Response of downstream migrating adult European eels (Anguilla anguilla) to bar racks under experimental conditions

Russon IJ, Kemp PS, Calles O. Response of downstream migrating adult European eels (Anguilla anguilla) to bar racks under experimental conditions.
Ecology of Freshwater Fish 2010: 19: 197–205. © 2010 John Wiley & Sons A/S Abstract – The behavioural response of downstream migrating nonsalmonid fish to hydraulic conditions associated with river infrastructure is poorly understood. The response of downstream migrating adult European eels (Anguilla anguilla) to bar racks (12 mm bar spacing) angled on the vertical and horizontal planes under different flow regimes and during periods of darkness was assessed. Eels predominantly moved along the channel floor and wall, tending to follow routes where turbulence intensity was high. Time taken to approach the racks was greater than expected if fish had moved passively with the flow. Eels did not exhibit clear avoidance behaviour prior to encountering the racks, instead marked changes in behaviour occurred only after physical contact was made with the structure. No impingement or passage through the racks occurred, and passes per approach were high (98.3%), when vertical racks were angled 15°, 30° or 45° relative to the flow. Impingement and passage through the racks only occurred when horizontally inclined racks were placed perpendicular to the flow. The time eels were impinged on the racks was negatively related to discharge when angled at 30° relative to the channel floor, and positively related when upright. Frequency of impingement was higher under low discharge (132.9 ± 16.6 l·s^?1). Impinged eels escaped from racks at approach velocities of 0.90 ± 0.05 m·s^?1. Passage through the upright rack was common under high discharge (278.9 ± 36.2 l·s^?1). The information presented will improve current fish passage criteria for European eels that are required to develop more effective fish passage facilities.     

Shipping canals on the downstream migration route of European eel (Anguilla anguilla): Opportunity or bottleneck?

Migrating fish species are worldwide in decline due to several global changes and threats. Among these causes are man‐made structures blocking their freshwater migration routes. Shipping canals with navigation locks play a dual role in this. These canals can serve as an important migration route, offering a short cut between freshwater and the sea. In contrast, the navigation locks may act as barriers to migration, causing delays and migration failures. To better understand these issues for downstream migrating fish, we studied the behaviour of European eels (Anguilla anguilla) in the Albert Canal at two scales. The mid‐scale contained a 27‐km canal pound confined by two navigation lock complexes, in which we released and tracked 86 silver eels. The small scale was a 200 × 150 m area just in front of the most downstream complex of the canal pound, where we analysed the behaviour of 33 eels in relation to the flow field resulting from a computational fluid dynamics (CFD) model. This paper discusses the factors influencing fish behaviour, and the relation between these behaviours on both scales. On the mid‐scale, migration efficiency resulted from a combination of intrinsic behaviour and flow in the canal pound. Also on the small scale, intrinsic behaviour influenced the success to pass the navigation lock. Increasing the flow would create more attraction and passage opportunities and hence facilitate migration through shipping canals, but only if this flow guides the fish through safe passage routes.     

Downstream passage of silver eels at a small hydroelectric facility

Abstract  A 100-mm diameter bypass was installed in the dam face at a small hydropower station on the Mokau River, New Zealand, and its use by downstream migrating silver eels, Anguilla spp., was monitored by trapping at the outlet in autumn 2002 and 2003. In addition, a passive integrated transponder system was used to monitor passage over the spillway. Migrant eels were able to find the bypass, with 544 and 744 eels recorded using it in 2002 and 2003 respectively. Although the bypass was the sole means of safe passage at low flow, migrant eels passed down the spillway in preference to the bypass when the dam was overtopped during floods. A combination of spilling and small diameter bypasses would provide safe downstream passage at hydroelectric facilities for silver eels, so long as entrainment and impingement at the intake screens can be prevented.     

Migration of silver eel,Anguilla anguilla,through three water pumping stations in The Netherlands

European eel, Anguilla anguilla (L.), migrating to sea encounter many man-made structures that can hamper and delay migration or induce mortality. Three pumping stations in Friesland, the Netherlands, were covered with acoustic receivers. Ninety-three silver eels tagged with acoustic transmitters were released in the polders upstream of the stations and 89% were detected passing a pumping station. The majority of silver eels passed the stations within a day after arriving at the station. Four silver eels stayed for longer than 2 weeks before passage, and 18 were detected at the receiver downstream the pumping station for more than one day, with detections up to several weeks. These detections probably indicated a dead eel, but could also indicate a live eel remaining at the site. Most of the silver eels passed the pumping stations within a day after release, so fish-friendly pumps will benefit the migrating population most. In the Netherlands, there are several thousand pumping stations. Installing fish passages near these stations is not feasible due to high costs. Prioritising all these sites in relation to the degree of blockage, mortality rates and its relative importance for migratory fish, can maximise the effectiveness of measures and mitigation taken.     

A preliminary telemetry study of the migration of silver European eel (Anguilla anguilla L.) in the River Mosel, Germany

Abstract  – To study the behaviour of silver eels ( Anguilla anguilla L.) during their downstream migration, particularly near a hydroelectric power dam, we tagged nine eels with ultrasonic transmitters and tracked their paths in the River Mosel, Germany. The onset of migration coincided with the first flood event that followed the full moon but was independent of daytime, because migration and turbine passage occurred during both day and night. During migration eels swam actively downstream with a velocity of 0.3–1.2 m · s⁻¹. When migrating eels arrived at the dam, they either passed through the turbines immediately or stayed upstream of the powerhouse for up to 8 days, showing a characteristic circling behaviour. Circling eels repeatedly approached the trashrack, sprinted upstream, and finally passed through the turbines with the next high water discharge. These observations are discussed with regard to the design of appropriate downstream passage facilities.     

Selective opening of hydroelectric dam spillway gates for downstream migrant eels in New Zealand

Abstract  Selective opening of the Patea hydroelectric dam spillway gates was examined for its potential to provide safe downstream passage for sexually mature (silver) eels in New Zealand. During autumn 2000, one of the spillway gates at the 82-m high Patea Dam was opened for 2.5 h and a large fyke net set across the river about 1.5 km below the dam. Fifty one live endemic longfin eels Anguilla dieffenbachii (Gray), 60 live shortfin eels Anguilla australis (Richardson), and eight dead shortfin eels were captured. All but three of the eels caught were downstream migrants, and aside from the dead eel, only a small portion (5%) had external signs of injury. It was likely that the majority of the eels captured passed over the spillway, but some may have originated from the section of river between the dam and the net or gone through the turbines. To further test the ability of eels to survive passage over the spillway, three controlled spills were made in autumn 2001 and 102 eels released immediately upstream of the partially opened spillway gates. A net stretched across the base of the concrete spillway was used to recover the eels. Most of the eels recovered from the spillway had low levels of injury. However, during the third controlled spill, 10 wild migrant eels were also caught with none showing any external signs of injury. This indicated that the damage noted on the recovered eels were the result of handling rather than passage over the spillway. It was concluded that selective opening of hydroelectric dam spillway gates can provide safe downstream passage for migrant eels.     

Reservoir entrapment and dam passage mortality of juvenile Chinook salmon in the Middle Fork Willamette River

Abstract – High‐head dams in Oregon’s Willamette River basin inhibit seaward migration and present significant mortality risks to ESA‐listed juvenile Chinook salmon (Oncorhynchus tshawytscha). Over 7 years, we passively collected 13,365 downstream‐migrating juvenile salmon in rivers above and below Willamette dams. Most salmon emigrated from upstream sites in February–June, but passed dams in November–February when reservoirs were drawn down near annual lows, and access to deep‐water passage routes improved. Samples collected above reservoirs were dominated by subyearlings, whereas below‐dam samples were a phenotypically diverse mix of subyearling, yearling and older salmon. The life history data indicated that Willamette reservoirs seasonally entrap many salmon and some sea‐ready smolts probably residualise. Annual dam‐passage mortality estimates were 8–59% (mean = 26%). Individual salmon mortality risk increased significantly with body length and varied with reservoir elevation and discharge. Operational changes that allow timely volitional emigration and development of less hazardous passage routes would benefit this threatened population.     

First isolation of Flavobacterium psychrophilum associated with reports of moribund wild European eel (Anguilla anguilla) in Scotland

In late April 2015, the River Dee Trust informed Marine Scotland Science, Fish Health Inspectorate (FHI), that there had been observations of dead and moribund European eels on the River Dee. Later in May, the Spey Fishery Board also reported a number of moribund European eels in a rotary screw smolt trap on the River Spey. In total, 10 cases involving moribund eels were investigated in 2015 and one case in 2016. In addition, a health screen was conducted to investigate the potential presence of Flavobacterium psychrophilum in healthy eels and Atlantic salmon from the River Dee in 2015. Externally, the diseased eels demonstrated white patches in different locations of the body. In all cases, F. psychrophilum was detected by bacterial isolation and/or molecular methods. Three isolates were further characterized by whole‐genome sequencing (WGS) as belonging to sequence type 15 (ST15). Histological examination of diseased European eels revealed lesions at the level of the integument. The pathogen screen for F. psychrophilum in wild healthy fish tested negative by PCR. Further investigation is required to understand the pathogenicity of this bacterium on the health of eels and the potential impact on the wild salmonid population.     

Migratory delay leads to reduced passage success of Atlantic salmon smolts at a hydroelectric dam

Passage of fish through hydropower dams is associated with mortality, delay, increased energy expenditure and migratory failure for migrating fish and the need for remedial measures for both upstream and downstream migration is widely recognised. A functional fish passage must ensure safe and timely passage routes that a substantial portion of migrating fish will use. Passage solutions must address not only the number or percentage of fish that successfully pass a barrier, but also the time it takes to pass. Here, we used radiotelemetry to study the functionality of a fish bypass for downstream‐migrating wild‐caught and hatchery‐released Atlantic salmon smolts. We used time‐to‐event analysis to model the influence of fish characteristics and environmental variables on the rates of a series of events associated with dam passage. Among the modelled events were approach rate to the bypass entry zone, retention rates in both the forebay and the entry zone and passage rates. Despite repeated attempts, only 65% of the tagged fish present in the forebay passed the dam. Fish passed via the bypass (33%), via spill (18%) and via turbines (15%). Discharge was positively related to approach, passage and retention rates. We did not detect any differences between wild and hatchery fish. Even though individual fish visited the forebay and the entry zone on multiple occasions, most fish passed during the first exposures to these zones. This study underscores the importance of timeliness to passage success and the usefulness of time‐to‐event analysis for understanding factors governing passage performance.