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.     

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.     

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.     

River connectivity reestablished: Effects and implications of six weir removals on brown trout smolt migration

Today's river systems have been extensively modified, requiring us to rethink how we approach the management of these important ecosystems. We evaluated the effects of removing 6 weirs in River Villestrup (Jutland, Denmark) on the smolt run of brown trout (Salmo trutta) over the course of 12 years. During 5 of these years, we evaluated the number, size, and timing of smolts during their downstream migration. We found an increase in smolt output following the weir removals, along with a decrease in average length and indications of an earlier peak migration. Our results suggest that barrier removal has led to an increase in spawning success by adults, fry survival, recruitment, and smolt migration success. Weir removal is therefore a viable management approach to restore connectivity in freshwater streams and rivers, which promotes the passage of smolts as they migrate to marine environments.     

Effect of decreased temperature on growth and smoltification of juvenile atlantic salmon (salmo salar) and brown trout (salmo trutta) in a norwegian regulated river

The lowermost 20 km of the River Surna, northwestern Norway receives cool water during summer from a hypolimnetic release mountain reservoir. In this part of the river, yearlings of both Atlantic salmon and brown trout are significantly smaller compared with those in the upper section of the river, which is unaffected by the cold water release. The slower growth below the power station causes both Atlantic salmon and anadromous brown trout to smoltify one year later compared with fish in the upper section of the river. This leads to higher mortality and therefore lower production of both adult salmon and trout.     

Survival of Migrating Sea Trout (Salmo trutta) Smolts During Their Passage of an Artificial Lake in a Danish Lowland Stream

Artificial lake development is often used as a management tool to reduce nutrient runoff to coastal waters. Denmark has restored more than 10 000 ha of wetlands and lakes in the last 14 years as a consequence of ‘Action Plans for the Aquatic Environment’, which aim to meet the demands of the European Union's Water Framework Directive. Juvenile, seaward migrating salmonids are highly affected by impounded waterbodies, as they are subjected to extraordinary high mortalities due to predation and altered habitat. From 2005 to 2015, survival and migration patterns of wild brown trout (Salmo trutta) smolts were investigated by using radio, acoustic and Passive Integrated Transponder telemetry both before and after the development of an artificial lake in a small Danish lowland stream. In 2005 and 2006, before the lake developed, survival was estimated to be 100% in the river stretch where the lake later developed. In 2007 and in the period between 2009 and 2015, mean yearly survival decreased to 26%. Mean time for passing the area increased significantly after the development of the lake from 0.42 to 5.95 days. Generalized additive models were used to model the probability of a successful passage. Water temperature and discharge were key environmental factors affecting survival of the smolts during the passage of the lake. Furthermore, smolt survival was negatively correlated with condition factor. This elevated level of smolt mortality may seriously compromise self‐sustaining anadromous salmonid populations when artificial lakes are developed in connection with rivers. 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.     

Low potential for restraint of anadromous salmonid reproduction by beaver Castor fiber in the Numedalslågen River catchment,Norway

After a long absence, beaver Castor fiber are rapidly returning to Europe. Their dam‐building and tree‐felling behaviour may have consequences for salmon Salmo salar and sea trout Salmo trutta management. In 2003 we investigated the parallel use of stream sections by beaver, sea trout and salmon and determined the potential hindrance that beaver dam‐building presented for reproducing salmon and sea trout along 65 km of the Numedalslågen River and tributaries, a major Norwegian catchment. We also surveyed landowner attitude to having beaver on salmon and sea trout streams. Most salmon spawned in the river and most sea trout in 51 tributaries. Nine of these tributaries also hosted spawning salmon. 15 (29%) of the 51 tributaries with spawning sea trout and six (67%) of the nine with spawning salmon had intermittently been occupied by beaver. Though beaver preferred to colonize the same sections of stream used for spawning, only 15% of the stream length navigable by salmon and sea trout on the 51 tributaries had actually been used by beaver, and only three colonies were occupied autumn 2003 (1 colony/25.0 km). Five dams were functioning during autumn 2003 on the 51 tributaries (1 dam/14.3 km). These potentially hindered sea trout and salmon from reaching 18% and 3%, respectively of their potential spawning habitat, though all dams were low (≤0.5 m). Though the autumn density of occupied beaver colonies along the river (1 colony/2.5 km) was 10.0 times the density on the 51 tributaries, no dams were built on the river. Thus most salmon reproduction in the catchment was unhindered by beaver. Nine of 14 landowners were unequivocally positive about having beaver together with salmon and sea trout. We conclude that the presence of beaver on similar catchments will likely have only an insignificant negative impact on the reproduction of sea trout and salmon. Copyright © 2007 John Wiley & Sons, Ltd.     

Movement and summer habitat of brown trout (Salmo trutta) below a pulsed discharge hydroelectric generating station

Radiotelemetry was used to investigate detailed movement and summer habitat of brown trout Salmo trutta (size range 157–488 mm TL, n=18) in the Kananaskis River, Alberta. Flows in the Kananaskis River respond to pulsed daily discharge from an upstream hydroelectric generating facility (range 0.15–25 m³ s⁻¹). Wetted area available for brown trout doubled during periods of high flow. Fluctuating river levels did not appear to influence the degree to which brown trout moved within the study site. However, there was evidence that brown trout used cover and pools more as discharge increased. During high flow conditions, brown trout used similar depths (63 cm), and significantly lower surface water velocities than during low flow conditions. Brown trout also moved closer to shore into interstitial spaces among woody debris and root complexes during high flow. Pool habitats were used most often compared with all other habitat types combined. Pools with large woody debris accounted for 75% of all habitat observations. Woody debris was used more often than all other cover types. Results of the study indicate that the effects of river regulation on brown trout appear to have been moderated by woody debris in pools and along river banks, which provided refuge from high water velocities during periods of high flow. Copyright © 1999 John Wiley & Sons, Ltd.     

Assessing the occurrence of egg stranding for trout and salmon in a regulated river

A key challenge in many regulated rivers is to define adequate flow levels to protect aquatic organisms. Provisioning of suitable flow can be pivotal bottlenecks for fishes such as salmon and trout that use the riverbed as an incubation habitat. Additionally, the locations where females spawn will define the probability that embryos will be dewatered during the incubation period in a regulated flow regime. We investigated the water flow, dewatering, and incubation mortality in Atlantic salmon and brown trout in natural nests over 19 years in the regulated Bjoreio River in western Norway. During the study period, different flow strategies were applied to mitigate the dewatering of incubating salmon and trout embryos. Average survival in nests sampled in late winter ranged from 54% to 92% among years and was significantly correlated with the minimum water flow occurring during the incubation period. Mortality was significantly higher in nests in shallow areas, reflecting nests exposed to dewatering. The results demonstrate a strong link between incubation mortality and managed flow regimes for river spawning salmonids. Using detailed information on the nest location and incubation mortality, we estimate minimum flow requirements for this river and demonstrate an approach to effectively mitigate the impact of river regulations on embryo survival in Atlantic salmon and brown trout.     

Effect of stream regulation on population parameters of atlantic salmon [Salmo salar L.] in the river Lærdalselva,western Norway

The river Lærdalselva, West Norway, was regulated in the autumn of 1974. Regulation led to an increase in winter flow and a decrease in summer flow in a section where there was natural production of salmon. A slight decrease in summer temperature was also recorded in the uppermost part of this section. No data existed on juvenile Atlantic salmon before regulation and the regulation effect on juvenile fish population parameters is therefore based on samples taken from adult salmon in the period 1969 to 1984. No differences in growth, smolt age, and smolt size which could be related to the regulation of the river were found. The smallest mean sizes of yearlings (0+) were found in 1964 and 1967. After regulation the mean size was never lower than before. Mean smolt age was between 3–1 and 3–9 years, but after regulation never exceeded that found before. River growth was slow, three year old smolts growing faster than four year old smolts. However, there were no differences in river growth before and after regulation.     

Effect of aquatic mosses on juvenile fish density and habitat use in the regulated River Suldalslågen,western Norway

Two morphologically distinct moss communities were found in the River Suldalslågen. The liver moss community consists of species which form a dense mat on the bottom, while the river moss (Fontinalis) community forms long tufts. Moss growth has increased since hydropower regulations due to reduced floods and increased winter flows. Increased moss cover affects the bottom structure, as well as intra‐gravel and near‐bottom hydraulics. We studied densities of juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) by electrofishing and habitat selection by direct underwater observation, in areas with natural moss cover compared with areas where mosses were experimentally removed. Areas with dense mats of liver mosses held lower densities of young of year (YoY) and older salmon parr than areas where liver moss had been removed. No differences in densities of YoY salmon were found between areas with and without Fontinalis. For older salmon, parr results were inconclusive. In some samples more and in others fewer fish were found in areas with Fontinalis moss removed. For trout, densities were higher in areas with Fontinalis, while results for liver moss were inconclusive. No major differences were found with regard to microhabitat selection between areas with and without river moss, suggesting that habitat quality in these areas was similar during summer, except with respect to substrate. Salmon held more exposed positions in areas without liver moss, but this is mainly attributed to different habitat availabilities. It is concluded that the relative increase in liver mosses in the River Suldalslågen has a negative impact on juvenile Atlantic salmon fish density. Copyright © 2002 John Wiley & Sons, Ltd.     

Migratory patterns and return to the catch site of adult brown trout (Salmo trutta L.) in a regulated river

The spawning migration and local homing of adult brown trout was analysed using radio telemetry in a regulated river in central Norway. Twenty‐eight large (37–64 cm) brown trout (Salmo trutta L.) were tracked before, during and after spawning in the River Nea, a watercourse with several obstructions, including an outlet tunnel from a power station and a regulated stretch (26 km) with 45 weirs. Two major patterns of spawning migration were found: (1) about half (n = 16; 57%) of the trout moved very little and remained in the deeper pools of the river from June until November; (2) about half (n = 12; 43%) of the trout migrated relatively long distances (12.5–28 km) up the river prior to the spawning period where they stayed in the outlets of small tributaries, or in rapids on the main river during the spawning period. We assume that these trout belong to a population of lake‐run migratory trout using the River Nea for spawning. There was no significant difference in body length of migratory and stationary brown trout and no significant difference in total distance moved by migratory males (30.5 km, n = 6) and females (20.5 km, n = 6, p > 0.05). Among migratory trout, we found no correlation between body length and migrated distance. Of the 12 migratory trout, nine undertook fast upward migration in periods of high water flow (> 100 m³/s). They passed the outlet tunnel from the power station and negotiated two to 35 weirs before reaching their main reproduction areas. Three trout crossed several weirs when the discharge was low (10–40 m³/s). When there was low discharge, fish remained at the outlet tunnel for up to four weeks and showed a high level of activity. Postspawning downstream migration started between 25 September and 25 October. Most migratory trout (n = 9) wintered in pools on the lower part of the river or in weir basins; only two trout descended to the lake (Selbusjøen) in late autumn. Copyright © 2004 John Wiley & Sons, Ltd.     

Effects of river regulation on the structure of a fast-growing brown trout (Salmo trutta L.) population

During the last 70 years, the Norwegian lake Mjøsa and its inflowing rivers have been subjected to serious changes due to hydroelectric power development. Regulation of the main inlet river, Gudbrandsdalslagen, started in 1919. The river power station at the Hunder fall was completed in 1964. This resulted in a reduction of winter water flow below the Hunder dam from approximately 26m³s^?1 to 2m³s^?1, which affected the most important spawning area of the fast-growing population of brown trout, Salmo trutta L. The population was investigated in detail in 1907, 1909, 1961, and 1985, and river growth, smolt age, and growth in Lake Mjøsa are compared. Only wild fish were included in the study. The main pattern throughout this period shows an increased river growth rate before smoltification and reduced smolt age. The average smolt age dropped from 4.7 years in 1909 to 4.1 years in 1985, and at the same time smolt size decreased from 26.8 cm to 25.1 cm. Considering the major changes in abiotic factors in the river spawning section, the changes in age structure and growth of brown trout smolt are comparatively small. In Lake Mjøsa, increased productivity due to input of nutrients has obviously favoured forage fish such as smelt (Osmerus eperlanus (L.)) and vendace (Coregonus albula (L.)). The growth rate of brown trout in the lake has improved from 1909 to 1961 and 1985, followed by a reduced spawning age. However, due to increased human exploitation the average length of ascending fish (approximately 68 cm) and condition factor ( K = 1.14–1.16) have altered little.     

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.     

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.     

Effects of a small hydropower station upon brown trout Salmo trutta L. in the River Hoz Seca (Tagus basin,Spain) one year after regulation

A small hydroelectric power station was built in 1993 on the River Hoz Seca (Tagus basin, central Spain). Pre‐ and post‐regulation studies provided the opportunity to test the early effects of this disturbance on the brown trout Salmo trutta L. population. Before and after comparisons of population density and biomass, age composition, growth and production were made upstream and downstream of the diversion dam. The effects of disturbance on benthic macroinvertebrates were also analysed but no changes in abundance were detected. The downstream estimated population densities and biomass of trout showed a decrease of about 50 and 43%, respectively, following regulation. Examination of length‐for‐age tables revealed no obvious change in growth but a significant difference in age structure. The main consequence of the imposed fluctuating flow regime was a serious reduction in trout production caused by a loss of suitable habitat and a loss of juveniles. Copyright © 1999 John Wiley & Sons, Ltd.     

Characterizing physical habitat preferences and thermal refuge occupancy of brook trout (Salvelinus fontinalis) and Atlantic salmon (Salmo salar) at high river temperatures

Anthropogenic influences, including climate change, are increasing river temperatures in northern and temperate regions and threatening the thermal habitats of native salmonids. When river temperatures exceed the tolerance levels of brook trout and Atlantic salmon, individuals exhibit behavioural thermoregulation by seeking out cold‐water refugia – often created by tributaries and groundwater discharge. Thermal infrared (TIR) imagery was used to map cold‐water anomalies along a 53 km reach of the Cains River, New Brunswick. Trout and salmon parr did not use all identified thermal anomalies as refugia during higher river temperature periods (>21°C). Most small‐bodied trout (8–30 cm) were observed in 80% of the thermal anomalies sampled. Large‐bodied trout (>35 cm) required a more specific set of physical habitat conditions for suitable refugia, that is, 100% of observed large trout used 30% of the anomalies sampled and required water depths >65 cm within or adjacent to the anomaly. Densities of trout were significantly higher within anomalies compared with areas of ambient river temperature. Salmon parr were less aligned with thermal anomalies at the observed temperatures, that is, 59% were found in 65% of the sampled anomalies; and densities were not significantly different within/ outside anomalies. Salmon parr appeared to aggregate at 27°C, and after several events over 27°C variability in aggregation behaviour was observed – some fish aggregated at 25°C, others did not. We stipulate this is due to variances of thermal fatigue. Habitat suitability curves were developed for velocity, temperature, depth, substrate, and deep water availability to characterize conditions preferred by fish during high‐temperature events. These findings are useful for managers as our climate warms, and can potentially be used as a tool to help conserve and enhance thermal refugia for brook trout and Atlantic salmon in similar systems.     

Scour depths at sites selected for spawning by brown trout (Salmo trutta) along a longitudinal gradient of a North American mountain river

In the Intermountain West, USA, fry of fall‐spawning brown trout (Salmo trutta) are susceptible to scour‐related mortality because they are still in the gravel during spring snowmelt run‐off events. The goal of our research was to understand patterns of gravel scour on the Logan River, Utah, in relation to brown trout spawning and whether mobility of spawning gravels could explain the absence of brown trout from higher elevations. We collected data to characterize local entrainment potential at spawning gravels longitudinally on the Logan River during 2009 and 2010 spring flood events. We used scour chains to measure scour depth at spawning locations, and we also examined the position of redds in channel cross sections in relation to the centre line. The flood magnitude in both years approximated the 2‐year flood magnitude, but the flood in 2009 was much longer in duration. Scour at 27% of scour chain locations exceeded the estimated median upper limit of developing fry in 2009, whereas scour at 0% of locations exceeded this depth in 2010. Brown trout spawned in locations with similar entrainment potential at both mid and high elevations, which contributed to a lack of trend in scour depth with elevation. In addition, the majority of areas chosen for spawning were channel margins. The relationship between local entrainment potential at spawning gravels and scour depth was similar for the mid‐elevation canyon zone with medium brown trout density and the high‐elevation noncanyon zone with low brown trout density. In a low‐elevation backwater zone containing high densities of brown trout, scour was high despite low levels of entrainment potential. Overall, findings suggest that spawning gravel scour is not limiting brown trout abundance at high elevations in this system given shallow scour depths overall and a general lack of increase in scour depth with increasing elevation/distance upstream.     

Seasonal Effects of Hydropeaking on Growth,Energetics and Movement of Juvenile Atlantic Salmon (Salmo Salar)

The performance and movements of juvenile Atlantic salmon Salmo salar exposed to variable water discharge (simulating hydropeaking) but with a stable water‐covered area were studied in six experimental stream channels, both during the winter and summer. Thirty fish were stocked into each channel, and the growth, body fat and movements of the fish were followed for about 2.5 months in each season. During the winter, no effect of hydropeaking was documented on performance or movement. In the summer, fish experiencing hydropeaking had lower body mass, lower body fat, and higher movement rates than the control fish. In general, effect sizes were small, and the rapid and frequent changes in water discharge and water level in the present study had small effects on the performance of juvenile Atlantic salmon. The cumulative long‐term effect at the population level is unknown, but a reduced growth rate of 10% and a reduction in body fat of 16% in the hydropeaking experiments in the summer might to some extent translate into increased smolt age and lower overwintering survival. Copyright © 2014 John Wiley & Sons, Ltd.