1,720,973 research outputs found
Quantifying the movement and behaviour of migratory European eel (Anguilla anguilla) in relation to physical and hydrodynamic conditions associated with riverine structures
No full textAnthropogenic structures such as dams, weirs, sluices, and hydropower facilities fragment river networks and restrict the movement of aquatic biota. The critically endangered European eel (Anguilla anguilla) migrates between marine and freshwater habitats and has undergone severe population decline. Barriers to migration are one of the negative impacts to be addressed for compliance with the EC Council Regulation for recovery of eel stocks. This thesis examines measures to reduce the effects of riverine structures on eel and improve passage facilities for both juvenile upstream and adult downstream migrating lifestages of this comparatively understudied species. The influence of turbulent attraction flow on eels ascending passage facilities was quantified at an intertidal weir. Plunging flow resulted in a two-fold increase in the number of eels using a pass. The behavioural mechanisms underlying this attraction, and wider questions of how eels respond to elevated water velocity and turbulent conditions found at barriers and fish passes were further investigated within a field flume. Eels showed a similarly strong attraction to turbulent areas, though adopted an energy conservation strategy by adjusting swim path to reduce the magnitude of velocity and turbulence encountered. Compensatory swimming speed was also used to reduce exposure to energetically expensive environments. Management recommendations are made to optimise the attraction of eels to pass facilities, yet ensure hydrodynamic conditions within the pass do not deter ascent. Legislative drivers also stipulate targets for seaward escapement of adult spawner stock. The impacts of multiple low head barriers and water abstraction intakes on route choice, delay, entrainment and escapement were quantified in a heavily regulated sub-catchment using telemetry. Entrainment loss at a single abstraction point was the biggest cause of reduced escapement, and was influenced by pumping regimes and management of intertidal structures. Delays at some structures were substantial (up to 68.5days), and reflected water management practices and environmental conditions. Sub-metre positioning telemetry allowed detailed behaviour of adult eel to be further quantified in relation to physical and hydrodynamic features at a hydropower intake. There was predominance of milling and thigmotactic behaviours at lower velocities (0.15 – 0.71 m s-1), whereas rejection occurred on encountering the higher water velocities and abrupt velocity gradients associated with flow constriction near the intake entrance. Information presented has implications for wider catchment management and highlights the potential to reduce barrier impacts through manipulation of structures and abstraction regimes. Quantifying eel behaviour in response to physical and hydrodynamic environments will aid the development of attraction, guidance and passage technologies
Escapement, route choice, barrier passage and entrainment of seaward migrating European eel, Anguilla anguilla, within a highly regulated lowland river
Full text linkFluvial disconnectivity can have important impacts on fish populations, including hindering movement between habitats required for different ontogenic stages. Recruitment of the European eel (Anguilla anguilla) has reduced by over 90% since the early 1980's, in part due to the effect of riverine barriers on its catadromous migration. There is a legislative requirement to restore free passage, increase habitat availability, and limit anthropogenic losses at intakes to aid eel recovery and good ecological status; necessitating an improved understanding of underlying processes. Escapement, route choice, delay at structures, and entrainment at water abstraction points of downstream migrating silver eels were examined using acoustic and Passive Integrated Transponder (PIT) telemetry in the heavily regulated lower river Stour, UK. Downstream migrating adult eel (n = 69) were trapped approximately 10 km upstream of the tidal limit, surgically implanted with an acoustic transducer and PIT transponder, and released between October and December in 2009 and 2010. Movements of tagged individuals were monitored by a linear array of 19 fixed acoustic receivers extending from the release site, through the last 9.2 km of the freshwater catchment. Three groups of water control structures, two water abstraction intakes and several possible routes of migration are present in the reach. Seventy six and 65% of tagged eels escaped from the study reach in 2009 and 2010, respectively. Entrainment at a single intake was the principal cause of loss and positively related to rapid increases in abstraction whilst eels were in the vicinity of the intake. Route choice into the estuary was dependent on discharge over a large intertidal weir; opening regimes of a tidal gate at the termination of the alternative channel; and abstraction rate at a nearby water intake. Long delays (up to 68.5 days) and recurrent behaviour were associated with several structures in the study reach; high variability between individuals reflected the management of spill at weirs. Potential scenarios for minimising entrainment and delay through integrated management of water level control structures and abstraction rates are discussed
The influence of attraction flow on upstream passage of European eel (Anguilla anguilla) at intertidal barriers
No full textRiver structures can delay or prevent upstream migration of the critically endangered European eel Anguilla anguilla (L.). Eel ladders are frequently installed to mitigate for the impacts of barriers. There has been little quantitative testing to optimise attraction to pass facilities. The effect of plunging and streaming flow on pass efficiency was tested within field trials using four eel ladders at an intertidal weir with little seaward freshwater discharge. Eel passage was 2 fold higher in the presence of plunging flow. Water temperature and height of tide were also significant factors influencing daily catch. A strong ‘edge effect’ influenced route choice, with greatest catches in traps positioned at the channel sides. Route choice was related to body size with largest size classes (>121 mm) mostly passing towards the centre of the channel. The findings show that simple manipulation of hydrodynamic conditions at the entrance to upstream eel passes can improve passage efficiency for both juvenile and adult life-stages
Using ‘trap and transport’ to facilitate seaward migration of landlocked European eel (Anguilla anguilla) from lakes and reservoirs
No full text‘Trap and transport’ (T&T) is employed to facilitate the seaward migration of European eel (Anguilla anguilla) past obstacles such as hydropower facilities. Hence, previous studies assessing its efficacy have focussed on disrupted fluvial systems. The transferability of findings from lotic to lentic systems is uncertain because many of the environmental cues that trigger eel migration in rivers are lacking from reservoirs and lakes, particularly those with limited flow connection to the surrounding catchment. We used acoustic telemetry to compare the migration behaviour of T&T adult A. anguilla which were fyke netted and transported from two disconnected reservoirs (n = 80) to a control group of resident river eel (n = 30) during their migration through the lower River Stour, UK, to the North Sea. Migration patterns and behaviour were broadly similar between the reservoir T&T eel and river eel with 86 and 90% of each group successfully reaching the sea, respectively. Reservoir eel were larger and at a more advanced stage of migratory readiness (silvering) and commenced migration sooner after release than the river eel, but they descended the catchment at a slower rate. Behaviour in the estuary was highly variable between individuals with residence times ranging from 5 h to 83 days (median = 1.4 days) across all groups. Only one individual failed to migrate through the estuary and most (75%) reached the sea within five days of entering the estuary. Findings indicate that T&T of adult eel from reservoirs represents a feasible method to allow landlocked individuals to migrate and potentially contribute to the spawning stock, either now or in the future. Results also highlighted the high capture effort that may be required to implement an effective T&T programme. Gaining a thorough understanding of eel abundance and population structure in the source waterbody is desirable before implementation
Data from: Response of seaward migrating European eel (Anguilla anguilla) to manipulated flow fields
No full textAnthropogenic structures (e.g. weirs and dams) fragment river networks and restrict the movement of migratory fish. Poor understanding of behavioural response to hydrodynamic cues at structures currently limits the development of effective barrier mitigation measures. This study aimed to assess the effect of flow constriction and associated flow patterns on eel behaviour during downstream migration. In a field experiment, we tracked the movements of 40 tagged adult European eels (Anguilla anguilla) through the forebay of a redundant hydropower intake under two manipulated hydrodynamic treatments. Interrogation of fish trajectories in relation to measured and modelled water velocities provided new insights into behaviour, fundamental for developing passage technologies for this endangered species. Eels rarely followed direct routes through the site. Initially, fish aligned with streamlines near the channel banks and approached the intake semi-passively. A switch to more energetically costly avoidance behaviours occurred on encountering constricted flow, prior to physical contact with structures. Under high water velocity gradients, fish then tended to escape rapidly back upstream, whereas exploratory 'search' behaviour was common when acceleration was low. This study highlights the importance of hydrodynamics in informing eel behaviour. This offers potential to develop behavioural guidance, improve fish passage solutions and enhance traditional physical screening.</span
Data for 'Response of seaward-migrating European eel (Anguilla anguilla) to an infrasound deterrent'
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Response of seaward-migrating European eel (Anguilla anguilla) to an infrasound deterrent
No full textBehavioural guidance technologies that employ stimuli to attract or repel fish offer potential to enhance, or even replace, costly physical and mechanical screens traditionally used to protect fish at river infrastructure such as hydropower and water intakes. At these structures, eel can suffer high rates of damage and mortality if entrained in pumps or turbines, or impinged on screens intended to protect them. This study used acoustic telemetry to quantify the behavioural response of adult European eel (Anguilla anguilla) to infrasound (12 Hz) under field settings. Eel (n = 50) were tracked after release immediately upstream of the forebay of a redundant hydropower facility. An infrasound deterrent located at the water intake either emitted continuously (ON) or was switched OFF. Treatment (ON/OFF) was alternated nightly over 10 consecutive nights with five eel released during a single trial conducted each night. Seventy eight percent of the 41 eel that moved downstream into the forebay passed the intake. Although the infrasound deterrent had no effect on passage rate, fine-scale differences in movement patterns were determined. When the infrasound was ON, eel trajectories were on average over twice as long with frequent erratic turns and milling behaviour (i.e. repeated lateral movements perpendicular to the principal flow direction), and they were less active within the intake channel close to the device. Infrasound deterrents that induce avoidance could be used to protect eel, either through enhancing the efficiency of physical screens or as part of multimodel behavioural guidance systems. It is important to consider the influence of site characteristics, especially water velocities that dictate the potential for eel to respond to an acoustic field created
Response of seaward migrating European eel (Anguilla anguilla) to manipulated flow fields
Full text linkAnthropogenic structures (e.g. weirs and dams) fragment river networks and restrict the movement of migratory fish. Poor understanding of behavioural response to hydrodynamic cues at structures currently limits the development of effective barrier mitigation measures. This study aimed to assess the effect of flow constriction and associated flow patterns on eel behaviour during downstream migration. In a field experiment, we tracked the movements of 40 tagged adult European eel (Anguilla anguilla) through the forebay of a redundant hydropower intake under two manipulated hydrodynamic treatments. Interrogation of fish trajectories in relation to measured and modelled water velocities provided new insights into behaviour, fundamental for developing passage technologies for this endangered species. Eels rarely followed direct routes through the site. Initially, fish aligned with streamlines near the channel banks, and approached the intake semi-passively. A switch to more energetically costly avoidance behaviours occurred on encountering constricted flow, prior to physical contact with structures. Under high water velocity gradients, fish then tended to escape rapidly back upstream, whereas exploratory ‘search’ behaviour was common when acceleration was low. This study highlights the importance of hydrodynamics in informing eel behaviour. This offers potential to develop behavioural guidance, improve fish passage solutions and enhance traditional physical screening
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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