Not to be cited without permission of the author

International Council for North Atlantic Salmon the Exploration of the Sea Working Group Working Paper 43/2008

Downstream migration of salmon smolt in the River : results of a transponder study 2007 by

Detlev Ingendahl *, Gerhard Feldhaus *, Gerard de Laak §, Tim Vriese $ and André

Breukelaar &.

*Landesamt für Natur, Umwelt und Verbraucherschutz Nordrhein-Westfalen, Fisheries, Heinsbergerstr. 53, 57399 Kirchhundem, $Sportvisserij Nederland, $Visadvies, &RIZA

Running headline: Downstream migrating salmon smolt in the River Rhine

Key words: Salmon smolt, downstream migration, River Rhine, Nedap Trail system, Delta

Abstract

Downstream migration of Atlantic salmon smolt was studied in the River Rhine in 2007 using the NEDAP Trail system. In spring 2007, 78 fish were released into a tributary of the R. Rhine, the River Wupper and one of its smaller affluents (River Dhünn). The two years old hatchery smolts (> 150 g) were tagged with a transponder (length 3.5 cm, weight 11.5 g) by surgery and introduction into the body cavity. After a recovery period of ten days in the hatcheries the fish were released into the river in the first half of March. The transponder equipped fish can be detected by fixed antenna stations when leaving the R. Wupper and during their migration through the Rhine delta to the sea. The NEDAP trail system is based on inductive coupling between an antenna loop at the river bottom and the ferrite rod antenna within the transponders. Each transponder gives its unique ID- number when the fish is passing a detection station. Overall 60 fish left the river of release (77 % of tagged fish). Out of this 60 individuals about 36 (46 % of tagged fish) reached the sea after passing the Rhine delta. The study aims to investigate migration routes and the success of downstream migration in the partly dammed Rhine delta (especially at the Haringvliet sluices). It may be repeated after re-opening the Haringvliet dam which is scheduled by the end of 2008 establishing better conditions for migratory fish species during their passage from freshwater to sea and vice versa.

1 Introduction In the River Rhine, a former important salmon river, Atlantic salmon was extinct by late 1950. When starting an ambitious program for the ecological rehabilitation of the Rhine the member states of the International Commission for the Protection of the R. Rhine (ICPR) agreed that migratory fish species as Atlantic salmon should again colonize the River and its tributaries (ICPR 2004). To achieve that goal a restocking program was started in several areas of the Rhine basin especially in Germany, France and Switzerland. Since 1990 adult salmon are regularly recorded in some tributaries of the R. Rhine (e.g. R. ) and at the first dam in the , 600 km upstream from the sea (fishpass Iffezheim). But numbers and return rates are probably to low for the establishment of a self-sustaining salmon population (MUNLV 2004). There are several potential bottle-necks identified actually hindering a sufficiently high number of salmon to return to the rivers where stocking takes place (Neumann et al. 1998).

One potential bottle neck is free migration of fish from freshwater to sea and vice versa in the Delta part of the Rhine in the Netherlands. During last decades the Delta has been partly dammed from the sea especially in the Ijsselmeer in the northern part and in the Haringvliet in the southern part of the Delta. Both water bodies have turned into complete freshwater and salt water is no longer allowed to penetrate into the Delta at both locations. Especially on the sea side of the Haringvliet adult sea trout and salmon are known to aggregate searching their way into the Delta when the sluices are open to discharge freshwater from the Rhine to the sea. In an earlier transponder study a high proportion of sea trout tagged on the sea side of Haringvliet finally moved further north and migrated via the “Nieuwe Waterweg” the only open migration pathway into the delta (bij de Vaate et al. 2002).

The present study aims to follow the downstream migration of Atlantic salmon smolts from a potential spawning tributary through the Rhine and its Delta to the sea. Downstream migration is a crucial moment in the life cycle of salmon when in spring juveniles are leaving their freshwater habitats in smaller tributaries to migrate out and to adapt to the particular conditions in the sea. Hatchery reared two year old smolts have been tagged with transponders using the NEDAP trail system. The study takes advantage of a network of detection stations strategically chosen along the River Rhine (mainly in the Delta) and already established to study upstream migration of adult sea trout and salmon (Breukelaar et al. 1998). The transponder gives its unique ID-codes to a data storage unit when the fish is passing a detection station. The presence of a detection station at the confluence of the R. Wupper with the Rhine enables the detection of tagged fish when leaving the river and starting their downstream migration.

With regard to the reintroduction of salmon into the R. Rhine it is important to investigate downstream migration, especially migration routes, losses in different parts of the Delta and the proportion of migrants successfully reaching the sea. In the near future, the opening regime of the Haringvliet sluices will be modified. Actually the sluices are only opened to discharge at low tide a surplus of freshwater from the Rhine into the sea and are closed to avoid salt water penetrating into the freshwater area of the Haringvliet. From 2009 onwards, at least two sluices out of 18 will be permanently kept open on a slit

2 to allow fish passing from both sides of the Haringvliet dam. This measure will re- establish a small mixing zone for freshwater and saltwater in the vicinity of the delta side of the Haringvliet and is therefore intended to partly recreate the estuary functions of the Haringvliet and to contribute to the ecological rehabilitation of the Rhine delta. After re- opening of the Haringvliet, studies such as the downstream migration of smolts will be replicated to evaluate the consequences of this rehabilitation measure and the possible improvement of fish migration in the Rhine Delta.

Material and Methods Study area The River Rhine has catchment area of 185.000 km 2 and a mean discharge at the German-Dutch boarder of about 2.280 m 3/s. The greater part of the River is situated in Germany and there are several big tributaries joining the Rhine (e.g. , and ). The transponder study started in one of the smaller tributaries, the River Wupper, located in the federal state of Northrhine-Westfalia. It enters the R. Rhine about 20 km downstream of the city of Cologne (Rhine km 705). At Rhine km 850, the Rhine enters the Netherlands and splits into three main branches to flow to the : a) the Waal, b) the Nederrijn and Lek, and c) the Ijssel (Fig. 1). Most water flow through the free-flowing Waal. It bifurcates in its downstream part to join the Lek and the River Meuse and discharge to the Sea via the Haringvliet and the free-flowing “Nieuwe Waterweg”. The proportion of the discharge going each way is regulated by the management of the sluices in the Haringvlietdamm. The Nederrijn is regulated by three weirs, of which two are equipped with a hydropower scheme. The Ijssel is free flowing and via the Ijsselmeer discharge at ebb tide through two sluices at the Afsluitdijk into the Waddensea.

Telemetry system and surgical implantation The NEDAP trail detection system is used for the study of downstream migrating salmon smolts in the River Rhine. It is based on inductive coupling between an antenna loop on the river bottom and a ferrite rod antenna within the transponder. A number of detection stations are situated at strategically chosen locations along the River Rhine (Fig. 1). When a fish passes a detection station its unique ID-codes is registered by a data storage unit (see Breukelaar et al. 1998; bij de Vaate et al. 2002).

Due to technical reasons (especially the size of the antenna coil within the transponder) the size of the tags could not further reduced so far (length: 3.5 cm, diameter: 13 mm and weight: 11.5 g in air, Fig 2). Therefore hatchery-reared 2 years old smolts from three salmon rearing stations were selected with at least a minimum weight and length of 150 g and 25 cm, respectively. These fish had not smoltified at age 1. According to Lacroix et al. (2004) tag length should not exceed 16 % of the body length and tag weight should be less than 8 % which was respected during the study.

The surgical procedure followed was described by Vriese (1995) and already used for adult salmon and sea trout. Fish had not been fed 24 h before tagging. They were individually anesthetized using a 40 mg/l benzocain solution. After surgery the fish was shortly hold for observation in a small recovery tank and later on in a larger holding tank.

3 78 smolts were released 10 days after surgery in 2007, 28 in the R. Wupper, and 50 in the R. Dhünn, a smaller tributary to the R. Wupper, Germany.

Results In 2007 three batches of smolts were released into the R Wupper. One batch with 28 individuals went directly into the Wupper approx. 20 km upstream of the mouth to the Rhine. Two batches, 25 fish each, were transported to the River Dhünn, a small tributary of the Wupper and released about 7 km upstream of the Rhine. No mortality was observed for tagged fish when released ten days after tagging. All fish were liberated during the first half of March (2 nd , 8 th and 13 th of March 2007).

In total, 62 smolts out of 78 tagged fish have been detected with the Nedap Trail system during the whole study. Two fish were located downstream the release area using a mobile detection unit and these fish did not leave the R. Dhünn until the end of the study. Detection of the first fish at the detection station in R. Wupper started on the first day after release of the first batch (2 nd of March) and the last smolt was detected on April 22th in the Delta.

The mean duration of downstream migration was approximately 17 days with three fish reaching the sea within three days and one fish migrating 39 days. The average duration was 11, 23 and 29 days for the three different batches, respectively. The fish released to the Wupper were taking less time than fish released in the R. Dhünn. During the migration period there were two peaks in discharge occurring. The smolt migration seemed to intensify several days later as the number of detection increased notably.

The rate of detection was not the same for three batches as all the fish released into the Wupper were detected on their way out to the R. Rhine (Tab. 1) but detection rate of the R. Dhünn batches only reached between 64 and 72 %. 16 out of 50 fish released into the R. Dhünn were never detected during the study. A total of 60 smolts migrated out of the Wupper system as they have been detected on their way downstream 1 km upstream the confluence with the Rhine.

At the first detection station in the Rhine about 140 km downstream the River Wupper 46 fish have passed down (station 13, Fig. 1) from March 2 nd to April 14th. 14 individuals that left the R. Wupper did not reach Xanten (18 % of released fish). In the river section between Xanten and following detection stations situated further downstream in the Netherlands another 5 smolts were lost (6 % of released fish).

38 smolts out of the remaining 41 fish entered the Waal towards the southern part of the Delta. One fish migrated into the Ijssel and probably arrived into Lake Ijsselmeer (detection at station 2, figure 1) without reaching the North Sea . Two other fish travelled along the Nederrijn and the Lek and one fish probably migrated successfully to the sea via the “Nieuwe Waterweg”.

4 23 salmon out of 38 that chose the Waal reached the sea by passing the Haringvliet sluices (64 % of all fish having reached the sea). This passage and their progression through the Delta coincided with high water discharge of the River Rhine. Therefore the Haringvliet sluices were kept open over a longer time period (during low tide) to evacuate the freshwater supply from the Rhine. The most important migration route in 2007 to the sea was: Wupper-Xanten-Waal-Haringvliet .

Another 12 fish that entered the Waal were registered at other detection stations in the “Benedenrivierengebied” (station No 7, 8 and 14, fig. 1) before arriving at sea. Three of the 38 “Waal” fish were not reported to get to the sea. All together 36 out of 78 tagged fish are assumed to have reached the sea (46 %, Tab. 2). The losses in the R. Wupper amounted 23 %. About 18 % of tagged fish disappeared in the German part of the Rhine between the R. Wupper and Xanten whereas 13 % of smolts arriving in the Delta (the Netherlands) did not successfully migrate to the sea.

Migration success differed markedly between the three batches. 21 out of 28 smolts (75%) released in the R. Wupper reached the sea. From the two batches stocked into the River Dhünn a fairly comparable number of 7 and 8 individuals, respectively, migrated successfully to the sea (28% and 32%, see Tab. 2). This difference was partly due to a higher number of fish that did not leave the Dhünn whereas all fish left the Wupper.

Discussion In 2006 a pilot study started with only 10 tagged fish. The first essay was performed to test the new available smaller NEDAP transponder and to evaluate if two year hatchery smolts could be used to study downstream migration. Half of the tagged smolts left the Sieg River and three fish were detected 180 km downstream in the lower part of the Rhine indicating an active downstream migration. At least two fish migrated into the Delta and one successfully arrived at the North Sea. These results confirmed the feasibility of the study using the NEDAP trail system (Vriese & Breukelaar. 2006).

In 2007, three batches with a total of 78 fish were tagged and released in the R. Wupper after a recovery of ten days in the hatchery. Post tagging mortality was not observed during that period and all tagged fish could be released to the river. 60 fish (77 %) left the R. Wupper, a higher percentage than observed in 2006 in the R. Sieg. All fish of the Wupper batch were recorded leaving the river whereas only 18 (72%) and 14 (56%) of the Dhünn batches migrated into the R. Rhine. Of the 18 non-migrating fish, only two were located downstream the release area two times by walking along the river using a mobile receiver. In the opposite direction, a weir without fish way probably disabled the upstream migration of tagged smolts. A certain number of herons were observed predating in some shallow river stretches. This species is known to predate on migrating smolts (Koed et al. 2002). Although a non detection of fish or transponder in the small river cannot completely ruled out, the hypothesis of predation by birds is advanced as they take the prey and the transponder out of the water (Koed et al. 2002). Predation by heron is more likely to occur in the smaller R. Dhünn than in the bigger R. Wupper with higher discharge and generally a greater water depth.

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More than half of the tagged fish (59 %) migrated down the Rhine and was detected about 140 km downstream of the R. Wupper on their way to the Delta. The loss in this part of the Rhine was 18 % of all tagged fish. As some smolts took more than 20 days to reach the German-Dutch boarder there is ample time for predation to take place. The R. Rhine has a cyprinid dominated fish community with pike, pike-perch and asp as potential predators for tagged salmon. Nevertheless post-tagging mortality can not be excluded being responsible for some losses but most investigations showed only minor effects of tagging on swimming performance, general behaviour and mortality of salmonids (Lacroix et al. 2004; Koed et al. 2002; Connors et al. 2002).

In the period of release high discharge especially in the R. Rhine potentially favoured a rapid downstream migration and the first fish arrived in the Delta within a few days. Migration activity seemed to increase shortly after peak discharge as indicated by the raising number of detections. So most smolts took advantage of higher discharge but tended to migrate during the falling lamb of peak events. But other fish needed more than 30 days to reach the sea. Some studies have indicated that the positive response to environmental factors (e.g. changes in discharge) needs a proper physiological status of the smolts (higher Na +, K + -ATPase activity). Therefore individual behaviour in migration might be depending of differences in the physiological status of individual tagged fish which was not investigated during this study (Koed et al. 2002). This may also explain the difference observed in migration duration between the three batches which ranged from 11 to 29 days. As the batches were kept at three hatcheries with different water quality and temperature regimes differences in the physiological conditions seems likely to have been occurred.

In the Delta area of R. Rhine (the Netherlands), 10 out of 36 fish did not complete the migration to the sea which is a lower loss compared to the German stretch of the Rhine (13 % versus 18% of tagged fish) for a slightly longer distance (180km versus 140km). Migration through the Nederrijn-Lek and the Ijssel was only observed for 3 out of 41 fish detected within the Delta. As the discharge is at least two times higher in the Waal, most fish entered this branch and arrived in the southern part of the Delta.

As freshwater was discharged through the Haringvliet sluices during a long period, a high percentage of tagged smolts probably following the strong currents leading into the Haringvliet area and passed through the open sluices (64%, Fig. 3). Only 13 out of 36 fish that reached the sea migrated via the only free migration route left which is the “Nieuwe Waterweg”. This is in considerably difference to results from a study on tagged silver eel where the Nieuwe Waterweg was by far the most prominent route for sea migration (Klein Breteler et al. 2007). During migration of silver eel in late autumn (2004-2006) water discharge in the R. Rhine was much lower in comparison to spring 2007 strongly reducing the opening of the sluices in Haringvlietdam. The re-opening of the Haringvliet scheduled for 2009 should improve migration of fish to the sea and vice versa. Eel and salmon seem two appropriate fish species for studying the benefits of this change as they are migrating at different seasons with potentially different discharge regime in the River Rhine.

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References Bij de Vaate ………… J. Fish Biol. 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 Connors, K.B., Scruton, D., Brown, J.A. & R.S. McKinley (2002): The effects of surgically-implanted dummy radio transmitters on the behaviour of wild Atlantic salmon smolts. Hydrobiologia 483, 231-237. ICPR, Internationale Kommission zum Schutze des Rheins (2004): Rhein & Lachs 2020, Programm für Wanderfische im Rheinsystem, 31 S. Klein Breteler, J., Vriese, T., Borcherding, J., Breulelaar, A., Jörgnsen, L., Staas, S., de Laak, G. & D. Ingendahl (2007): Asssement of population size and migration routes of silver eel in the River Rhine based on a 2-year combined mark-recapture and telemetry study. Journal of Marine Science, 64, 1450-1456. Koed, A., Jepsen, N., Aarestrup, K. & C. Nielsen (2002): Initial mortality of radio-tagged Atlantic salmon ( Salmo salar L.) smolts following release downstream of a hydropower station. Hydrobiologia 483, 31-37. Lacroix, G.L., Knox, D. & P. McCurdy (2004): Effects of implanted dummy acoustic transmitters on juvenile Atlantic salmon smolts. J. Trans. Am. Fish. Soc. 133: 211-220 Limnoplan (2006): Entwicklung eines standardisierten Smoltmonitorings mit Hilfe einer rotary-screw-trap an der Sieg. – Unveröffentlichte Studie im Auftrag der Sieg Fischerei- Genossenschaft, Hennef, als Beitrag zum Wanderfischprogramm NRW, 34 S. + Anhang MUNLV (2004): Jahresbericht zum Wanderfischprogramm 2004 – Arbeitsbericht, Hrsg.: Ministerium für Umwelt und Naturschutz, Landwirtschaft und Verbraucherschutz Nordrhein-Westfalen, 81 S. Neumann, D., Ingendahl, D., Molls, F. & A. Nemitz (1998): Lachswiedereinbürgerung in NRW-Biologische Engpässe und Vorschläge für zukünftige Maßnahmen. LÖBF- Mitteilungen, 20-25. Vriese, F.T. (1995): Implanting van transponders in salmoniden. Organisatie ter Verbetering van de Binnenvisserij, Nieuwegein. OVB-Onderzoeksrapport 1995-26, 59 p Vriese, F.T. & A. Breukelaar (2006): Pilot study smolt tagging 2006. Visadvies, Utrecht, VA2006_66, 8 pp.

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8 Figures:

Figure 1: Map of the Rhine delta indicating the location of transponder detection stations.

Figure 2: Size of transponder used for migration studies, smolt transponder on the right.

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River Wupper 78

River Rhine 60

Rhine Xanten 46

Waal Lek IJssel Vuren Nieuwegein Kampen 38 2 1

Haringvliet Nieuwe Ijsselmeer Sluices Waterweg Afsluitdijk 23 13 0

Fig. 3: Overview on migration routes and number of tagged smolts detected in different river sections (Haringvliet and Nieuwe Waterweg = smolts presumably reached the sea).

10 Tables:

Origin of smolts Stock River N N detected % detected Hagen-Haspe Ätran Dhünn 25 18 72 Kirchhundem-Albaum Ätran Dhünn 25 16 64 Stukenbrock Burrishoole Wupper 28 28 100 Sum 78 62 79

Tab. 1: Number of tagged fish released and percentage of detected fish from three batches.

Origin of smolts Stock Released Left Release River Sea (% of tagged) Hagen-Haspe Ätran 25 18 7 (28) Kirchhundem-Albaum Ätran 25 14 8 (32) Stukenbrock Burrishoole 28 28 21 (75) Sum 78 60 36 (46)

Tab. 2: Percentage of detected fish out of three batches reaching the Sea.

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