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.