Big dams and salmon evolution: changes in thermal regimes and their potential evolutionary consequences

The influence of climate changes on heavy metals accumulation in Alosa immaculata from the Danube River Basin

This research report provides a comprehensive overview of the historical trends in heavy metal concentrations in the Pontic shad (Alosa immaculata) populations from both the Danube River and the Black Sea, while also exploring the potential influence of global warming on metal accumulation. Through bibliometric modeling analysis, it reveals significant limitations in existing international research, particularly the lack of comprehensive data on the impact of hydroclimatic changes on heavy metal accumulation in Alosa immaculata. Recognizing the critical importance of studies on heavy metal bioaccumulation in Danube shad, this research underscores their significance in defining tolerance thresholds, quantifying the impact of toxic elements along the aquatic food chain, and enhancing the economic sustainability of ichthyofauna monitoring efforts. Furthermore, these studies contribute invaluable insights into the complex dynamics of aquatic ecosystems, offering essential decision-making support for optimizing commercial fishing management practices on the Danube and ensuring robust support systems for industrial fishing endeavors.

Reservoirs regulated by small dams have a similar warming effect than lakes on the summer thermal regime of streams

Small dams account for the majority of reservoirs throughout the world, yet little is known about their effects on stream temperatures. Given that water temperature is vital for maintaining the integrity of aquatic ecosystems, studying the effects of small dams is important. This study aims to understand the effect of small dams on summer stream temperatures in a protected area in southern Quebec, Canada. We assessed the effect of small surface-release dams on four attributes of the thermal regimes (magnitude, frequency and duration of warm events, and rate of change) of streams by comparing water temperature measured in the main tributary upstream and in the main outlet downstream of the reservoir. We also compared the thermal effects of reservoirs to those of natural lakes of similar size. Using a generalized additive model, we identified key determinants of stream temperature to assess the influence of reservoir and natural lake characteristics on the thermal regime of streams. In August 2020, we observed an average warming of 3.7 °C downstream of reservoirs regulated by small dams compared to conditions upstream of the reservoir. During this period, the warming effect of reservoirs was not significantly different from the warming effect of natural lakes (3.4 °C). In addition to the drainage area, distance to an upstream water body, and the proportion of the watershed occupied by water bodies were the primary determinants of stream temperature in August, demonstrating the importance of nearby water bodies on stream thermal regimes. Given their warming effect, small waterbodies may limit the available habitat for species that are sensitive to warm temperatures. As the construction of small dams is accelerating at the global scale, a clear understanding of the cumulative effects of small lakes and reservoirs on stream temperature is required to ensure the sound management of aquatic ecosystems.

Pre- and post-dam river water temperature alteration prediction using advanced machine learning models

Dams significantly impact river hydrology by changing the timing, size, and frequency of low and high flows, resulting in a hydrologic regime that differs significantly from the natural flow regime before the impoundment. For precise planning and judicious use of available water resources for agricultural operations and aquatic habitats, it is critical to assess the dam water's temperature accurately. The building of dams, particularly several dams in rivers, can significantly impact downstream water. In this study, we predict the daily water temperature of the Yangtze River at Cuntan. Thus, this work reveals the potential of machine learning models, namely, M5 Pruned (M5P), Random Forest (RF), Random Subspace (RSS), and Reduced Error Pruning Tree (REPTree). The best and effective input variables combinations were determined based on the correlation coefficient. The outputs of the various machine learning algorithm models were compared with recorded daily water temperature data using goodness-of-fit criteria and graphical analysis to arrive at a final comparison. Based on a number of criteria, numerical comparison between the models revealed that M5P model performed superior (R² = 0.9920, 0.9708; PCC = 0.9960, 0.9853; MAE = 0.2387, 0.4285; RMSE = 0.3449, 0.4285; RAE = 6.2573, 11.5439; RRSE = 8.0288, 13.8282) in pre-impact and post-impact spam, respectively. These findings suggest that a huge wave of dam construction in the previous century altered the hydrologic regimes of large and minor rivers. This study will be helpful for the ecologists and river experts in planning new reservoirs to maintain the flows and minimize the water temperature concerning spillway operation. Finally, our findings revealed that these algorithms could reliably estimate water temperature using a day lag time input in water level. They are cost-effective techniques for forecasting purposes.

North American diadromous fishes: Drivers of decline and potential for recovery in the Anthropocene

Diadromous fishes migrate between freshwater and marine habitats to complete their life cycle, a complexity that makes them vulnerable to the adverse effects of current and past human activities on land and in the oceans. Many North American species are critically endangered, and entire populations have been lost. Major factors driving declines include overfishing, pollution, water withdrawals, aquaculture, non-native species, habitat degradation, over-zealous application of hatcheries designed to mitigate effects of other factors, and effects of climate change. Perhaps, the most broadly tractable and effective factors affecting diadromous fishes are removals of the dams that prevent or hinder their migrations, alter their environment, and often favor non-native biotic communities. Future survival of many diadromous fish populations may depend on this.

Genome-wide analysis reveals demographic and life-history patterns associated with habitat modification in landlocked, deep-spawning sockeye salmon (Oncorhynchus nerka)

Human-mediated habitat fragmentation in freshwater ecosystems can negatively impact genetic diversity, demography, and life history of native biota, while disrupting the behavior of species that are dependent on spatial connectivity to complete their life cycles. In the Alouette River system (British Columbia, Canada), dam construction in 1928 impacted passage of anadromous sockeye salmon (Oncorhynchus nerka), with the last records of migrants occurring in the 1930s. Since that time, O. nerka persisted as a resident population in Alouette Reservoir until experimental water releases beginning in 2005 created conditions for migration; two years later, returning migrants were observed for the first time in ~70 years, raising important basic and applied questions regarding life-history variation and population structure in this system. Here, we investigated the genetic distinctiveness and population history of Alouette Reservoir O. nerka using genome-wide SNP data (n = 7,709 loci) collected for resident and migrant individuals, as well as for neighboring anadromous sockeye salmon and resident kokanee populations within the Fraser River drainage (n = 312 individuals). Bayesian clustering and principal components analyses based on neutral loci revealed five distinct clusters, largely associated with geography, and clearly demonstrated that Alouette Reservoir resident and migrant individuals are genetically distinct from other O. nerka populations in the Fraser River drainage. At a finer level, there was no clear evidence for differentiation between Alouette Reservoir residents and migrants; although we detected eight high-confidence outlier loci, they all mapped to sex chromosomes suggesting that differences were likely due to uneven sex ratios rather than life history. Taken together, these data suggest that contemporary Alouette Reservoir O. nerka represents a landlocked sockeye salmon population, constituting the first reported instance of deep-water spawning behavior associated with this life-history form. This finding punctuates the need for reassessment of conservation status and supports ongoing fisheries management activities in Alouette Reservoir.

Classifying California's stream thermal regimes for cold-water conservation

Stream temperature science and management is rapidly shifting from single-metric driven approaches to multi-metric, thermal regime characterizations of streamscapes. Given considerable investments in recovery of cold-water fisheries (e.g., Pacific salmon and other declining native species), understanding where cold water is likely to persist, and how cold-water thermal regimes vary, is critical for conservation. California’s unique position at the southern end of cold-water ecosystems in the northern hemisphere, variable geography and hydrology, and extensive flow regulation requires a systematic approach to thermal regime classification. We used publicly available, long-term (> 8 years) stream temperature data from 77 sites across California to model their thermal regimes, calculate three temperature metrics, and use the metrics to classify each regime with an agglomerative nesting algorithm. Then, we assessed the variation in each class and considered underlying physical or anthropogenic factors that could explain differences between classes. Finally, we considered how different classes might fit existing criteria for cool- or cold-water thermal regimes, and how those differences complicate efforts to manage stream temperature through regulation. Our results demonstrate that cool- and cold-water thermal regimes vary spatially across California. Several salient findings emerge from this study. Groundwater-dominated streams are a ubiquitous, but as yet, poorly explored class of thermal regimes. Further, flow regulation below dams imposes serial discontinuities, including artificial thermal regimes on downstream ecosystems. Finally, and contrary to what is often assumed, California reservoirs do not contain sufficient cold-water storage to replicate desirable, reach-scale thermal regimes. While barriers to cold-water conservation are considerable and the trajectory of cold-water species towards extinction is dire, protecting reaches that demonstrate resilience to climate warming remains worthwhile.

Evolution is a double-edged sword, not a silver bullet, to confront global change

Although there is considerable optimism surrounding adaptive evolutionary responses to global change, relatively little attention has been paid to maladaptation in this context. In this review, we consider how global change might lead populations to become maladapted. We further consider how populations can evolve to new optima, fail to evolve and therefore remain maladapted, or become further maladapted through trait-driven or eco-evo-driven mechanisms after being displaced from their fitness optima. Our goal is to stimulate thinking about evolution as a "double-edged sword" that comprises both adaptive and maladaptive responses, rather than as a "silver bullet" or a purely adaptive mechanism to combat global change. We conclude by discussing how a better appreciation of environmentally driven maladaptation and maladaptive responses might improve our current understanding of population responses to global change and our ability to forecast future responses.

A probabilistic modeling framework for assessing the impacts of large reservoirs on river thermal regimes - A case of the Yangtze River

Hydrological and thermal river regimes have changed greatly due to the construction of reservoirs and dams. Water temperature changes have important significance for aquatic habitats and freshwater ecosystems. To investigate the impact of large reservoirs on the water temperature regime along the middle reach of the Yangtze River, we present a probabilistic modeling framework to ascertain the joint dependence structures of air-water temperature and discharge-water temperature between pre-reservoir and post-reservoir periods based on the copula theory. The results show that the principle of maximum entropy (POME) method can better estimate the marginal distributions of temperature regimes. Reservoir operation disturbed the dependence structures of air-water temperature, especially after the Three Gorges Reservoir (TGR) was put into operation. Different shifts in the occurrence probabilities of high or low water temperatures at the downstream and upstream stations under extreme air temperature and discharge are observed, indicating the great effects of reservoirs on the dependence structures of the downstream river flow and thermal regime. Relying on the developed model, we propose the appropriate ranges of air temperature and discharge to maintain a suitable water temperature for Chinese sturgeon (Acipenser sinensis) spawning activity. The results of this study demonstrate the influence of dams on the thermal regime and can be helpful for optimizing reservoir operations to enhance biological conservation in the Yangtze River.

Effect of Cascading Reservoirs on the Flow Variation and Thermal Regime in the Lower Reaches of the Jinsha River

We analyzed the alteration of discharge and water temperature caused by two newly established reservoirs in the lower reaches of the Jinsha River. In comparison to longer-term observations from the pre-impoundment period, the seasonal flow variability was significantly affected by the larger, upstream-located Xiluodu reservoir, with higher discharge in spring and reduced discharge in summer. The smaller, downstream located Xiangjiaba reservoir did not contribute significantly to the total hydrological alteration caused by the reservoir cascade. Thermal stratification occurred in spring and summer in the Xiluodu reservoir, but was not observed in the Xiangjiaba reservoir. The vertical structure and seasonal dynamics of thermal stratification were mainly governed by the water temperature of the inflow and the depth of the water outlet. Despite the different thermal structure, both reservoirs reduced the amplitude of annual temperature variations and delayed the seasonal temperature cycle in the downstream river water. In contrast to discharge variation, thermal effects were cumulative along the cascading reservoirs. Homogenization and delay effects can be expected to increase further with the completion of ongoing reservoir construction upstream of the two studied reservoirs. Based on our findings, we discuss the larger-scale impacts of cascading hydropower developments and emphasize the need for taking water temperature and its variation into account when developing optimized operation or mitigation strategies for these systems.

Thermal exposure of adult Chinook salmon and steelhead: Diverse behavioral strategies in a large and warming river system

Rising river temperatures in western North America have increased the energetic costs of migration and the risk of premature mortality in many Pacific salmon (Oncorhynchus spp.) populations. Predicting and managing risks for these populations requires data on acute and cumulative thermal exposure, the spatio-temporal distribution of adverse conditions, and the potentially mitigating effects of cool-water refuges. In this study, we paired radiotelemetry with archival temperature loggers to construct continuous, spatially-explicit thermal histories for 212 adult Chinook salmon (O. tshawytscha) and 200 adult steelhead (O. mykiss). The fish amassed ~500,000 temperature records (30-min intervals) while migrating through 470 kilometers of the Columbia and Snake rivers en route to spawning sites in Idaho, Oregon, and Washington. Spring- and most summer-run Chinook salmon migrated before river temperatures reached annual highs; their body temperatures closely matched ambient temperatures and most had thermal maxima in the lower Snake River. In contrast, many individual fall-run Chinook salmon and most steelhead had maxima near thermal tolerance limits (20–22 °C) in the lower Columbia River. High temperatures elicited extensive use of thermal refuges near tributary confluences, where body temperatures were ~2–10 °C cooler than the adjacent migration corridor. Many steelhead used refuges for weeks or more whereas salmon use was typically hours to days, reflecting differences in spawn timing. Almost no refuge use was detected in a ~260-km reach where a thermal migration barrier may more frequently develop in future warmer years. Within population, cumulative thermal exposure was strongly positively correlated (0.88 ≤ r ≤ 0.98) with migration duration and inconsistently associated (-0.28 ≤ r ≤ 0.09) with migration date. All four populations have likely experienced historically high mean and maximum temperatures in recent years. Expected responses include population-specific shifts in migration phenology, increased reliance on patchily-distributed thermal refuges, and natural selection favoring temperature-tolerant phenotypes.

Heating Impact of a Tropical Reservoir on Downstream Water Temperature: A Case Study of the Jinghong Dam on the Lancang River

Reservoirs change downstream thermal regimes by releasing water of different temperatures to that under natural conditions, which may then alter downstream biodiversity and ecological processes. The hydropower exploitation in the mainstream Lancang-Mekong River has triggered concern for its potential effects on downstream countries, especially the impact of the released cold water on local fishery production. However, it was observed recently that the annual water temperature downstream of the Jinghong Reservoir (near the Chinese border) has increased by 3.0 °C compared to its historical average (1997–2004). In this study, a three-dimensional (3D) model of the Jinghong Reservoir was established to simulate its hydro- and thermodynamics. Results show that: (1) the impoundment of the Jinghong Reservoir contributed about 1.3 °C to the increment of the water temperature; (2) the solar radiation played a much more important role in comparison with atmosphere-water heat exchange in changing water temperatures; and (3) the outflow rate also imposed a significant influence on the water temperature by regulating the residence time. After impoundment, the residence time increased from 3 days to 11 days, which means that the duration that the water body can absorb solar radiation has been prolonged. The results explain the heating mechanism of the Jinghong Reservoir brought to downstream water temperatures.

Dam operations may improve aquatic habitat and offset negative effects of climate change

Dam operation impacts on stream hydraulics and ecological processes are well documented, but their effect depends on geographical regions and varies spatially and temporally. Many studies have quantified their effects on aquatic ecosystem based mostly on flow hydraulics overlooking stream water temperature and climatic conditions. Here, we used an integrated modeling framework, an ecohydraulics virtual watershed, that links catchment hydrology, hydraulics, stream water temperature and aquatic habitat models to test the hypothesis that reservoir management may help to mitigate some impacts caused by climate change on downstream flows and temperature. To address this hypothesis we applied the model to analyze the impact of reservoir operation (regulated flows) on Bull Trout, a cold water obligate salmonid, habitat, against unregulated flows for dry, average, and wet climatic conditions in the South Fork Boise River (SFBR), Idaho, USA.

Thermal adaptation and phenotypic plasticity in a warming world: Insights from common garden experiments on Alaskan sockeye salmon

An important unresolved question is how populations of coldwater-dependent fishes will respond to rapidly warming water temperatures. For example, the culturally and economically important group, Pacific salmon (Oncorhynchus spp.), experience site-specific thermal regimes during early development that could be disrupted by warming. To test for thermal local adaptation and heritable phenotypic plasticity in Pacific salmon embryos, we measured the developmental rate, survival, and body size at hatching in two populations of sockeye salmon (Oncorhynchus nerka) that overlap in timing of spawning but incubate in contrasting natural thermal regimes. Using a split half-sibling design, we exposed embryos of 10 families from each of two populations to variable and constant thermal regimes. These represented both experienced temperatures by each population, and predicted temperatures under plausible future conditions based on a warming scenario from the downscaled global climate model (MIROC A1B scenario). We did not find evidence of thermal local adaptation during the embryonic stage for developmental rate or survival. Within treatments, populations hatched within 1 day of each other, on average, and among treatments, did not differ in survival in response to temperature. We did detect plasticity to temperature; embryos developed 2.5 times longer (189 days) in the coolest regime compared to the warmest regime (74 days). We also detected variation in developmental rates among families within and among temperature regimes, indicating heritable plasticity. Families exhibited a strong positive relationship between thermal variability and phenotypic variability in developmental rate but body length and mass at hatching were largely insensitive to temperature. Overall, our results indicated a lack of thermal local adaptation, but a presence of plasticity in populations experiencing contrasting conditions, as well as family-specific heritable plasticity that could facilitate adaptive change.

From population genomics to conservation and management: a workflow for targeted analysis of markers identified using genome-wide approaches in Atlantic salmon Salmo salar

A genotyping assay for the Ion Torrent Ion PGM platform was developed for fast and cost-effective targeted genotyping of key single nucleotide polymorphisms (SNPs) earlier identified using a genome-wide SNP array in Atlantic salmon Salmo salar. The method comprised a simple primer design step for multiplex-polymerase chain reaction (PCR), followed by two rounds of Ion Torrent Ion PGM sequencing to empirically evaluate marker efficiency in large multiplexes and to optimise or exclude them when necessary. Of 282 primer pairs initially tested, 217 were successfully amplified, indicating good amplification success (>75%). These markers included the sdy partial gene product to determine genetic sex, as well as three additional modules comprising SNPs for assessing neutral genetic variation (N_SNP = 150), examining functional genetic variation associated with sea age at maturity (N_SNP = 5), and for performing genetic subpopulation assignment (N_SNP = 61). The assay was primarily developed to monitor long-term genetic changes in S. salar from the Teno River, but modules are likely suitable for application in a wide range of S. salar populations. Furthermore, the fast and versatile assay development pipeline offers a strategy for developing targeted sequencing assays in any species.

Freshwater fishes of Patagonia: conservation and fisheries

The absence of much literature on the Patagonian fish fauna in comparison with that of the neotropics, has previously been blamed on its poor species diversity. Knowledge of the fishes of Patagonia, however, rose sharply at the beginning of the present century, allowing for an understanding of the complex biogeographical history that has led to the present diversity and distribution patterns. There are several new and potential threats to biodiversity and conservation of Patagonian fishes, such as the introduction of exotic species, damming, climate change and changes geared to safeguard economic interests, often acting synergistically. A great amount of new information is now available and the aim of the present review is to articulate this knowledge in a comprehensive way in order to aid in the development of tools to face the increasing challenges posed by environmental change and human activity. Knowledge about fishes of Patagonia has grown at the same time as human actions, and presence.

Special Issue: Evolutionary perspectives on salmonid conservation and management

This special issue of Evolutionary Applications comprises 15 papers that illustrate how evolutionary principles can inform the conservation and management of salmonid fishes. Several papers address the past evolutionary history of salmonids to gain insights into their likely plastic and genetic responses to future environmental change. The remaining papers consider potential evolutionary responses to climate warming, biological invasions, artificial propagation, habitat alteration, and harvesting. All of these papers consider how such influences might alter selective regimes, which should then favour plastic or genetic responses. Some of the papers then go on to document such responses, at least some of which are genetically based and adaptive. Despite the different approaches and target species, all of the papers argue for the importance of evolutionary considerations in the conservation and management of salmonids.

Thermal exposure of adult Chinook salmon in the Willamette River basin

Radiotelemetry and archival temperature loggers were used to reconstruct the thermal experience of adult spring Chinook salmon (Oncorhynchus tshawytscha) in the highly regulated Willamette River system in Oregon. The study population is threatened and recovery efforts have been hampered by episodically high prespawn mortality that is likely temperature mediated. Over three years, 310 salmon were released with thermal loggers and 68 were recovered in spawning tributaries, primarily at hatchery trapping facilities downstream from high-head dams. More than 190,000 internal body temperature records were collected (mean ~2800 per fish) and associated with 14 main stem and tributary reaches. Most salmon experienced a wide temperature range (minima ~8-10 °C; maxima ~13-22 °C) and 65% encountered potentially stressful conditions (≥18 °C). The warmest salmon temperatures were in lower Willamette River reaches, where some fish exhibited short-duration behavioral thermoregulation. Cumulative temperature exposure, measured by degree days (DD) above 0 °C, varied more than seven-fold among individuals (range=208-1498 DDs) and more than two-fold among sub-basin populations, on average. Overall, ~72% of DDs accrued in tributaries and ~28% were in the Willamette River main stem. DD differences among individuals and populations were related to migration distance, migration duration, and salmon trapping protocols (i.e., extended pre-collection holding in tributaries versus hatchery collection shortly after tributary entry). The combined data provide spatially- and temporally-referenced information on both short-duration stressful temperature exposure and the biologically important total exposure. Thermal exposure in this population complex proximately influences adult salmon physiology, maturation, and disease processes and ultimately affects prespawn mortality and fitness. The results should help managers develop more effective salmon recovery plans in basins with marginal thermal conditions.

Sensitivity of salmonid freshwater life history in western US streams to future climate conditions

We projected effects of mid-21st century climate on the early life growth of Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) in western United States streams. Air temperature and snowpack trends projected from observed 20th century trends were used to predict future seasonal stream temperatures. Fish growth from winter to summer was projected with temperature-dependent models of egg development and juvenile growth. Based on temperature data from 115 sites, by mid-21st century, the effects of climate change are projected to be mixed. Fish in warm-region streams that are currently cooled by snow melt will grow less, and fish in suboptimally cool streams will grow more. Relative to 20th century conditions, by mid-21st century juvenile salmonids' weights are expected to be lower in the Columbia Basin and California Central Valley, but unchanged or greater in coastal and mountain streams. Because fish weight affects fish survival, the predicted changes in weight could impact population fitness depending on other factors such as density effects, food quality and quantity changes, habitat alterations, etc. The level of year-to-year variability in stream temperatures is high and our analysis suggests that identifying effects of climate change over the natural variability will be difficult except in a few streams.

Evolutionary change driven by metal exposure as revealed by coding SNP genome scan in wild yellow perch (Perca flavescens)

Pollution can drive rapid evolutionary change in wild populations. This study targets functional polymorphisms of chronically metal-contaminated wild yellow perch (Perca flavescens). A de novo transcriptome scan contrasted subsets of individuals from clean (n = 16) and contaminated (n = 16) lakes to identify 87 candidate annotated coding SNPs. Candidate genotypes and liver [metal] were obtained in 10 populations (n = 1,052) and a genome scan distinguished outliers: one nuclear (cyclin G1 gene) and two mitochondrial (cytochrome b and NADH dehydrogenase subunit 2 genes) also displaying allelic correlation to mean population [cadmium]. Whole mtDNA and 17 kb surrounding cyclin G1 were characterised through 454 sequencing thus revealing two non-synonymous substitutions involving dissimilar amino acids. Based on associated functions and inter-population differentiation, contaminated perch may have been selected for fast life cycle completion (p53 pathway) and memorization impairment mitigation (long-term potentiation pathway). In accordance with predicted evolutionary trajectory for stressed and energy deprived organisms, adapted perch would not compensate for repair mechanism inhibition, instead reallocating energy towards growth and favouring inexpensive impairment mitigation adaptations over costly detoxification. Overall, 85 years of selection could have driven rapid, potentially adaptive evolution by selecting alleles increasing perch fitness in polluted environments.

Provenance matters: thermal reaction norms for embryo survival among sockeye salmon Oncorhynchus nerka populations

Differences in thermal tolerance during embryonic development in Fraser River sockeye salmon Oncorhynchus nerka were examined among nine populations in a controlled common-garden incubation experiment. Forcing embryonic development at an extreme temperature (relative to current values) of 16° C, representing a future climate change scenario, significantly reduced survival compared to the more ecologically moderate temperature of 10° C (55% v. 93%). Survival at 14° C was intermediate between the other two temperatures (85%). More importantly, this survival response varied by provenance within and between temperature treatments. Thermal reaction norms showed an interacting response of genotype and environment (temperature), suggesting that populations of O. nerka may have adapted differentially to elevated temperatures during incubation and early development. Moreover, populations that historically experience warmer incubation temperatures at early development displayed a higher tolerance for warm temperatures. In contrast, thermal tolerance does not appear to transcend life stages as adult migration temperatures were not related to embryo thermal tolerance. The intra-population variation implies potential for thermal tolerance at the species level. The differential inter-population variation in thermal tolerance that was observed suggests, however, limited adaptive potential to thermal shifts for some populations. This infers that the intergenerational effects of increasing water temperatures may affect populations differentially, and that such thermally mediated adaptive selection may drive population, and therefore species, persistence.

Parental identity influences progeny responses to incubation thermal stress in sockeye salmon Onchorhynchus nerka

The influence of individual parentage on progeny responses to early developmental temperature stress was examined in a cross-fertilization experiment using sockeye salmon Oncorhynchus nerka. Differences in survival, hatch timing and size were examined among five paternally linked and five maternally linked offspring families (Weaver Creek population, British Columbia, Canada) incubated at 12, 14 and 16° C from just after fertilization to hatch. Mean embryonic survival was significantly lower at 14 and 16° C; however, offspring families had substantially different survival responses across the thermal gradient (crossing reaction norms). Within temperature treatments, substantial variation in embryonic survival, alevin mass, time-to-hatch and hatch duration were attributable to family identity; however, most traits were governed by significant temperature-family interactions. For embryonic survival, large differences between families at 16° C were due to both female and male spawner influence, whereas inter-family differences were obscured at 14° C (high intra-family variation), and minimal at 12° C (only maternal influence detected). Despite post-hatch rearing under a common cool thermal regime, persistent effects of both temperature and parentage were detected in alevin and 3 week-old fry. Collectively, these findings highlight the crucial role that parental influences on offspring may have in shaping future selection within salmonid populations exposed to elevated thermal regimes. An increased understanding of parental and temperature influences and their persistence in early development will be essential to developing a more comprehensive view of population spawning success and determining the adaptive capacity of O. nerka populations in the face of environmental change.

Competitive interactions shape offspring performance in relation to seasonal timing of emergence in Atlantic salmon

1. Timing of birth/hatching may have strong effects on offspring fitness. Breeding time is generally considered to have evolved to match offspring arrival with optimal seasonal environmental conditions, though this is rarely tested experimentally and factors shaping the relations between timing of birth and reproductive success are often poorly understood. 2. By manipulating incubation temperature of Atlantic salmon embryos, and hence controlling for maternal and genetic effects, we obtained offspring emerging from nests prior to (accelerated), during and after (decelerated) normal emergence times, and accordingly experienced widely different seasonal environmental conditions at emergence (stream temperature range 4-16 °C). The accelerated group emerged at temperatures that are generally considered to be highly sub-optimal for growth and likely imposes strong constraints on feeding and activity, and during a peak in water discharge which is expected to negatively influence habitat availability. 3. In the wild, overall mortality during the period after emergence was 79%, and was significantly affected by both release density and emergence timing. Accelerated offspring, which emerged earliest and experienced the harshest environmental conditions, had both highest survival and largest final body size. The effect was particularly strong at the high density release site, where survival of accelerated offspring was significantly higher than both the normal and decelerated groups. 4. In more controlled semi-natural environments, all developmental groups were able to perform well, but accelerated offspring had a relatively better performance than the later emerging offspring when density was high. Furthermore, the relative performance of the different groups was not sensitive to water discharge regimes (temporally stable vs. fluctuating). 5. These results suggest that the performance of offspring in relation to seasonal timing of emergence is highly affected by competitive interactions in Atlantic salmon. Although a match between phenology and optimal seasonal environmental conditions may be highly important for organisms depending on specific resources that are only available during a limited period of the season, such resources may be available in variable amounts year around for many organisms. For these, offspring success may to a larger degree be shaped by the timing of their hatching/birth relative to each other, and particularly so under high population densities.

A review of the likely effects of climate change on anadromous Atlantic salmon Salmo salar and brown trout Salmo trutta, with particular reference to water temperature and flow

The present paper reviews the effects of water temperature and flow on migrations, embryonic development, hatching, emergence, growth and life-history traits in light of the ongoing climate change with emphasis on anadromous Atlantic salmon Salmo salar and brown trout Salmo trutta. The expected climate change in the Atlantic is for milder and wetter winters, with more precipitation falling as rain and less as snow, decrease in ice-covered periods and frequent periods with extreme weather. Overall, thermal limits for salmonids are species specific. Scope for activity and growth and optimal temperature for growth increase with temperature to an optimal point before constrain by the oxygen content of the water. The optimal temperature for growth decreases with increasing fish size and varies little among populations within species, whereas the growth efficiency may be locally adapted to the temperature conditions of the home stream during the growth season. Indirectly, temperature influences age and size at smolting through its effect on growth. Time of spawning, egg hatching and emergence of the larvae vary with temperature and selective effects on time of first feeding. Traits such as age at first maturity, longevity and fecundity decrease with increasing temperature whilst egg size increases with temperature. Water flow influences the accessibility of rivers for returning adults and speed of both upstream and downstream migration. Extremes in water flow and temperature can decrease recruitment and survival. There is reason to expect a northward movement of the thermal niche of anadromous salmonids with decreased production and population extinction in the southern part of the distribution areas, migrations earlier in the season, later spawning, younger age at smolting and sexual maturity and increased disease susceptibility and mortality. Future research challenges are summarized at the end of the paper.