Cooler,bigger; warmer,smaller: Fine-scale thermal heterogeneity maps age class and species distribution in behaviourally thermoregulating salmonids

Behavioural thermoregulation is a survival strategy that occurs in response to an exceedance of thermal stress-inducing thresholds. When salmonids experience exceedance of these thresholds, they seek regions of colder water, known as thermal refuges. During an extreme temperature event of summer 2021 (main stem ~31.5°C) a large aggregation of Atlantic salmon (Salmo salar—all age classes) and brook trout (Salvelinus fontinalis—>20 cm) was observed on the Little Southwest Miramichi River in New Brunswick, Canada. Using a drone-mounted thermal infrared (TIR) sensor, fine-scale TIR imagery of the occupied refuge was acquired. Polarized glasses were worn by an onshore observer to make visual observations. Constructing maps from these data we examined the spatial distribution of fish, and the corresponding temperature of the areas they occupied. Salmonids were found to be distributed by age class and species, with the distribution driven by the mosaic of temperatures in the refuge. Young of the year (YOY), 1+, 2+ and adult Atlantic salmon occupied areas with average temperatures ~30.1, 28.8, 25.7 and 21.9°C, respectively; whilst mature brook trout occupied areas ~21.8°C. Noteworthy is the observation of thermally aggregating young of the year Atlantic salmon, and the range in temperatures they occupied (~8°C). One isolated, shallow, cold water patch (~22°C) exclusively held YOY Atlantic salmon. Our findings highlight the importance of several different thermal characteristics of thermal refuges and their link to salmonid occupancy and can aid the design of ecologically meaningful thermal refuge augmentation/restoration projects.     

Juvenile coho salmon behavioural characteristics in Klamath river summer thermal refugia

During the summer in the main‐stem Klamath River, juvenile salmonids respond positively to cooler tributary temperatures by congregating in large schools at the mouths of these tributaries, referred to as thermal refugia. The purpose of this paper is to summarize results from coho salmon (Oncorhynchus kisutch) thermal refugia studies conducted since 2006 in the Klamath River. Results showed that juvenile coho salmon started using thermal refugia when the Klamath River main‐stem temperature approached approximately 19 °C. The majority of the juvenile coho salmon within the studied thermal refugia were found in the slower velocity habitat associated with cover. Juvenile coho salmon counts in the studied thermal refugia dramatically decreased at temperatures >22–23 °C, suggesting that this approximates their upper thermal tolerance level. Although some juvenile coho salmon were very mobile, others chose, until smoltification, to rear in the mouths and lower reaches of non‐natal tributaries where thermal refugia occurred, apparently because of suitable habitat conditions. Information gained from these investigations will improve our knowledge of the life history of coho salmon in the Klamath River drainage and how they use the main‐stem river. Copyright © 2010 John Wiley & Sons, Ltd.     

Contrasting temperatures,waterflows, and light: seasonal habitat selection by young Atlantic salmon and brown trout in a boreonemoral river

Habitat use and habitat selection by young Atlantic salmon and brown trout were investigated by direct underwater observation. We sampled during winter and summer water temperatures (low: 3–7°C; high: 9–12°C) coinciding with low and high waterflows (12–20 and 60–80 m³ s^?1), and during day and night in winter, and on six selected stations in the river. Observations of 396 salmon and 120 trout indicated a distinct seasonal pattern in behaviours and habitat selection. Feeding was the dominant behaviour at high water temperatures during summer. In winter, there was a diurnal pattern in behaviour; both species sheltered in interstitial spaces in the substrate during daylight, but during night held positions on or close to the substrate in slower flowing stream areas. Coarse substrate providing cover was therefore an important habitat factor during daylight at low water temperatures, while slow‐flowing water was important during night. Although spatial niche overlap was considerable both in summer and winter, salmon and trout segregated with respect to meso‐ and microhabitat selection, and relatively more at low temperatures. Both species changed their use of mesohabitats towards more slow‐flowing glide/flat habitats in winter. Irrespective of season, trout preferred in general more slow‐flowing water than salmon did, but the difference was more pronounced in winter. Salmon used a wider range of water depths and in particular water velocities, than did trout. Both species were less tolerant of high water velocities at low water temperatures. The seasonal and diurnal pattern in habitat selection reported have important implications for habitat research and habitat‐hydraulic modelling. Copyright © 2001 John Wiley & Sons, Ltd.     

Temperature-dependent Effects of Rainbow Trout on Growth of Atlantic Salmon Parr

Temperature may influence interactions between species by regulating energy balances of individuals. We conducted a laboratory study to determine whether temperature influenced the effects exerted by large rainbow trout on the growth of Atlantic salmon parr. Bioenergetic models were used to predict maintenance rations so that food resources were limiting over a range of temperatures; equal biomasses of rainbow trout were substituted for Atlantic salmon to evaluate the relative effect of interspecific interactions on Atlantic salmon growth. In the presence of rainbow trout, salmon growth increased as temperatures increased from 15°C to 25°C; no such temperature effect occurred for salmon maintained alone. Growth differences between salmon maintained with and without trout were highly significant at 25°C but not at 15°C. We conclude that the presence of trout depressed salmon growth at 15°C but not at higher temperatures, most likely a result of differences in thermal optima between these two species. Field data show that the proportion of stocked Atlantic salmon to wild rainbow trout coexisting in natural streams is a function of mean summer temperature. As stream temperatures increased, Atlantic salmon became increasingly favored over rainbow trout, but with a concomitant decrease in total salmonine biomass. We suggest that Atlantic salmon restoration programs focus more attention on relatively warm streams in watersheds where interactions with naturalized rainbow trout may occur.     

Atlantic salmon (Salmo salar) in the regulated River Alta: effects of altered water temperature on parr growth

The chief objective of this study was to analyse the effects of altered water temperature, due to the hydropower regulation of the River Alta, on growth of Atlantic salmon parr. The river was developed for hydroelectric purposes in 1987. A 110 m high concrete dam was built in the main river 49 km upstream from the outlet to the sea. The outlet of the power station is located 2.5 km downstream from the dam. The annual regime of water temperature has been altered downstream from the power station because of the regulation. It has decreased 1–2° C during June, July and the first half of August, while it has increased up to 3° C during late summer. During winter, water temperature has increased from 0° C to about 0.3–0.4° C. Atlantic salmon is the predominant fish species in the river. They can penetrate 46 km from the sea, up to the outlet of the power station. In this paper I have studied the relationship between growth of juvenile Atlantic salmon and water temperature in the upper part of the river. At similar temperatures, the growth rate of salmon parr in the River Alta is higher in early summer than later in the growing season. In early summer the salmon grew faster than the maximum rate predicted by a recently published model. Therefore, I adjusted the model to describe growth rates of salmon in early summer (ice break to mid‐August), using data derived prior to the hydropower development (1981–1986). The new model proved effective at describing growth rates of fish in early summer following the hydropower development (1987–1996). After development, growth rates decreased during early summer, but increased correspondingly later in the season. There was close agreement between these growth changes and the altered annual regime of river temperature. Overall, only minor changes in annual growth rates have been observed after the hydropower development. Copyright © 2003 John Wiley & Sons, Ltd.     

Growth and Smoltification of Three Norwegian Strains of Atlantic Salmon Salmo salar Reared under Different Thermal Regimes

A comparative study was performed with juvenile Atlantic salmon Salmo salar from three stocks in Western Norway that differ in their natural conditions. One is from warm, lowland river conditions (Årdal), one is from cold glacial river conditions (Stryn) and the last one (Suldalslågen) is from a hydropower‐regulated river. The salmon parr were tagged and reared at 4, 5.5 and 7 °C and simulated natural water temperature (SNT) for river Suldalslågen. Size distribution was unimodal at 4 °C, with a change to a bimodal distribution, representing potential 1+ and 2+ smolts, at the other temperature regimes. The relative biomass of 1+ smolts varied between the stocks as Stryn (cold glacial river) stock had the highest number of smolts at 7 °C and the Suldalsågen stock (hydropower‐regulated river) displaying the highest number at the SNT regime. Overall, the Stryn stock, originating from cold river conditions, seemed to be well adapted to growth and smoltification at cold temperatures, whereas salmon parr from river Suldalslågen seem to be better adapted to the natural temperature regime (SNT) of this river than the other two stocks. This was reflected in the gill Na⁺,K⁺‐ATPase as the Suldalslågen stock showed increasing activity from 16 April (4.2 µmol ADP mg protein^?1 h^?1) to 10 May (9.2 µmol ADP mg protein^?1 h^?1), and at the end of the experiment, enzyme activity in Suldalslågen stock was significantly higher than both Stryn (5.7) and Årdal (5.9 µmol ADP mg protein^?1 h^?1) stock. In contrast, the warm lowland stock, Årdal, fish had low Na⁺,K⁺‐ATPase activity with no distinct peak at any of the sampled dates from March through May. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of elevated water temperature on fish and macroinvertebrate communities below small dams

Many studies have investigated the ecological changes that occur below dams that release cold, hypolimnetic water, but very few studies have looked at the effects of the release of warm, surface waters. The effect of small, surface release dams on downstream thermal regimes is a major habitat concern for many cold‐water systems, however. The objective of this study was to examine the effects of summer temperature increases due to impoundment on downstream fish and macroinvertebrate communities in cold‐water streams. We sampled fish, macroinvertebrates and habitat upstream and downstream of dams on ten rivers during the summers of 1998 and 1999. Changes in mean summer temperature downstream varied from a cooling of 1 °C to an increase of more than 5 °C. Increasing temperatures downstream coincided with lower densities of several cold‐water fish species, specifically brown trout (Salmo trutta), brook trout (Salvelinus fontinalis) and slimy sculpin (Cottus cognatus) while overall fish species richness increased downstream. Density of mottled sculpin (Cottus bairdi), another cold‐water species, was not related to temperature changes below the dams. Macroinvertebrates showed shifts in community composition below dams that increased temperature. This study provides information useful for determining the extent of impact of these small, surface release dams, which are abundant across the country. Copyright © 2003 John Wiley & Sons, Ltd.     

Thermal habitat assessment of alternative flow scenarios in a tailwater fishery

Tailwaters below hydropower dams can create desirable coldwater trout fisheries; however, a flow regime ideal for hydropower often presents challenges for management of the fishery. The Smith River tailwater (Henry County, VA) offers a self‐sustaining brown trout fishery managed for trophy trout (≥ 406 mm), yet trophy‐sized fish are rare. Slow growth and small size are likely caused by any one or a combination of thermal habitat, limited food resources, and/or physical habitat. To evaluate the potential for thermal habitat improvement, temperature changes resulting from alternative flows were assessed with a one‐dimensional hydrodynamic model coupled with a water temperature model. Simulated temperatures from each flow scenario were assessed every 2 river kilometres over a 24 kilometre river section below the dam for occurrence of optimal growth temperatures, as well as compliance with Virginia Department of Environmental Quality hourly temperature change and daily maximum temperature standards. The occurrence of optimal growth temperatures increased up to 11.8% over existing conditions by releasing water in the morning, decreasing the duration of release, and not increasing baseflow. Incidences of hourly temperature changes greater than 2°C were reduced from 4% to 0–1.2% by non‐peaking releases, increasing baseflow, morning releases, and decreasing the duration of release. Maximum temperature occurrence (> 21°C) decreased from 1.3% to 0–0.1% by releasing flows daily to prevent elevated temperatures on non‐generation days, increasing baseflow, increasing duration of release, and releasing in the morning rather than evening. Despite conflicting adjustments to best improve all thermal criteria concurrently, a 7‐day/week, morning, one hour release regime was determined to improve all criteria throughout the tailwater compared to existing conditions. Copyright © 2005 John Wiley & Sons, Ltd.     

Habitat utilization and preferences in juvenile atlantic salmon (salmo salar) in streams

Young Atlantic salmon appear to occupy similar suitable stream habitats year round. The salmon is stationary, often associated with “home stones”. At low water temperatures in winter, the fish seek shelter in the substrate, which has to be coarse enough to provide interstices for the fish to hide in. In summer, salmon select habitats within tolerable ranges of habitat variables, rather than narrow optima. It is suggested that a genetic basis to habitat evaluation allows incorporation of the full range of behavioural responses. This is important as partially different habitats may be selected in diverse streams. Habitat availability influences both habitat use and habitat preferences. Suitable summer habitats have depths in the range 5–90 cm; mean water velocities 10–80 cm s^?1, and gravel-to-boulder substratum. Fish size affects habitat use, as young of the year are found in the more shallow habitats closer to the stream banks, whereas the older parr use a wider range of habitats. In the absence of brown trout, the Atlantic salmon parr, and especially young of the year, use habitats otherwise inhabited by brown trout. Water velocities are in many cases the principal physical habitat variable determining the distribution of Atlantic salmon in streams, but other variables are also important. Depth is more important in small streams than in large streams.     

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.     

Mitigation of cold‐water thermal pollution downstream of a large dam with the use of a novel thermal curtain

Hypolimnial releases from dams during periods of thermal stratification modify the downstream riverine thermal regime by decreasing water temperature and reducing natural diel thermal variability. This cold‐water thermal pollution in rivers can persist for hundreds of kilometres downstream of dams and impact important ecological processes such as fish spawning. To mitigate this problem, a first‐of‐its‐kind thermal curtain was fitted to the large bottom release Burrendong Dam on the Macquarie River, Australia. The thermal curtain acts by directing warmer, near‐surface epilimnial water to the low‐level hypolimnial offtake. This study aimed to test the efficacy of the thermal curtain by measuring temperatures before and after the curtains installation, quantifying the magnitude and extent of cold‐water thermal pollution along the Macquarie River downstream of Burrendong Dam. Epilimnial releases with use of the curtain increased diel temperature ranges and the mean monthly water temperature below the dam. Epilimnial releases with use of the curtain increased diel temperature ranges from 0.9°C to 2.5°C and reduced the difference between the mean monthly water temperature of an upstream control and a downstream site by up to 3.5°C. A comparison of the monthly temperature means along the river, indicated that thermal recovery, whereby temperatures returned to within the natural range of upstream temperatures occurred 45 km downstream of the dam during summer when the thermal curtain was deployed, compared with approximately 200 km prior to deployment of the curtain. Our study suggests that the use of thermal curtains can reduce cold‐water thermal pollution and improve ecological outcomes for river ecosystems downstream of dams.     

Large‐scale spatial variability of riverbed temperature gradients in Snake River fall Chinook salmon spawning areas

In the Snake River basin of the Pacific northwestern United States, hydroelectric dam operations are often based on the predicted emergence timing of salmon fry from the riverbed. The spatial variability and complexity of surface water and riverbed temperature gradients results in emergence timing predictions that are likely to have large errors. The objectives of this study were to quantify the thermal heterogeneity between the river and riverbed in fall Chinook salmon spawning areas and to determine the effects of thermal heterogeneity on fall Chinook salmon emergence timing. This study quantified river and riverbed temperatures at 15 fall Chinook salmon spawning sites distributed in two reaches throughout 160 km of the Snake River in Hells Canyon, Idaho, USA, during 3 different water years. Temperatures were measured during the fall Chinook salmon incubation period with self‐contained data loggers placed in the river and at three different depths below the riverbed surface. At all sites, temperature increased with depth into the riverbed, including significant differences (p < 0.05) in mean hourly water temperature of up to 3.8°C between the river and the riverbed among all the sites. During each of the 3 water years studied, river and riverbed temperatures varied significantly among all the study sites, among the study sites within each reach and between sites located in the two reaches. Considerable variability in riverbed temperatures among the sites resulted in fall Chinook salmon emergence timing estimates that varied by as much as 36 days within a reach, depending on the source of temperature data used for the estimate. Monitoring of riverbed temperature gradients at a range of spatial scales throughout the Snake River would provide better information for managing hydroelectric dam operations, and would aid in the design and interpretation of future empirical research into the ecological significance of physical riverine processes. Published in 2007 by John Wiley & Sons, Ltd.     

Effects of Environmental Water Transfers on Stream Temperatures

Low streamflows and warm stream temperatures currently limit habitat and productivity of trout, including native Lahontan cutthroat trout in Nevada's Walker Basin. Environmental water transfers, which market water from willing sellers to instream uses, are evaluated to improve instream habitat. We use River Modelling System, an hourly, one‐dimensional hydrodynamic and water quality model, to estimate current and potential environmental water transfer effects on stream temperatures. Model runs simulate a range of environmental water transfers, from 0.14 to 1.41 cms, at diversions and reservoirs for wet year 2011 and dry year 2012. Results indicate that critically warm stream temperatures generally coincide with low flows, and thermal refugia exist in East Walker River, a tributary of the Walker River. Environmental water transfers reduce maximum stream temperatures by up to 3 °C in dry years and are more effective in dry years than wet years. This research suggests that environmental water transfers can enhance instream habitat by improving water quality as well as increasing instream flow. Copyright © 2015 John Wiley & Sons, Ltd.     

Understanding the Thermal Regime of Rivers Influenced by Small and Medium Size Dams in Eastern Canada

Although small and medium‐size dams are prevalent in North America, few studies have described their year‐round impacts on the thermal regime of rivers. The objective of this study was to quantify the impacts of two types of dams (run‐of‐river, storage with shallow reservoirs) on the thermal regime of rivers in eastern Canada. Thermal impacts of dams were assessed (i) for the open water period by evaluating their influence on the annual cycle in daily mean water temperature and residual variability and (ii) for the ice‐covered winter period by evaluating their influence on water temperature duration curves. Overall, results showed that the run‐of‐river dam (with limited storage capacity) did not have a significant effect on the thermal regime of the regulated river. At the two rivers regulated by storage dams with shallow reservoirs (mean depth < 6 m), the annual cycle in daily mean water temperature was significantly modified which led to warmer water temperatures in summer and autumn. From August to October, the monthly mean water temperature at rivers regulated by storage dams was 1.4 to 3.9°C warmer than at their respective reference sites. During the open water period, the two storage dams also reduced water temperature variability at a daily timescale while increased variability was observed in regulated rivers during the winter. Storage dams also had a warming effect during the winter and the winter median water temperature ranged between 1.0 and 2.1°C downstream of the two storage dams whereas water temperature remained stable and close to 0°C in unregulated rivers. The biological implications of the altered thermal regimes at rivers regulated by storage dams are discussed, in particular for salmonids. Copyright © 2016 John Wiley & Sons, Ltd.     

Der Einfluss der Ufervegetation auf die Wassertemperatur unter gewässertypspezifischer Berücksichtigung von Fischen und benthischen Evertebraten am Beispiel von Lafnitz und Pinka

This study is based on the results of the transdisciplinary research project BIO_CLIC. The aim of this study was (1) to synthesize and reflect the scientific knowledge, (2) to understand the potential of riparian vegetation on water temperature and (3) to ameliorate the impacts on the aquatic habitats of benthic invertebrates and fish at the rivers Lafnitz and Pinka. These objectives had been achieved by detailed field investigations, the assessment of abiotic environmental parameters (water temperature, riparian vegetation, shading and morphology), the comparison of effects of dynamic processes (incl. water temperature, riparian vegetation, change of river morphology) and biotic habitat use of benthic invertebrates and fish assemblages. The results provide an environmental and biological overview of potential local impacts on water temperature during heat wave periods and additionally taking into account diverse climate scenarios. Three hotspots at each river were selected to characterize specific river types with respect to river morphology, riparian vegetation, thermal regime, as well as the biocoenosis of fish and benthic invertebrates. The temperature regime influences all life stages of fish species and benthic invertebrates. They prefer different temperature regimes along a river continuum that correspond with typical species assemblages. Our evaluation of water temperatures for longitudinal biozenotic zones showed significant differences for shaded and unshaded river reaches. The river type specific mean water temperature for trout and grayling zone in summer is between 11 °C and 16 °C and for barbel and nase above 16 °C. Temperature changes of 2 °C lead to a shift of species composition preferring ?warm-water“ species. River reaches with functioning riparian vegetation are able to mitigate these effects of extreme water temperature increase.     

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.     

Spawning habitat of Atlantic salmon and brown trout: general criteria and intragravel factors

Generalized habitat criteria for spawning sites of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) using depth, water velocity and substrate size were created based on published information. In addition, information on critical intragravel conditions for egg development was summarized. Salmon spawned mostly in relatively deep, swift‐velocity habitats (20–50 cm, 35–65 cm s^?1), whereas trout selected slightly shallower and slower flowing spawning sites (15–45 cm, 20–55 cm s^?1). Salmon and trout preferred pebbles (16–64 mm) for spawning. The minimum oxygen concentration for successful incubation of eggs varies with the developmental stage of eggs, and supply of it may be reduced by deposited fine sediment. Habitat criteria for spawning sites are narrower than those for small juveniles; therefore the use of separate criteria is recommended. In addition to the traditional habitat criteria variables (depth, water velocity, substrate), the critical intragravel factors affecting egg survival should be incorporated in biologically meaningful criteria for spawning habitat modelling. Copyright © 2008 John Wiley & Sons, Ltd.     

Salmonid Spawning Runs and Estimated Ova Production in Normandale Creek of Lake Erie

The Normandale Creek study area (2,531 m²)provides spawning and nursery grounds for lake-run coho salmon (Oncorhynchus kisutch), rainbow trout (Salmo gairdneri), and brown trout (Salmo trutta), In 1973–74, 59 adult salmonids (58% rainbow trout, 39% coho salmon, and 3% brown trout) ascended the stream between 31 October and 12 May. Upstream movements were significantly related to peak stream discharge (r = 0.21) and highly significantly related to discharge occurring on the day following the peak freshet (r = 0.34). Despite a highly significant correlation (r = 0.29) between flow and water temperature, the latter factor is not significantly related with upstream movement of adult fish. Coho salmon spawned at water temperatures of 1 to 10° C between 2 November and 19 December, rainbow trout at 1 to 15° C between 9 November and 14 May, and brown trout at 7° C between 4 and 5 November. Of 86 nests constructed, 60% were disturbed by re-use or sand deposition. From a calculated deposition of 90,403 ova, it is estimated that some 22% survived to emergence.     

DEVELOPMENT OF A STOCHASTIC WATER TEMPERATURE MODEL AND PROJECTION OF FUTURE WATER TEMPERATURE AND EXTREME EVENTS IN THE OUELLE RIVER BASIN IN QUÉBEC,CANADA

A stochastic model is proposed to reproduce daily water temperature at 18 observation sites (11 main stem and 7 tributary sites) in the Ouelle River basin located in southern Quebec, Canada, using meteorological variables as predictors. A random sampling procedure without replacement was adopted for the model calibration and validation to overcome the limited length of the observed water temperature series. The predicted water temperature series were then submitted to variance inflation to reproduce the observed variability of the water temperature series. Historical water temperature series were obtained from observed meteorological predictors, whereas reference and future water temperature series were obtained from stochastic water temperature model using five reference (1970–1999) and future (2046–2065) meteorological predictors simulated by five different climate model runs. The reference series reproduced summer mean water temperature and the number of consecutive days with water temperature higher than 21 °C or 25 °C fairly well. On the basis of the historical series, it can be assumed that the seven tributaries of the Ouelle River provided thermal refugia for native salmon between 1970 and 1999. Future water temperature series projected by the stochastic model show that the seven tributaries could still be used as refugia to prevent lethal stress, whereas the temperature in the main stem and in three tributaries will be high enough to constitute stressful conditions for feeding juvenile Atlantic salmon. Copyright © 2012 John Wiley & Sons, Ltd.     

Tributary confluences are dynamic thermal refuges for a juvenile salmonid in a warming river network

As rivers warm, cold‐water fish species may alleviate thermal stress by moving into localized thermal refuges such as cold‐water plumes created by cool tributary inflows. We quantified use of two tributary confluence plumes by juvenile steelhead, Oncorhynchus mykiss, throughout the summer, including how trout positioned themselves in relation to temperature within confluence plumes. At two confluences, Cedar and Elder creeks, along the South Fork Eel River, California, USA, we monitored temperatures using in situ logger grids throughout summer 2016. Fish were counted within confluences via snorkel surveys five times a day on 5 days at each site. We found diel and seasonal dependence on confluence use by steelhead, especially at the Cedar Creek confluence, where mainstem temperatures exceeded 28°C. At this site, fish moved into the confluence on the warmest days and warmest times of the day. Fish observed within the Cedar Creek confluence plume were most common in locations between 20–22°C, rather than the coldest locations (14.5°C). At Elder Creek, where mainstem temperatures remained below 24°C, there was little relationship between mainstem temperature and steelhead presence in the confluence plume. At both sites, steelhead distribution within plumes was influenced by spatial variation of temperature and mean temperature in surveyed grid cells. Our results show that cool tributaries flowing into warmer mainstem reaches (over 24°C) likely create important thermal refuges for juvenile steelhead. As mainstem rivers warm with climate change, cool‐water tributary inputs may become more important for sustaining cold‐water salmonids near the southern end of their range.