8 16 25 135 63 150 9 29 24 216 106 217 Total 275 208 1.954 654 1724 Calculated 204 160 1.485 469 Measured 71 48 469 185 Grand 3.091

total

Results

4 Results

4.1 Telemetry

4.1.1 Eel detection and migration activity In total 150 eels were implanted with a transponder. Of these 150 implanted eels 71 individuals were detected on the NEDAP TRAIL system® with in total 170 different registrations. The detected amount up till the end of January 2007 (47%) is slightly lower than in 2004 and 2005, 55% and 53% respectively. After the 9 th of January until the end of that month no eels were detected on the system, possibly because the discharge was rather low in that period. Table 5 gives the statistics on the registrations of the eels tagged in 2006. Of the detected group 10% were judged to be intermediate at tagging and 90% were silver.

Table 5 Registration statistics of tagged eels Tagging date Tagging location Tagged (n) Detected eels Detected (%) 16-aug Siegburg 6 5 83% 17-aug Cologne 26 10 38% 7-sep Cologne 30 14 47% 13-sep Siegburg 5 2 40% 14-sep Cologne 27 8 30% 23-nov Cologne 43 26 60% 24-nov Cologne 13 6 46% 150 71 47%

In total 11 eels were tagged and released in the Sieg. As the Sieg has its own detection station (near Menden), the start of their downstream migration on the Rhine can be detected. Out of the 11 eels, 10 eels left the Sieg successfully (4 in August, 2 in September and 4 in November 2006). Of these 10 eels, 7 were detected in the Netherlands.

It is generally assumed that the time of detection of an eel on a station is indicative for it’s migration activity. It is also assumed that eels migrate preferably during the night. This was not supported by our findings in the years 2004 and 2005 of the experiment, as can be seen in the figures 2 and 3 (on the next page). In 2004, of the 212 detections, 112 were during night time (between 6 pm until 6 am) and 100 during day time (between 6 am until 6 PM). In 2005 these figures were 93 detections (night time) and 89 detections (day time). From the 170 detections of tagged eels in 2006 however, 103 were during night time and 67 were during day time, which suggests a preference for migrating during nocturnal hours. Probably, the exact conditions (e.g. discharge, water clarity, light conditions) during downstream migration determine when eels are most active. In figure 4 (on the next page) the time of detections of 2006 eels are displayed. In figure 5 the percentage of detections in the course of 24 hours are given; the percentage of detections is clearly rising during afternoon hours.

Results

time of detection

0:00

18:00

h 12:00

6:00

0:00 0 50 100 150 200 250

number of detections

Figure 2. Time of detection of eels on detection stations of 2004 eels (n=212 detections; 112 during night time and 100 during day time)

time of detection

0:00

18:00

h 12:00

6:00

0:00 0 20 40 60 80 100 120 140 160 180 200 number of detections

Figure 3. Time of detection of eels on detection stations of 2005 eels (n=182 detections; 93 during night time and 89 during day time)

Results

time of detection

0:00

18:00

h 12:00

6:00

0:00 0 20 40 60 80 100 120 140 160 180 number of detections

Figure 4. Time of detection of eels on detection stations of 2006 eels (n=170 detections; 103 during night time and 67 during day time)

6 % of detections total

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time (hours)

Figure 5. Percentage of detections during 24 hours.

There seems to be a tendency that migration activity is more intense in the days around new moon and during periods with a higher discharge (near Lobith). This can be seen in figures 6, 7 and 8 of the years 2004, 2005 and 2006 respectively, although the effect is not very pronounced.

Results

40 6000

35 5000 30 4000 25 20 3000

15 2000

m3/s discharge 10

Number of detections 1000 5 0 0

1-9-04 1-1-05

1-10-04 1-11-04 1-12-04

number of detections new moon m3/s Figure 6. Migration activity (nr. of detections) through time in 2004

Figure 7. Migration activity (nr. of detections) through time in 2005

4000 14 3500 12 3000 10 2500 8 2000 6

number 1500 4 1000 2 500 Lobith discharge 0 0 1-aug 15-okt 30-okt 14-nov 29-nov 14-dec 29-dec 16-aug 31-aug 15-sep 30-sep

number of detections new moon m3/s

Figure 8. Migration activity (nr. of detections) through time in 2006

Results

4.1.2 Migration routes of eel in the Netherlands, escapement to sea and losses in the river branches The different migration routes of eel are given in Annex 7.3 in the form of a migration map. In the following tables the individual migration routes are given, sorted by date of implantation. Some remarks regarding the information in the tables. When a transponder number is highlighted in red it is believed that the particular eel succeeded in reaching the North Sea.

Up till now (in this year of the experiment) no eel has entered the North Sea by way of Lake IJsselmeer, although at least 6 eels migrated via the IJssel. One eel migrated through the Haringvliet and was detected on the stations Haringvliet Stellendam Noord (station no. 21) and a day later on Haringvliet Stellendam Zuid (station no. 22) and succeeded in reaching the North Sea. Probably, the seaward door at station 21 was closed and the eel had to swim to the most southern part of the dam, where the doors are open more often, to escape to sea. The actual reaching of the North Sea of the other eels (transponder no. highlighted in red) can not be confirmed by detection of the eel on the station as there is no detection station at the only open exit to the sea (Nieuwe Waterweg near Rotterdam).

Between brackets in the table the number of days is mentioned, either between release in Germany and the detection on the station River Rhine Xanten or between stations in river branches in the Netherlands. When a detection station in the table is in italics it means that there has been no actual detection of the eel on the station but that the eel must have passed the station because it was detected on the following station. When there are more possible migration routes in such case the most likely route is mentioned in the table.

Table 6 Tagging date: 16 August 2006 in Siegburg (tagged=6, detected=5)

Days between Transponder Migration route release and last nr. registration 2083 Sieg →Sieg Menden (13 days) 13 2085 Sieg → Sieg Menden (12 days)→ River Rhine- 40 Cologne →River Rhine Xanten →Waal (28 days) 2097 Sieg → Sieg Menden (13 days)→ River Rhine- 24 Cologne →River Rhine Xanten →Waal (4 days) →de Noord(8 days) →North Sea 2098 Sieg → Sieg Menden (7 days)→ River Rhine-Cologne →River 85 Rhine Xanten →Waal →Dortsche Kil(34 days)→ Dortsche Kil(43 days) →Oude Maas (1 day) →North Sea 2123 Sieg → Sieg Menden → River Rhine-Cologne →River Rhine 101 Xanten(101)

Table 7 Tagging date: 17 August 2006 in Cologne (tagged=26, detected=10)

Days between Transponder Migration route release and last nr. registration 2082 River Rhine-Cologne →River Rhine-Xanten (101 days) →Waal 103 (1 day) 2088 River Rhine-Cologne →River Rhine-Xanten (144 days) 144

Results

Days between Transponder Migration route release and last nr. registration 2093 River Rhine-Cologne →River Rhine-Xanten →Nederrijn(36 38 days) →Lek (2 days) →North Sea 2095 River Rhine-Cologne →River Rhine-Xanten →Waal (51 77 days) →Beneden Merwede →Dordtsche Kil (1 day) →Spui (10 days) →Oude Maas (14 days) →North Sea 2096 River Rhine-Cologne →River Rhine-Xanten (91 days) →Waal 117 (1 day) →Beneden Merwede →De Noord (25 days) →North Sea 2127 River Rhine-Cologne →River Rhine-Xanten →Waal (11 12 day) →Beneden Merwede →De Noord (1 day) →North Sea 2128 River Rhine-Cologne →River Rhine-Xanten →IJssel (54 days) 54 2130 River Rhine-Cologne →River Rhine-Xanten (114 days) 114 2131 River Rhine-Cologne →River Rhine-Xanten (101 days) →Waal 109 (5 days) →Dordtsche Kil (0 days) →Waal (2 days) →Dordtsche Kil (1 day →Oude Maas (0 days) →North Sea 2132 River Rhine-Cologne →River Rhine-Xanten →Waal (4 days) 4

Table 8 Tagging date: 7 september 2006 in Cologne (tagged=30, detected=14)

Days between Transponder Migration route release and last nr. registration 2072 River Rhine-Cologne →River Rhine-Xanten →Waal (5 days) 5 2073 River Rhine-Cologne →River Rhine-Xanten →Nederrijn Arnhem 40 (37 days) → Nederrijn Maurik (3 days) → Nederrijn Hagestein (0 days) → Lek (0 days) →North Sea 2074 River Rhine-Cologne →River Rhine-Xanten →Waal (11 days) 39 →Dortsche Kil (1 day) → Dortsche Kil (23 days) →Haringvliet Stellendam Noord (3 days) → Haringvliet Stellendam Zuid (1 day) →North Sea 2171 River Rhine-Cologne →River Rhine-Xanten →Waal (5 6 days) →Beneden Merwede →De Noord (1 day) →North Sea 2173 River Rhine-Cologne →River Rhine-Xanten →Waal (32 62 days) →Beneden Merwede →De Noord (30 day) →North Sea 2176 River Rhine-Cologne →River Rhine-Xanten (56 days) →IJssel 61 (5 days) 2181 River Rhine-Cologne →River Rhine-Xanten (79 days) →Waal 92 (13 days) 2199 River Rhine-Cologne →River Rhine-Xanten →Waal (10 days) 10 2205 River Rhine-Cologne →River Rhine-Xanten →Waal (7 days) → 7 Beneden Merwede →De Noord (0 days) →North Sea 2206 River Rhine-Cologne →River Rhine-Xanten →Waal → 18 Beneden Merwede → De Noord (18 days) →Waal (0 days) 2207 River Rhine-Cologne →River Rhine-Xanten (70) →Waal (1 123 day) →Waal (53 days) 2210 River Rhine-Cologne →River Rhine-Xanten →Waal (40 47 days) →Beneden Merwede →De Noord (7 day) →North Sea 2214 River Rhine-Cologne →River Rhine-Xanten →IJssel (96 days) 96 2217 River Rhine-Cologne →River Rhine-Xanten (62 days) →IJssel 65 (3 days)

Results

Table 9 Tagging date: 13 september 2006 in Siegburg (tagged=5, detected=2)

Days between Transponder Migration route release and last nr. registration 2078 Sieg →Sieg Menden → River Rhine-Cologne →River Rhine- 57 Xanten →Waal (30 days) →Dortsche Kil (1 day) →Oude Maas (26 days) →North Sea 2188 Sieg →Sieg Menden → River Rhine-Cologne →River Rhine- 55 Xanten

Table 10 Tagging date: 14 september 2006 in Cologne (tagged=27, detected=8)

Days between Transponder Migration route release and last nr. registration 2145 River Rhine-Cologne →River Rhine-Xanten →IJssel (25 25 days) 2152 River Rhine-Cologne →River Rhine-Xanten →Waal (64 64 days) 2156 River Rhine-Cologne →River Rhine-Xanten →Nederrijn (22 23 days) →Lek (1 day) 2158 River Rhine-Cologne →River Rhine-Xanten →Waal (18 18 days) 2159 River Rhine-Cologne →River Rhine-Xanten →IJssel (25 25 days) 2185 River Rhine-Cologne →River Rhine-Xanten (72 days) →Waal 88 (16 days) 2196 River Rhine-Cologne →River Rhine-Xanten (57 days) 57 2197 River Rhine-Cologne →River Rhine-Xanten (72 days) →Waal 81 (9 days)

Table 11 Tagging date: 23 november 2006 in Cologne (tagged=43, detected=26)

Days between Transponder Migration route release and last nr. registration 2142 River Rhine-Cologne →River Rhine-Xanten (2 days) →Waal 16 (14 days) 2148 River Rhine-Cologne →River Rhine-Xanten (2 days) →Waal 3 (1 day) 2160 River Rhine-Cologne →River Rhine-Xanten (5 days) →Waal 7 (2 days) 2163 River Rhine-Cologne →River Rhine-Xanten (31 days) →Waal 42 (11 days) 2168 River Rhine-Cologne →River Rhine-Xanten (2 days) 2 2221 River Rhine-Cologne →River Rhine-Xanten (1 day) →Waal 4 (2 days) →Dordtsche Kil (2 days) →Oude Maas (0 days) →North Sea 2223 River Rhine-Cologne →River Rhine-Xanten (4 days) →Waal 17 (13 days) 2226 River Rhine-Cologne →River Rhine-Xanten (7 days) 7 2227 River Rhine-Cologne →River Rhine-Xanten (13 days) →Waal 22 (9 days)

Results

Days between Transponder Migration route release and last nr. registration 2232 River Rhine-Cologne →River Rhine-Xanten (5 days) →Waal 21 (16 days) →Dordtsche Kil (0 days) 2237 River Rhine-Cologne →River Rhine-Xanten (1 day) →Waal 12 (11 days) 2239 River Rhine-Cologne →River Rhine-Xanten (5 days) →Waal 21 (16 days) 2240 River Rhine-Cologne →River Rhine-Xanten (1 day) →Waal 19 (18 days) 2244 River Rhine-Cologne →River Rhine-Xanten (12 days) 12 2245 River Rhine-Cologne →River Rhine-Xanten (38 days) 38 2246 River Rhine-Cologne →River Rhine-Xanten (15 days) 15 2249 River Rhine-Cologne →River Rhine-Xanten (4 days) →Waal 17 (13 days) 2252 River Rhine-Cologne →River Rhine-Xanten (3 days) →Waal 18 (7 days) →Dordtsche Kil (of Beneden Merwede of Spui) →Oude Maas (11 days) →North Sea 2253 River Rhine-Cologne →River Rhine-Xanten (4 days) →Waal 15 (11 days) 2259 River Rhine-Cologne →River Rhine-Xanten (3 days) →Waal 22 (19 days) 2262 River Rhine-Cologne →River Rhine-Xanten (1 day) 1 2263 River Rhine-Cologne →River Rhine-Xanten (1 day) →Waal 17 (16 days) 2264 River Rhine-Cologne →River Rhine-Xanten (29 days) 29 2265 River Rhine-Cologne →River Rhine-Xanten (16 days) 16 2266 River Rhine-Cologne →River Rhine-Xanten (3 days) →Waal 20 (17 days) 2267 River Rhine-Cologne →River Rhine-Xanten (1 day) →Waal 4 (1 day) →Dordtsche Kil (1 day) →Oude Maas (1 day) →North Sea

Table 12 Tagging date: 24 november 2006 in Cologne (tagged=13, detected=6)

Days between Transponder Migration route release and last nr. registration 2228 River Rhine-Cologne →River Rhine-Xanten (17 days) 17 2248 River Rhine-Cologne →River Rhine-Xanten (2 days) →Waal 29 (27 days) 2255 River Rhine-Cologne →River Rhine-Xanten (1 day) →Waal 33 (32 days) 2268 River Rhine-Cologne →River Rhine-Xanten (35 days) 35 2269 River Rhine-Cologne →River Rhine-Xanten (6 days) 6 2277 River Rhine-Cologne →River Rhine-Xanten (1 day) →Waal 21 (20 days)

As can be concluded from the tables above, 23 times an eel was not detected on the station River Rhine Xanten. At the station Beneden Merwede detection failure was 8 times (probably 9, taken into account the misdetection of transponder no. 2252). The tagging of eel was discontinued after 13 September 2006, as it became clear that

Results

station River Rhine Xanten was not working properly. When the necessary repairs had been done, the tagging continued on the 23 rd of November. After that date, there were no malfunctions in the NEDAP system and the picture of the migration of eel is believed to be accurate.

Of the 71 eels detected, 18 eels probably have reached the North Sea, of which one by way of the Haringvliet. Of the other 17 escapement is not totally certain, as they could have been caught by professional fishermen downstream of last detection stations. However, none of these transponders numbers were returned, although there were some recaptures in the area. The escapement of eel based on these numbers is, 25% ((18/71)*100). This is in close accordance with the results of the experiments in 2004 and 2005 (escapement respectively 32% and 27%). The slightly lower number in 2006 might be related to the cessation of the transpondering activities (malfunctioning of station River Rhine Xanten), as a result of which quite large numbers of silver eels were tagged late in the migration season (end of November). It is likely that several eels are still present in the higher parts of the river system.

Table 13 gives an overview of the order of importance of the different migration routes that eels have taken in the Rhine system and where eels have been lost in the various branches of the river. Although not that massively chosen as in the years before, the most important route to reach the sea is: River Rhine Cologne → River Rhine Xanten → Waal → Beneden Merwede → De Noord → North Sea . Of the 18 eels that reached the sea, 6 eels followed this migration route. The next most important route to the sea is: River Rhine Cologne → River Rhine Xanten → Nederrijn → Lek → North Sea. In total, 3 eels followed this route, which is higher than in the years before. The explanation is that the during this migration season the discharge of the Rhine was relatively high, compared to 2004 and 2005, with several peaks in discharge. The amount of water that flows through the Nederrijn/Lek is also higher in this situation (the same goes for the IJssel). As a consequence these river branches are more attractive as migration routes. Remarkable is that one of these 3 eels migrated downstream using all the three fish passages that are present at the sluice complexes in this river stretch. The migration routes of the other eels that have reached the sea are more or less a variation on a theme (the most important migration route) in which eels take different branches in the Benedenrivieren area ( Dordtse Kil, Oude Maas, Spui). Partly, this is caused by changing river current direction as a consequence of the tide. The eel with transponder number 2131 is clearly an example of this ( Waal → Dordtse Kil → Waal → Dordtse Kil → Oude Maas etc).

Table 13 Migration routes in order of importance

Days between Transponder Route no. Migration route release and last nr. registration 1 River Rhine-Cologne →River Rhine- 2082 103 Xanten→Waal 2132 4 2072 5 2181 92 2199 10 2207 123 2152 64

Results

Days between Transponder Route no. Migration route release and last nr. registration 2158 25 2185 88 2197 81 2142 16 2148 3 2160 7 2163 42 2223 17 2227 22 2237 12 2239 21 2240 19 2249 17 2253 15 2259 22 2263 17 2266 20 2248 29 2255 33 2277 21 2 River Rhine-Cologne → River Rhine-Xanten 2088 144 2130 114 2196 57 2168 2 2226 7 2244 12 2245 38 2246 15 2262 1 2264 29 2265 16 2228 17 2268 35 2269 6 3 River Rhine-Cologne → River Rhine-Xanten 2096 117 → Waal → Beneden Merwede → De Noord 2127 12 → North Sea 2171 6 2173 62 2210 47 2205 7 4 River Rhine-Cologne → River Rhine-Xanten 2128 54 → IJssel 2176 61 2214 96 2217 65 2145 25 2159 25 5 River Rhine-Cologne → River Rhine-Xanten 2093 38 →Nederrijn → Lek →Northsea 2073 40 2156 23 6 River Rhine-Cologne → River Rhine-Xanten 2221 4 → Waal → Dordtsche Kil → Oude Maas → 2267 4 North Sea 7 River Rhine-Cologne → River Rhine-Xanten 2131 109 → Waal → Dordtsche Kil → Waal → Dordtsche Kil → Oude Maas → North Sea 8 River Rhine-Cologne → River Rhine-Xanten 2074 39 → Waal → Dordtsche Kil → Dordtsche Kil → Haringvliet Stellendam noord →Haringvliet Stellendam zuid → North Sea

Results

Days between Transponder Route no. Migration route release and last nr. registration 9 River Rhine-Cologne → River Rhine-Xanten 2206 18 → Waal →de Noord → Waal 10 River Rhine-Cologne → River Rhine-Xanten 2095 77 → Waal →Beneden Merwede → Dordtsche Kil → Spui →Oude Maas → North Sea 11 River Rhine-Cologne → River Rhine-Xanten 2232 21 → Waal → Dordtsche Kil 12 River Rhine-Cologne → River Rhine-Xanten 2252 18 → Waal → Dordtsche Kil (of Beneden Merwede of Spui) → Oude Maas (11 days) → North Sea 13 Sieg →Sieg Menden → River Rhine- 2078 57 Cologne → River Rhine-Xanten → Waal → 2098 85 Dortsche Kil → Oude Maas → North Sea 14 Sieg → Sieg Menden → River Rhine- 2123 101 Cologne → River Rhine-Xanten 2188 55 15 Sieg → Sieg Menden → River Rhine- 2085 40 Cologne → River Rhine Xanten → Waal 16 Sieg → Sieg Menden → River Rhine- 2097 24 Cologne → River Rhine Xanten → Waal → de Noord → North Sea 17 Sieg → Sieg Menden 2083 13

Up till now the number of recaptured transponders (8) is slightly lower than in 2005, when 10 transponders were returned. Also, one of the returned transponders originates from an eel released in 2005 (no. 1777). Table 14 gives the data on transponder returns.

Table 14 Returned transponders in 2006/2007. Transponder no. Date of release Location of release Date of recapture Location of recapture River brance 1777 7-9-2005 Siegburg xxx xxx IJsselmeer Hardinxveld- Beneden 2072 7-9-2006 Cologne 10-9-2006 Giessendam Merwede 2130 17-8-2006 Cologne 5-1-2007 Ooipolder Waal 2137 17-8-2006 Cologne 28-8-2006 Ochten Waal 2186 7-9-2006 Cologne 17-9-2006 Varikse veer Waal 2190 13-9-2006 Siegburg 13-9-2006 Rees Rijn 2200 14-9-2006 Cologne 18-11-2006 Cologne Rijn 2216 14-9-2006 Cologne 4-12-2006 Gameren Waal

Of the transponders, 3 were returned by professional fishermen (2 in fykes in the Netherlands; 1 in the eelschokker in Germany). The other transponders were found at the shore line of the various river branches. The eel with transponder number 2072 was detected once at station Waal Vuren and recaptured in the Beneden Merwede. The eel with transponder number 2130 was only detected once at station Rhine Xanten and lost in the Waal. The eel with transponder number 2137 was never detected, just as the eel with transponder number 2186, although they were

Results

recaptured (found) in the Netherlands (both Waal). Transponders 2190 and 2200 were recaptured/found in Germany. The eel with transponder number 2216 was also not detected in the Netherlands but found in the Waal area. It is rather remarkable that of the 5 eels (not counting the one from 2005) recaptured/found in the Netherlands 3 eels were never detected by station Xanten. It is however, consistent with station Rhine Xanten not functioning properly and, all eels were found in the upper stretch of the Waal, so could not be detected by other, more downstream located stations.

The malfunctioning of station Rhine Xanten could also be the cause of migration route River Rhine Cologne → River Rhine Xanten → Waal being the most “popular”. In total 27 eels were detected in this way and lost after station Waal Vuren, were as only 14 eels were detected on the stretch River Rhine Cologne → River Rhine Xanten and lost after Xanten. Of the 27 eels of the first route, 5 were not detected by station Rhine Xanten. Of the 14 eels of the second route all were detected by station Rhine Xanten. In both cases the majority of the eels detected on station Rhine Xanten were tagged on or after the 23rd of November (route 1: 17 of 26; route 2: 9 of 14), when station Xanten was functioning properly again.

Assuming all eels not reaching the North Sea, either being caught or dead in the different branches of the system, the losses over the different routes are: 14 eels after Xanten: almost 20% ((14/71) * 100), being the largest part of the river system in the Netherlands (Xanten to the detection stations Kampen (IJssel), Nieuwegein (Lek) and Vuren (Waal); 27 eels after station Waal Vuren: 38% ((27/71) * 100); 6 eels after station Kampen (IJssel): almost 9% ((6/71) * 100) and 6 eels after various detection stations (Beneden Merwede, Spui, Dordtse Kil and Oude Maas): almost 9%. So, a total of 75% of the 71 eels detected is lost in the river branches in the Netherlands (under the assumption that no eels escape undetected via fish or shipping sluices).

4.1.3 Swimming speed during downstream migration

Table 15 gives an overview of the duration of the downstream migration of eel and possible swimming speeds on different routes. As can be seen, there is a lot of variation in the duration of the downstream migration. A maximum migration of 154 km/day up to a slowest migration of 1 km/day is registered. The actual swimming speed can be faster in the situation that eel is not swimming continuously and has periods of inactivity. With a speed of 154 km/day and an average river flow velocity of 1 m/s, eel is migrating downstream actively (0,72 m/s above river flow). There is a large similarity (in the variations) in the downstream migration of eels over the years 2004, 2005 and 2006.

Table 15 Duration of migration (days). The migration is ex. the flow velocity (1m/sec). Route Km Number of Quickest duration Longest duration Avr. duration of observations of migration of migration migration

days days days km/day km/day km/day

Results

Cologne-Xanten 154 46 1 154 144 1 32 5 Cologne-Vuren 278 44 2 139 123 2 31 9 Cologne-Kampen 317 6 25 13 96 3 55 6 Sieg-Buisdorf-Sieg- Menden 7 4 7 1 13 1 11 1 Cologne-Spijkenisse 335 7 4 84 109 3 50 7 Cologne-Arnhem 215 3 22 10 37 6 32 7 Cologne-Maurik 254 1 40 6 40 6 40 6 Cologne-Hagestein 279 1 40 7 40 7 40 7 Cologne-Nieuwegein 283 3 23 12 40 7 34 8 Cologne-Zuidland 340 1 62 5 62 5 62 5 Cologne-'s Gravendeel 314 8 3 105 106 3 34 9 Cologne-Stellendam 357 1 38 9 38 9 38 9 Cologne-Kinderdijk 321 8 6 54 117 3 37 9

4.2 Mark recapture

In the tables below (table 16 and 17) once again the batch mark statistics and the recapture samples are given. Figure 9 gives the length distribution of recaptured eels.

Table 16 Floy tagging statistics 2006

Table 17 Composition of recapture samples taken in 2006 in the Waal Waal Intermediate 50-70 Intermediate >70 Silver eel 50-70 Silver eel >70 Total 275 208 1.954 654 Total intermediate Total silver 483 (16%) 2608 (84%) Grand total 3.091

silvereel % intermediate

0 50 60 70 80 90 100 length in cm

Figure 9. Percentage of intermediate and silver eel per length class

Results

Recapture samples were only taken in the river Waal. From previous research in 2004 and 2005 it was concluded that the majority of the Silver eels take that route to sea. Nevertheless, during the research period in 2006 it became obvious that eels with floy tags were recaptured in all the branches of the river Rhine in the Netherlands and in the Rhine in Germany (see table 19 and figure 9). These recaptures could, however, not be used for the population estimate of the eel because data regarding total catch, weight and length composition were not available.

In the recapture samples (1.724 kg; n = 3.091) checked by VisAdvies (table 17) no floy tagged eels were found. In the same period fisherman Klop found 6 floy tagged eels in his own catches (1.050 kg; n = 1702). For the population estimate in the river Waal the recapture samples of VisAdvies and the catches of fisherman Klop were totalized, under the assumption of equal length and weight composition. The number of tagged eels travelling through the Waal was derived from telemetry data (150 eels with transponders; 71 eels detected; 53% not migrating ((79/150)*100), 63% migrating via the Waal (of 71 eels 6 via IJssel, 3 via Nederrijn/Lek, 17 not detected after station Xanten: (((71-26)/71)*100)) and calculated as follows: 2.627 floy tagged eel * 53% not migrating * 63% migrating via the river Waal = M = 876 eels. Table 18 gives the results from the population estimate in the Waal in 2006. The number of silver and intermediate eels is calculated as 0.7 million (interval 0.5 million) with an weight of almost 0.4 million kg (using an average weight of 0.569 kg). In 2004 the number of eels was calculated as between 0.4 and 1.8 million eels, depending underlying assumptions. In 2005 the number of eels calculated was 1.1 million (Vriese et al . 2006).

Table 16 Population estimate 2006 (Waal) silver and intermediate eels.

Population estimate 2006 (Waal) silver and intermediate eels M C R N Interval Kg 876 4.793 6 700.097 552.924 398.355

Looking at the locations of the recaptured floy tagged eels, it can be concluded that 8 were caught in Germany, 5 were caught on lake IJsselmeer/Ketelmeer (migrating via IJssel), 1 was caught in the Kromme Rijn (migrating via Nederrijn/Lek) and 21 were caught in the Benedenrivieren area (migrating via Waal). When looking at the recapture percentages in the various river branches (excluding recaptures in Germany: 78% via Waal ((21/27)*100), 4% via Nederrijn/Lek ((1/27)*100) and 19% via IJssel ((5/27)*100)), there is quite a similarity with the telemetry data (83% migrating via Waal ((45/54)*100), 6% via Nederrijn/Lek ((3/54)*100) and 11% via IJssel ((6/54)*100), which indicates that the picture of the eel migration derived by both methods (telemetry and mark recapture) is quite consistent.

Of the 8 recaptures in Germany, 2 were made by the eel schokker near Kalkar (river km 844,6) and 6 eels were caught by 3 anglers (one of them caught 4 floy tagged eels) in the vicinity of Cologne near the release location. Rod catches were made 2-3 weeks after release of the eels, indicating that the eels started feeding again. The recaptures made in the Netherlands were all by professional fishermen for the most part fishing with fykes, although one eels was caught by elektrofishing.

Results

Table 179 Recaptured eels with floy tags. Location of recapture Sequence number Floy number tag Weekof tagging number Weekof recapture number 1 667 35 35 Ketelmeer 2 526 35 36 Waal (Woudrinchem) 3 566 35 37 Rhine, Kalkar, R-km 844,6 4 441 33 37 Rhine, Cologne 5 259 32 38 Rhine, Cologne (km 684) 6 506 33 38 Rhine, Cologne 7 686 35 38 Rhine, Cologne (km 686) 8 1030 36 38 Rhine, Cologne (Severinsbrücke) 9 24 32 39 Hollands Diep 10 645 35 39 IJsselmeer (Lelystad) 11 555 35 39 Sliedrecht 12 1795 39 40 Rhine, Kalkar, R-km 844,6 13 636 35 40 Beneden Merwede 14 1472 39 40 Beneden Merwede 15 959 36 41 Haringvliet 16 117 32 41 Caland kanaal 17 385 33 41 Caland kanaal 18 1944 39 41 Noord Hollands Diep 19 1723 39 41 Nieuwe Waterweg (km 1015) 20 578 35 41 IJsselmeer (Afsluitdijk Kornwerderzand) 21 882 36 42 Caland kanaal 22 1026 36 42 IJsselmeer (Hoornse Hop) 23 1719 39 42 Noord Hollands Diep 24 695 35 43 Haringvliet 25 1715 39 43 Beneden Merwede 26 463 33 43 Rhine, Cologne (Porz-Westhoven) 27 1168 36 44 Beneden Merwede 28 778 36 45 Sliedrecht 29 1801 39 45 Kromme Rijn 30 284 33 46 Oude Maas 31 1495 39 46 IJsselmeer (Afsluitdijk Oost) 32 2307 43 47 Haringvliet 33 1045 36 48 Nieuwe Waterweg 34 891 36 51 Beneden Merwede 35 1901 39 50 Beneden Merwede 36 777 Data missing

As can be seen in table 19, the recaptures are also indicative of the large variation of the duration of the downstream migration. One eel was caught in Ketelmeer in the same week that it was released. Other eels are caught months after being released (up to 4 months).

Results

Figure 9. Map with recaptures of eel (transponder and floy tags)

Discussion and conclusions

5 Discussion and conclusions

5.1 Discussion

When comparing the results of 2006 with the results of 2004 and 2005, the detected number of eels up till the end of January 2007 (47%) is slightly lower than in 2004 and 2005, being 55% and 53% respectively. The slightly lower number in 2006 might be related to the cessation of the transpondering activities (malfunctioning of station River Rhine Xanten), as a result of which quite large numbers of silver eels (55 of the total of 150) were tagged late in the migration season (end of November). In 2005 almost 10% of the 2004 eels with transponders migrated downstream in the period of January 2005 – August 2005, bringing the total amount of detected migrating eels up to 63%. In the period end of January 2006 up to October 2006 10 eels of the 2005 group were detected (see annex 7.5), bringing the total amount of detected eels up to 60%. It is likely that several eels tagged in 2006 are still present in the higher parts of the river system and will migrate downstream in the months to come.

It is generally assumed that the time of detection of an eel on a station is indicative for it’s migration activity. It is also assumed that eels migrate preferably during the night. This was not supported by our findings in the years 2004 and 2005 of the experiment. In 2004, of the 212 detections, 112 were during night time (between 6 pm until 6 am) and 100 during day time (between 6 am until 6 PM). In 2005 these figures were 93 detections (night time) and 89 detections (day time). From the 170 detections of tagged eels in 2006 however, 103 were during night time and 67 were during day time, which suggests a preference for migrating during nocturnal hours. Probably, the exact conditions (e.g. discharge, water clarity, light conditions) during downstream migration determine when eels are most active.

Of the 71 eels detected, 18 eels probably have reached the North Sea, of which one by way of the Haringvliet. Of the other 17 escapement is not totally certain, as they could have been caught by professional fishermen downstream of last detection stations. However, none of these transponders numbers were returned, although there were some recaptures in the area. The escapement of eel based on these numbers is, 25% ((18/71)*100). This is in close accordance with the results of the experiments in 2004 and 2005 (escapement respectively 32% and 27%). Again, just as in 2005, there was no escapement from detected transpondered eels migrating via the IJssel and lake IJsselmeer. In 2004 one eel escaped via this route. These results are consistent with the high fishing pressure on lake IJsselmeer.

The most important migration route over the three years of research is River Rhine Cologne → River Rhine Xanten → Waal → Beneden Merwede → De Noord → North Sea although the numbers of eel that take this route varies. In 2004 15 out of 30 eels took this route (whole migration period). In 2005 20 out of 24 eels (whole migration period), and in 2006 6 eels out of 18 (up till now). It seems that when the overall discharge of the Rhine during the migration period of silver eel is lower, a larger amount of eels take this route. Normally, 2/3 rd of the discharge flows via the Waal en 1/3 rd enters the Pannerdensch Kanaal, which splits up in the river IJssel and the Nederrijn/Lek. The Nederrijn/Lek receives 2/9 th of the discharge, leaving the

Discussion and conclusions

remainder for the IJssel (1/9 th ). However when the total discharge is low, the river IJssel receives more water to ensure enough depth for shipping purposes. In the second half of 2006 there were several periods with high discharge, making it easier for eel to use al three river branches.

In 2006 a maximum migration speed of 154 km/day was registered, and a slowest migration of 1 km/day. With a speed of 154 km/day and an average river flow velocity of 1 m/s, eel is migrating downstream actively (0,72 m/s above river flow). There is a large similarity (in the variations) in the speed of the downstream migration of eels over the years 2004, 2005 and 2006.

Using floy tags in 2006 was a favourable decision in terms of recapture of eels. When using a batch mark (heliogen blue) in 2004 and 2005, numbers of recaptures were 6 and 5 respectively, due to the poor visibility of the mark (it was not detected by fishermen). In 2006 the number of recaptures was maximized to 36, with all floy tags being detected by fishermen. Although this was a success, it has not led to a better population estimate. In the recapture samples checked by VisAdvies no floy tagged eels were found and only because fisherman Klop found 6 floy tags himself in other batches of eel an estimate could be made. The number of silver and intermediate eels in the Waal in 2006 is calculated as 0.7 million (interval 0.5 million) with an weight of almost 0.4 million kg (using an average weight of 0.569 kg). In 2004 the number of eels was calculated as between 0.4 and 1.8 million eels, depending underlying assumptions. In 2005 the number of eels calculated was 1.1 million. Although the uncertainties are large, it is remarkable that the estimates in the three years are so alike. Bruijs et al (2003) calculates for the Meuse a number of 225,000 silver eel passing Alphen between autumn and December 2002. However, the river basin of the Rhine is 5 times larger than that of the Meuse.

Other recaptures of floy tagged eels could not be used for the estimate because data on the total catches (in terms of weight and length composition) were not available. In the near future however, it will be mandatory to register these data (EU data collection regulation) and the prospects for making a precise estimate using floy tags will be much better. There are, however, some questions about the longevity of floy tags in eel. On one occasion it was possible to see a recaptured eel (figure 10) with a floy tag. The location of the insertion of the floy tag was inflamed and the tag could be removed quite easily.

Figure 10. Recaptured eel with floy tag

Discussion and conclusions

5.2 Conclusions

The NEDAP TRAIL System®, although not flawless, has proved to be an important and appropriate system to monitor the downstream migration of silver eel.

The detected number of 2006 eels up till the end of January 2007 (47%) is slightly lower than in 2004 and 2005, being 55% and 53% respectively.

Of the 71 eels detected up till now, 18 eels probably have reached the North Sea, of which one by way of the Haringvliet. The escapement of eel based on these numbers is, 25%.

This is in close accordance with the results of the experiments in 2004 and 2005 when the escapement was 32% and 27% respectively.

Again, just as in 2005, there was no escapement from detected transpondered eels migrating via the IJssel and lake IJsselmeer. In 2004 one eel escaped via this route.

In 2006 a maximum migration speed of 154 km/day was registered, and a slowest migration of 1 km/day.

With a speed of 154 km/day and an average river flow velocity of 1 m/s, eel is migrating downstream actively (0,72 m/s above river flow).

There is a large similarity (in the variations) in the speed of the downstream migration of eels over the years 2004, 2005 and 2006.

By using floy tags in 2006 a total number of 36 recaptures (up till now) could be made. It has, however, not lead to a better population estimate because data on total catch could not be obtained.

The number of silver and intermediate eels in the Waal in 2006 is calculated as 0.7 million (interval 0.5 million) with an weight of almost 0.4 million kg (using an average

Discussion and conclusions

weight of 0.569 kg). In 2004 the number of eels was calculated as between 0.4 and 1.8 million eels, depending underlying assumptions. In 2005 the number of eels calculated was 1.1 million.

Recaptured floy tagged eels were taken in Germany (N = 8), in lake IJsselmeer/ Ketelmeer (N = 5), in the Kromme Rijn (N = 1) and 21 were caught in the Beneden- rivieren area.

When looking at the recapture percentages in the various river branches (excluding recaptures in Germany: 78% via Waal ((21/27)*100), 4% via Nederrijn/Lek ((1/27)*100) and 19% via IJssel ((5/27)*100)), there is quite a similarity with the telemetry data (83% migrating via Waal ((45/54)*100), 6% via Nederrijn/Lek ((3/54)*100) and 11% via IJssel ((6/54)*100), which indicates that the picture of the eel migration derived by both methods (telemetry and mark recapture) is quite consistent.

References

6 References

Baras, E., & D. Jeandrain, 1998. Evaluation of surgery procedures for tagging eel Anguilla anguilla (L.) with biotelemetry transmitters. Hydrobiologia 371/372: 107-111.

Bij de Vaate, A. & A.W. Breukelaar (eds.), 2001. De migratie van zeeforel in Nederland. Rijksinstituut voor Integraal Zoetwaterbeheer & Afvalwaterbehandeling, rapport nr. 2001.046. ISBN 9036954037.

Breukelaar, A.W., A. bij de Vaate & K.T.W. Fockens, 1998. Inland migration study of sea trout (Salmo trutta) into the rivers Rhine and Meuse (The Netherlands), based on inductive coupling radio telemetry. Hydrobiologia 371/372: 29-33.

Breukelaar, A.W., F.T.W. Fockens & A. bij de Vaate, 2000. Technical aspects of the NEDAP TRAIL system used in a sea trout (Salmo trutta L.) migration study. In: Moore, A & I. Russel, Proc. Third Conference on Fish Telemetry in Europe, Norwich (UK), 20-25 June, 1999: 7-11.

Bruijs, M.C.M., H.J.G. Polman, G.H.F.M. van Aerssen & R.H. Hadderingh (KEMA), H.V. Winter, C. Deerenberg, & H.M. Jansen (RIVO), U. Silvwevers & B. Adam (IFÖ), U. Dumont & N. Kessels (FM), 2003. Management of silver eel: Human impact on downstream migrating eel in the river Meuse. 50180283-KPS/MEC 03-6183. Final Report Contract Q5RS-2000-31141. KEMA Nederland B.V. Arnhem, 105 p.

Lucas, M., 1989. Effects of implanted dummy transmitters on mortality, growth and tissue reaction in rainbow trout, (Salmo gairdneri, Richardson). J. Fish. Biol. 35: 577- 587.

Moore, A., I.C. Russell & E.C.E. Potter, 1990. The effects of intraperitoneally implanted dummy acoustic transmitters on the behaviour and physiology of juvenile Atlantic salmon (Salmo salar L.). J. Fish. Biol. 37: 713-721.

McKinley, R.S., G. Power & H.E. Kowalyk, 1994. Transmitter attachment/implant: laboratory manual. Report Ontario Hydro Research, Toronto, and Univ. Waterloo, Biol. Dept., Waterloo (Canada). 18 pp.

F.-W. Tesch, 1978. Telemetric observations on the spawning migration of eel (Anguilla anguilla) west of the European continental shelf. Environ. Biol. Fish., volume 3, number 2, 1978.

Vriese, F.T., 1995. Implantering van transponders in salmoniden. Report no. 26, Organisation for the Improvement of Inland Fisheries, Nieuwegein [in Dutch].

Vriese, F.T , J.C.A. Merkx.(VisAdvies) & A.W. Breukelaar (RWS-RIZA), 2006. Population study of female downstream migrating silver eel Anguilla anguilla in the Rhine system in 2005. VisAdvies BV, Utrecht. Project VA2005_06, 72 pages.

Annexes

7 Annexes

7.1 Transponder implantation data of eel Sequencenumber Transpondernumber Silver/Interm.ermediate Weight(g) Length(cm) Date tagging of Location Fishinggear Time release of Eyehoriz.(mm) Eyevert.(mm) Lengthpect.fin (mm) 1 2115 Silver 1343 87 16-aug-06 Sieg Electro 15:30 109 88 399 2 2123 Silver 1709 97 16-aug-06 Sieg Electro 15:30 115 110 421 3 2098 Interm. 918 74 16-aug-06 Sieg Electro 15:30 100 91 348 4 2097 Silver 894 78 16-aug-06 Sieg Electro 15:30 91 88 396 5 2085 Silver 645 69 16-aug-06 Sieg Electro 15:30 93 79 322 6 2083 Silver 772 72 16-aug-06 Sieg Electro 15:30 93 88 378 7 2150 Silver 964 78 17-aug-06 Mosel Fyke 11:30 89 72 330 8 2093 Silver 1526 88 17-aug-06 Mosel Fyke 11:30 81 78 428 9 2130 Silver 1027 81 17-aug-06 Mosel Fyke 11:30 70 69 472 10 2133 Silver 942 81 17-aug-06 Mosel Fyke 11:30 81 82 388 11 2131 Silver 1137 87 17-aug-06 Mosel Fyke 11:30 100 95 373 12 2099 Interm. 910 81 17-aug-06 Mosel Fyke 11:30 81 75 414 13 2096 Silver 1045 80 17-aug-06 Mosel Fyke 11:30 0 0 0 14 2128 Silver 975 77 17-aug-06 Mosel Fyke 12:30 79 73 393 15 2129 Silver 966 77 17-aug-06 Mosel Fyke 12:30 84 79 404 16 2084 Silver 964 81 17-aug-06 Mosel Fyke 12:30 70 68 360 17 2137 Silver 1076 83 17-aug-06 Mosel Fyke 12:30 90 83 359 18 2086 Silver 1078 88 17-aug-06 Mosel Fyke 12:30 93 92 399 19 2127 Silver 2735 107 17-aug-06 Mosel Fyke 15:00 89 89 497 20 2136 Silver 918 78 17-aug-06 Mosel Fyke 15:00 108 90 345 21 2138 Silver 981 83 17-aug-06 Mosel Fyke 15:00 100 89 409 22 2082 Silver 986 85 17-aug-06 Mosel Fyke 15:00 72 68 342 23 2132 Silver 1541 90 17-aug-06 Mosel Fyke 15:00 99 81 429 24 2134 Silver 792 73 17-aug-06 Mosel Fyke 15:00 81 80 370 25 2135 Interm. 828 76 17-aug-06 Mosel Fyke 15:00 75 59 318 26 2088 Silver 1145 80 17-aug-06 Mosel Fyke 15:00 91 78 401 27 2095 Interm. 891 78 17-aug-06 Mosel Fyke 15:00 61 58 390 28 2094 Silver 811 76 17-aug-06 Mosel Fyke 15:00 88 76 352 29 2087 Interm. 961 81 17-aug-06 Mosel Fyke 15:00 87 69 375 30 2139 Silver 1010 81 17-aug-06 Mosel Fyke 15:00 98 81 400 31 2179 Silver 990 78 17-aug-06 Mosel Fyke 15:00 77 63 348 32 2180 Silver 898 75 17-aug-06 Mosel Fyke 15:00 70 68 344 33 2193 Silver 1423 88 7-sep-06 Mosel Fyke 11:00 99 85 445 34 2199 Silver 1751 91 7-sep-06 Mosel Fyke 11:00 105 90 420 35 2171 Silver 1714 98 7-sep-06 Mosel Fyke 11:00 100 76 436 36 2217 Silver 1535 91 7-sep-06 Mosel Fyke 11:00 97 80 422 37 2209 Silver 1740 91 7-sep-06 Mosel Fyke 11:00 108 90 482

Annexes

38 2214 Silver 1118 82 7-sep-06 Mosel Fyke 11:00 92 88 425 39 2208 Silver 1350 90 7-sep-06 Mosel Fyke 11:00 99 100 428 40 2213 Silver 1122 83 7-sep-06 Mosel Fyke 11:00 81 72 428 41 2195 Silver 1050 83 7-sep-06 Mosel Fyke 11:00 80 74 364 42 2182 Silver 1000 79 7-sep-06 Mosel Fyke 11:00 77 75 370 43 2206 Silver 877 82 7-sep-06 Mosel Fyke 11:00 108 90 391 44 2070 Interm. 1560 90 7-sep-06 Mosel Fyke 11:00 91 85 411 45 2176 Silver 1049 76 7-sep-06 Mosel Fyke 12:30 99 82 418 46 2076 Silver 1282 85 7-sep-06 Mosel Fyke 12:30 96 80 368 47 2215 Silver 1555 92 7-sep-06 Mosel Fyke 12:30 100 97 424 48 2187 Silver 2005 95 7-sep-06 Mosel Fyke 12:30 109 92 496 49 2073 Interm. 1345 86 7-sep-06 Mosel Fyke 12:30 102 80 422 50 2173 Silver 1100 82 7-sep-06 Mosel Fyke 12:30 88 88 440 51 2177 Silver 1040 82 7-sep-06 Mosel Fyke 12:30 99 89 401 52 2071 Silver 1259 88 7-sep-06 Mosel Fyke 12:30 100 89 412 53 2181 Silver 1720 97 7-sep-06 Mosel Fyke 12:30 97 84 369 54 2072 Silver 1581 89 7-sep-06 Mosel Fyke 12:30 80 78 433 55 2207 Silver 1184 85 7-sep-06 Mosel Fyke 12:30 99 85 420 56 2210 Silver 1910 99 7-sep-06 Mosel Fyke 12:30 110 91 472 57 2186 Silver 1622 92 7-sep-06 Mosel Fyke 12:30 105 92 430 58 2205 Silver 1315 88 7-sep-06 Mosel Fyke 14:30 84 79 391 59 2203 Silver 1285 89 7-sep-06 Mosel Fyke 14:30 102 95 462 60 2170 Interm. 935 77 7-sep-06 Mosel Fyke 14:30 91 72 400 61 2178 Silver 1545 93 7-sep-06 Mosel Fyke 14:30 99 88 453 62 2074 Silver 1481 96 7-sep-06 Mosel Fyke 14:30 100 92 388 63 2190 Silver 1650 86 13-sep-06 Sieg Electro 17:00 107 99 397 64 2188 Silver 785 71 13-sep-06 Sieg Electro 17:00 91 85 374 65 2198 Interm. 790 74 13-sep-06 Sieg Electro 17:00 74 64 374 66 2078 Interm. 685 74 13-sep-06 Sieg Electro 17:00 72 59 347 67 2189 Silver 720 69 13-sep-06 Sieg Electro 17:00 100 92 357 68 2174 Silver 1316 87 14-sep-06 Mosel Fyke 11:15 99 90 431 69 2077 Interm. 842 76 14-sep-06 Mosel Fyke 11:15 70 59 320 70 2196 Silver 1461 89 14-sep-06 Mosel Fyke 11:15 100 89 419 71 2197 Silver 1258 90 14-sep-06 Mosel Fyke 11:15 92 98 498 72 2216 Silver 1149 86 14-sep-06 Mosel Fyke 11:15 100 86 360 73 2211 Silver 1690 93 14-sep-06 Mosel Fyke 11:15 108 96 432 74 2192 Silver 1109 80 14-sep-06 Mosel Fyke 11:15 98 82 406 75 2185 Silver 1442 88 14-sep-06 Mosel Fyke 11:15 108 90 445 76 2146 Silver 1140 84 14-sep-06 Mosel Fyke 11:15 101 91 398 77 2162 Silver 1264 86 14-sep-06 Mosel Fyke 13:05 106 92 428 78 2184 Silver 1677 93 14-sep-06 Mosel Fyke 13:05 97 92 379 79 2183 Silver 1160 88 14-sep-06 Mosel Fyke 13:05 102 96 441 80 2200 Silver 1454 93 14-sep-06 Mosel Fyke 13:05 92 91 480 81 2159 Interm. 1388 91 14-sep-06 Mosel Fyke 13:05 82 78 434 82 2172 Silver 1268 84 14-sep-06 Mosel Fyke 13:05 74 88 439 83 2144 Silver 1456 91 14-sep-06 Mosel Fyke 13:05 104 94 378 84 2201 Silver 1068 83 14-sep-06 Mosel Fyke 13:05 81 85 404 85 2145 Silver 1244 87 14-sep-06 Mosel Fyke 13:05 81 81 362 86 2152 Silver 1737 92 14-sep-06 Mosel Fyke 13:05 101 82 447 87 2147 Silver 1256 85 14-sep-06 Mosel Fyke 13:05 72 69 369 88 2164 Silver 1163 86 14-sep-06 Mosel Fyke 13:05 87 83 414 89 2166 Interm. 1262 87 14-sep-06 Mosel Fyke 13:05 82 81 398

Annexes

90 2156 Interm. 938 79 14-sep-06 Mosel Fyke 13:05 82 78 378 91 2158 Silver 1321 89 14-sep-06 Mosel Fyke 13:05 80 78 451 92 2153 Silver 1484 91 14-sep-06 Mosel Fyke 13:05 99 92 514 93 2141 Interm. 939 79 14-sep-06 Mosel Fyke 13:05 75 72 346 94 2157 Interm. 1112 83 14-sep-06 Mosel Fyke 13:05 93 89 475 95 2253 Silver 957 78 23-nov-06 Mosel Fyke 10:15 82 78 410 96 2242 Silver 1361 87 23-nov-06 Mosel Fyke 10:15 71 65 358 97 2151 Silver 1498 87 23-nov-06 Mosel Fyke 10:15 90 88 327 98 2149 Silver 978 78 23-nov-06 Mosel Fyke 10:15 84 86 361 99 2168 Silver 1166 84 23-nov-06 Mosel Fyke 10:15 92 73 396 100 2143 Silver 1346 87 23-nov-06 Mosel Fyke 10:15 90 89 398 101 2148 Silver 1418 90 23-nov-06 Mosel Fyke 11:30 83 92 404 102 2155 Silver 1195 85 23-nov-06 Mosel Fyke 11:30 85 78 407 103 2142 Silver 1024 84 23-nov-06 Mosel Fyke 11:30 94 91 430 104 2160 Silver 1363 86 23-nov-06 Mosel Fyke 11:30 90 81 381 105 2163 Silver 1107 82 23-nov-06 Mosel Fyke 11:30 72 78 361 106 2240 Silver 1339 86 23-nov-06 Mosel Fyke 11:30 80 74 367 107 2224 Silver 1131 82 23-nov-06 Mosel Fyke 11:30 83 80 324 108 2218 Silver 950 75 23-nov-06 Mosel Fyke 12:30 82 82 325 109 2274 Silver 2328 107 23-nov-06 Mosel Fyke 12:30 107 112 438 110 2223 Silver 1734 94 23-nov-06 Mosel Fyke 12:30 87 85 390 111 2257 Silver 1589 89 23-nov-06 Mosel Fyke 12:30 104 99 440 112 2226 Silver 1082 82 23-nov-06 Mosel Fyke 13:15 82 80 366 113 2266 Silver 1490 89 23-nov-06 Mosel Fyke 13:15 83 77 396 114 2264 Silver 1012 82 23-nov-06 Mosel Fyke 13:15 83 75 387 115 2222 Silver 1079 81 23-nov-06 Mosel Fyke 13:15 88 71 380 116 2221 Silver 1233 84 23-nov-06 Mosel Fyke 13:15 86 85 417 117 2229 Silver 1112 80 23-nov-06 Mosel Fyke 13:15 88 77 377 118 2232 Silver 92 23-nov-06 Mosel Fyke 13:15 86 77 384 119 2225 Silver 85 23-nov-06 Mosel Fyke 15:00 80 75 366 120 2239 Silver 86 23-nov-06 Mosel Fyke 15:00 75 74 371 121 2236 Silver 86 23-nov-06 Mosel Fyke 15:00 75 65 383 122 2237 Silver 87 23-nov-06 Mosel Fyke 15:00 88 79 368 123 2249 Silver 86 23-nov-06 Mosel Fyke 15:00 76 69 387 124 2227 Silver 97 23-nov-06 Mosel Fyke 15:00 84 79 503 125 2250 Silver 85 23-nov-06 Mosel Fyke 15:00 74 68 332 126 2267 Silver 87 23-nov-06 Mosel Fyke 16:00 81 73 345 127 2259 Silver 98 23-nov-06 Mosel Fyke 16:00 95 88 419 128 2233 Silver 94 23-nov-06 Mosel Fyke 16:00 82 78 440 129 2252 Silver 82 23-nov-06 Mosel Fyke 16:00 81 77 350 130 2273 Silver 78 23-nov-06 Mosel Fyke 16:00 85 79 336 131 2231 Silver 83 23-nov-06 Mosel Fyke 16:00 82 78 328 132 2265 Silver 83 23-nov-06 Mosel Fyke 17:30 92 83 330 133 2263 Silver 87 23-nov-06 Mosel Fyke 17:30 83 77 403 134 2262 Silver 88 23-nov-06 Mosel Fyke 17:30 88 91 402 135 2245 Silver 91 23-nov-06 Mosel Fyke 17:30 87 84 387 136 2246 Silver 78 23-nov-06 Mosel Fyke 17:30 74 67 318 137 2244 Silver 84 23-nov-06 Mosel Fyke 17:30 78 78 350 138 2271 Silver 87 24-nov-06 Mosel Fyke 10:00 78 68 364 139 2275 Silver 89 24-nov-06 Mosel Fyke 10:00 84 78 374 140 2255 Silver 84 24-nov-06 Mosel Fyke 10:00 81 74 424 141 2254 Silver 85 24-nov-06 Mosel Fyke 10:00 74 70 365

Annexes

142 2260 Silver 93 24-nov-06 Mosel Fyke 10:00 80 68 380 143 2268 Silver 87 24-nov-06 Mosel Fyke 10:00 80 64 362 144 2269 Silver 97 24-nov-06 Mosel Fyke 10:00 91 87 453 145 2228 Silver 75 24-nov-06 Mosel Fyke 10:00 78 61 373 146 2248 Silver 90 24-nov-06 Mosel Fyke 11:30 81 73 400 147 2219 Silver 86 24-nov-06 Mosel Fyke 11:30 83 68 407 148 2247 Silver 84 24-nov-06 Mosel Fyke 11:30 83 78 337 149 2277 Silver 77 24-nov-06 Mosel Fyke 11:30 75 77 388 150 2220 Silver 86 24-nov-06 Mosel Fyke 11:30 76 86 402

Annexes

7.2 Registrations of eel on the NEDAP TRAIL system®

Registration nr. Detection station Transponder nr. Time Date 1 Waal_Vuren 2072 0:03 12-9-2006 2 Nederrijn_Arnhem 2073 22:23 14-10-2006 3 Nederrijn_Maurik 2073 2:42 17-10-2006 4 Nederrijn_Hagestein 2073 11:43 17-10-2006 5 Lek_Nieuwegein 2073 18:35 17-10-2006 6 Waal_Vuren 2074 15:02 18-9-2006 7 DKil_s'Gravendeel 2074 0:37 19-9-2006 8 DKil_s'Gravendeel 2074 1:32 19-9-2006 9 DKil_s'Gravendeel 2074 17:02 12-10-2006 10 DKil_s'Gravendeel 2074 19:36 12-10-2006 11 HV_Stellendam_noord 2074 20:27 15-10-2006 12 HV_Stellendam_noord 2074 19:01 16-10-2006 13 HV_Stellendam_noord 2074 19:07 16-10-2006 14 HV_Stellendam_zuid 2074 20:20 16-10-2006 15 Waal_Vuren 2078 11:41 13-10-2006 16 DKil_s'Gravendeel 2078 3:01 14-10-2006 17 OudeMaas_Spijkenisse 2078 1:30 9-11-2006 18 Rijn_Xanten 2082 6:22 26-11-2006 19 Waal_Vuren 2082 2:57 27-11-2006 20 Sieg_Menden 2083 22:52 29-8-2006 21 Sieg_Menden 2085 23:18 28-8-2006 22 Waal_Vuren 2085 16:20 25-9-2006 23 Rijn_Xanten 2088 15:04 7-1-2007 24 Nederrijn_Arnhem 2093 6:41 22-9-2006 25 Lek_Nieuwegein 2093 2:36 24-9-2006 26 Waal_Vuren 2095 17:23 7-10-2006 27 DKil_s'Gravendeel 2095 14:08 8-10-2006 28 DKil_s'Gravendeel 2095 17:41 8-10-2006 29 Spui_Zuidland 2095 9:04 18-10-2006 30 OudeMaas_Spijkenisse 2095 4:13 2-11-2006 31 Rijn_Xanten 2096 23:15 16-11-2006 32 Waal_Vuren 2096 23:52 17-11-2006 33 DeNoord_Kinderdijk 2096 1:37 12-12-2006 34 Sieg_Menden 2097 3:22 29-8-2006 35 Waal_Vuren 2097 22:14 2-9-2006 36 DeNoord_Kinderdijk 2097 23:26 10-9-2006 37 Sieg_Menden 2098 1:08 23-8-2006 38 DKil_s'Gravendeel 2098 0:03 26-9-2006 39 DKil_s'Gravendeel 2098 7:03 27-9-2006 40 DKil_s'Gravendeel 2098 22:27 8-11-2006 41 OudeMaas_Spijkenisse 2098 11:34 9-11-2006 42 Rijn_Xanten 2123 14:56 25-11-2006 43 Waal_Vuren 2127 7:27 28-8-2006 44 DeNoord_Kinderdijk 2127 4:14 29-8-2006 45 IJssel_Kampen 2128 2:40 10-10-2006 46 Rijn_Xanten 2130 19:51 9-12-2006 47 Rijn_Xanten 2131 19:38 26-11-2006 48 Waal_Vuren 2131 9:02 1-12-2006

Annexes

49 DKil_s'Gravendeel 2131 21:17 1-12-2006 50 Waal_Vuren 2131 15:44 3-12-2006 51 DKil_s'Gravendeel 2131 9:11 4-12-2006 52 OudeMaas_Spijkenisse 2131 19:14 4-12-2006 53 Waal_Vuren 2132 6:49 21-8-2006 54 Rijn_Xanten 2142 14:36 25-11-2006 55 Waal_Vuren 2142 2:06 9-12-2006 56 IJssel_Kampen 2145 17:45 9-10-2006 57 Rijn_Xanten 2148 8:45 25-11-2006 58 Waal_Vuren 2148 18:54 26-11-2006 59 Waal_Vuren 2152 20:01 17-11-2006 60 Nederrijn_Arnhem 2156 22:33 6-10-2006 61 Lek_Nieuwegein 2156 13:00 7-10-2006 62 Waal_Vuren 2158 23:59 1-10-2006 63 IJssel_Kampen 2159 3:05 9-10-2006 64 Rijn_Xanten 2160 8:57 28-11-2006 65 Waal_Vuren 2160 17:22 30-11-2006 66 Rijn_Xanten 2163 17:13 24-12-2006 67 Waal_Vuren 2163 3:31 4-1-2007 68 Rijn_Xanten 2168 20:32 25-11-2006 69 Waal_Vuren 2171 14:22 12-9-2006 70 DeNoord_Kinderdijk 2171 1:39 13-9-2006 71 Waal_Vuren 2173 22:26 9-10-2006 72 DeNoord_Kinderdijk 2173 23:11 8-11-2006 73 Rijn_Xanten 2176 9:09 2-11-2006 74 IJssel_Kampen 2176 20:34 7-11-2006 75 Rijn_Xanten 2181 2:51 25-11-2006 76 Waal_Vuren 2181 13:24 8-12-2006 77 Rijn_Xanten 2185 20:01 25-11-2006 78 Waal_Vuren 2185 6:01 11-12-2006 79 Rijn_Xanten 2188 15:39 17-11-2006 80 Rijn_Xanten 2196 17:28 10-11-2006 81 Rijn_Xanten 2197 3:13 25-11-2006 82 Waal_Vuren 2197 19:55 4-12-2006 83 Waal_Vuren 2199 19:22 17-9-2006 84 Waal_Vuren 2205 1:02 14-9-2006 85 DeNoord_Kinderdijk 2205 18:29 14-9-2006 86 DeNoord_Kinderdijk 2205 21:49 14-9-2006 87 DeNoord_Kinderdijk 2205 23:48 14-9-2006 88 DeNoord_Kinderdijk 2206 13:28 25-9-2006 89 Waal_Vuren 2206 17:05 25-9-2006 90 Rijn_Xanten 2207 0:35 16-11-2006 91 Waal_Vuren 2207 1:05 17-11-2006 92 Waal_Vuren 2207 2:30 19-11-2006 93 Waal_Vuren 2207 17:05 8-1-2007 94 Waal_Vuren 2210 22:56 17-10-2006 95 DeNoord_Kinderdijk 2210 13:10 23-10-2006 96 DeNoord_Kinderdijk 2210 16:51 23-10-2006 97 DeNoord_Kinderdijk 2210 22:31 23-10-2006 98 DeNoord_Kinderdijk 2210 9:36 24-10-2006 99 DeNoord_Kinderdijk 2210 9:40 24-10-2006 100 DeNoord_Kinderdijk 2210 11:30 24-10-2006

Annexes

101 IJssel_Kampen 2214 20:41 12-12-2006 102 Rijn_Xanten 2217 22:36 9-11-2006 103 IJssel_Kampen 2217 8:13 12-11-2006 104 Rijn_Xanten 2221 19:11 24-11-2006 105 Waal_Vuren 2221 21:58 25-11-2006 106 DKil_s'Gravendeel 2221 12:59 27-11-2006 107 OudeMaas_Spijkenisse 2221 17:30 27-11-2006 108 OudeMaas_Spijkenisse 2221 20:06 27-11-2006 109 OudeMaas_Spijkenisse 2221 23:52 27-11-2006 110 Rijn_Xanten 2223 14:39 27-11-2006 111 Waal_Vuren 2223 3:17 10-12-2006 112 Rijn_Xanten 2226 11:08 30-11-2006 113 Rijn_Xanten 2227 10:07 6-12-2006 114 Waal_Vuren 2227 21:15 15-12-2006 115 Rijn_Xanten 2228 13:35 11-12-2006 116 Rijn_Xanten 2232 0:11 28-11-2006 117 Waal_Vuren 2232 1:29 14-12-2006 118 DKil_s'Gravendeel 2232 20:12 14-12-2006 119 Rijn_Xanten 2237 22:29 24-11-2006 120 Waal_Vuren 2237 19:08 5-12-2006 121 Waal_Vuren 2237 19:29 5-12-2006 122 Waal_Vuren 2237 19:40 5-12-2006 123 Waal_Vuren 2237 19:44 5-12-2006 124 Waal_Vuren 2237 20:05 5-12-2006 125 Waal_Vuren 2237 20:08 5-12-2006 126 Waal_Vuren 2237 20:11 5-12-2006 127 Waal_Vuren 2237 20:15 5-12-2006 128 Waal_Vuren 2237 20:22 5-12-2006 129 Rijn_Xanten 2239 18:12 28-11-2006 130 Waal_Vuren 2239 2:05 14-12-2006 131 Rijn_Xanten 2240 18:01 24-11-2006 132 Waal_Vuren 2240 8:42 12-12-2006 133 Waal_Vuren 2240 8:49 12-12-2006 134 Rijn_Xanten 2244 5:17 5-12-2006 135 Rijn_Xanten 2245 14:03 31-12-2006 136 Rijn_Xanten 2246 17:07 8-12-2006 137 Rijn_Xanten 2248 11:42 26-11-2006 138 Waal_Vuren 2248 10:07 23-12-2006 139 Waal_Vuren 2248 10:10 23-12-2006 140 Rijn_Xanten 2249 2:40 27-11-2006 141 Waal_Vuren 2249 21:35 10-12-2006 142 Rijn_Xanten 2252 11:02 27-11-2006 143 Waal_Vuren 2252 13:58 30-11-2006 144 OudeMaas_Spijkenisse 2252 0:14 10-12-2006 145 OudeMaas_Spijkenisse 2252 18:42 11-12-2006 146 OudeMaas_Spijkenisse 2252 18:51 11-12-2006 147 OudeMaas_Spijkenisse 2252 21:04 11-12-2006 148 Rijn_Xanten 2253 2:47 27-11-2006 149 Waal_Vuren 2253 14:46 8-12-2006 150 Rijn_Xanten 2255 16:26 25-11-2006 151 Waal_Vuren 2255 8:44 27-12-2006 152 Rijn_Xanten 2259 6:46 26-11-2006

Annexes

153 Waal_Vuren 2259 21:52 15-12-2006 154 Rijn_Xanten 2262 20:19 24-11-2006 155 Rijn_Xanten 2263 12:57 24-11-2006 156 Waal_Vuren 2263 22:56 10-12-2006 157 Rijn_Xanten 2264 20:55 22-12-2006 158 Rijn_Xanten 2265 15:17 9-12-2006 159 Rijn_Xanten 2266 21:34 26-11-2006 160 Waal_Vuren 2266 21:23 13-12-2006 161 Rijn_Xanten 2267 18:04 24-11-2006 162 Waal_Vuren 2267 19:40 25-11-2006 163 DKil_s'Gravendeel 2267 16:56 26-11-2006 164 DKil_s'Gravendeel 2267 21:12 26-11-2006 165 DKil_s'Gravendeel 2267 1:20 27-11-2006 166 OudeMaas_Spijkenisse 2267 11:24 27-11-2006 167 Rijn_Xanten 2268 14:08 29-12-2006 168 Rijn_Xanten 2269 2:25 30-11-2006 169 Rijn_Xanten 2277 14:53 25-11-2006 170 Waal_Vuren 2277 20:29 15-12-2006

Annexes

7.3 Migration routes of eel in order of importance

River Rhine-Cologne →River Rhine- River Rhine-Cologne → River Rhine-Xanten Xanten →Waal

River Rhine-Cologne → River Rhine-Xanten River Rhine-Cologne → River Rhine-Xanten → Waal → Beneden Merwede → De Noord → IJssel → North Sea

River Rhine-Cologne → River Rhine-Xanten River Rhine-Cologne → River Rhine-Xanten

→Nederrijn → Lek →Northsea → Waal → Dordtsche Kil → Oude Maas →

Annexes

North Sea

River Rhine-Cologne → River Rhine-Xanten River Rhine-Cologne → River Rhine-Xanten → Waal → Dordtsche Kil → Dordtsche Kil → → Waal → Dordtsche Kil → Waal → Haringvliet Stellendam noord →Haringvliet Dordtsche Kil → Oude Maas → North Sea Stellendam zuid → North Sea

River Rhine-Cologne → River Rhine-Xanten River Rhine-Cologne → River Rhine-Xanten → Waal →Beneden Merwede → Dordtsche → Waal →de Noord → Waal Kil → Spui →Oude Maas → North Sea

Annexes

River Rhine-Cologne → River Rhine-Xanten

River Rhine-Cologne → River Rhine-Xanten → Waal → Dordtsche Kil (of Beneden

→ Waal → Dordtsche Kil Merwede of Spui) → Oude Maas (11 days)

→ North Sea

Sieg →Sieg Menden → River Rhine-Cologne Sieg → Sieg Menden → River Rhine-Cologn → River Rhine-Xanten → Waal → Dortsche → River Rhine-Xanten Kil → Oude Maas → North Sea

Annexes

Sieg → Sieg Menden → River Rhine- Sieg → Sieg Menden → River Rhine-Cologne Cologne → River Rhine Xanten → Waal → → River Rhine Xanten → Waal de Noord → North Sea

Sieg → Sieg Menden

Annexes

7.4 Data of floy tagged eel.

Tagging data of floy tagged eel < total of > 70 total of date 50 - 70 cm 50 marked eels cm counted eels cm

Floy-tag, dorsal 10-8-2006 total ( marked ) 295 44 251 tag - no.: week 32 silver 81 34 47 0 81 0001 - 0275 1st intermediant 214 10 204 0 214 no. of used tags: yellow - 0 8 8 16 275 sum 44 259 8 311

Floy-tag, dorsal 17-8-2006 total ( marked ) 257 57 200 tag - no.: week 33 silver 54 32 22 0 54 0251 - 0513 2nd intermediant 203 25 178 19 222 no. of used tags: yellow - 0 2 3 5 262 sum 57 202 22 281

Floy-tag, dorsal 31.08.206 total ( marked ) 198 105 93 tag - no.: week 35 silver 121 96 25 0 121 0514 - 0715 3rd intermediate 77 9 68 0 77 no. of used tags: yellow - 0 10 8 18 201 sum 105 103 8 216

Floy-tag, dorsal 7-9-2005 total ( marked ) 619 168 451 tag - no.: week 36 silver 448 120 328 0 448 0717 - 1237 4th intermediate 171 48 123 22 193 no. of used tags: yellow - 0 2 55 57 520 sum 168 453 77 698

Floy-tag, dorsal 13-9-2006 total ( marked ) 188 34 154 tag - no.: week 37 silver 60 15 45 1 61 1238 - 1376 5th intermediate 128 19 109 8 136 no. of used tags: yellow - 0 21 17 38 138 sum 34 175 26 235

Floy-tag, dorsal 27-9-2006 total ( marked ) 460 141 319 tag - no.: week 39 silver 288 130 158 5 293 1448 - 1963 6th intermediate 172 11 161 47 219 no. of used tags: yellow - 0 20 25 45 515 sum 141 339 77 557

Floy-tag, dorsal 18-10-2006 total ( marked ) 138 39 99 tag - no.: 7th silver 100 38 62 0 100 1665 - intermediate 38 1 37 10 48 no. of used tags: yellow - 0 0 3 3 sum 39 99 13 151

Floy-tag, dorsal 26-10-2006 total ( marked ) 302 202 100

Annexes

tag - no.: week 43 silver 255 196 59 6 261 2136 - 2494 8th intermediate 47 6 41 25 72 no. of used tags: yellow - 0 4 0 4 358 sum 202 104 31 337

Floy-tag, dorsal 22-11-2006 total ( marked ) 170 89 81 tag - no.: week 47 silver 153 89 64 3 156 2495 - 2683 9th intermediate 17 0 17 0 17 no. of used tags: yellow - 0 0 0 0 188 sum 89 81 3 173 Recaptured eels with a floy-tag Country Location Fishinggear Floytagnummer Date Date recapture of NL 667 1-aug-06 Ketelmeer Fyke NL 526 Beginning sept Waal (Woudrichem) Fyke Schokker D 566 13-sep-06 Rhine, Kalkar, R-km 844,6 (links) "Anita" D 441 15-sep-06 Rhine, Cologne Rod D 259 18-sep-06 Rhine, Cologne (km 684) Rod D 506 18-sep-06 Rhine, Cologne Rod D 686 19-sep-06 Rhine, Cologne (km 686) Rod D 1030 20-sep-06 Rhine, Cologne (Severinsbrücke) Rod NL 24 26-sep-06 Hollands Diep Fyke NL 645 27-sep-06 IJsselmeer (Lelystad) Fyke NL 555 30-sep-06 Sliedrecht Fyke Schokker D 1795 3-okt-06 Rhine, Kalkar, R-km 844,6 (links) "Anita" NL 636 6-okt-06 Beneden Merwede Fyke NL 1472 6-okt-06 Beneden Merwede Fyke NL 959 9-okt-06 Haringvliet Fyke NL 117 10-okt-06 Caland kanaal Fyke NL 385 10-okt-06 Caland kanaal Fyke NL 1944 10-okt-06 Noord Hollands Diep Fyke NL 1723 11-okt-06 Nieuwe Waterweg (km 1015) Fyke IJsselmeer (Afsluitdijk NL 578 12-okt-06 Kornwerderzand) Fyke NL 882 17-okt-06 Caland kanaal Fyke NL 1026 17-okt-06 IJsselmeer (Hoornse Hop) Fyke NL 1719 18-okt-06 Noord Hollands Diep Fyke NL 695 23-okt-06 Haringvliet Fyke NL 1715 26-okt-06 Beneden Merwede Fyke D 463 27-okt-06 Rhine, Cologne (Porz-Westhoven) Rod NL 1168 31-okt-06 Beneden Merwede Fyke NL 778 7-nov-06 Sliedrecht Fyke NL 1801 7-nov-06 Kromme Rijn Electro NL 284 15-nov-06 Oude Maas Fyke NL 1495 18-nov-06 IJsselmeer (Afsluitdijk Oost) Fyke NL 2307 23-nov-06 Haringvliet Fyke NL 1045 30-nov-06 Nieuwe Waterweg Fyke

Annexes

7.5 Telemetry data of eels transpondered in 2005 and detected in 2006 sequencenumber Station number of transponder Date 1 Waal_Vuren 1678 20-1-2006 2 BenMerwede_BHardinx 1755 22-2-2006 3 Rijn_Xanten 1900 20-2-2006 4 Waal_Vuren 1900 26-2-2006 5 Waal_Vuren 1918 16-9-2006 6 DeNoord_Kinderdijk 1918 28-9-2006 7 Rijn_Xanten 1927 11-12-2006 8 DKil_s'Gravendeel 1928 6-5-2006 9 OudeMaas_Spijkenisse 1928 7-5-2006 10 OudeMaas_Spijkenisse 1928 9-5-2006 11 DKil_s'Gravendeel 1928 10-5-2006 12 BenMerwede_BHardinx 1928 16-5-2006 13 Nederrijn_Arnhem 1930 9-6-2006 14 IJsselmeer_Kornwerd 1957 15-5-2006 15 Waal_Vuren 1966 23-7-2006 16 DeNoord_Kinderdijk 1966 24-7-2006 17 DeNoord_Kinderdijk 1966 26-7-2006 18 BenMerwede_BHardinx 1966 27-7-2006 19 DKil_s'Gravendeel 1966 29-9-2006 20 DKil_s'Gravendeel 1966 30-9-2006 21 Waal_Vuren 1986 20-2-2006 22 BenMerwede_BHardinx 1986 20-2-2006

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