The importance of warm habitat to the growth regime of cold-water fishes

Assessing the Role of Incubation Temperature as a Barrier to Successful Establishment of Coho Salmon (Oncorhynchus kisutch) in a Rapidly Warming Arctic

Warming associated with climate change is driving poleward shifts in the marine habitat of anadromous Pacific salmon (Oncorhynchus spp.). Yet the spawning locations for salmon to establish self-sustaining populations and the consequences for the ecosystem if they should do so are unclear. Here, we explore the role of temperature-dependent incubation survival and developmental phenology of coho salmon (Oncorhynchus kisutch) as a potential early life history barrier to establishment in an Arctic stream. We exposed embryos to temperatures previously recorded in the substrate of an Arctic groundwater spring-fed spawning environment. Using a common garden experimental design, coho salmon embryos were exposed to treatments that thermally mimicked four spawning dates from August 1 to October 1 (AUG1, SEPT1, SEPT15, and OCT1). Spawning temperatures were 6°C at the warmest (AUG1) and 1.25°C at the coldest (OCT1). We observed low survival rates in SEPT1 (41%) and OCT1 (34%) and near complete mortality in the other treatments. While far below what is considered normal in benign hatchery-like conditions, these rates suggest that temperatures experienced at these spawning dates are survivable. We detected differences in developmental rates across treatments; embryos developed 1.9 times faster in the warmest treatment (AUG1, 120 days) compared to the coldest (OCT1, 231 days). Differences in accumulated thermal units (ATUs) needed for hatching ranged from 392 ATUs in AUG1 to 270 ATUs in OCT1, revealing compensation in developmental requirements. Given these findings, the most thermally suitable spawning dates within our study are between September 15 and October 1, which facilitates hatching and projected nest emergence to occur in spring warming conditions (March-September). Broadly, our findings suggest that spawning sites within thermal tolerances that can support the survival and development of coho salmon exist in the North American Arctic. Whether the habitat is otherwise suitable for transitions through other life stages remains unknown.

Temperature-Induced Seasonal Dynamics of Brain Gangliosides in Rainbow Trout (Oncorhynchus mykiss Walbaum) and Common Carp (Cyprinus carpio L.)

This study aimed to determine the expression and distribution of gangliosides in specific regions of the brains of rainbow trout (Oncorhynchus mykiss Walbaum) and common carp (Cyprinus carpio L.) with regard to seasonal temperature changes. Seasonal changes in ganglioside expression and distribution within the species were expected. The natural ecosystems of these fishes differ significantly due to their distinct habitat preferences, geographic distributions, and environmental requirements. Based on the fact that the common carp is eurythermic and adapts to a wide range of temperatures, while the rainbow trout is stenothermic and thrives in a narrower temperature range, it was expected that these species would exhibit distinct patterns of ganglioside modification as part of their adaptive response to temperature fluctuations. Immunohistochemistry using specific antibodies for the major brain gangliosides (GM1, GD1a, GD1b, GT1b), along with the Svennerholm method for quantifying sialic acid bound to gangliosides, revealed that cold acclimatization led to an increase in polysialylated gangliosides in the common carp brain and an increase in trisialogangliosides in the rainbow trout brain. Immunohistochemical analysis also identified region-specific changes in ganglioside expression, suggesting specific functional roles in neuronal adaptation. These results supported the hypothesis that the composition and distribution of brain gangliosides change in response to seasonal thermal shifts as part of the adaptive response. The results underscore the importance of gangliosides in neuronal function and adaptation to environmental stimuli, with implications for understanding fish resilience to temperature changes. This study offers valuable insights into species' temperature adaptation, with implications for physiological and ecological management and improved aquaculture practices. Future research could expand the species scale, study molecular mechanisms and regulatory pathways in ganglioside metabolism, and examine ganglioside interactions with membrane proteins and lipids for a deeper understanding of thermal adaptation.

Stochastic process is main factor to affect plateau river fish community assembly

Plateau river ecosystems are often highly vulnerable and responsive to environmental change. The driving mechanism of fish diversity and community assembly in plateau rivers under changing environments presents a significant complexity to the interdisciplinary study of ecology and environment. This study integrated molecular biological techniques and mathematical models to identify the mechanisms influencing spatial heterogeneity of freshwater fish diversity and driving fish community assembly in plateau rivers. By utilizing environmental-DNA metabarcoding and the null model, this study revealed the impact of the stochastic process on fish diversity variations and community assembly in the Huangshui Plateau River of the Yellow River Basin (YRB) in China. This research identified 30 operational taxonomic units (OTUs), which correspond to 20 different fish species. The findings of this study revealed that the fish α-diversity in the upstream region of Xining is significantly higher than in the middle-lower reach (Shannon index: P = 0.017 and Simpson: P = 0.035). This pattern was not found to be related to any other environmental factors besides altitude (P = 0.023) that we measured. Further, the study indicated that the assembly of fish communities in the Huangshui River primarily depends on stochastic ecological processes. These findings suggested that elevation was not the primary factor impacting the biodiversity patterns of fish in plateau rivers. In plateau rivers, spatial heterogeneity of fish community on elevation is mainly determined by stochastic processes under habitat fragmentation, rather than any other physicochemical environmental factors. The limitations of connectivity in the downstream channel of the river could be taken the mainly responsibility for stochastic processes of fish community in Huangshui River. Incorporating ecological processes in the eDNA approach holds great potential for future monitoring and evaluation of fish biodiversity and community assembly in plateau rivers.

Diurnal patterns of spatial stream temperature variations reveal the need for integrating thermal heterogeneity in riverscape habitat restoration

Longer durations of warmer weather, altered precipitation, and modified streamflow patterns driven by climate change are expected to impair ecosystem resilience, exposing freshwater ecosystems and their biota to a severe threat worldwide. Understanding the spatio-temporal temperature variations and the processes governing thermal heterogeneity within the riverscape are essential to inform water management and climate adaptation strategies. We combined UAS-based imagery data of aquatic habitats with meteorological, hydraulic, river morphology and water quality data to investigate how key factors influence spatio-temporal stream heterogeneity on a diurnal basis within different thermal regions of a large recently restored Danube floodplain. Diurnal temperature ranges of aquatic habitats were larger than expected and ranged between 14.2 and 28.0 °C (mean = 20.7 °C), with peak median temperatures (26.1 °C) around 16:00 h. The observed temperature differences in timing and amplitude among thermal regions were unexpectedly high and created a mosaic pattern of temperature heterogeneity. For example, cooler groundwater-influenced thermal regions provided several cold water patches (CWP, below 19.0 °C) and potential cold water refuges (CWRs) around 12:00 h, at the time when other habitats were warmer than 21.0 °C, exceeding the ecological threshold (20.0 °C) for key aquatic species. Within the morphological complexity of the restored floodplain, we identified groundwater influence, shading and river morphology as the key processes driving thermal riverscape heterogeneity. Promoting stream thermal refuges will become increasingly relevant under climate change scenarios, and river restoration should consider both measures to physically prevent habitat from excessive warming and measures to improve connectivity that meet the temperature requirements of target species for conservation. This requires restoring mosaics of complex and dynamic temperature riverscapes.

Reservoir-derived subsidies provide a potential management opportunity for novel river ecosystems

Aquatic ecosystems world-wide are being irreversibly altered, suggesting that new and innovative management strategies are necessary to improve ecosystem function and sustainability. In river ecosystems degraded by dams environmental flows and selective withdrawal (SWD) infrastructure have been used to improve habitat for native species. Yet, few studies have quantified nutrient and food web export subsidies from upstream reservoirs, despite their potential to subsidize downstream riverine food webs. We sampled nutrient, phytoplankton, and zooplankton concentrations in outflows from the Shasta-Keswick reservoir complex in Northern California over a 12-month period to understand how SWD operation and internal reservoir conditions interact to influence subsidies to the Sacramento River. We found that nutrients, phytoplankton, and zooplankton were continuously exported from Shasta Reservoir to the Sacramento River and that gate operations at Shasta Dam were important in controlling exports. Further, our results indicate that gate operations and water-export depth strongly correlated with zooplankton community exports, whereas internal reservoir conditions (mixing and residence time) controlled concentrations of exported zooplankton biomass and chlorophyll a. These results demonstrate that reservoirs can be an important source of nutrient and food web subsidies and that selective withdrawal infrastructure may provide a valuable management tool to control ecosystem-level productivity downstream of dams.

Exploring metapopulation-scale suppression alternatives for a global invader in a river network experiencing climate change

Invasive species can dramatically alter ecosystems, but eradication is difficult, and suppression is expensive once they are established. Uncertainties in the potential for expansion and impacts by an invader can lead to delayed and inadequate suppression, allowing for establishment. Metapopulation viability models can aid in planning strategies to improve responses to invaders and lessen invasive species' impacts, which may be particularly important under climate change. We used a spatially explicit metapopulation viability model to explore suppression strategies for ecologically damaging invasive brown trout (Salmo trutta), established in the Colorado River and a tributary in Grand Canyon National Park. Our goals were to estimate the effectiveness of strategies targeting different life stages and subpopulations within a metapopulation; quantify the effectiveness of a rapid response to a new invasion relative to delaying action until establishment; and estimate whether future hydrology and temperature regimes related to climate change and reservoir management affect metapopulation viability and alter the optimal management response. Our models included scenarios targeting different life stages with spatially varying intensities of electrofishing, redd destruction, incentivized angler harvest, piscicides, and a weir. Quasi-extinction (QE) was obtainable only with metapopulation-wide suppression targeting multiple life stages. Brown trout population growth rates were most sensitive to changes in age 0 and large adult mortality. The duration of suppression needed to reach QE for a large established subpopulation was 12 years compared with 4 with a rapid response to a new invasion. Isolated subpopulations were vulnerable to suppression; however, connected tributary subpopulations enhanced metapopulation persistence by serving as climate refuges. Water shortages driving changes in reservoir storage and subsequent warming would cause brown trout declines, but metapopulation QE was achieved only through refocusing and increasing suppression. Our modeling approach improves understanding of invasive brown trout metapopulation dynamics, which could lead to more focused and effective invasive species suppression strategies and, ultimately, maintenance of populations of endemic fishes.

Behavioural thermoregulation in cold-water freshwater fish: Innate resilience to climate warming?

Behavioural thermoregulation enables ectotherms to access habitats providing conditions within their temperature optima, especially in periods of extreme thermal conditions, through adjustments to their behaviours that provide a "whole-body" response to temperature changes. Although freshwater fish have been detected as moving in response to temperature changes to access habitats that provide their thermal optima, there is a lack of integrative studies synthesising the extent to which this is driven by behaviour across different species and spatial scales. A quantitative global synthesis of behavioural thermoregulation in freshwater fish revealed that across 77 studies, behavioural thermoregulatory movements by fish were detected both vertically and horizontally, and from warm to cool waters and, occasionally, the converse. When fish moved from warm to cooler habitats, the extent of the temperature difference between these habitats decreased with increasing latitude, with juvenile and non-migratory fishes tolerating greater temperature differences than adult and anadromous individuals. With most studies focused on assessing movements of cold-water salmonids during summer periods, there remains an outstanding need for work on climatically vulnerable, non-salmonid fishes to understand how these innate thermoregulatory behaviours could facilitate population persistence in warming conditions.

Effects of climate on salmonid productivity: A global meta-analysis across freshwater ecosystems

Salmonids are of immense socio-economic importance in much of the world, but are threatened by climate change. This has generated a substantial literature documenting the effects of climate variation on salmonid productivity in freshwater ecosystems, but there has been no global quantitative synthesis across studies. We conducted a systematic review and meta-analysis to gain quantitative insight into key factors shaping the effects of climate on salmonid productivity, ultimately collecting 1321 correlations from 156 studies, representing 23 species across 24 countries. Fisher's Z was used as the standardized effect size, and a series of weighted mixed-effects models were compared to identify covariates that best explained variation in effects. Patterns in climate effects were complex and were driven by spatial (latitude, elevation), temporal (time-period, age-class), and biological (range, habitat type, anadromy) variation within and among study populations. These trends were often consistent with predictions based on salmonid thermal tolerances. Namely, warming and decreased precipitation tended to reduce productivity when high temperatures challenged upper thermal limits, while opposite patterns were common when cold temperatures limited productivity. Overall, variable climate impacts on salmonids suggest that future declines in some locations may be counterbalanced by gains in others. In particular, we suggest that future warming should (1) increase salmonid productivity at high latitudes and elevations (especially >60° and >1500 m), (2) reduce productivity in populations experiencing hotter and dryer growing season conditions, (3) favor non-native over native salmonids, and (4) impact lentic populations less negatively than lotic ones. These patterns should help conservation and management organizations identify populations most vulnerable to climate change, which can then be prioritized for protective measures. Our framework enables broad inferences about future productivity that can inform decision-making under climate change for salmonids and other taxa, but more widespread, standardized, and hypothesis-driven research is needed to expand current knowledge.

Lakes in Hot Water: The Impacts of a Changing Climate on Aquatic Ecosystems

Our planet is being subjected to unprecedented climate change, with far-reaching social and ecological repercussions. Below the waterline, aquatic ecosystems are being affected by multiple climate-related and anthropogenic stressors, the combined effects of which are poorly understood and rarely appreciated at the global stage. A striking consequence of climate change on aquatic ecosystems is that many are experiencing shorter periods of ice cover, as well as earlier and longer summer stratified seasons, which often result in a cascade of ecological and environmental consequences, such as warmer summer water temperatures, alterations in lake mixing and water levels, declines in dissolved oxygen, increased likelihood of cyanobacterial algal blooms, and the loss of habitat for native cold-water fisheries. The repercussions of a changing climate include impacts on freshwater supplies, water quality, biodiversity, and the ecosystem benefits that they provide to society.

Socioeconomic resilience to climatic extremes in a freshwater fishery

Heterogeneity is a central feature of ecosystem resilience, but how this translates to socioeconomic resilience depends on people's ability to track shifting resources in space and time. Here, we quantify how climatic extremes have influenced how people (fishers) track economically valuable ecosystem services (fishing opportunities) across a range of spatial scales in rivers of the northern Rocky Mountains, USA, over the past three decades. Fishers opportunistically shifted from drought-sensitive to drought-resistant rivers during periods of low streamflows and warm temperatures. This adaptive behavior stabilized fishing pressure and expenditures by a factor of 2.6 at the scale of the regional fishery (i.e., portfolio effect). However, future warming is predicted to homogenize habitat options that enable adaptive behavior by fishers, putting ~30% of current spending at risk across the region. Maintaining a diverse portfolio of fishing opportunities that enable people to exploit shifting resources provides an important resilience mechanism for mitigating the socioeconomic impacts of climate change on fisheries.

Population genomic monitoring provides insight into conservation status but no correlation with demographic estimates of extinction risk in a threatened trout

The current extinction crisis requires effective assessment and monitoring tools. Genetic approaches are appealing given the relative ease of field sampling required to estimate genetic diversity characteristics assumed related to population size, evolutionary potential, and extinction risk, and to evaluate hybridization with non-native species simultaneously. However, linkages between population genetic metrics of diversity from survey-style field collections and demographic estimates of population size and extinction risk are still in need of empirical examples, especially for remotely distributed species of conservation concern where the approach might be most beneficial. We capitalized on an exceptional opportunity to evaluate congruence between genetic diversity metrics and demographic-based estimates of abundance and extinction risk from a comprehensive Multiple Population Viability Analysis (MPVA) in a threatened fish, the Lahontan cutthroat trout (LCT). We sequenced non-native trout reference samples and recently collected and archived tissue samples of most remaining populations of LCT (N = 60) and estimated common genetic assessment metrics, predicting minimal hybridization with non-native trout, low diversity, and declining diversity over time. We further hypothesized genetic metrics would correlate positively with MPVA-estimated abundance and negatively with extinction probability. We uncovered several instances of hybridization that pointed to immediate management needs. After removing hybridized individuals, cautious interpretation of low effective population sizes (2-63) suggested reduced evolutionary potential for many LCT populations. Other genetic metrics did not decline over time nor correlate with MPVA-based estimates of harmonic mean abundance or 30-year extinction probability. Our results demonstrate benefits of genetic monitoring for efficiently detecting hybridization and, though genetic results were disconnected from demographic assessment of conservation status, they suggest reduced evolutionary potential and likely a higher conservation risk than currently recognized for this threatened fish. We emphasize that genetic information provides essential complementary insight, in addition to demographic information, for evaluating species status.

A melting cryosphere constrains fish growth by synchronizing the seasonal phenology of river food webs

Mountain watersheds often contain a mosaic of glacier-, snow-, and rain-fed streams that have distinct hydrologic, temperature, and biogeochemical regimes. However, as glaciers diminish and precipitation shifts from snow to rain, the physical and chemical characteristics that make glacial or snowmelt streams distinct from rain-fed streams will fade. Among the unforeseen consequences of this hydrologic homogenization could be the loss of unique food webs that sustain aquatic consumers. To explore the impacts of a melting cryosphere on stream food webs, we parameterized an aquatic food web model with empirical physicochemical data from glacier-, snow-, and rain-fed streams in southeast Alaska and used the model to simulate the seasonal biomass dynamics of aquatic primary producers and consumers and the growth of juvenile salmon. Model results suggest that glacier-, snow-, and rain-fed streams exhibit seasonal asynchronies in the timing of biofilm and aquatic invertebrate abundance. Although warmer rain-fed streams were more productive during the summer (June through September), colder glacial and snowmelt streams provided enhanced foraging and growth opportunities throughout the remainder of the year. For juvenile salmon that can track peaks in resource abundance within river networks, the loss of meltwater streams strongly constrained modeled growth opportunities by removing spatially and temporally distinct foraging habitats within a watershed. These findings suggest that climate change induced homogenization of high latitude river networks may result in the loss of unique food web dynamics, which could diminish the capacity of watersheds to sustain mobile consumers.

Do metapopulations and management matter for relict headwater bull trout populations in a warming climate?

Mountain headwater streams have emerged as important climate refuges for native cold-water species due to their slow climate velocities and extreme physical conditions that inhibit non-native invasions. Species persisting in refuges often do so as fragmented, relict populations from broader historical distributions that are subject to ongoing habitat reductions and increasing isolation as climate change progresses. Key for conservation planning is determining where remaining populations will persist and how habitat restoration strategies can improve biological resilience to enhance the long-term prospects for species of concern. Studying bull trout, a headwater species in the northwestern USA, we developed habitat occupancy models using a data set of population occurrence in 991 natal habitat patches with a suite of novel geospatial covariates derived from high-resolution hydroclimatic scenarios and other sources representing watershed and instream habitat conditions, patch geometry, disturbance, and biological interactions. The best model correctly predicted bull trout occupancy status in 82.6% of the patches and included effects for: patch size estimated as habitat volume, extent of within-patch reaches <9°C mean August temperature, distance to nearest occupied patch, road density, invasive brook trout prevalence, patch slope, and frequency of high winter flows. The model was used to assess 16 scenarios of bull trout occurrence within the study streams that represented a range of restoration strategies under three climatic conditions (baseline, moderate change, and extreme change). Results suggested that regional improvements in bull trout status were difficult to achieve in realistic restoration strategies due to the pervasive nature of climate change and the limited extent of restoration actions given their high costs. However, occurrence probabilities in a subset of patches were highly responsive to restoration actions, suggesting that targeted investments to improve the resilience of some populations may be contextually beneficial. A possible strategy, therefore, is focusing effort on responsive populations near more robust population strongholds, thereby contributing to local enclaves where dispersal among populations further enhances resilience. Equally important, strongholds constituted a small numerical percentage of patches (5%-21%), yet encompassed the large majority of occupied habitat by volume (72%-89%) and their protection could have significant conservation benefits for bull trout.

How riparian and floodplain restoration modify the effects of increasing temperature on adult salmon spawner abundance in the Chehalis River, WA

Stream temperatures in the Pacific Northwest are projected to increase with climate change, placing additional stress on cold-water salmonids. We modeled the potential impact of increased stream temperatures on four anadromous salmonid populations in the Chehalis River Basin (spring-run and fall-run Chinook salmon Oncorhynchus tshawytscha, coho salmon O. kisutch, and steelhead O. mykiss), as well as the potential for floodplain reconnection and stream shade restoration to offset the effects of future temperature increases. In the Chehalis River Basin, peak summer stream temperatures are predicted to increase by as much as 3°C by late-century, but restoration actions can locally decrease temperatures by as much as 6°C. On average, however, basin-wide average stream temperatures are expected to increase because most reaches have low temperature reduction potential for either restoration action relative to climate change. Results from the life cycle models indicated that, without restoration actions, increased summer temperatures are likely to produce significant declines in spawner abundance by late-century for coho (-29%), steelhead (-34%), and spring-run Chinook salmon (-95%), and smaller decreases for fall-run Chinook salmon (-17%). Restoration actions reduced these declines in all cases, although model results suggest that temperature restoration alone may not fully mitigate effects of future temperature increases. Notably, floodplain reconnection provided a greater benefit than riparian restoration for steelhead and both Chinook salmon populations, but riparian restoration provided a greater benefit for coho. This pattern emerged because coho salmon tend to spawn and rear in smaller streams where shade restoration has a larger effect on stream temperature, whereas Chinook and steelhead tend to occupy larger rivers where temperatures are more influenced by floodplain connectivity. Spring-run Chinook salmon are the only population for which peak temperatures affect adult prespawn survival in addition to rearing survival, making them the most sensitive species to increasing stream temperatures.

Contribution of warm habitat to cold-water fisheries

A central tenet of landscape ecology is that mobile species depend on complementary habitats, which are insufficient in isolation, but combine to support animals through the full annual cycle. However, incorporating the dynamic needs of mobile species into conservation strategies remains a challenge, particularly in the context of climate adaptation planning. For cold-water fishes, it is widely assumed that maximum temperatures are limiting and that summer data alone can predict refugia and population persistence. We tested these assumptions in populations of redband rainbow trout (Oncorhynchus mykiss newberrii) in an arid basin, where the dominance of hot, hyperproductive water in summer emulates threats of climate change predicted for cold-water fish in other basins. We used telemetry to reveal seasonal patterns of movement and habitat use. Then, we compared contributions of hot and cool water to growth with empirical indicators of diet and condition (gut contents, weight-length ratios, electric phase angle, and stable isotope signatures) and a bioenergetics model. During summer, trout occurred only in cool tributaries or springs (<20 °C) and avoided Upper Klamath Lake (>25 °C). During spring and fall, ≥65% of trout migrated to the lake (5-50 km) to forage. Spring and fall growth (mean [SD] 0.58% per day [0.80%] and 0.34 per day [0.55%], respectively) compensated for a net loss of energy in cool summer refuges (-0.56% per day [0.55%]). In winter, ≥90% of trout returned to tributaries (25-150 km) to spawn. Thus, although perennially cool tributaries supported thermal refuge and spawning, foraging opportunities in the seasonally hot lake ultimately fueled these behaviors. Current approaches to climate adaptation would prioritize the tributaries for conservation but would devalue critical foraging habitat because the lake is unsuitable and unoccupied during summer. Our results empirically demonstrate that warm water can fuel cold-water fisheries and challenge the common practice of identifying refugia based only on summer conditions.

Assessing climate change impacts on Pacific salmon and trout using bioenergetics and spatiotemporal explicit river temperature predictions under varying riparian conditions

Pacific salmon and trout populations are affected by timber harvest, the removal and alteration of riparian vegetation, and the resulting physical changes to water quality, temperature, and associated delivery of high-quality terrestrial prey. Juvenile salmon and trout growth, a key predictor of survival, is poorly understood in the context of current and future (climate-change mediated) conditions, with resource managers needing information on how land use will impact future river conditions for these commercially and culturally important species. We used the Heat Source water temperature modeling framework to develop a spatiotemporal model to assess how riparian canopy and vegetation preservation and addition could influence river temperatures under future climate predictions in a coastal river fed by a moraine-dammed lake: the Quinault River in Washington State. The model predicted higher water temperatures under future carbon emission projections, representative concentration pathway (RCP) 4.5 and 8.5, with varying magnitude based on different riparian vegetation scenarios. We used the daily average temperature output from these scenarios to predict potential juvenile fish growth using the Wisconsin bioenergetics model. A combination of riparian vegetation removal and continued high carbon emissions resulted in a predicted seven-day average daily maximum temperature (7DADM) increase of 1.7°C in the lower river by 2080; increases in riparian shading mitigate this 7DADM increase to only 0.9°C. Under the current thermal regime, bioenergetics modeling predicts juvenile fish lose weight in the lower river; this loss of potential growth worsens by an average of 20–83% in the lower river by 2080, increasing with the loss of riparian shading. This study assess the impact of riparian vegetation management on future thermal habitat for Pacific salmon and trout under warming climates and provide a useful spatially explicit modeling framework that managers can use to make decisions regarding riparian vegetation management and its mechanistic impact to water temperature and rearing juvenile fish.

Intraspecific variability in thermal tolerance: a case study with coastal cutthroat trout

Fish physiological performance is directly regulated by their thermal environment. Intraspecific comparisons are essential to ascertain the vulnerability of fish populations to climate change and to identify which populations may be more susceptible to extirpation and which may be more resilient to continued warming. In this study, we sought to evaluate how thermal performance varies in coastal cutthroat trout (Oncorhynchus clarki clarki) across four distinct watersheds in OR, USA. Specifically, we measured oxygen consumption rates in trout from the four watersheds with variable hydrologic and thermal regimes, comparing three ecologically relevant temperature treatments (ambient, annual maximum and novel warm). Coastal cutthroat trout displayed considerable intraspecific variability in physiological performance and thermal tolerance across the four watersheds. Thermal tolerance matched the historical experience: the coastal watersheds experiencing warmer ambient temperatures had higher critical thermal tolerance compared with the interior, cooler Willamette watersheds. Physiological performance varied across all four watersheds and there was evidence of a trade-off between high aerobic performance and broad thermal tolerance. Given the evidence of climate regime shifts across the globe, the uncertainty in both the rate and extent of warming and species responses in the near and long term, a more nuanced approach to the management and conservation of native fish species must be considered.

Hydroclimatic Conditions, Wildfire, and Species Assemblages Influence Co-Occurrence of Bull Trout and Tailed Frogs in Northern Rocky Mountain Streams

Although bull trout (Salvelinus confluentus) and tailed frogs (Ascaphus montanus) have co-existed in forested Pacific Northwest streams for millennia, these iconic cold-water specialists are experiencing rapid environmental change caused by a warming climate and enhanced wildfire activity. Our goal was to inform future conservation by examining the habitat associations of each species and conditions that facilitate co-occupancy. We repurposed data from previous studies in the northern Rocky Mountains to assess the efficacy of bull trout electrofishing surveys for determining the occurrence of tailed frogs and the predictive capacity of habitat covariates derived from in-stream measurements and geospatial sources to model distributions of both species. Electrofishing reliably detected frog presence (89.2% rate). Both species were strongly associated with stream temperature and flow regime characteristics, and less responsive to riparian canopy cover, slope, and other salmonids. Tailed frogs were also sensitive to wildfire, with occupancy probability peaking around 80 years after a fire. Co-occupancy was most probable in locations with low-to-moderate frequencies of high winter flow events, few other salmonids, a low base-flow index, and intermediate years since fire. The distributions of these species appear to be sensitive to environmental conditions that are changing this century in forests of the northern Rocky Mountains. The amplification of climate-driven effects after wildfire may prove to be particularly problematic in the future. Habitat differences between these two species, considered to be headwater specialists, suggest that conservation measures designed for one may not fully protect the other. Additional studies involving future climate and wildfire scenarios are needed to assess broader conservation strategies and the potential to identify refuge streams where both species are likely to persist, or complementary streams where each could exist separately into the future.

Quantification of thermal impacts across freshwater life stages to improve temperature management for anadromous salmonids

Water temperature is the major controlling factor that shapes the physiology, behaviour and, ultimately, survival of aquatic ectotherms. Here we examine temperature effects on the survival of Chinook salmon (Oncorhynchus tshawytscha), a species of high economic and conservation importance. We implement a framework to assess how incremental changes in temperature impact survival across populations that is based on thermal performance models for three freshwater life stages of Chinook salmon. These temperature-dependent models were combined with local spatial distribution and phenology data to translate spatial-temporal stream temperature data into maps of life stage-specific physiological performance in space and time. Specifically, we converted temperature-dependent performance (i.e. energy used by pre-spawned adults, mortality of incubating embryos and juvenile growth rate) into a common currency that measures survival in order to compare thermal effects across life stages. Based on temperature data from two abnormally warm and dry years for three managed rivers in the Central Valley, California, temperature-dependent mortality during pre-spawning holding was higher than embryonic mortality or juvenile mortality prior to smolting. However, we found that local phenology and spatial distribution helped to mitigate negative thermal impacts. In a theoretical application, we showed that high temperatures may inhibit successful reintroduction of threatened Central Valley spring-run Chinook salmon to two rivers where they have been extirpated. To increase Chinook salmon population sizes, especially for the threatened and declining spring-run, our results indicate that adults may need more cold-water holding habitat than currently available in order to reduce pre-spawning mortality stemming from high temperatures. To conclude, our framework is an effective way to calculate thermal impacts on multiple salmonid populations and life stages within a river over time, providing local managers the information to minimize negative thermal impacts on salmonid populations, particularly important during years when cold-water resources are scarce.

Climate change and expanding invasive species drive widespread declines of native trout in the northern Rocky Mountains, USA

Climate change and invasive species are major threats to native biodiversity, but few empirical studies have examined their combined effects at large spatial and temporal scales. Using 21,917 surveys collected over 30 years, we quantified the impacts of climate change on the past and future distributions of five interacting native and invasive trout species throughout the northern Rocky Mountains, USA. We found that the occupancy of native bull trout and cutthroat trout declined by 18 and 6%, respectively (1993–2018), and was predicted to decrease by an additional 39 and 16% by 2080. However, reasons for these occupancy reductions markedly differed among species: Climate-driven increases in water temperature and decreases in summer flow likely caused declines of bull trout, while climate-induced expansion of invasive species largely drove declines of cutthroat trout. Our results demonstrate that climate change can affect ecologically similar, co-occurring native species through distinct pathways, necessitating species-specific management actions.

Temperature, emergence phenology and consumption drive seasonal shifts in fish growth and production across riverscapes

Changes in biophysical conditions through time generate spatial and temporal variability in habitat quality across landscapes. For river ecosystems, researchers are increasingly able to characterize spatial and temporal patterns in habitat conditions, referred to as shifting habitat mosaics, yet rarely demonstrate how this translates into corresponding biological processes such as organism growth and production. We assessed spatial patterns and processes determining seasonal changes in juvenile Chinook Salmon Oncorhynchus tshawytscha size, growth and production over 30-40 km in two NE Oregon subbasins. We quantified seasonal patterns of growth by combining estimated emergence dates and body size distributions in July and September. We then used analysis of bioenergetics, empirical fish diets and spatial models incorporating temperature, habitat and population density to evaluate mechanisms driving spatiotemporal patterns of growth. Lastly, we quantified seasonal contributions to individual fish growth and to total production as a function of position within the stream network. Spatial heterogeneity in incubation temperatures corresponded to later estimated emergence timing with distance upstream in both subbasins. During spring, estimated growth rates decreased with distance upstream, and coupled with emergence patterns, resulted in pronounced longitudinal gradients in body size by July. During summer, spatial patterns of growth reversed, with greater diet ration sizes and growth efficiencies upstream than downstream. These opposing spatiotemporal patterns of emergence timing and seasonal growth rates produced longitudinal gradients in the proportion of fish growth achieved in spring versus summer, with up to 80% of an individual's growth occurring in spring at downstream sites but as low as 10% at upstream sites. Coupling longitudinal patterns of fish density and growth revealed that in one subbasin the majority (65%) of total production occurred in spring, while in the other, in which fish were concentrated in headwaters, the majority (60%) of production occurred in summer. While recent work has emphasized inter-annual shifts in fish production across large spatial scales, this study demonstrates that longitudinal gradients of fish growth and production can reverse across seasons, and reveals important contributions of warmer, downstream habitats to overall production that occurred during cooler times of the year.