Assessing migratory bottlenecks and escapement of silver eel (Anguilla anguilla) in the highly urbanized North Sea Canal basin,the Netherlands

Habitat fragmentation and migration barriers have attributed to the decline of European eel (Anguilla anguilla). The migration of silver eels through pumping stations, ship locks, and migration facilities within the North Sea Canal basin using acoustic telemetry and overall escapement by mark–recapture was studied. A network of 61 acoustic receivers and three PIT-tag stations was built, and 305 silver eels were tagged with acoustic transmitters and 3923 with PIT-tags. Of all the silver eels that were detected, 55% passed barriers between the polders and the canal and 46% also passed the barrier complex at IJmuiden to sea, mainly via the ship locks. Overall, silver eel escapement to the North Sea per year averaged 81,629 silver eels and 14.3% suffered mortality in the pumping station at IJmuiden. Migration speed was lower for silver eels that initiated their migration upstream compared with silver eels further downstream, higher for silver eels passing barriers later in the migration period, and highest at sea. This study of silver eel movement and escapement in the North Sea canal basin indicated several bottlenecks. Passage success along barriers varied strongly between sites and types of barriers. While at two smaller locations, silver eels migrated through the pedal valves in the ship lock gates, a small passage facility and the pedal valves in a ship lock at complex IJmuiden did not enhance silver eel migration. The barriers and unnatural canal system caused additional delay during silver eel migrations. Mitigation measures for management could include installing fish-friendly pumps, using pumping stations only during the day, and in addition opening ship locks and pedal valves at the beginning of the night.     

Effects of a surface oriented travelling screen and water abstraction practices on downstream migrating salmonidae smolts in a lowland stream

Downstream migration of immature salmonids (smolts) may be associated with severe mortalities in anthropogenically altered channels. In Pacific salmon, several investigations have suggested the use of the dominating surface orientation of smolts to improve fish by‐pass structures in large and deep hydroelectric reservoirs. The present study tested the use of a surface orientated travelling screen to guide Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts past a water abstraction site in a shallow lowland stream. The percentage of total discharge abstracted from the stream was included in the analyses. Indigenous migrating smolts were trapped, PIT tagged and subsequently released upstream of the water abstraction site. Releases shifted between a present or absent travelling screen. The migration success of the released smolts was evaluated using a trap situated downstream of the water abstraction site. There was no evidence that the surface oriented travelling screen had any influence on the probability of fish passing the water abstraction site. However, for both species, the probability of successful migration past the water abstraction site correlated negatively with the abstracted percentage of the total daily stream discharge. These findings may have important management implications because they suggest that short term changes in the percentage of total stream discharge abstracted may have consequences for the downstream migration success of smolts. Copyright © 2009 John Wiley & Sons, Ltd.     

MULTIPLICATIVE LOSS OF LANDLOCKED ATLANTIC SALMON Salmo salar L. SMOLTS DURING DOWNSTREAM MIGRATION TROUGH MULTIPLE DAMS

Relatively little is known about the downstream migration of landlocked stocks of Atlantic salmon Salmo salar L. smolts, as earlier migration studies have generally focused on upstream migration. However, in watersheds with many hydroelectric plants (HEPs), multiplicative loss of downstream‐migrating salmon smolts can be high, contributing to population declines or extirpations. Here we report the results from a study of wild landlocked Atlantic salmon smolts in the River Klarälven. Salmon smolts, tagged with acoustic transmitters, were released at different locations and followed as they passed 37 receivers along a 180‐km‐long river segment, including eight dams as well as free‐flowing control stretches. We found that 16% of the smolts successfully migrated along the entire river segment. Most losses occurred during HEP passages, with 76% of the smolts being lost during these passages, which contrasts with the 8% smolt loss along unregulated control stretches. Migration speed was 83% slower along regulated stretches than along unregulated stretches. The observed lower migration speed at regulated stretches was dependent on fish size, with large fish moving slower than small fish. Discharge affected migration speed but not losses. As previously shown for anadromous populations, our study of landlocked salmon demonstrates similar negative effects of multiple passages of HEPs by downstream‐migrating smolts. On the basis of this and previous migration studies, we advocate using a holistic approach in the management and conservation of migratory fish in regulated rivers, which includes safe passage for both upstream‐ and downstream‐migrating fish. Copyright © 2012 John Wiley & Sons, Ltd.     

Downstream Migration of the European Eel (Anguilla Anguilla) in the Elbe River,Germany: Movement Patterns and the Potential Impact of Environmental Factors

Recruitment of European eels (Anguilla anguilla) has declined to the extent that they have been added to the IUCN Red List of Threatened Species. Therefore, it is critical to ensure that eels complete their outward river migration in order to contribute to the available spawning stock. We conducted a 4‐year (2007–2011) telemetry study to understand the migratory behaviour and potential impact of environmental factors on the eel during this critical life stage. Out of 399 female eels tagged with acoustic transmitters, only 28% demonstrated clear downstream migratory behaviour. Fifty‐five percent were detected exhibiting no downstream migration behaviour and 17% were not detected at any monitoring station. Movement patterns of downstream‐migrating (silver) eels were characterized by nocturnal activity and seasonal migration, with distinct peaks in autumn and spring. Migration was often discontinuous and exhibited phases of active locomotion and expanded stopovers. The most important determinants of movement activity were water temperature, cumulative precipitation and moonlight, although the significance varied by season and location in the river basin. Our results evidence a discontinuous, stepwise migration over an extended period. Furthermore, our findings indicate that migration success depends on holding duration prior to tagging and environmental predictors with varying importance depending on the season, as well as the locations of capture, tagging and release. Copyright © 2015 John Wiley & Sons, Ltd.     

Naturalised Atlantic salmon smolts are more likely to reach the sea than wild smolts in a lowland fjord

The survival rates of three groups of seaward‐migrating Salmo salar smolts were investigated in 2005, 2016, and 2017 in the River Skjern and River Omme, as well as in the Ringkøbing Fjord using acoustic telemetry. Ringkøbing Fjord extends for approximately 300 km², and has a narrow, regulated outlet to the sea. Smolts of three different origins: (a) wild smolts, (b) hatchery‐reared smolts previously released at half‐year‐old, and (c) hatchery‐reared smolts previously released at 1‐year‐old were captured in rotary screw traps and surgically implanted with acoustic transmitters. The progress during seaward migration was monitored with a network of automatic listening stations deployed in the river estuary, fjord mouth and sea opening. The smolts' probability of survival in the river was related to their length, with larger smolts being more likely to reach the fjord. Once in the fjord, the probability of reaching the sea was related with the smolt's group, with smolts previously released at half‐year‐old being more likely to succeed than wild smolts. However, none of the biometric or behavioural variables explained the difference between the studied smolt groups, masking the potential reasons behind this difference in survival probability. Overall, approximately 47% of the tagged smolts were registered at the last array of automatic listening stations (i.e., entered the sea), demonstrating the early migration as a critical bottleneck for the local Atlantic salmon population. Ultimately, this limits the number of Atlantic salmon that survive to adulthood and return to River Skjern and River Omme for spawning.     

Connectivity is a two‐way street—the need for a holistic approach to fish passage problems in regulated rivers

We evaluated the effects of a rehabilitation project, whose goal was to re‐establish longitudinal connectivity for anadromous trout in the regulated river Emån. We used a holistic approach, by tagging and following both upstream‐migrating spawners (N = 348) and downstream‐migrating smolts (N = 80) and kelts as they passed two hydroelectric plants (HEP 2‐3) with nature‐like fishways. When migrating upstream, 84–88% of the spawners stopped, primarily at spawning grounds, before reaching HEP2. The proportion of stoppers was lower (56%) for fish that had been to the fishways in previous years, indicating that the recolonization rate is likely to increase over time. Of the spawners that approached the fishway at HEP2, 77% rapidly located the fishway situated next to the tail‐race, resulting in an attraction efficiency of 81% and a passage efficiency of 95%. The time required to locate the fishway inside the former channel at HEP3 was substantial, but the attraction efficiency (89%) and passage efficiency (97%) were nevertheless high. The kelts swam downstream mainly in spring, using spill gates and the fishways, to swim past HEP2 and 3 and continue downstream to the Baltic Sea. Iteroparity was confirmed by the fact that 20% of the spawners were tagged in previous years. Smolt loss was about 30% for both HEPs, with a higher turbine‐induced loss 30% for fish passing through Francis runners than a Kaplan runner. Fifteen per cent of the tagged smolt reached the sea and none of these fish had swum through the Francis runners. It will probably take many years before longitudinal connectivity is fully re‐established in the river Emån, due to substantial losses of both upstream‐migrating spawners (35% loss) and downstream‐migrating smolts (50%) and kelts. In addition, smolt production in areas upstream of HEP3 is far below carrying capacity. Thus, additional measures that not only facilitate movement of upstream spawners, but also reduce mortality and injuries of downstream migrants are urgently needed to create a self‐sustaining fish population. Copyright © 2009 John Wiley & Sons, Ltd.     

Downstream Swimming Behaviour of Catadromous and Potamodromous Fish Over Spillways

Flow characteristics associated with spillways are important to restore ecological connectivity because spillways can either constrain or offer safe routes for downstream passage of fish. We studied the hydrodynamics of flow and downstream movement behaviour of the catadromous European eel (Anguilla anguilla) and the potamodromous Iberian barbel (Luciobarbus bocagei) in spillways with upstream face inclinations of θ = 90° (standard) 45° and 30° (modified). The standard spillway was tested for two water depths (H = 32 and 42 cm). Modified spillways facilitated downstream passage and reduced delay times of passage of the European eel. Upstream of the 90° spillway, distinct recirculation areas were observed, and associated turbulence strongly hampered downstream passage of fish. Both species were found to avoid turbulence, but barbel displayed stronger avoidance for areas of rapid changes in flow velocity when comparing to eels. Overall, eels were faster in passing the spillway and had a higher downstream passage success (80%) than barbel (32%). Eels were predominantly thigmotactic in contrast to barbel, which showed limited contact with structures. The results suggest that modified spillways may enhance passage survival. Copyright © 2015 John Wiley & Sons, Ltd.     

Spawning Migration of Wild and Supplementary Stocked Landlocked Atlantic Salmon (Salmo Salar)

Upstream migration by adult salmonids is impeded by dams in many regulated rivers, as is the case for landlocked Atlantic salmon, Salmo salar, in the River Klarälven, Sweden. There, the salmon cannot reach the spawning grounds due to the presence of eight dams. Hence, hatchery‐reared smolts are released downstream of the dams, and upstream migrating spawners are caught in a trap at the lowermost dam before transported by truck to the spawning grounds past the dams. To identify the spawning grounds and compare the behavior of wild and hatchery‐reared Atlantic salmon during upstream migration and spawning, 34 wild and 28 hatchery‐reared, radio‐tagged Atlantic salmon were followed during their spawning migration from August to October 2011. Half (50%) of the hatchery fish, but only 11,8% of the wild fish ended up as fallbacks, i.e. they migrated past the first downstream power station, and did not spawn. A significantly higher proportion (21.4%) of hatchery‐ reared salmon moved in an erratic way, with several up and down stream movements, when compared to the wild salmon (5.9%). When looking at the salmon that stayed in the river (exc. fallbacks), wild individuals exhibited a holding behavior (little or no movements before presumed spawning) more often (86.7%) than the reared ones (50%). The wild salmon also held position (and presumably spawned) for longer time (25.4 days) than the reared salmon (16.1 days). Reared salmon held position, on average, 10 km further upstream than wild salmon, passing the presumed best‐quality spawning habitat. The migration speed (average 17.4 km/day) between two logger stations did not differ between wild and reared fish or between sexes. Our results suggest that the reproductive success of hatchery‐reared Atlantic salmon is relatively low and their capacity as supplementary spawners to the wild population in the Klarälven, is probably small. Copyright © 2015 John Wiley & Sons, Ltd.     

EUROPEAN SILVER EEL MIGRATION AND FISHERIES‐INDUCED MORTALITY IN THE HAVEL RIVER SYSTEM (GERMANY)

By using acoustic telemetry, the behaviour and fishery mortality of silver eels Anguilla anguilla (L.) were studied during the early phase of the spawning migration through the Havel River downstream to the river Elbe. A total of 99 female silver eels were implanted with acoustic transmitters and released at two locations in the Havel River system in autumn 2007. Tag recaptures by fisheries were used to assess fishery mortality. Most eels continued their downstream migration immediately after release during October–November in the year of release. However, some eels continued their downstream migration the following spring or autumn; only a few eels migrated in winter and summer. Tagged eels reached the river Elbe from 2 days to more than 1 year after tagging. Fishing efficiency was very high, and by the end of December 2008, only 8% of the tagged eels had reached the river Elbe, whereas 25% were reported caught by commercial fishers and 2% by recreational fishers. The remaining 65% of the tagged eels were unaccounted for at the end of the study period. Copyright © 2011 John Wiley & Sons, Ltd.     

Tests of two types of bypass for downstream migration of eels at a small hydroelectric power plant

Efficiencies of two types of bypass, a surface and a bottom sluice, were tested for the natural downstream migration of silver eels Anguilla anguilla at a small hydroelectric power plant at Halsou, on the River Nive in France. Naturally migrating eels were caught after their passage through either bypass. A total of 637 eels were trapped during the three‐year study. Total efficiency for both bypasses, evaluated on the basis of downstream movement of radiotagged eels, ranged from 56% to 64%. Given a bias due to hydrological conditions at the time of the runs, the precise efficiency of each separate bypass was not calculated. However, preferred passage through the bottom bypass for both tagged and untagged eels was confirmed by telemetry, as three to four times as many eels transited through the bottom bypass compared to the surface one. The behaviour of 74 individuals released in the forebay was observed by radiotelemetry. Close to half of the radiotagged eels returned up the headrace after their release, and most eventually migrated downstream over the dam with appropriate environmental conditions. Upon arrival at the power plant, eels displayed foraging behaviour in the forebay with frequent displacement interrupted by long resting periods in zones with low current. The repulsive effect of the trashrack located in front of the turbine intake increased with increasing turbined discharge. The study indicated that a trashrack with a smaller bar‐spacing (around 20 mm), associated with an appropriate bypass, could deflect a large proportion of the female eels from the turbines. However, this solution needs to be tested on site to quantify the risk of mortality due to impingement on the trashrack. Copyright © 2005 John Wiley & Sons, Ltd.     

Poor Performance of a Retrofitted Downstream Bypass Revealed by the Analysis of Approaching Behaviour in Combination with a Trapping System

The implementation of fish downstream migration bypass systems is still a major challenge, and there is interest in validating the adequacy of different configurations of bypass devices. In the Amblève River (Belgium), a mobile 3.3‐m height dam feeds two principal Francis and one Francis micro‐turbine and is equipped with a modern vertical slot fish pass and a downstream bypass. The aim of this study was to test the bypass attraction and efficiency (i.e. percentage of fish that approach the entrance and use the bypass) for Atlantic salmon smolts. During three consecutive years, a total of 1346 smolts were equipped with a radio frequency identification tag and released from March to May upstream of the dam of Lorcé. The entrance of the downstream migration bypass was equipped with a radio frequency identification antenna in order to detect the smolts approaching. In 2014, a capture cage was also placed downstream the bypass to evaluate its efficiency. The mean percentage of detected smolts at the entrance varied from 26.2 to 39.7%. In 2014, 16.5% of the released smolts entered the bypass and were finally caught in the cage, representing 39% of the smolts detected at the entrance. More than 98% of the detections occurred during night (mainly between 9 pm and 3 am ). The searching delay near the bypass entrance varied from less than 5 min to more than 5 days (median 4.3 min). Visual observation indicated a behavioural reluctance before entering the bypass, with a shift from positive to negative rheotaxy. Our results underline the difficulty to install retrofitted bypass system on old existing hydropower plants. Copyright © 2016 John Wiley & Sons, Ltd.     

Behavioural interactions between prey (trout smolts) and predators (pike and pikeperch) in an impounded river

Movements of radio‐tagged pike Esox lucius (L.), pikeperch Stizostedion lucioperca (L.) and outward migrating sea trout smolts Salmo trutta (L.) were studied in a shallow Danish reservoir to obtain information of predator–prey interactions between these species. Twenty pikeperch (55–74 cm) and 19 pike (52–72 cm) were tagged. Female pikeperch spent more time near the outlet sluice during the smolt run (May) than at other times of the year, apparently actively hunting the smolts delayed in this area. In contrast, male pikeperch did not seem to participate in the smolt predation but remained stationary during the smolt run, presumably guarding their nests. Most tagged pike were present at the spawning grounds during the peak of the smolt run, where they had little chance of smolt encounter. Twenty migrating trout smolts were radio‐tagged in the river upstream of the reservoir. Ten of these were located in the vicinity of the outlet sluice at least once, but were unwilling or unable to find and enter the sub‐surface outlet sluice. Only one tagged smolt left the reservoir. After 1–12 days in the reservoir, the remaining smolts were eaten by pikeperch or pike and the results indicate that female pikeperch and few female pike have adjusted their behaviour to predation on smolts during the smolt run. The smolt predation in this man‐made reservoir is higher than in natural lakes, probably due to the changed physical environment and introduced predators, such as pikeperch. The outlet sluice practice and the temporal overlap between smolt run and predator‐spawning may be key factors in smolt survival. Copyright © 2000 John Wiley & Sons, Ltd.     

Causes of mortality of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts in a restored river and its estuary

During October 2000 to August 2002, the River Skjern Nature Project was implemented by removing dykes and re‐meandering 20.5 km of the lower canalized river. As a consequence the length of the river stretch increased to 23 km. A lake of 250 ha developed in the river valley 5 km upstream from the river mouth because of subsiding soils caused by reclamation and drainage since the 1960s. Using radiotelemetry, the mortality of wild Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts in the River Skjern and its estuary was investigated prior to and after the implementation of the project. Altogether, 77 Atlantic salmon and 66 brown trout smolts were caught, tagged and released in the river upstream of the restoration project during the spring of 2000 and 2002. The in‐river smolt mortality was more than double in 2002 for both Atlantic salmon and brown trout compared with 2000. This was primarily due to bird predation in 2002 which was not observed in 2000. The in‐river bird predation in 2002 was mediated by the new lake, which quickly became an important bird rest area. Estuarine mortality mainly caused by cormorants (Phalacrocorax carbo sinensis) differed significantly between species, but was high for both Atlantic salmon (39%) and brown trout (12%) in both years of investigation. The aggregated smolt mortality in the river and in the estuary (48%) may threaten an indigenous self‐sustaining Atlantic salmon population in the River Skjern. When planning river restoration projects, caution should be used, especially where permanently flooded floodplains (lakes) develop due to subsiding soil. In situations where rivers pass directly through newly developed lakes, migratory species such as Atlantic salmon and brown trout may be severely affected due to increased exposure to predation from predatory fish and birds. Copyright © 2006 John Wiley & Sons, Ltd.     

Mortality of silver eels migrating through different types of hydropower turbines in Lithuania

Hydropower plants (HPP) are considered to be one of the major threats to the survival of European eels when they migrate downstream along inland water bodies during the early part of their annual journey to the spawning area in the Sargasso Sea. There are 98 HPPs in Lithuania and thousands throughout Europe. Numerous studies describe HPP induced mortality rates among European eels as variable depending on local, environmental, and technical factors. This heterogeneity in effect complicates theoretical extrapolation to eel mortality arising from specific types of HPP, necessary for effective management of local stocks. Silver eel mortality was estimated for 4 different HPPs in Lithuania. Mortality was estimated using RFID (Radio Frequency Identification, passive integrated transponders) tags and acoustic telemetry in a large HPP (>100 MW) with Kaplan turbines, a small HPP (<1 MW) with a Kaplan turbine and a fish passage, and for the first time in two small HPPs (<1 MW) with CINK turbines. The results supported a hypothesis that the mortality rate of migrating eels depends mainly on the type and size of the turbine. HPP induced mortality varied from 100% in a small CINK turbine down to 25% in the large HPP with Kaplan turbines. The importance of simple mitigation measures was highlighted by 34% of all tagged eels bypassing one of the HPP via an adjacent fish passage constructed for upstream migration of salmonids. The observed differences in mortality provide essential information for long term strategies designed to restore depleted eel populations in Lithuania and other European countries.     

First record of migrating silver American eels (Anguilla rostrata) in the St. Lawrence Estuary originating from a stocking program

In fall 2009, six migrating silver American eels (Anguilla rostrata) originating from a stocking program were caught in the brackish waters of the St. Lawrence Estuary. These maturing eels were all females with gonads developed at a similar stage as other migrating eels in the estuary. Fluorescent oxytetracycline marks observed on the otoliths allowed us to assert without any doubt that they came from glass eels caught in Nova Scotia and stocked 4 years earlier in the Richelieu River, 500 km upstream from the recapture location. Their length varied between 570 and 668 mm, which is within the size range of naturally recruited female silver eels in Nova Scotia, while silver eel are most generally longer than 80 cm in the St. Lawrence estuary. Their growth rate was also exceptionally fast. This direct observation is the first evidence that American eels stocked as glass eels can migrate seaward at least as far as the estuary in synchrony with naturally recruited female silver eels en route to their spawning grounds in the Sargasso Sea.     

SHIP‐LOCK–INDUCED SURGES IN AN IMPOUNDED RIVER AND THEIR IMPACT ON SUBDAILY FLOW VELOCITY VARIATION

The majority of large rivers are fragmented by dams, and navigation is often supported by the installation of ship locks. Despite their ubiquitous existence, the effect of ship locks on river basin hydrodynamics is rarely considered in an environmental context. Ship‐lock operation induces single‐wave crests or troughs called surges, which propagate along the basin and are subject to reflection at the up‐ and downstream impoundments. We used pressure sensors and acoustic Doppler current profiler measurements to investigate the effects of up‐ and downstream ship locking on the water level and the current velocity dynamics in a 12.9‐km‐long basin of the impounded river Saar (Germany). Ship lifting at the lower dam and the associated water export from the basin results in a negative surge propagating upstream, whereas a descending ship‐lock operation at the upper dam of the basin creates a positive surge propagating downstream. Both types of waves are subject to positive reflection at the opposing dams. Frequent lock operations lead to a complex pattern of multiple superimposing surges. The resulting water level fluctuations are comparable in magnitude with those associated with discharge variations due to hydropower peaking but occur at much shorter timescales. Associated with the water surface displacement during wave passage is a corresponding increase or decrease of the longitudinal current velocity. The magnitude of wave‐induced velocity fluctuations can exceed mean flow velocities by a factor of three and, depending on wave type and direction, can result in a reversal of the main flow direction of the river. Because of their longevity of several hours and superposition effects, ship‐lock–induced surges govern 66.3% and 45.4% of the subdaily variations in flow velocity and water level, respectively. This article concludes with a discussion of the potential effects of lock‐induced flow dynamics in impoundments on oxygen dynamics and methane ebullition. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluating upstream passage and timing of approach by adult bigheaded carps at a gated dam on the Illinois River

Dams are a conservation threat because they function as barriers to native fish movement; however, they may prevent the spread of invasive species. Invasive bigheaded carps (Hypophthalmichthys spp.) threaten the Great Lakes ecosystem and are advancing towards Lake Michigan via the Illinois River. Navigation dams on the Illinois River may deter bigheaded carps' upstream movement. We investigated the permeability of the Starved Rock Lock and Dam (SRLD), the most downstream gated Illinois River dam, to bigheaded carps' migration by examining the timing of individuals approaching and passing through SRLD in relation to gate openness, tailwater elevation, and water temperature. Using acoustic telemetry of (N = ~104 per year) tagged fish, 13 upstream passages of bigheaded carps occurred through SRLD between 2013 and 2016. Eleven passages occurred through the dam gates and 2 through the lock chamber, indicating deterrents (e.g., CO₂) placed in SRLD lock chamber may only limit passage of a small proportion of all fish passing through the lock‐and‐dam structure. Passages were documented only in 2013 and 2015. Most of the dam gate passages occurred during high water when gates were completely out of the water. Timing of bigheaded carps approaching SRLD was positively correlated with rising water temperature and high tailwater elevation, and all fish approached during late March through mid‐September. Movement through dams is rare; modifying gate operations to reduce gate openness during late spring and summer could further reduce the permeability of gated dams such as SRLD to bigheaded carps, slowing their upstream advance.     

Performance of a prototype surface collector for juvenile salmonids at Bonneville Dam's first powerhouse on the Columbia River,Oregon

During April–July 2000, we radio‐tagged and released juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) to evaluate a prototype surface flow bypass at Bonneville Dam on the Columbia River. The mock bypass, called a prototype surface collector (PSC), had six vertical slot entrances that were each 6 m wide and 12 m deep. The PSC was retrofitted to the upstream face of Bonneville Dam's First Powerhouse. Our objectives were to: (1) assess species‐specific differences in movement patterns and behaviour of fish within 6 m of the face of the PSC, (2) estimate the efficiency and effectiveness of the PSC and (3) evaluate factors affecting the performance of the PSC. We found that 60–72% of the fish, depending on species, detected within 6 m of the PSC entered it. Of the fish that passed the First Powerhouse at turbines 1–6, 79–83% entered the PSC. Diel period was a significant contributor to PSC performance for all species, and day of year was a significant contributor to PSC performance for subyearling Chinook salmon. The PSC was twice as effective (%fish/%flow) as the spillway, passing 2.5:1 steelhead and subyearling Chinook salmon and 2.4:1 yearling Chinook salmon per unit of water. If fully implemented, the PSC would increase the percentage of fish that pass the First Powerhouse through non‐turbine routes from 65–77% (without the PSC) to 76–85% (with the PSC), depending on species. Published in 2008 by John Wiley & Sons, Ltd.     

The behaviour of Pacific salmonid smolts during passage over two experimental weirs under light and dark conditions

Little is known of how fish respond to the hydraulic environment associated with diversion or bypass structures at hydroelectric power installations. To address this lack of knowledge, this paper presents results from a study to assess how three species of Pacific salmonid smolt (Oncorhynchus spp.) responded to distinct gradients of velocity and depth associated with two submerged weirs as they passed through an experimental flume at McNary Dam (Columbia River, USA) under illuminated and dark conditions. Migrating smolts entered one of two available treatment channels as coherent schools from which individuals would either disassociate from the group and pass over the weirs, or would reject them by swimming upstream. Alternatively, fish maintained position at the upstream end of the flume by swimming into the flow. The response of smolts to velocity and depth gradient and light condition varied between species, and route of passage was influenced by fork length. Initial channel selection and school size was not influenced by weir type, although schools resided longer within the short‐weir channel. The majority of smolts (70%) entered the treatment channels facing downstream (negative rheotaxis), but switched orientation at the crests of the weirs. This switch in orientation occurred farther downstream in the short‐weir treatment and for the largest smolts. The variation in response of different species of smolts to hydraulic gradients has important implications for the design of screening mechanisms used at hydroelectric power installations to divert migrant juvenile salmonids. Published in 2006 by John Wiley & Sons, Ltd.     

Telemetry observations of predation and migration behaviour of brown trout (Salmo trutta) smolts negotiating an artificial lake

Eutrophication of coastal areas as a consequence of the agricultural use of fertilizers is a widespread problem. The development of artificial lakes and constructed wetlands in nutrient‐rich rivers is a widely used management tool in the fight to decrease eutrophication. Juvenile salmonids that have to negotiate these lakes during their downstream migration to the sea are commonly subjected to high mortality due to increased predation pressure and delayed passage. In this study, we double tagged 39 brown trout smolts with passive integrated transponder and radio tags to gain further insight into predation rates and migration patterns during their passage of an artificial lake in a Danish lowland stream in the spring of 2016. Thirty‐four of the tagged smolts, caught and released upstream, entered the lake, of which 22 (65%) successfully exited the lake. Four smolts (12%) returned upstream to the river. Three smolts were predated in the lake by two northern pike (Esox lucius). Three tags were recovered from the lake bottom, and two disappeared out of the study area after a last detection in the lake. Tracking the smolts manually and by automatic listening stations showed highly erratic movement patterns during lake passage. Further, we observed long delays of up to 27 days after the smolts reached the river mouth and before they entered the sea, potentially due to low sea water temperature or due to the stocking of a large amount of hatchery‐reared brown trout smolts. The results are discussed in the context of abiotic and biotic factors, which differed considerably in the year 2016 compared with previous years.