Estuarine behaviour of European silver eel (<i>Anguilla anguilla</i>) in the Scheldt estuary

Estuaries are among the most productive ecosystems in the world and are characterised by high habitat diversity. As transition areas between inland rivers and the open sea, they function as transport zones for diadromous species like the European eel (Anguilla anguilla), a catadromous fish species that migrates to the Sargasso Sea for spawning. However, information on the migratory behaviour of eel in estuaries is scarce. Therefore, more insight is needed to efficiently restore and conserve the species. We tracked 47 eels with acoustic telemetry between July 2012 and October 2015 and analysed their behaviour from the Braakman creek into the Scheldt Estuary, separated by a tidal barrier. Eels arrived in the Braakman between mid-summer and early winter and stayed there on average 44 days (0 - 578 days). As such, arrival in the Scheldt Estuary was much later: between early autumn and early winter. The average residence time in the Scheldt Estuary was considerably shorter than in the Braakman, and was only five days (0 - 64 days). The long residence time in the Braakman was probably due to the discontinuous operation of the tidal barrier, which is used to control the water level in the upstream wetland area. This resulted in a discontinuous flow conditions, leading to searching behaviour in eels. Eventually 37 eels did pass the sluice and reached the Scheldt Estuary; the 10 eels which did not pass the sluice were probably caught by a commercial eel fisherman in the Braakman creek. In the Scheldt Estuary, 26 eels migrated towards the sea, whereas eight took the opposite direction and three were only detected at the first receivers downstream of the sluice. The eight eels that did not migrate towards the sea showed estuarine retention behaviour. They could have been injured by the tidal barrier or missed the right moment to migrate, and could be waiting in the estuary until favourable conditions are met to proceed their journey. Our results indicate that eel migration is obstructed by a tidal barrier, which resulted in delayed eel migration. As the migratory period occurred from mid-summer to early winter, this information can be implemented in management plans such as environmental windows to open the sluice during eel migration if circumstances allow such measurements

Fish Behaviour in the Vicinity of a Navigation Lock Complex: the Challenges

Hydraulic structures such as navigation locks, pumping stations and hydropower plants play an important role in navigation, water management and sustainable energy production. However, these structures may severely impact the aquatic ecosystem and freshwater fish in particular. In Belgium, the Albert Canal connecting the river Meuse to the river Scheldt, is an important migration route for European eel (Anguilla anguilla, critically endangered) and Atlantic salmon (Salmo salar, vulnerable). Both species have a downstream migrating phase in their lifecycle (respectively silver eels and salmon smolts), during which they are hampered by hydraulic structures. In the coming years, Archimedes screws are to be installed at the navigation lock complexes present in the Flemish part of the canal, which can function both as pumping stations and hydropower generators. A first installation is already present at the navigation lock complex of Kwaadmechelen. Before fish mitigation measures can be implemented, it is important to gain understanding on how the downstream migrating fish are affected by hydrodynamics around the complex. In this paper, we focus on the challenges in investigating fish behaviour, related to the acoustic telemetry used to determine fish positions, as well as on the complexity of a hydrodynamic CFD model for the studied site. Additionally, we present some preliminary results. In the next phase of the research, observed fine-scale behaviour of the fish in front of the navigation lock complex will be compared with predicted flow patterns by means of a CFD model

First detection and energy measurement of recoil ions following beta decay in a Penning trap with the WITCH experiment

The WITCH experiment (Weak Interaction Trap for CHarged particles) will search for exotic interactions by investigating the beta-neutrino angular correlation via the measurement of the recoil energy spectrum after beta decay. As a first step the recoil ions from the beta-minus decay of 124In stored in a Penning trap have been detected. The evidence for the detection of recoil ions is shown and the properties of the ion cloud that forms the radioactive source for the experiment in the Penning trap are presented.Comment: 9 pages, 6 figures (9 figure files), submitted to European Physical Journal

Onderzoek naar de trekvissoorten in het Schelde-estuarium. Voortplantings-en opgroeihabitat van rivierprik en fint

Migratory fish such as river lamprey and twaite shad are important indicators of ecosystem functioning. Over the past century, most migratory fish have disappeared from the river Scheldt due to human impacts. The previous study on migratory fishes in the Scheldt showed however that most species show the first signs of recovery (Stevens et al., 2009). For both river lamprey and twaite shad there are strong indications that they reproduce in the Scheldt. However, the spawning and nursery habitats of both species are unknown and it is unclear whether the preconditions for a sustainable recovery are met. The spawning and nursery habitat of river lamprey can be located through targeted sampling of the larvae in the sediment. Sampling with fyke nets showed that adult river lamprey migrate mainly to the Bovenschelde and Zwalmbeek. In both rivers a number of locations were selected, which are, according to the literature, expected to be suitable habitats for the larvae of river lamprey. Wadable sites were sampled with a specially designed sediment pump and the deeper sites with a Van Veen grab. In neither of these rivers, however, river lamprey larvae could be found and no spawning sites could be identified. Possible reasons for the lack of larvae in the samples are (1) that no suitable larval habitat is present in the studied areas, (2) that the larval density in the investigated habitats is low and hence sampling frequency should be increased, (3) that the River Bovenschelde and the River Zwalm are not the main spawning grounds for river lamprey in the Scheldt. Telemetry of adult river lamprey could be a possible solution to locate the spawning grounds. In order to improve the reproduction and survival of river lamprey in the River Bovenschelde, the migration barriers in the Scheldt and its tributaries should be cleared and sufficient larval habitat should be availability. Larval habitat could be created in the River Zwalm and other tributaries through the restoration of natural banks. In addition, mud and sand banks in the Bovenschelde should be protected as much as possible as potential larval habitat.The population of twaite shad in the Scheldt is too small to identify the critical habitats by sampling in the field. Therefore, a habitat suitability model for spawning and larval shad was constructed based on literature data. Hereto, we first selected the environmental variables that determine habitat suitability. Next, for each variable the tolerance range was determined. Finallly, the variables were combined using fuzzy logic in order to determine the degree of suitability of a habitat. The model predicts the presence of suitable spawning habitat in the Upper Zeeschelde, upstream of the River Durme. Later in the season, when the water temperature rises, suitable spawning habitat is also present in the Rivers Kleine Nete and Grote Nete. Suitable habitat for larval shad is located mainly in the Upper Zeeschelde upstream Rupelmonde and in the River Rupel. Spawning of twaite shad takes place in the main channel and during their ontogeny the larvae migrate to the edges of the main channel and to side channels.Therefore, in areas with suitable spawning and larval habitat, both the main and side channels need protection. In particular mudflats, sand flats and subtidal low dynamic habitats should be safeguarded. Dredging of these habitats thus mortgages the recovery of the twaite shad population in the Scheldt. The oxygen concentration in the estuary has been greatly improved in recent years.However, in summer a low-oxygen zone in the freshwater area persists, comprising the upstream migration of adults and the survival of larvae. Periodic hypoxic conditions should therefore be avoided and a minimum oxygen content of 5 mg / l is essential for both adults and larvae. During the last century, hydrodynamics in the estuary has increased markedly. As a result, larvae have more difficulties in maintaining their position in suitable habitat. Actions that increase the river/tidal flow or eliminate local retention areas should therefore be avoided