ICES CM 2006/J:04
Paper presented at the ICES Annual Science Conference, Session J: Is there more to eels than SLIME?,17-26 Sepember 2006, Maastricht, The Netherlands
Title: Assessment of population size and migration routes of silver eel in the river Rhine: a two-year combined mark-recapture and telemetry study.
Authors: Jan Klein Breteler1, Tim Vriese2, Jost Borcherding3, André Breukelaar4, Lothar Jörgensen5, Stefan Staas6, Gerard de Laak7 & Detlev Ingendahl8
Abstract The German-Dutch project “Rhine Silver Eel” started in 2004 and focuses at quantification of the female silver eel escapement of the whole Rhine system, the largest West-European river basin, by mark recapture and telemetry. By using telemetry the migration pathways of the downstream migrating eels released in Cologne (300-350 km from the sea, depending on the migration route) were tracked in the three main branches of the Rhine (Waal, Nederrijn/Lek, IJssel + Lake IJsselmeer) and in the complex network of river branches of the Rhine delta. Over 3,000 female silver eels > 50 cm were marked and released in Cologne in 2004 and in 2005, more than 4,000 and 6,000 respectively were checked for marks in the different Rhine branches close to the sea and 6 and 5 eels were recaptured in these years. Estimates of the population size of the downstream migrating female (> 50 cm) Rhine silver eel population in 2004 and 2005 were at most 2.0.106 and 3.5.106 respectively with corresponding biomasses of 1.0.106 and 1.9.106 kg and 95% confidence levels in the same order of magnitude. The silver eels migrated all day long and only in 2005 seemed to peak in migrational activity in the early evening. Downstream migration up to the sea took less than 2 days up to more than one year and generally occurred in October and November of the year of release. Most of the escapees seem to find their way to the sea via the Nieuwe Waterweg and not via Lake IJsselmeer or Haringvliet. The total percentage of escapees is 34% of the migrating eels of the 2004 cohort and 27% of the 2005 cohort (both up to 2006) passing Xanten. The telemetry data suggest that the Nederrijn/Lek, the only location where hydropower stations have been built in the lower Rhine system, might be relevant for downstream migration of eels only in years with higher discharges.
Introduction The European eel (Anguilla anguilla) stock is in strong decline and recruitment is as low as 1% of historic levels (Dekker 2004; ICES 2004). Therefore the European Commission has proposed a regulation establishing measures for the recovery of the European eel stock (EU, 2005). The regulation aims at 40% escapement of adult silver eel from each river basin, defined according to the Water Framework Directive (WFD) and measured with respect to undisturbed conditions. Monitoring programs of eel in the EU generally were focused on glass eel and yellow eel in the past and did not focus on silver eel however. Most of these also were not fishery independent. Actually there are only a few data rich local water bodies, the large lakes IJsselmeer (Dekker 2000, 2004) and Lough Neigh (Rosell et al. 2006) e.g., from which these targets could be derived from historical scientific data for the respective local water bodies; only for Lake IJsselmeer these data are fishery independent. Specific local targets could theoretically be derived from historical local data of yellow eel or glass eel abundance and biomass and information on local growth and mortality. The modelling and validation needed for that purpose, however will take some more time in the EU project SLIME, and in many cases local calibration would only be possible with new data of these life stages to be obtained in the
1 [email protected] 2 [email protected] 3 [email protected] 4 [email protected] 5 [email protected] 6 [email protected] 7 [email protected] 8 [email protected] future. Complicating is that it is generally not known in specific cases on a river basin level which factors (turbine mortalities or fisheries e.g.) are responsible for the losses of eels during lifetime and downstream migration. This limits the effectiveness of potential management measures, reduces the acceptation of fisheries measures by fisheries managers and makes the use of the SLIME models questionable. In some recent studies, in the small river Frémur (Feunteun et al. 2000) and in the larger river the Meuse (Winter et al. 2006), the current silver eel escapement has been quantified fishery independently on a river basin level, but real large river basins have not been studied for eel escapement as yet. Historical or actual data of the escapement of silver eels from the whole Rhine system that includes Lake IJsselmeer and is the largest West-European river basin, was lacking until now for instance. Therefore several German states (NorthRhine-Westfalia and Rhineland-Pfalz) and The Netherlands started the Rhine Silver Eel (RSE) project in 2004 (Klein Breteler et al. 2005). One of the targets of this project is to make fishery independent estimates of the yearly total Rhine silver eel run by mark-recapture and the use of telemetry. The RSE project also aims at a quantitative evaluation of the downstream migration routes for the silver eels in the Rhine delta because this will support optimizing the efficiency of the measures to be taken at a local scale. The full project not only deals with the quantity of silver eels migrating downstream the Rhine, but also with its health status (Haenen et al. 2006). From a viewpoint of conservation, large female eels (> 70 cm or > 0.7 kg) are more important than smaller ones or than males (EELREP 2005). Therefore this paper focuses on the quantification of the female part of the whole downstream migrating Rhine silver eel population in 2004 and 2005, and the different migration routes of these female migrants in the lower Rhine.
Material and methods
Study area
With a catchment area of 185,000 km2, a length of 1,320 km and an average discharge of more than 2,000 m3/s the Rhine is one of the longest and most important rivers in Europe. The greater part of the river (1,000 km) is situated in Germany where it is joined by several large tributaries like the Neckar, the Main and later the Moselle, the latter adding an average discharged of 150 m3/s to the total volume. Entering the Netherlands at Spijk the Rhine has a width of more than 300 m. The Rhine divides into the Waal and via the Pannerdens kanaal into the other two distributaries Nederrijn (which becomes the Lek near Wijk bij Duurstede) and IJssel, which flows into lake IJsselmeer. The greater part of the discharge flows via the Waal (two thirds), the IJssel receiving one ninth and the Nederrijn two ninths of the discharge under normal conditions. The Waal, which becomes Boven Merwede near Gorichem, flows via the Nieuwe Merwede and the Beneden Merwede, merging with the Meuse, through the Hollands Diep and Haringvliet estuary and the Oude Maas, which flow into the Nieuwe Waterweg, into the North Sea. Part of the Beneden Merwede branch continues as the Noord to join the Lek near the village of Kinderdijk and forms the Nieuwe Maas, which flows past Rotterdam, continues as the Nieuwe Waterweg and enters the North Sea. In the river stretch Nederrijn/Lek there are three large weirs with shipping sluices near Driel, Amerongen and Hagestein. Fish ladders are present at the weirs. There are two hydropower stations, one near Amerongen, with a maximum power output of 10 Mwe and a smaller one at Hagestein with a capacity of 1.8 Mwe. Both are fitted with horizontal Kaplan bulb turbines, the entrances being protected by trash racks with 10 cm spacing. Fisheries in the Rhine are most intensive in the downstream sections of the river. Mostly large fykenets set on poles near the shore are used especially for silver eel, but also small fykenets set in trains on the bottom are used. In the upstream sections of the river there is also some electrofishing. Although there is insight in the numbers of professional fishermen present, the exact fishing pressure is not known. Currently there is a study underway to establish the fishing pressure in the coastal area’s and in the various river stretches which also focuses on the bycatch of salmonids.
Eel in the experiment
The research strategy is based on the use of routine catches of silver (and intermediate) eels in the Moselle. The downstream migrating silver eels from the Moselle are caught annually upstream from the hydropower stations by the Struktur und Genehmigungsbehörde Nord, Rheinland-Pfalz. These eels, 3,000-5,000 kg/year, are trucked and released routinely downstream of these obstructions in the migration routes or directly into the Rhine near the confluence of Rhine and Moselle. As most of these eels are longer than 50 cm and males always are smaller than that size, the project focuses at the female part of the population > 50 cm.
Mark recapture
The eels were batch marked at the Moselle and at the Rhine near Cologne in various weeks spread through August – November, starting in the beginning of August. In 2004 the target was the batch marking of 2,000-4,000 eels. In 2005 the target was set at 6,000 eels. The batch mark used in this study was heliogenblue, applied with a panjet inoculator. The location of the batch mark is at the ventral side of the eel. In 2004 distinct batch marks for successive months were used (near the anal fin, mid ventral between pectoral and anal fin, near the pectoral fins). In 2005 batch marks were applied at the ventral side, 2 cm caudal from the pectoral fins. In both years eels were classified as silver, intermediate and yellow on external appearance. Only silver eels and intermediate eel were marked. Size was estimated by eye as >70, 50-70 and <50 cm. The last group was not marked. All handling and making was done without anesthetization.
Table 1. Numbers of marked eels > 50 cm, silver or intermediate, in 2004 and 2005. August September October November Totals 2004 1,793 763 373 252 3,181 2005 984 1,126 780 376 3,266
In total 3,181 silver and intermediate eels were marked in 2004, thus the target more or less reached (table 1). In 2005, 3,266 individuals were marked which is slightly more than half of the target in 2005, due to low catches of eel in the Moselle. Recapture of eels was done by checking the catches of commercial fishermen fishing with fykenets in the most downstream parts of the Rhine branches in the Netherlands. Recapture locations were selected in downstream sections of [1] Waal (Boven Merwede, Nieuwe Merwede and Beneden Merwede), [2] Lek and [3] IJsselmeer near the Afsluitdijk, which separates Lake IJsselmeer from the Waddensea. When catches were landed, they were sorted by hand for silvering status (silver or intermediate) and for length (50-70 and >70 cm) and checked for the presence of batch marks applied either in 2004 or in 2005 and for the presence of tags (transponders). A sub sample of eels was measured each time.
Table 2. Numbers of eels > 50 cm, silver and intermediate, checked for marks in the most downstream parts of the Waal, Lek and IJsselmeer in 2004 and 2005. Location August September October November December Totals 2004 Waal 0 518 1,094 0 109 1,721 Lek 212 491 0 66 769 IJsselmeer 276 842 685 0 1,803 Total 4,293
2005 Waal 1,124 924 1,779 417 11 4,255 Lek 0 165 257 0 100 522 IJsselmeer 0 361 989 393 0 1,743 Total 6,520
In 2004, 20 visits were made to professional fishermen in the period September – December and in total 4,239 silver and intermediate eels were checked for batch marks (table 2). In 2005 30 visits were made to professional fishermen in the period August – December and in total 6,520 silver and intermediate eels were checked for batch marks.
Telemetry system
In this study the NEDAP TRAIL System® was used (Breukelaar et al., 1998), which is based on the inductive coupling between an antenna loop and ferrite rod antenna within transponders. The tracking system can be used in rivers, canals and estuaries. The system consists of detection stations (each of these stations contains three parallel antenna cables on the river bottom covering the entire width) and implantable transponders. The network of detection stations is shown in figure 1 (situation on 1 October 2005) and the station names are given in table 3. Every 4 seconds the transmitter at the detection station 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 cylindrical 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 and a length of 10 mm, ensuring a lifetime of at least one year. Weight of a transponder is 26.5 g in air, 16.0 g in water and its volume is 9.5 cm3. 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 does not affect transmission when the distance between the antenna and the transponder is < 15 m (Breukelaar et al., 1998). The lifetime of the used transponders is 1.5 – 2 years.
Figure 1. Network of detection stations
Table 3. Names and locations of the detection stations in Rhine and Meuse Detection River (branch) or Location Start date station lake nr. 2 IJssel Kampen 1-12-1996 5 Spui Zuidland 1-12-1996 6 Dordtsche Kil ‘s-Gravendeel 1-12-1996 7 De Noord Alblasserdam 1-12-1996 8 Beneden Boven Hardinxveld 1-12-1996 Merwede 9 Lek Nieuwegein 1-12-1996 10 Waal Vuren 1-12-1996 11 Bergsche Maas Capelse Veer 1-12-1996 (Dussen) 12 Grensmaas Stevensweert 1-12-1996 13 Rhine Xanten (BRD) 22-5-1997 14 Oude Maas Spijkenisse 3-11-1997 15 Roer St. Odiliënberg 19-6-1998 16 Sieg Menden (BRD) 1-7-1998 19 IJsselmeer Den Oever 24-2-2000 18 IJsselmeer Kornwerderzand 24-2-2000 25 Meuse Sambeek 21-9-2000 23 Meuse Balgoij 21-9-2000 26 Meuse Afferden 21-9-2000 24 Meuse Sambeek 1-10-2001 downstreams weir 22 Haringvliet Stellendam zuid 1-11-2001 21 Haringvliet Stellendam noord 3-12-2001 28 Meuse Borgharen 10-10- 2001 27 Meuse Itteren 1-5-2003 K4 Meuse Linne wkc 4-9-2002 R2 Meuse Linne vistrap 9-9-2003 K3 Meuse Linne dorp 27-8-2002 K2 Meuse Alphen wkc 4-9-2002 R1 Meuse Alphen vistrap 9-9-2003 K1 Meuse Lith dorp 27-8-2002 31 Nederrijn Arnhem 24-2-2005 30 Nederrijn Maurik 16-3-2005 29 Nederrijn Hagestein 17-3-2005
Surgical procedures and implantations
The eels used for the telemetry part of this study also originated from the Moselle and were 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”), ≥ 64 cm in 2004 and ≥73 cm in 2005 were used for implanting transponders. In several other studies transmitters or transponders have been successfully implanted in eels (Baras & Jeandrain, 1998; Winter et al., 2006). In this study the same surgical implanting procedure was used as in aforementioned study, with several slight changes. As an anaesthetic benzocain (80 mg/l) was used instead of 2-phenoxy-ethanol (0.9 ml/l). The gills of the anaesthetised eel were permanently flown through with oxygen saturated river water. In 2005, a tissue adhesive for humane purposes was used (Histoacryl®) instead of LoctiteTM glue. Additionally the glued incision was strengthened with one suture (Vicryl 3-0, V452G, FS-1 24,0 mm, 3/8c). By making a mid ventral incision, 3 cm long, the transponder was surgically implanted in the posterior quarter of the body cavity. The incision was closed using Histoacryl® tissue glue and one suture was applied. After the surgical procedure was finished and the measurements were taken, the eel was released individually into the river. Tagged eel had the same external marking as the marked eels in the mark-recapture part of the study. Inside the transponder there was a label with a telephone number, a forwarding address and the promise of a reward. For every tag returned a reward of € 35 was offered. In 2004 150 silver eels were implanted at the boathouse in Cologne in September (N = 25), October (N = 88) and November (N = 37). Of the 150 eels tagged 74% were classified as intermediate and 26% as silver. In 2005 157 silver eels were tagged. Implantation took place at the same location in August (N = 30), September (N = 81) and October (N = 46). Of the 157 tagged eels 68% were classified as intermediate and 32% as silver.
Data analysis
With regards to the mark recapture part of the experiment, it originally was the intention to estimate the amount of silver eel migrating along the different branches of the Rhine system and to estimate the total population in the Rhine. As the number of recaptures was very low, only a very tentative estimate of the total population in the Rhine was made, using the unbiased modified Lincoln-Petersen method, which assumes that the ratio of marked individuals (M) to population size (N) is equal to the ratio of recaptured fish (R) to the catch taken for census (C) (Ricker, 1975; Pollock et al, 1990):
N = (M + 1) * (C + 1) * (R + 1)-1
To calculate SD, R was treated as a binomial variable (low numbers of eels recaptured, < 25) and the variance V (with SD = √ (v)) was estimated according to Seber (1982):
V = ((M + 1) * (C + 1) * (M – R) * (C – R)) / ((R + 1)2 * (R + 2))
Mark recapture data were corrected for the proportion of silver eels not migrating, as concluded from the results of the telemetry part of the study.
Data, collected by using the NEDAP TRAIL System®, up to 15 January 2006 were used. From the collected data each individual eel was followed on its migration to the sea or to the detection station it was detected for the last time. In some cases it became clear from more downstream detections 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. These were repaired and missing detections were corrected for the downstream detections. 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 of the same transponder 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 with salmonids 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). When transponders were returned, transponder number, date of catch, fishing gear, location, name and address was registered. In a single case a transponder was returned anonymously. Also very seldom, catch of eel was reported from a very different region (Lake IJsselmeer), when last detections were from the lower stretches of the Rhine. When the location of catch was not known, the river stretch downstream of the station of last detection was assigned to it. Based on the detection data the relative importance of different routes, the timing and swimming speed of migration, and the mortality and escapement of eel were determined. The number of escapees was derived from the detections in the stations close to the sea and the information on seaward migration from individual eel detections.
Results
Mark recapture
The statistics of the recaptures of eel batch marked in 2004 en 2005 are given in Table 4.
Table 4. Statistics of recaptured silver eel Year Date Number Length Location (river branch) 2004 22nd September 3 67, 73, 85 cm Boven Merwede 15th October 2 82, 86 cm Beneden Merwede 16th October 1 76 cm Lek 2005 4th October 2 74, 91 cm Beneden Merwede 20th October 2 60, 74 cm Beneden Merwede 8th November 1 71 cm Nieuwe Merwede
It was originally the intention to estimate the number of silver eel migrating along the different branches of the Rhine system and to estimate the total population in the Rhine. The numbers of recaptures were to low to achieve this, so only a very tentative estimate was made, using the unbiased modified Lincoln-Petersen method. Two calculations were made, one using the total number of batch marked individuals in the years 2004 (N = 3,180) and 2005 (N = 3,266) and one correcting for the total number of downstream migrants as revealed in the telemetry study. In 2004 and 2005 the percentage of downstream migrants, as measured at station 13 (Xanten), was respectively 62% and 53% (Ncorr. 2004 = 1,972; Ncorr. 2005 = 1,731). In Table 5 the results of population estimates are given for both years (not corrected and corrected for the amount of downstream migrants). Estimates of the population size of the silver eel population > 50 cm of the whole Rhine system in 2004 range from 1.2–1.9 million individuals with an associated biomass of 0.6-1.0 million kg, depending on the underlying assumptions. For the year 2005 the range is 1.9-3.6 million individuals with an associated biomass of 1.0–1.8 million kg. The biomass has been calculated by multiplying the numbers with the average weight of the eels checked for batch marks in 2004.
Table 5. Marked and released female eels (> 50 cm) at Cologne (M), number of eels > 50 cm checked for marks in downstream sections of the Rhine (C) and recaptures (R) and population estimates of the total Rhine silver eel population > 50 cm in numbers (N) and biomass (B) in 2004 and 2005. Not corrected for non migrants
Year M C R N SDN B (kg) 2004 3,180 4,239 6 1,926,777 679,907 1,011,558 2005 3,266 6,520 5 3,550,685 1,340,183 1,864,109
Corrected for non migrants
Year M C R N SDN B (kg) 2004 1,972 4,239 6 1,194,832 421,338 627.287 2005 1,731 6,520 5 1,882,374 709,910 988,246
Telemetry
Eel detection and migration activity
Up till January the 15th 2006, 93 eels out of the 150 (62%) tagged in 2004 were detected (212 different registrations) on the NEDAP TRAIL stations. Of the 157 eels tagged in 2005, 83 eels (53%) were detected (182 different registrations) up till then. Comparing the detected amount of the 2005 group with the number of detections of the 2004 group on January the 15th 2005, it is almost the same (55% in 2004; 53% in 2005). Also in both years, one silver eel was caught by a recreational fisherman (2004: Cologne, river km 684, 2005: Düsseldorf, river km 747), using dead fish as bait. It is generally assumed that the time of detection of an eel on a station is indicative for its migration activity. It is also assumed that eels migrate preferably during the night. In our study the differences in activity during night and day were only slight. From the 212 detections of eel in 2004, 100 were between 6:00 and 18:00 hours (day) and 112 were between 18:00 and 6:00 hours (night). From the 182 detections of eel in 2005 these figures were respectively 89 (day) and 93 (night). Figure 2 shows the percentage per hour of total detections for both years. Only in 2005 there was a higher number of detections in the early evening (19:00 hours). In both years the downstream migration occurred predominantly in October and November, as can be seen in figure 3. The migration in the fall of 2004 seems more concentrated, probably due to higher peaking discharge events in this period, compared to the rather low and stable discharge in the same period in 2005 (roughly half of that in 2004). In the spring of 2005 there is again a small peak in migration activity, corresponding with a high discharge event.
12 2004 2005 10
8
6
4 % of total detections
2
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Time (hours)
Figure 2. Percentage per hour of total detections for 2005 and 2005
120
100
80 Released 2004 Detections 2004 60 Released 2005 Number Detections 2005 40
20
0 Jul-05 Jun-05 Oct-05 Jan-06 Oct-04 Jan-05 Feb-05 Mar-05 Apr-05 Sep-05 Nov-05 Dec-05 Sep-04 Nov-04 Dec-04 Aug-05 May-05 Months
Figure 3. No. of silver eels released for the telemetry part of the study and no. of silver eels detections in 2004 and 2005. Migration routes of eel in the Netherlands, mortality and escapement to sea.
Figure 4 shows the individual movement patterns of silver eel in time along the Rhine and its different branches. Each line represents one individual, starting at the release site, connecting its consecutive detections on the various stations at a certain distance from the sea. A large variation in the individual migration patterns occured, from very fast downstream movement (from release site to sea in under 2 days) to delayed migrations starting months after being released. Also, eels interrupt their migration and stay for months in certain river sections. The longest time (almost 10 months) in the river system was for one eel that was tagged and released on the 21 October 2004 and detected for the last time on August the 6th 2005, which is thought to have reached the North Sea.
Three eels in 2004 and one in 2005 travelled the distance between the release site (Cologne) to Xanten (154 km) within 24 hours, which equals an average migration speed of 1.78 m/s, well above the average flow speed of the Rhine (approx. 1 m/s) indicating an active downstream migration or selective use of the fast currents. The most important migration route in both years is: release site Cologne (Rhine) → Xanten (Rhine) → Vuren (Waal) → Boven Hardinxveld (Beneden Merwede) → Alblasserdam (De Noord) → North Sea (15 eels of the 2004 group; 19 eels of the 2005 group). Of the 2004 group more eels migrated via the Nederrijn/Lek (7 eels) and only 4 via the IJssel. Of the 2005 group 9 eels preferred the IJssel and 3 eels initially chose for the Nederrijn/Lek but returned for the weir at Driel (2 finally migrated via the IJssel (11 total) and one via the Waal). The choice of migration routes was clearly discharge related. Normally the total discharge of the Rhine is divided in 2/3 via the Waal, 2/9 via the Nederrijn/Lek and 1/9 via the IJssel. However, in the relevant months of 2005 there was very little discharge in which situation the sluices in the Nederrijn/Lek are closed to preserve water for shipping purposes in the IJssel. Figures 5 and 6 show the numbers of eels that migrated respectively in 2004 and 2005 along the different branches in the system.
Considering the 2004 group, 57 eels (38%) were not ready to migrate or suffered some kind of mortality (tagging or fisheries) in the upstream parts of the river. Of the 93 eels entering the Netherlands, 34 individuals (37%) are supposed to have reached the sea, almost all via the Nieuwe Waterweg near Rotterdam (one eel via the IJssel, Lake IJsselmeer and the Afsluitdijk, detected on the station Den Oever). Downstream from Xanten 25 eels (27%) are lost in the river stretches between this location and the detection stations on the IJssel (Kampen), the Nederrijn (Arnhem) and the Waal (Vuren). Of the eels (4) entering Lake IJsselmeer only one individual succeeds in escaping to the Waddensea. Quite a high number of eels (57) is detected on station Vuren (Waal), of which 16 eels (28%) are never detected again. No eels were detected escaping via the Haringvlietdam.
Of the 2005 group, up till the 15th of January 2006, 83 eels (53%) were detected, passing station Xanten on the Rhine. Of the total group 47% did not migrate as of yet. After the 15th January several eels were detected, but these results could not be incorporated in this paper. Nevertheless the results of this group are quite similar to the 2004 group. Of the 83 individuals entering the Netherlands, 23 individuals (28%) are supposed to have reached the sea, all via the Nieuwe Waterweg. It is believed that this number will rise when the total period until the fall of 2006 is considered. Downstream from Xanten 29 eels (35%) are lost in the river, which is slightly higher than in the 2004 group. Although the number of eels reaching Lake IJsselmeer is more than double compared with the 2004 group (probably due to the discharge situation), no escapees are registered on the detection stations in the Afsluitdijk. Of the 2005 group the number of reported recaptures was relatively high, 10 transponders were returned back of which 4 came from professional fishermen from Lake IJsselmeer. Also in 2005/2006 the largest numbers of eels are passing detection station Vuren on the Waal (44 individuals). Of these group 15 eels are not detected again (34%). Two returned transponders came from Hollands Diep / Haringvliet, one was reported anonymously, one was reported from Germany (fish trader) which was alleged to come from Lake IJsselmeer but was last detected on the Beneden Merwede and two transponders were found near the shoreline.
4 4 5 0 -04 0 05 0 -05 05 05 -05 05 p- c- -05 -05 t- i 05 c- -06 kt-04 b r pr- e p- kt-05 aug-04 se o nov de jan fe m a m jun- jul- aug-05 se o nov de jan 370 Release Release Rhine, Cologne 350
330
310
290
270
250
230 Rhine, Xanten 210
190
170
Rhine, Arnhem 150
Distance to sea (km) sea to Distance 130
110
Rhine, Nieuwegein 90 IJssel, Kampen Waal, Vuren Beneden Merwede 70 Dordtsche Kil De Noord 50 Oude Maas North Sea 30 Spui/Zuidland 10 Lake IJssel, Den Oever Waddensea -10
Figure 4. Individual movement patterns of eel (August 2004 – January 2006) from site of release to point of last detection (detections on various stations are connect by a line). Eels last detected on the stations 9 (Lek Nieuwegein), 7 (De Noord Alblasserdam) and 14 (Oude Maas Spijkenisse) and moving in downstream direction are supposed to have reached the North Sea as there is no detection station present at Nieuwe Waterweg, Rotterdam
Figure 5, Numbers of eels migrating along different Rhine branches tagged in 2004 (including double counts due to migration of the eel between stations), starting with 150 in Cologne, and numbers reaching the sea (escapees).
Figure 6. Numbers of eels migrating along different Rhine branches in tagged 2005 2004 (including double counts due to migration of the eel between stations), starting with 157 in Cologne, and number reaching the sea (escapees).
Discussion
The estimates of the population size of the downstream migrating female Rhine silver eel population in 2004 and 2005 were of the same order of magnitude which suggests that the results are reliable unless systematic errors occurred. Mark-recapture studies for estimating the size of a population rely on the assumptions that all marked specimen have the same chance of being (re)captured as the unmarked ones (including equal mortalities), that the population is closed (or immigration and emigration terms are known and compensated for) and that there is no mark loss and full mark detection. In this study we have treated the downstream migrating female (> 50 cm) part of the silver eel population of the whole Rhine system as a population that was closed in time (not in space, because many of them escape to the sea) during the downstream migration season. The marked eels were caught in the Moselle and released 350 km upstream in the River Rhine, allowing them to mix with the downstream migrating population, and recapture efforts were made at a distance of 50 km or less from the sea. The population studied therefore does not include the female silver eels living closer to the sea or in the coastal zones belonging to the Rhine system according to the Water Framework Directive (WFD).
The recapture efforts in 2005 were scheduled proportionally distributed over the different river branches according to the normal discharge conditions. Two thirds of the fishing effort for recaptures was concentrated in the Waal in 2005 therefore, because the Waal receives two thirds of the total Rhine discharge under normal conditions. This was based on the pilot data from 2004 (Klein Breteler et al. 2005) which seemed to illustrate that the migrating silver eels distributed according to the discharges in the different river branches. The recapturing effort also was concentrated (70%) in the weeks between the last moon quarter and full moon in 2005, anticipating on peak migration during these periods (Tesch 1999). Therefore we believe that the marked silver eels mixed randomly in the population and that we additionally sampled for recapture purposes proportionally to the mixed population in different water bodies and in different time periods.
A number of silver (and intermediate) eels that were marked and released in this study most probably did not migrate downstream during the migration season however. Several studies (Durif 2004, Durif et al. 2005) show that the silvering state of an eel easily can be misinterpreted from external appearance and that this may result in up to 3 years of delay in the downstream migration (Winter et al. 2006). Such eels can be considered as emigrants in our mark-recapture study because they did not make part of the population sampled for recaptures in the downstream parts of the Rhine. We compensated for these by combining the telemetry part of this study with the mark-recapture part and made separate estimates of the population size corrected for the number of marked individuals for non-migrants as evidenced by our telemetry data. These are approximately 50% lower than the uncorrected estimates. Our corrected estimates of the population size might be biased if eels provided with a transponder suffer selective mortality due to the implantation of the transponder. In that case the population estimates will fall between the uncorrected and the corrected ones. The corrected estimates might also be biased by differences in (migration) behavior of larger females as compared with smaller ones, because the mark-recapture study focused on the female population > 50 cm and in the telemetry study only females > 64 cm (2004) or > 70 cm (2005) were used. Most of the eels marked and released were of the size class 50-70 cm. The chance of downstream migration of female eels increases amongst others with size (Durif et al. 2005, EELREP 2005). Such possible size dependent behavioral differences therefore will induce an overestimation of the actual population size but cannot result in higher estimates than the uncorrected ones.
Mark-recapture studies of the population size also are sensitive for the assumption of equal mortality of marked and unmarked individuals. A random sample of the eels caught in November 2004 and provided with comparable heliogenblue marks was tested therefore for survival and mark loss in tanks flown trough with pond water in a fish culture station during 40 days. Most of the eels got diseased and 38% of the control group, provided with the heliogenblue mark only, died. There was no mark loss in the control group. The crowded conditions and low prevailing water temperatures in the holding tanks probably negatively effected the survival of the eels (Klein Breteler et al. in prep.) and we do not believe that such disease occurred in the marked or unmarked population in this study. Anyhow, we did not compensate for selective mortality of the marked eels and the population size of the female silver eels in the Rhine basin may be overestimated therefore in this study. As the detection and recovery of marked eels was done by trained and fishery-independent personnel, we assume that the population estimate is not biased by unreported marks. Overlooking all possible errors and biases in the population estimates, we conclude that the population size of the downstream migrating female Rhine silver eel population in 2004 and 2005 was not exceeding 2.0.106 and 3.5.106 respectively with corresponding biomasses of 1.0.106 and 1.9.106 kg and 95% confidence levels in the same order of magnitude.
The telemetry part of the study showed that most of the eels provided with a transponder migrated in the same year. Some eels marked in 2004 were detected downstream in 2005 however and 38% and 47% of the marked individuals in the respective years were not detected at all. The latter may be due to delayed migration (Durif 2004, Durif et al. 2005, Winter et al. 2006) but also could be caused by mortality or battery lifetime of the transponders. Since the transponders used with full batteries have a lifetime between 1.5 – 2 years this can be excluded as a possible cause. Effects of transponder implantation in silver eels on mortality and behavior were tested in other research (Winter et al., 2005). No differences were found in mortality between eels with or without transponders. No loss of transponder, expulsion or encapsulation was found. But the eels with transponders showed a lower activity level than the control group. Klein Breteler et al. (in prep.) showed that implantation of a transponder in silver eels caught in 2004 in the river Moselle indeed did not significantly induce mortality, but that the surgery wounds of many eels were more or less severely inflamed and sometimes spontaneously reopened after some time period (without resulting in loss of transponders). Also Winter et al. (2004) found inflamed surgery wounds in 41% of the eels with a transponder. Therefore the surgery procedure in 2005 was improved by using Histoacryl® instead of LoctiteTM and the glued incision was strengthened with one suture. We also heightened the minimal size of the silver eels provided with a transponder in 2005 (from 64 to 70 cm) because it appeared to be physically difficult to close the surgical wound after implantation of the transponder in some eels of even 70 cm or 700 g (Klein Breteler et al. in prep.). The effectiveness of these improvements in the methods has not been evaluated as yet.
Most of the silver eels migrated via the Waal and not via the Nederrijn/Lek or via the IJssel and Lake IJsselmeer. The migrating numbers in the IJssel were higher in 2005 (11) than in 2004 (4) and only 1 eel of these probably escaped to the Waddensea as evidenced by the detections in stations 18 and 19. Although this meant a mean loss of 93%, probably due to the high fishing pressure in this area (Dekker 2000, 2004), many more silver eels with a transponder released in Cologne disappeared in the southern Dutch parts of the Rhine system (in the Waal and in the lower branches finally flowing into Haringvliet and Nieuwe Waterweg) and in the German part of the Rhine between Cologne and station 13 (Xanten, near the Dutch Border). For the present we assume that the latter probably results from delay in the migration up to the first downstream detection station (13, Xanten) due to an incomplete silvering process in the eels. From the viewpoint of a migrating eel the situation in the lower branches of the Rhine (Spui, Dortsche Kil and the Noord) is rather complex. Because of tidal influences and management of the sluices of Haringvliet the direction of the flow in these branches changes regularly, resulting in conflicting migrational clues for the eel. We tracked the migration pathways of all eels individually and observed that they sometimes travelled upstream and downstream these branches albeit always against the current. This type of behaviour might result in prolonged stay in these branches and hence a risk for increased fisheries or natural mortality. We did not track any silver eel in station 21 and 22 (Haringvliet, at the sluices), but know that there is a large eel fishery in Haringvliet and it is suspected therefore that fisheries mortality also plays a significant role in Haringvliet. Anyhow, most of the escapees seem to find their way to the sea via the Nieuwe Waterweg and not via Lake IJsselmeer or Haringvliet. The total percentage of escapees is 34% of the migrating eels of the 2004 cohort and 27% of the 2005 cohort (both up to 2006) passing Xanten. But there is some uncertainty about the number of escapees presented here because of the fact that there is no detection station present near the mouth of the Nieuwe Waterweg (physically impossible) and because of the presence of some fisheries downstream from the detection stations 7, 9, and 14. Thus, the actual entering of the sea can not be observed in a comparable way as at the Waddensea (stations 18 and 19) and Haringvliet (stations 21 and 22). The total percentage of escapees of the 2004 cohort is nearly identical with the 37% found for migrating silver eels in the Meuse system (Winter et al. 2006), which also discharges in Haringvliet. There are 2 hydropower stations in the Meuse accounting for comparable mortality as for fisheries. Mortality due to hydropower in the lower Rhine system only may occur in the Nederrijn/Lek and not more than 7 eels tried to pass this branch of the river Rhine in 2004 (6 successfully). In 2005 only 3 eels were detected upstream the weir at Driel (station 31 built in February 2005) but all returned and continued their way via the IJssel (2) and Waal (1). Under relatively low discharge conditions of the River Rhine, such as occurred in 2004 and 2005, the discharge via Nederrijn/Lek is kept low by the weir at Driel for shipping purposes in the River IJssel. This seems to reduce the risk of turbine mortality for downstream migrating silver eels in the lower Rhine system in such years.
The practical use of the data presented in this paper for the management of the European eel is that the estimates of the female Rhine silver eel population allow for a first comparison with data on fisheries mortality. More than 50% of The Netherlands belongs to the Rhine river basin. The total Dutch annual inland silver eel catch is, except for the Scheldt system (province Sealand), 1.0.105 kg in the large rivers (Meuse included), 0.4.105 kg in Lake IJsselmeer and 1.4.105 kg in the other inland waters (estimate from Aalcomité 2005). Because the River Meuse is included in the latter, this is in total less than 28% and 15% of the estimated female silver eel run in 2004 and 2005 respectively. The fishery independent telemetry data show however that 65% of the silver eels passing Xanten seems to disappear somewhere in The Netherlands. We suppose that continuing of the Rhine Silver Eel project in the next years and the registration of the fisheries data according to the EU-regulation (EU 2005) both will help to explain this discrepancy and will give support to effective management measures. The mark-recapture estimates of the whole Rhine silver eel population also may be compared with similar estimates from other large river basins and may help then to prioritize management measures, for instance with the Loire system (Feunteun et al., 2003) or the whole Baltic. The telemetry data presented show that most of the Rhine silver eels from the upstream parts of the Rhine migrate via the Waal. In reducing impacts on this population priorities should be given to measures in the Waal and more downstream from the Waal lying sections of the Rhine. We expect that turbine mortality in the Nederrijn/Lek possibly only is important in years with higher discharges.
Acknowledgements
The project was made possible by using the eels caught in the yearly trap-and-release actions by the Struktur und Genehmigungsbehörde Nord (Rheinland-Pfalz), intended to help the silver eels with passing the hydropower stations in the river Moselle. The Rheinfischereigenossenschaft of Northrhine- Westfalia, the Landesanstalt für Ökologie, Bodenordnung und Forsten NRW (LÖBF), the Rheinischer Fischereiverband Nordrhein e.V., the Organisation ter Verbetering van de Binnenvisserij (OVB) and Sportvisserij Nederland all contributed to the field work. The Combinatie van Beroepsvissers and the PO Nederlandse Vissersbond-IJsselmeer mediated in cooperation of the Dutch professional fishermen. The Universität zu Köln (Zoologisches Institut, Allg. Ökologie und Limnologie) made the Bootshaus in Cologne available as a field and laboratory facility. The Rijksinstituut voor Integraal Zoetwaterbeheer en Afvalwaterbehandeling made the antenna infrastructure available for the telemetry and analysed the primary telemetry data. OVB and LÖBF coordinated in 2004 and 2005 respectively and are acknowledged for partly funding the project. The study was partly funded by EU through the Financial Instrument for Fisheries Guidance (FIFG) to Rheinfischereigenossenschaft. The authors acknowledge the participation of staff of all partners to the field work, especially G. Feldhaus, A. Hehenkamp and J. Merkx.
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