ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on September 5, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(7):1450-1456; doi:10.1093/icesjms/fsm130
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Assessment of population size and migration routes of silver eel in the River Rhine based on a 2-year combined mark-recapture and telemetry study
1 VIVION BV, Händelstraat 18, 3533GK Utrecht, The Netherlands
2 Visadvies BV, Vondellaan 14, 3521 GD Utrecht, The Netherlands
3 Zoologisches Institut, Universität zu Köln, Allgemeine Ökologie und Limnologie, Ökologische Forschungsstation, 46459 Rees-Grietherbusch, Germany
4 Rijksinstituut voor Integraal Zoetwaterbeheer en Afvalwaterbehandeling, Postbus 17, 8200 AA Lelystad, The Netherlands
5 Struktur und Genehmigungsbehörde Nord Rheinland-Pfalz, Postfach 200361, 56003 Koblenz, Germany
6 Rheinfischereigenossenschaft im Lande Nordrhein-Westfalen, Römerhofweg 12, 50374 Erftstadt, Germany
7 Sportvisserij Nederland, Postbus 162, 3720 AD Bilthoven, The Netherlands
8 Bezirksregierung Arnsberg, Heinsbergerstr. 53, 57399 Kirchhundem-Albaum, Germany
Correspondence to D. Ingendahl: tel: +49 272377 940; fax: +49 272377 977; e-mail: detlev.ingendahl{at}bezreg-arnsberg.nrw.de
Klein Breteler, J., Vriese, T., Borcherding, J., Breukelaar, A., Jörgensen, L., Staas, S., de Laak, G., and Ingendahl, D. 2007. Assessment of population size and migration routes of silver eel in the River Rhine based on a 2-year combined mark-recapture and telemetry study. – ICES Journal of Marine Science, 64: 1450–1456.More than 3000 female silver eels >50 cm were marked and released in the River Rhine at Cologne in 2004 and 2005, and more than 4000 and 6000 per year, respectively, were checked for marks in the different Rhine branches close to the sea. Migration pathways of downstream-migrating eels were also tracked by telemetry from the point of release (300–350 km from the sea, depending on the migration route) through the three main branches of the Rhine (Waal, Nederrijn + Lek, IJssel + Lake IJsselmeer) to the sea. Downstream migration to the sea took from <2 d to more than a year, but was generally in October and November of the year of release. Most successful migrators seemed to find their way to the sea via the Nieuwe Waterweg rather than via Lake IJsselmeer or Haringvliet. Some 23% of released eels of the 2004 cohort and 15% of the 2005 cohort made it to the sea in less than 2 years. The telemetry data suggest that the Nederrijn + Lek watercourse, the only location where hydropower stations have been built in the lower Rhine system, might be important for downstream migration of eels only in the years with greater discharges, suggesting that management measures should concentrate on the Waal and downstream sections to improve spawning escapement of the silver eel population of the Rhine system.
Keywords: Anguilla anguilla, eel, mark-recapture, migration, population size, Rhine, silver eel, telemetry
Received 3 January 2007; accepted 13 July 2007; advance access publication 5 September 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The European eel (Anguilla anguilla) stock has declined to the point where recruitment is as low as 1% of historical levels. The European Commission has proposed a regulation (EU, 2005) that aims at a minimum 40% escapement of adult silver eel from each river basin, defined according to the Water Framework Directive (WFD), with respect to undisturbed conditions.
Until now, monitoring programmes for eels in the EU have depended mainly on fisheries-dependent data, and have focused on glass eels and yellow eels rather than silver eels (ICES, 2004; Dekker, 2005). Moreover, detailed data on a river basin level are usually missing, specifically on those factors that are responsible for the losses of eels during downstream migration (e.g. turbine mortalities or fisheries). This limits the development and effectiveness of potential management measures and reduces their acceptance by fisheries managers.
In studies on the small River Frémur (Feunteun et al., 2000) and in the medium River Meuse (Winter et al., 2006), escapement of silver eels has been quantified by fishery-independent means on a river basin level. Large river systems, however, have not yet been studied for eel escapement. The River Rhine system is the largest West European river basin, and there are no data on the escapement of silver eels from the whole system. Therefore, the German states North Rhine–Westphalia and Rhineland–Palatinate, and the Netherlands started the Rhine Silver Eel project in 2004 to (i) quantify the female part of the whole downstream migrating Rhine silver eel population independently from fisheries, and (ii) determine the relevance of the different migration routes of these female migrants in the Lower Rhine. Here, we give the results of the project in 2004 and 2005.
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Material and methods |
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Study area
The River Rhine has a catchment area of 185 000 km2, a length of 1250 km (excluding the "Alpine Rhine") and an average discharge of 2280 m3 s–1 at Rees, close to the Dutch border (Brenner et al., 2004). The greater part of the river (1000 km) is situated in Germany, where it is joined by several large tributaries, e.g. the River Moselle. The Moselle adds an average of 315 m3 s–1 to the total discharge of the Rhine and is regulated by 14 weirs associated with hydropower stations (Fichtner, 2003). Farther downstream, in the Netherlands, there are three main routes for Rhine water to flow to the North Sea: (i) the Waal, (ii) the Nederrijn and Lek, and (iii) the IJssel, which includes Lake IJsselmeer (Figure 1). Most water flows via the Waal. The Nederrijn is regulated by three weirs, two of which are combined with a hydropower station. The IJssel is free-flowing and discharges at ebb tide into the Dutch Wadden Sea by two sluices in the Afsluitdijk, in the northern part of Lake IJsselmeer. The Waal is also free-flowing and bifurcates in its most downstream section to rejoin the Lek and the River Meuse, which together discharge to the North Sea via the Haringvliet and the free-flowing Nieuwe Waterweg, in proportions that depend on the management of the sluices in the Haringvlietdam.
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Although there are some alternative escapement routes to the North Sea, for instance via IJmuiden (Figure 1), these seem to be of minor importance because of their size, discharge, and the presence of several ship locks. It is assumed, therefore, that migrating silver eels from the Rhine system generally escape to the sea either via the sluices in the Afsluitdijk, or through the Nieuwe Waterweg, or via the Haringvliet sluices.
Eel fisheries are intensive in the downstream sections of the River Rhine and in Lake IJsselmeer. Generally, large fykenets set on poles near the shore and small fykenets set in trains on the bottom are used. In the upstream sections of the River Rhine, there is also some electrical fishing. Although the numbers of professional fishers are known, the fishing pressure is only known in Lake IJsselmeer (Dekker, 2000).
Eels used in the experiment
To reduce the mortality at hydropower stations in the Moselle, downstream-migrating eels are caught annually with fykenets upstream from the hydropower stations (3000–5000 kg year–1) and translocated downstream of these obstructions. Most of these eels are >50 cm and are therefore females (Tesch, 1999). Some of these eels were used either for mark-recapture or for telemetry in this study. Only silver eels or eels intermediate between silver and yellow, evidenced by external characteristics (silvering, large eyes), were used.
Mark and recapture
The captured eels were batch-marked from August to November of both 2004 and 2005 (Table 1) before being transferred and released at Cologne, 
700 km from the sources of the Rhine. We assumed that this allowed them to mix with the rest of the downstream-migrating population before recapture. The batch mark used in this study was Heliogen blue (F.W.576.08, Aldrich Chemical Company Inc, Milwaukee), applied with a Panjet inoculator (Hart and Pitcher, 1969) on 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 on the ventral side, 2 cm behind the pectoral fins. The size of the eels was estimated by eye as >70, 50–70, and <50 cm. The last group was not used in the study and was therefore not marked. All handling and marking was done without anaesthesia because we assumed that this would benefit eel survival in such large-scale operations.
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Catches by commercial eel fishers using fykenets in the most downstream parts of the Rhine branches in the Netherlands were checked for marked eels. Recapture locations were selected in downstream sections of the Waal, the Lek, and in Lake IJsselmeer near the sluices in the Afsluitdijk. Detection and recovery of marked eels was by trained fishery-independent personnel. In 2004, 20 visits were made to professional fishers in the period September–December, the main season of the silver eel fishery, focusing mainly on the Waal and Lake IJsselmeer. In an attempt to sample proportional to the population in 2005, most of the effort of the 30 visits made between August and December was put on the Waal at the period between the last quarter and the new moon, because two-thirds of the total discharge of the Rhine runs via the Waal in normal years and peak migration of eel seems to be during that lunar period (Tesch, 1999). In all, 4293 and 6520 eels (silver and intermediate) were checked for marks and tags in 2004 and 2005, respectively (Table 1).
When catches were landed, they were sorted by hand into silver or intermediate, and checked for length (50–70 cm, and >70 cm) and the presence of externally visible Heliogen blue marks from batch-marking and surgical implantation of transponders. In both 2004 and 2005, the fraction of eels >70 cm was somewhat lower in the downstream parts of the Rhine where the recaptures were made than in the batch-marked eels from the Moselle (Table 2). In contrast, the fraction of eels that were silver was higher in the downstream Rhine sections in 2005. There are missing data on the silvering status of the eels in the downstream Rhine sections in 2004 because of miscommunication in this first year of measurements between the many participants involved in the study.
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Telemetry system
In this study, the NEDAP TRAIL System® was used, as described by Breukelaar et al. (1998) and Bij de Vaate and Breukelaar (2001). The system consisted of active transponders, each with a unique code, implanted in the eels, and a network of detection stations, as shown in Figure 1 (which reflects the situation on 1 October 2005). The transponder consisted of a cylindrical biocompatible glass tube of diameter 15 mm and length 65 mm, weighing 26.5 g in air and 16.0 g in water, with a lifetime of 1.5–2 years. Each of the detection stations was provided with antenna cables across the entire width of the river on the river bottom, which allowed us to monitor the individual passage of a large number of eels remotely.
Field tests showed that the detection system still functioned well with a maximum antenna length of 550 m, a water depth of 15 m, and a passing speed of up to 5–6 m s–1. The effects of ship engine noise are negligible, and conductivities up to 6000 µS cm–1 do not affect transmission when the distance between the antenna and the transponder is <15 m (Breukelaar et al., 1998).
Surgical procedures and implantations
The eels used for the telemetry part of this study also originated from the Moselle. Only eels classified as "silver" and "intermediate" (not "yellow") 
64 cm in 2004 and 73 cm in 2005 were used. We followed the surgical implanting procedure used by Baras and Jeandrain (1998) and Winter et al. (2006). Benzocain (80 mg l–1) was used as an anaesthetic instead of 2-phenoxy-ethanol (0.9 ml l–1), and the gills of the anaesthetized eel were irrigated with oxygen-saturated river water. In 2005, a tissue adhesive for humane purposes (Histoacryl®) was used instead of LoctiteTM glue, and the glued incision was strengthened with a single suture (Vicryl 3-0, V452G, FS-1 24.0 mm, 3/8c).
The transponder was surgically implanted in the posterior quarter of the body cavity through a mid-ventral incision 3 cm long. After the surgical procedure each eel was measured and, when it had recovered sufficiently, released into the river. The treatment of the eels was not considered to be an experiment on animals according to German Animal Protection Law (50.203.2 SO 04/04-LÖBF for 2004 and 50.203.2 SO 03/05-LÖBF for 2005).
Tagged eels had the same external marking (Heliogen blue dye) 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 offer of a reward (
35 per tag). The tags could only be found when degutting a tagged eel or when the tags had been lost by the eels. Returns of tags came from commercial fishers, anglers, traders, and other persons.
Data analysis
The mark-recapture part of the study was used to make estimates of the size of the population of eels >50 cm from the whole Rhine system that were migrating downstream. The telemetry part of the study permitted us to make corrections for non-migrants in the mark-recapture study. Moreover, it provided information on the migration routes of the eels (the river courses taken) and on how many and where losses of eels occurred during downstream migration.
As the number of batch-marked recaptures was very low, only a very tentative estimate of the total silver eel escapement 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 marked fish that were recaptured (R) to the catch taken for census (C) (Ricker, 1975; Pollock et al., 1990):
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To calculate the standard deviation and the variance V, R was treated as a binomial variable, because of the low numbers of eels recaptured (<25). V was estimated according to Seber (1982):
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Two different estimates of the population size were made, one using the total number of batch-marked individuals and one correcting for the percentage of downstream migrants carrying transponders as measured at the detection station in Xanten (Germany, close to the Dutch border) in the telemetry study. The corrected estimate of the population size was obtained by replacing M by Mc in the formulae above, where Mc=M*dX*d–1R, dX being the number of eels detected with a transponder in Xanten, and dR the number of eels with a transponder released. Separate estimates were made for the migrating cohorts of 2004 and 2005, using the values for dX and dR in Figure 2. The population biomass was calculated by multiplying the numbers by the average weight of the eels checked in 2004 for batch marks.
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In the telemetry study, data up to 15 January 2006 were used. Each eel was followed on its migration to the sea or to the detection station where it was detected for the last time. It became clear, from more downstream detections, that many eels were not detected at one or more stations during their migration. Missing detections at a station occurred in 20–32% of the passages, depending on the station. The signals were analysed according to the method of Breukelaar et al. (1998) and Bij de Vaate and Breukelaar (2001).
The numbers of escapees to the sea at the Afsluitdijk and the Haringvliet were determined at the sluices where they entered the sea. It is physically impossible to locate a detection station near the mouth of the Nieuwe Waterweg, so we used the combined detections of the three most downstream stations instead (De Noord, Oude Maas, and Lek), covering all eels migrating to the Nieuwe Waterweg.
In cases where transponders were returned, transponder number, date of catch, fishing gear, location, name, and address were registered, and the information was used in this study in the same manner as the detection data. If the location of catch was unknown, the river stretch downstream of the station of last detection was assigned. The recaptures of the transponders are not suitable for making estimates of population size by mark-recapture, because the number of eels checked for the presence of transponders is not known. These recapture data were only used as an indication of sources of losses (commercial fishers, anglers).
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Results |
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Mark and recapture
In 2004, the migration peak of silver eels in the Moselle was in August. In 2005, it was more evenly spread from August to October. Totals of 3181 and 3266 silver and intermediate (silvering) eels were marked and released in 2004 and 2005, respectively (Table 1). The recaptures of marked eels were few in both years (six in 2004, five in 2005) and were mainly in the Waal (ten) and the Lek (one), but not in Lake IJsselmeer. Estimates of the silver eel population >50 cm in the whole Rhine system depend on the correction for downstream-migrating individuals. For 2004, they range between 1.2 and 1.9 million eels, with an estimated biomass of 0.6–1.0 million kg (Table 3). For 2005, the range was 1.9–3.6 million eels with an estimated biomass of 1.0–1.8 million kg.
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Telemetry
In 2004 and 2005, respectively, 150 and 157 eels were tagged with transponders. Most eels were >70 cm. Some 26% of these eels in 2004 and 32% in 2005 were classified as silver (in the mark-recapture programme, 18% of eels >70 cm were silver in 2004, and 91% in 2005).
Two transponders from the 2004 group and ten transponders of the 2005 group were returned. In 2004, one came from an angler near Cologne and one was reported by a trader near Lake IJsselmeer. In 2005, four came from professional fishers from Lake IJsselmeer, two were from Hollands Diep/Haringvliet, one was reported anonymously, one was reported from Germany (by a fish trader), and two transponders were said to have been found near the shore.
The most frequented migration route in both years was via the Waal, and escapement to the North Sea via the Nieuwe Waterweg (Figure 2). No eel seemed to escape through the Haringvliet sluices and only one through the Afsluitdijk in 2004. Although the telemetric data showed that just four eels from the 2004 cohort and ten from the 2005 group migrated via the IJssel, nearly all appeared to have been lost in Lake IJsselmeer between Kampen and the Afsluitdijk. A few eels tried to migrate via the Nederrijn + Lek. All three eels of the 2005 cohort that entered the Nederrijn appeared to have turned back at the first weir and proceeded either via the Waal or the IJssel.
By 15 January following the date of release, the percentage of detected eels was nearly equal each year, 55% of the 2004 cohort and 53% of the 2005 cohort, and another 7% of the 2004 cohort was detected by 15 January 2006. On the basis of the telemetric detections up to January 2006, only 23% and 15% of the silver eels released in Cologne in 2004 and 2005, respectively, were classified as escapees to the sea (Figure 2). In both years, the downstream migration of the eels with a transponder took place predominantly in October and November, about 1 month after release (Figure 3).
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Most silver eels migrated at average speeds of <0.2 m s–1 over a distance of 154 km from the point of release (Cologne) to Xanten (Figure 4). However, some eels moved very fast downstream, from the point of release to the sea in <2 d, whereas others started their migration months after release. Also, some eels interrupted their migration and stayed several months in certain river sections. The longest time a tagged eel remained in the river system was an individual released on 21 October 2004 that was detected for the last time on 6 August 2005, almost 10 months later.
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Discussion |
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Based on a combination of mark-recapture experiments and telemetry studies, it was possible for the first time to obtain a rough estimate of the numbers and biomass of migrating silver eels in the River Rhine, the largest West European river system, and also to identify the most important migrating routes through the heavily modified branches of the Rhine delta.
Estimating the size of a population by mark-recapture relies on assumptions that all marked fish have the same chance of being (re)captured as 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. Here, we 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 downstream in the River Rhine, still 350 km upstream from the sea, and recapture efforts were made at a distance of 50 km or less from the sea at locations not influenced by other water systems, such as the Meuse. The population studied could therefore be said to represent the whole Rhine population of female silver eels, other than those living closer to the sea or in the coastal zones belonging to the Rhine system, according to the WFD.
On the basis of the telemetry data from 2004, the recapture effort in 2005 was proportionally distributed over the different river branches according to the normal discharge conditions, and was concentrated in the anticipated peak migration during these periods (Tesch, 1999). Therefore, although we had to assume that the marked silver eels mixed randomly in the population, we believe that we sampled for recaptures proportionally to the mixed population in different waterbodies 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 (Durif, 2004; Durif et al., 2005; Winter et al., 2006). We compensated for these by combining the results of telemetry with the mark-recapture results, and made separate estimates of population size corrected for the number of marked eels for non-migrants as evidenced by telemetry data. For this, we assumed that the eels with surgically implanted transponders would show the same behaviour and survival as the marked individuals. We did not compensate for possible selective mortality of the eels with a transponder or for differences in behaviour, because selective mortality of these eels seems to be negligible and there were no diurnal effects or effects in the timing of activity during an 11-week period (Winter et al., 2005).
We tried to make estimates of post-release mortality of tagged eels in a separate 40-d study in tanks at a fish culture station (unpublished data). Statistical effects of the implantation of transponders on survival could not be proven (
2 = 4.47, p = 0.107), but we observed that not all surgery wounds healed well (inflammations) and some were found open at the end of the 40-d experiment. This may have been due to an outbreak of disease in both the control group (n = 34) and the treatment group with implanted transponders (n = 30), probably related to the holding of the eels under crowded conditions. We could therefore not use these results for estimating tagging mortality in the Rhine silver eel study, although they did make us change our surgical procedure in 2005 from that in 2004, described earlier.
The uncorrected estimates of population size, 2.0 x 106 and 3.6 x 106 in 2004 and 2005, respectively (Table 3), can be considered to be maximum estimates of the downstream-migrating female Rhine silver eel population, with corresponding biomasses of 1000 and 1900 t, and 95% confidence intervals of the same order of magnitude. If all census data are excluded for Lake IJsselmeer, where no marked eels were recaptured, these population estimates would have been lower in 2004 and 2005 by 42% and 27%, respectively, although these values are within the 95% confidence intervals of the maximum estimates.
The telemetry part of the study indicates that most eels released with a transponder migrated downstream in the same year. However, 38% and 47% in 2004 and 2005, respectively, were not detected at all. Before escaping to the sea, an eel released in Cologne had to pass at least three detection stations. As the detection stations failed to register 20–32% of the eels passing, depending on the station, the chance for an eel of being undetected before escaping to the sea will not be >3%, which does not explain the high percentage of undetected eels. The latter may be partly due to delayed migration (Durif, 2004; Durif et al., 2005; Winter et al., 2006), because a few eels of the 2004 cohort were also detected in summer 2005.
Based on the high degree of reliability of the NEDAP-system and the lifetime of the transponders of 1.5–2 years, we suggest that methodological problems were not the reason that some tagged eels were not detected, and that mortality could be one valid reason. Winter et al. (2005) found no loss of transponders and no differences in mortality between eels with or without transponders. Mortality of silver eels in the Meuse system was 16–26% at hydropower stations (Winter et al., 2006). Mortality attributable to hydropower in the Lower Rhine system, however, may occur only in the Nederrijn, and just seven eels tried to pass the Nederrijn in 2004 (all successfully) and none in 2005. In years with low discharge of the Rhine, when the discharge of the Nederrijn is kept as low as possible for water-management purposes, the risk of turbine mortality seems to be hardly relevant for silver eels migrating downstream on the Rhine. We expect that turbine mortality in the Nederrijn + Lek may only be important in years with greater discharges, so we recommend that additional protective measures be applied for such years.
Most of the Rhine silver eels migrated via the Waal and not via the Nederrijn + Lek or via the IJssel + Lake IJsselmeer. The numbers detected in the IJssel were higher in 2005 (ten) than in 2004 (four), and only one of these probably escaped to the Wadden Sea. Although this meant a mean loss of 93% in Lake IJsselmeer, probably because of the high fishing pressure (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. We assume that the reason for the last probably results from delay in the migration to the first downstream detection station.
We did not detect any silver eels at the Haringvliet sluices, but know that there is a large eel fishery in Haringvliet and around its sluices and suspect that fishery mortality may play a significant role there. Winter et al. (2006) suggested 22–26% fishing mortality in the Netherlands for the River Meuse eel population.
The practical use of estimates of the female Rhine silver eel population for the management of the European eel is that they allow a first comparison with data on fishing mortality. More than 50% of the Netherlands lies within the River Rhine basin. Except for the River Scheldt system (province of Sealand), the total reported Dutch annual inland silver eel catch is 100 t in the large rivers (River Meuse included), 40 t in Lake IJsselmeer, and 140 t in other inland waters (Aalcomité, 2005). Because the River Meuse is included in this total catch of 280 t, the catch in total is <28% and 15% of the estimated female silver eel runs in the River Rhine in 2004 and 2005, respectively.
From the telemetry detections, the total percentage of escapees related to the eels passing Xanten (near the Dutch border) up to 2006 was 37% of the 2004 cohort and 28% of the 2005 cohort (Figure 2). Correcting for 20–32% missing detections at the last station before the eels enter the sea, we estimate the real escapement to be no greater than 46% of the 2004 cohort or 37% of the 2005 cohort. These values are equal to or slightly higher than the 37% found for migrating silver eels in the River Meuse system (Winter et al., 2006), which also discharges into the Haringvliet. The fishery-independent telemetry data, therefore, show that at least 54–63% of the silver eels passing Xanten seem to disappear somewhere in the Netherlands. This contrasts with the 15–28% based on the catch data from Aalcomité (2005).
We hope that continuing the Rhine silver eel project in the next few years and registering the fisheries data according to the EU regulation (EU, 2005) will help to explain this discrepancy and will support effective management. The telemetry data clearly showed that most silver eels from the upstream parts of the Rhine migrate via the Waal. Concentrating the management measures on the Waal and downstream sections may therefore help to reduce substantially the impact on the eel population escaping from the upstream River Rhine.
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Acknowledgements |
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This project was facilitated by making use of the eels caught in the annual trap-and-release programme of the Struktur und Genehmigungsbehörde Nord (Rheinland–Pfalz). The Rheinfischereigenossenschaft, LÖBF-NRW (at present: Bezirksregierung Arnsberg), the Rheinischer Fischereiverband, Sportvisserij Nederland, and the former OVB all contributed to the fieldwork. The Combinatie van Beroepsvissers and the PO Nederlandse Vissersbond–IJsselmeer mediated in cooperation of Dutch professional fishers. The Universität zu Köln made a field and laboratory facility available in Cologne. RIZA made the antenna infrastructure available for the telemetry and analysed the primary telemetry data. OVB and LÖBF coordinated the project and are acknowledged for partly funding the project. For another part, the project was funded by the EU through the FIFG to Rheinfischereigenossenschaft. We acknowledge the participation of Erwin Winter and the staff of all partners to the fieldwork (specifically G. Feldhaus, A. Hehenkamp, and J. Merkx), and are most grateful for the helpful comments made by Mike Pawson and an anonymous reviewer on an earlier version of the manuscript.
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References |
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