Size Of Trout Research Articles

Shrinking sizes of trout and salamanders are unexplained by climate warming alone

Decreases in body sizes of animals related to recent climate warming can affect population persistence and stability. However, direct observations of average sizes over time and their interrelationships with underlying density-dependent and density-independent processes remain poorly understood owing to the lack of appropriate long-term datasets. We measured body size of two species common to headwater streams in coastal and Cascades ecoregions of the Pacific Northwest of North America over multiple decades, comparing old-growth and managed forests. We found consistent decreases in median length of Coastal Cutthroat Trout Oncorhynchus clarkii clarkii, but a coexisting species, the Coastal Giant Salamander Dicamptodon tenebrosus, appears to be more resilient to size changes over time. Based on observed trends, adult trout have decreased in length by 6–13% over the last 30 years. Length decreased more in larger compared to smaller animals, suggesting that these effects reflect changes in growth trajectories. Results from a model-selection approach that included hydroclimatic and biological information as covariates in one of our study ecoregions demonstrated that stream temperature alone did not explain observed length reductions. Rather, a combination of density-dependent (animal abundances) and local density-independent factors (temperature, habitat, and streamflow) explained observed patterns of size. Continued decreases in size could lead to trophic cascades, biodiversity loss, or in extreme cases, species extirpation. However, the intricate links between density-independent and density-dependent factors in controlling population-level processes in streams need further attention.

Stream groundwater inputs generate fine‐scale variation in brook trout phenology and growth across a warming landscape

Abstract Climate change is increasing global atmospheric temperatures, which can reduce abundance and cause range shifts in species that are sensitive to warming. However, fine‐scale thermal heterogeneity can drive highly variable local responses to climate change, especially in freshwater environments that differ in groundwater inputs and geomorphology. We used temperature data collected during 2012–2021 from 10 small, pristine streams in eastern Canada to characterise thermal variation at a small spatial scale (~25 km2). We then used relationships between daily air and stream temperatures to reconstruct stream temperature since 1980, and assessed how thermal variation influenced the phenology and growth of brook trout (Salvelinus fontinalis). Air–stream temperature relationships varied considerably among streams despite their close proximity, with predicted summer temperatures differing up to 9.5°C between warmer rainfall‐dominated streams and cooler groundwater‐dominated streams. Rainfall‐dominated streams warmed more than twice as fast as groundwater‐dominated streams across all seasons since 1980, with nearly four‐fold differences in rates of warming evident during summer months. Fine‐scale thermal heterogeneity also shaped brook trout phenology, as juveniles in rainfall‐dominated streams were estimated to hatch and emerge much later (~70 and 40 days, respectively) and experience faster phenological shifts than in groundwater‐dominated streams. Relationships between juvenile brook trout size and accumulated degree‐days were positive, but slopes differed over two‐fold and did not vary systematically based on stream hydrology, suggesting more idiosyncratic impacts of warming on early growth. Collectively, our study illustrates how species responses to climate change in freshwater environments can be consistent in direction but vary substantially in magnitude owing to the influence of groundwater. Future climate change is likely to increase the thermal stress experienced by brook trout populations in warmer rainfall‐dominated streams, while potentially benefiting those in groundwater‐dominated streams where current temperatures are often suboptimal. Observed differences in the rates and ecological impacts of warming among streams suggest that fine‐scale thermal variation must be considered when forecasting effects of future climate change on stream fish population dynamics.

Climate vulnerability for a desert fish: integrating hydrologic exposures, adaptive capacity, and growth potential

Climate vulnerability can be evaluated by multiple organismal responses. We developed a climate vulnerability framework focused on growth potential of redband trout ( Oncorhynchus mykiss newberri). We employed a bioenergetics model to evaluate spatial variability in growth potential in relation to constraints on body size imposed by stream flow, physiological responses linked to variable thermal regimes, and variation in physiological adaptive capacity inferred from field respirometry. Results indicate that maximum size (g) of redband trout increases with stream discharge. Growth potential is strongly linked to body size, with smaller individuals performing better relative to larger fish in cooler thermal regimes. Annual patterns of growth varied among sites and were related to body size and physiological adaptive capacity. Putatively cold-adapted fish were more likely to exhibit bimodal growth with peaks in spring and autumn, whereas warm-adapted fish exhibit a summer peak in growth potential. These findings offer insights into how climate vulnerability of stream-living fish can be conditioned on both stream flow and thermal regimes, and the potential for physiological adaptive capacity to influence vulnerability.

Can thermal refuges save salmonids? Simulation of cold‐pool benefits to salmonid populations

AbstractObjectiveThe literature on thermal refuges has focused on when fish use what kinds of refuge, but we address how refuge availability and characteristics affect population abundance and persistence under warming.MethodsPopulation benefits of thermal refuges are very difficult to assess empirically, so we used InSTREAM, an individual‐based trout population model. InSTREAM represents several ways temperature affects survival and growth, as mediated by behaviors such as deciding when and where to feed. The model’s credibility was established by showing that it closely reproduced 10 observed patterns in how refuge use varied with temperature, time of day, and trout size and in how refuge availability affects populations. We then simulated four levels of refuge availability in four increasingly warm temperature regimes, examining (1) survival and growth over summer periods of prolonged high temperatures and (2) population abundance and persistence over 22 years.ResultIn the simulation experiments, summer refuge use acted as a population bottleneck: the number of surviving trout increased with refuge area and decreased with temperature. During prolonged summer high temperatures, mortality was high until abundance dropped to a level sustainable by the refuges. The model predicted reduced but persistent populations under high warming when thermal refuges made up 2% or more of stream area.ConclusionThe concept of salmonids “hanging on” in refuges, losing weight but surviving peak temperatures, was not supported by our experiment. The value of potential refuges to salmonid populations appears determined not just by temperature and area, but also by other characteristics that affect growth and survival, such as the availability of food and habitat for feeding and predator avoidance.

Factors Influencing Early Growth of Juvenile Tiger Trout Stocked into Subalpine Lakes as Biocontrol and to Enhance Recreational Angling

Tiger trout (Salmo trutta × Salvelinus fontinalis) are sterile hybrids often stocked as a biocontrol agent for undesirable fishes and to enhance recreational angling. Yet, how different ecological processes affect their post-stocking performance remain poorly understood. Rapid growth early in life can foster rapid transitions to piscivory, and improve survival. Identifying factors that benefit early growth can help managers optimize tiger trout stocking for meeting multiple fisheries management objectives. Here, we characterized the trophic ecology and growth of tiger trout stocked at varying densities into seven lentic subalpine systems in Colorado, USA. Study systems supported different species of undesirable fish (e.g., minnows or suckers). We used stable isotopes of carbon and nitrogen to quantify trophic relationships among tiger trout and other fishes in each system. We then evaluated several system-specific attributes as predictors for the size of tiger trout at age-1 using a Random Forest model. Stable isotopes demonstrated the potential for resource competition among tiger trout and other fishes, but potential varied by system. Indices of resource competition ranked highest in the Random Forest model, but the stocking density of tiger trout was most important, suggesting that intraspecific competition outweighed interspecific competition in driving early growth. These processes were mediated by system productivity. Thus, stocking density in combination with the realized carrying capacities of systems should be considered when making management decisions for tiger trout.

Wildfire in western Oregon increases stream temperatures, benthic biofilms, and juvenile coastal cutthroat trout size and densities with mixed effects on adult trout and coastal giant salamanders

Wildfire has become increasingly common and severe across forested landscapes. Shortly after wildfire, loss of riparian cover along streams and subsequent increases in light can elevate stream temperatures, a key control on metabolic rates of biota. Increased light can also increase autotrophic basal resource availability with potential bottom-up effects. We evaluated wildfire impacts on aquatic ecosystems in a replicated Before-After Control-Impact study 1 year after a severe wildfire in western Oregon, U.S. Stream temperature, chlorophyll a accrual, and age-0 coastal cutthroat trout size, density, and biomass increased in all three burned streams relative to changes in three unburned references. When streams were evaluated collectively, fire did not have an effect on larger vertebrate density or biomass. However, considering streams individually, two severely burned sites had substantial temperature increases and declines in larger vertebrate density and biomass, but the moderately burned site had modest temperature increases and adult cutthroat trout and Pacific giant salamanders increased. The loss of riparian canopies post-fire increased temperature and algae, but fish responses varied with age class and larger vertebrate responses were inconsistent.

Isolation, culture and bacterial contamination of oogonial stem cells of Brown trout, <em>Salmo trutta macrostigma</em> (Dumeril, 1858)

Increased commercial value of brown trout, Salmo trutta macrostigma has lead to its declined natural stock due to over exploitation, and thereby made it an endangered species. Cryopreservation of spermatogonial and oogonial stem cells may help in protecting this species. In this context, we have earlier isolated and cultured spermatogonial stem cells from male S.t. macrostigma. In this study, we report isolation and culture of oogonial stem cells from brown trout (S. t. macrostigma).In addition, bacterial contamination in oogonial cell culture media were identified and described. Wild females wereobtained from Kılıç Trout Fish Farm (Kahramanmaraş, Turkey). In order to identify the appropriate size, age and ovarystructure for oogonial stem cell isolation and culture, the ovary structure was morphologically and histologically studied.Fish were anesthetized with 0.04% 2-phenoxethanol. The ovary tissue were digested by 0.25% trypsin-EDTA.HBSS, with1.0 μg/mL NaHCO3, antibiotics were used to maintain cells in a viable stage. The concentration of cells was measured byhemocytometer. Antibiogram and Gram staining techniques were applied to the culture media contaminated with bacteria.Appropriate age, size and weight of trout for oogonial stem cell isolation and culture were identified as 7+ month old,14.6±1.6 cm, 28.2±7.7 g, respectively. The highest oogonial stem cells were measured in the perinucleolar stage of theovary. Enterobacter cloacae and Acinetobacter baumannii were identified in the contaminated cell media. Density ofoogonial stem cells were measured as 5.4×105±2.6×105 cells/mL. In this study, germ cell isolation and culture technique wasdeveloped for S. t. macrostigma.

A catastrophic coal mine spill in the Athabasca River watershed induces isotopic niche shifts in stream biota including an endangered rainbow trout ecotype

Freshwater biodiversity is declining from impacts associated with anthropogenic stressors. Here, we use carbon (δ13C) and nitrogen (δ15N) stable isotopes to assess food web effects following a coal mine spill that displaced biota and altered biophysical stream characteristics. We compared isotopic niche metrics of benthic macroinvertebrates and the fish community, including non-native brook trout (Salvelinus fontinalis) and endangered Athabasca rainbow trout (Oncorhynchus mykiss), to infer spatial differences in site-specific resource use along a habitat disturbance gradient. Predatory benthic macroinvertebrate trophic position was elevated where impacts from the spill were most pronounced. Autochthonous carbon contribution to consumer diets was lowest in biota sampled at the most highly impacted site from the mine spill, leading to an unexpected expansion of the isotopic niche size of rainbow trout and the aquatic invertebrate community. Collectively, our results suggest variation in trophic resource assimilation across multiple levels of the food web, fueled by the allochthonous energy pathway in highly impacted study sites. We conclude this reflects a biotic response to altered basal aquatic resources following a major industrial disturbance.

Retention of Postocular Visible Implant Elastomer in Two Sizes of Adult Brown Trout and Rainbow Trout

AbstractRetention of visible implant elastomer (VIE) was assessed during an evaluation of put‐grow‐and‐take Brown Trout Salmo trutta and Rainbow Trout Oncorhynchus mykiss stockings in Apalachia Reservoir, North Carolina. Visible implant elastomer was injected in the postocular adipose tissue of two sizes (approximately 254 [small] and 356 [large] mm TL) of Brown Trout and Rainbow Trout in November 2012–2015. Each year, 1,000 large and 1,500 small trout of each species were marked with VIE. Mark retention was assessed after 30 d in a hatchery and during poststocking collection events in Apalachia Reservoir. The recaptured trout were examined for VIE marks during annual spring electrofishing, fall gill‐net samples, and a year‐long creel survey. The 30‐d VIE retention rate for all trout and all years was 99.9%. Following stocking, both sizes of Rainbow Trout were collected up to 17 months after marking and VIE retention was 100%. Large and small Brown Trout were collected up to 33 and 34 months after marking, and the VIE retention rates were 100% and 99.3%, respectively. This study found visible implant elastomer to be an effective and simple batch mark that remained visible on Rainbow Trout for up to 17 months and Brown Trout for up to 34 months postmarking in the wild.

Early-Middle Pleistocene freshwater ecosystems in the Sierra de Atapuerca (northern Iberia) based on the Gran Dolina fish record

Fish bones are common in Pleistocene cave deposits in Europe. In this paper, we report on fish remains from the Gran Dolina cave (Trinchera del Ferrocarril) in the Sierra de Atapuerca in Spain, to increase what is known of the freshwater ecosystems close to the cave. The 19-m-thick section, divided into 11 stratigraphic levels, represents an Early to Middle Pleistocene time span as dated by biostratigraphy, ESR, U-series, and magnetostratigraphy. We focus on the Sondeo South site, excavated from 1993 to 1999, which has yielded 1087 fish bones comprising the following taxa: brown trout (Salmo trutta), the common European eel (Anguilla anguilla) and chub (Squalius sp.). Taphonomic studies suggest that the European eagle owl (Bubo bubo) was the most likely predator responsible for the accumulation. Changes observed in the body size of brown trout through the sequence cannot be attributed to climate change (contravening Bergmann's rule). Our study documents the presence of a pre-mountain river system characterized by permanent, oxygen-rich, relatively cold flowing waters around the Sierra de Atapuerca during Early-Middle Pleistocene times.

Supplemental Feeding as a Tool in the Development of a Quality Wild Rainbow Trout Fishery

AbstractSouthwest Virginia offers a plethora of fishing opportunities for wild trout (Brook Trout Salvelinus fontinalis, Brown Trout Salmo trutta, and Rainbow Trout Oncorhynchus mykiss) in headwater and large valley floor streams. However, low productivity in most area streams, including the South Fork Holston River, limits growth leading to populations where individuals seldom exceed 25 cm. The Buller Fish Hatchery, located in Smyth County, Virginia, offers anglers a unique 1.6‐km special‐regulation catch‐and‐release trout fishery on the South Fork Holston River limited to single‐hook artificial lures. Historically, this section was dominated by wild Rainbow Trout that seldom reached 25 cm. A supplemental feeding program was initiated by the Virginia Department of Wildlife Resources in March 2015 in hopes of significantly increasing the size distribution of Rainbow Trout. This limited feeding program improved the size distribution within the first year, and 24% of Rainbow Trout exceeded 30 cm. Annual monitoring (2015–2018) documented that 17–34% of Rainbow Trout collected at the treatment reach exceeded 30 cm, with densities varying from 215 to 554 fish/km for fish greater than 30 cm. Relative weight greater than 100 shifted from 0% of the population in 2014 to 8–25% of the population in 2015–2018. Rainbow Trout size structure and relative weight remained unchanged during the survey period at the control reach and were lower than at the treatment reach once supplemental feeding began. Annual costs associated with supplemental feeding at this reach were calculated at US$1,271/year and averaged $2.34/fish (>30 cm) within the catch‐and‐release area. Results suggest that supplemental feeding can be an effective tool for improving wild trout size structure in low‐productivity waters. Feeding was not cost prohibitive and was within Virginia Department of Wildlife Resources budget guidelines.

Development of a risk assessment method for sea trout in coastal areas exploited for aquaculture

The regulation of aquaculture production in Norway considers the potential impact of salmon lice on wild fish. However, most attention has been focused on impacts on wild Atlantic salmon, despite the fact that anadromous brown trout spend the majority of their marine phase in coastal waters, where salmon lice have the highest impact. In the present study, we first suggest changes in marine living area and marine feeding time as sustainability indicators for first-time migrant sea trout, as high salmon lice densities may exclude sea trout from otherwise usable habitat and force them to return early to freshwater. Further, a method based on a bio-hydrodynamic model was developed to serve as a proxy for these indicators. The method accounted for the size, migration timing and spatial extent of sea trout and was demonstrated in 2 Norwegian salmon aquaculture production areas, Hardangerfjord (PO3) and Romsdalsfjord (PO5), and 2 focal rivers from within each fjord. Based on these comparisons, we exemplify how the change in marine living area and marine feeding time differed between PO3 and PO5 and within the areas. Sea trout migrating to sea late (June 5) were always more affected by lice than those migrating early (April 24) or at intermediate dates (May 15). Our estimates revealed dramatic potential impacts of salmon lice on sea trout populations, which were greatly influenced by spatial and temporal aspects. Considering the negative impacts of salmon lice on sea trout, a holistic view of environmental interactions between aquaculture and wild species that depend on habitats exploited for production is necessary.

Using simulation to understand annual sea lamprey marking rates on lake trout

Sea lampreys attack fish, killing some and leaving marks on others. Great Lakes fishery managers rely on observed marking rates to assess the success of the sea lamprey control program and estimate sea lamprey-induced mortality of lake trout. Because marking rates are only observed on survivors of sea lamprey attacks, they may not provide a reliable index of actual attack or mortality rates. To investigate the effect of survivor bias, we developed a simulation model representing a single season (June–December) of sea lamprey attacks. Simulated attack rates varied with month and lake trout size; simulated pierce and lethality rates varied with month alone. Surveyed marking rates were represented by simulated survivors in October; true rates were calculated from all simulated lake trout (dead and alive) in December. Simulation results were subsetted to include only those within the range of marking rates actually observed in the Great Lakes. Type A (piercing) marking rates were a good index of the sea lamprey attack rate and the sea lamprey-induced mortality rate if annual lethality rates were relatively constant. Type B (non-piercing) marking rates were a good index of the sea lamprey attack rate and the sea lamprey-induced mortality rate if annual pierce rates were relatively constant. Due to the uncertainty surrounding the pierce and lethality rates, we recommend that sea lamprey abundance information be incorporated in existing lake trout statistical catch-at-age models via a functional response component relating sea lamprey feeding to lake trout abundance, if possible.

Variations in flavor according to fish size in rainbow trout (Oncorhynchus mykiss)

Rainbow trout are a widely consumed fish species. However, the effects of fish size on flavor have not been fully elucidated. Here, the flavor qualities of the dorsal, tail, and abdomen of four sizes of rainbow trout (average weights 0.25, 1.14, 2.26, and 2.89 kg, respectively) were investigated by measuring fatty acids, nonvolatile compounds, and volatile compounds and by performing electronic tongue analysis. 5′-Nucleotide levels were higher in the dorsal and tail regions than in the abdomen of fish. Additionally, the heaviest fish had the highest contents of lactic acid and succinic acid, whereas fish weighing 1.14 kg had the highest levels of total free amino acids. Additionally, 0.25 and 1.14 kg fish had higher concentrations of aldehydes. t-Distributed stochastic neighborhood embedding and correlation analyses revealed that the overall flavor profiles were reflected by electronic-tongue analysis. Overall, our findings showed that fish size affected flavor in different parts of rainbow trout.

Growth, osmoregulation and energy budget of rainbow and steelhead trout under different salinity acclimation methods and the best transition size of steelhead trout

In order to meet the demand of salmon market, Chinese scientists and entrepreneurs are working on salmon mariculture far offshore in the Yellow Sea, China. Rainbow, steelhead trout and Atlantic salmon were selected as the main culture species. The aims of the present study were as follows: (a) investigate the effect of the salinity acclimation method on the growth, osmoregulation and energy budget in two forms of Oncorhynchus mykiss, rainbow and steelhead trout and (b) explore the optimal size of steelhead trout for the seawater entry. In trial I, rainbow (mean = 99.44 g) and steelhead trout (mean = 99.01 g) were reared for 40 days after undergoing salinity acclimation at three rates: an abrupt increase in salinity from 0 to 30 g/L (T30); an abrupt increase in salinity to 14 g/L, followed by a daily increase of 2 g/L (T2) or 6 g/L (T6) until reaching 30 g/L; and no salinity exposure (control treatment) (T0). In trial II, steelhead trout with body weights of approximately 100 and 400 g were cultured for 60 days with two treatments, T0 and T2, and the specific growth rate (SGR) was calculated every 10 days. In trial I, in both kinds of fish, the optimal growth performance, survival rate, osmoregulation and energy budget were observed in the T0 treatment, followed by the T2 treatment. These results indicate that O. mykiss with a body weight of approximately 100 g can adapt to sea water with a gradual transition (T2), but they are still not suitable for the seawater entry because of low growth. Based on the recorded SGRs in trial II, our formulated regression formula revealed that approximately 200 g is the optimal size of steelhead trout for the transition to sea water.

Springtime Exploitation of Brook Trout by Anglers in Remote Headwater Streams of Central Appalachia

AbstractIn the central Appalachian Mountains, Brook Trout Salvelinus fontinalis are a popular target of anglers, but given the remoteness of many of these systems traditional creel methods of evaluating angling effects are impractical. We used a combination of angler and fish survey sampling methods to determine the sizes and numbers of fish harvested. Fish harvest information was reported by anglers, trout size structures were determined by electrofishing, and angler effort was identified via surveys and remote camera sampling in six streams in West Virginia. On average, anglers retained 3.7 ± 0.2 fish (mean ± SE) ≥ 170 mm TL per trip. Between March 13 and May 29, we estimated that anglers harvested from 0.1 to 2.3 ± 0.3 fish per 100 m in the six streams. Applying these harvest rate estimates to Brook Trout > 170 mm TL, densities in 25 headwater streams in West Virginia yielded a mean of 14.5 angler‐days to deplete harvestable‐sized fish in these streams. Anglers appeared knowledgeable of local fish populations and focused greater effort on streams with larger populations of harvestable‐sized fish. While Brook Trout populations in low‐productivity streams may be particularly sensitive to harvest, anglers appear to use them less often. However, in low‐productivity streams where fewer harvestable‐size fish occur, anglers may still have noticeable effects on mortality through postrelease stress on abundant smaller‐sized fish. This study identifies the localized effects that a small subsection of anglers may have on remote populations of sensitive fish and should be carefully considered as a way to understand more widespread effects on popular sport fishes that exist in remote areas.

Effects of fish size and route of infection on virulence of a Danish Yersinia ruckeri O1 biotype 2 strain in rainbow trout (Oncorhynchus mykiss)

The comparative virulence of Yersinia ruckeri serotype O1 biotype 2 strain 07111224 from a Danish outbreak of enteric redmouth disease in rainbow trout (Oncorhynchus mykiss) was investigated in different sizes of rainbow trout (5, 19, 25 and 52 g) using a panel of challenge methods (bath, intraperitoneal (IP) injection, anal intubation and cohabitation).The results show that Y. ruckeri 07111224 is virulent enough to successfully cause mortality in 5–52 g rainbow trout through three different infection routes (bath, IP injection and anal intubation), and strongly suggests that susceptibility to Y. ruckeri infection is dependent on fish size. Bath challenge (108 CFU/ml) caused 79% mortality in 5 g fish, while the mortality in 19–52 g fish was approximately 26%. IP injection challenges (105–106 CFU/individual) resulted in high mortalites in fish of all sizes. Anal intubation challenges (109 CFU/individual) induced 82% mortality in 5 g fish and decreased with increasing fish weight. Finally, cohabitation challenges induced 45% mortality in 5 g test fish when test fish were cohabitated 1:1 with shedder fish, while mortalities in test fish of five other challenge groups were lower (5–25%). Statistical models were fit to the data from each challenge model. The model parameter predictions suggests statistically significant effects of fish weight for bath, IP and anal intubation challenges. Furthermore, they highlight the importance of the ratio of shedders to test fish in cohabitation challenges and of the challenge doses, in particular for the IP challenge model.

Egg size and lipid content of lake trout (Salvelinus namaycush) in the wild and in captivity

Egg quality influences early life survival in fishes, but drivers of egg quality variation remain poorly understood. I examined egg quality of a long-lived, iteroparous salmonid (lake trout (Salvelinus namaycush)) with respect to maternal traits and environmental conditions in wild and captively reared populations. Variation was stronger and more consistent for egg size than for lipid content. Among females, egg size was most strongly related to maternal age, in both wild and captive populations, and faster-growing females tended to produce larger eggs. Among wild populations, maternal growth was more strongly associated with indices of ecosystem size or productivity than climate, whereas the opposite was observed for egg size. Egg size and, to a lesser extent, maternal growth in captive populations were positively correlated with egg size and growth, respectively, in their wild source populations. However, captive females grew faster and produced larger eggs at age, but smaller eggs at length, than their wild counterparts. Lake trout egg quality variation has both maternal and environmental components, and captive rearing appears to alter the ontogenetic progression of egg size.

Warm water temperatures and shifts in seasonality increase trout recruitment but only moderately decrease adult size in western North American tailwaters

Dams throughout western North America have altered thermal regimes in rivers, creating cold, clear “tailwaters” in which trout populations thrive. Ongoing drought in the region has led to highly publicized reductions in reservoir storage and raised concerns about potential reductions in downstream flows. Large changes in riverine thermal regimes may also occur as reservoir water levels drop, yet this potential impact has received far less attention. We analyzed historic water temperature and fish population data to anticipate how trout may respond to future changes in the magnitude and seasonality of river temperatures. We found that summer temperatures were inversely related to reservoir water level, with warm temperatures associated with reduced storage and with dams operated as run-of-river units. Variation in rainbow trout (Oncorhynchus mykiss) recruitment was linked to water temperature variation, with a 5-fold increase in recruitment occurring at peak summer temperatures (18 °C vs. 7 °C) and a 2.5-fold increase in recruitment when peak temperatures occurred in summer rather than fall. Conversely, adult trout size was only moderately related to temperature. Rainbow and brown trout (Salmo trutta) size decreased by ~24 mm and 20 mm, respectively, as mean annual and peak summer temperatures increased. Further, rainbow trout size decreased by ~29 mm with an earlier onset of cold winter temperatures. While increased recruitment may be the more likely outcome of a warmer and drier climate, density-dependent growth constraints could exacerbate temperature-dependent growth reductions. As such, managers may consider implementing flows to reduce recruitment or altering infrastructure to maintain coldwater reservoir releases.

Hydropower impacts on reservoir fish populations are modified by environmental variation

Global transition towards renewable energy production has increased the demand for new and more flexible hydropower operations. Before management and stakeholders can make informed choices on potential mitigations, it is essential to understand how the hydropower reservoir ecosystems respond to water level regulation (WLR) impacts that are likely modified by the reservoirs' abiotic and biotic characteristics. Yet, most reservoir studies have been case-specific, which hampers large-scale planning, evaluation and mitigation actions across various reservoir ecosystems. Here, we investigated how the effect of the magnitude, frequency and duration of WLR on fish populations varies along environmental gradients. We used biomass, density, size, condition and maturation of brown trout (Salmo trutta L.) in Norwegian hydropower reservoirs as a measure of ecosystem response, and tested for interacting effects of WLR and lake morphometry, climatic conditions and fish community structure. Our results showed that environmental drivers modified the responses of brown trout populations to different WLR patterns. Specifically, brown trout biomass and density increased with WLR magnitude particularly in large and complex-shaped reservoirs, but the positive relationships were only evident in reservoirs with no other fish species. Moreover, increasing WLR frequency was associated with increased brown trout density but decreased condition of individuals within the populations. WLR duration had no significant impacts on brown trout, and the mean weight and maturation length of brown trout showed no significant response to any WLR metrics. Our study demonstrates that local environmental characteristics and the biotic community strongly modify the hydropower-induced WLR impacts on reservoir fishes and ecosystems, and that there are no one-size-fits-all solutions to mitigate environmental impacts. This knowledge is vital for sustainable planning, management and mitigation of hydropower operations that need to meet the increasing worldwide demand for both renewable energy and ecosystem services delivered by freshwaters.