Juvenile Emigration Research Articles

How do Management Goals for Wild Chinook Salmon Align with Feasibility?

Evaluation of goals is crucial for effective management to conserve viable and diverse fish populations and to support harvest. Management of wild Chinook Salmon Oncorhynchus tshawytscha in the Snake River basin exemplifies the struggle to meet fisheries goals. Considering contemporary production of juvenile emigrants, it is imperative to determine the survival from emigration to adult (Se‐a) required to meet abundance goals. Increased anthropogenic impact on spawning and rearing habitats leads to higher Se‐a required to attain goals. Wilderness populations exhibit viability and could sustain fisheries with Se‐a lower than required elsewhere. Conversely, populations targeted for habitat restoration demand higher Se‐a to fulfill goals, indicating a need to enhance egg‐to‐smolt survival. However, Se‐a has fallen below the threshold needed for replacement in all populations, even at current low abundances. Despite these challenges, most populations still possess the potential to achieve abundance goals, emphasizing the importance of strategic interventions to bolster their resilience.

“Les Miserables” Children of Great Britain in Canada: Maria Rye’s Migration Programme

In this paper, the author explores Maria Susan Rye’s emigration society. She initiated the relocation of British children from workhouses and orphanages to Canada in 1869. Documentary and narrative sources have rarely been used to explore this “inconvenient” topic of British history. Few works by Western researchers of child migration appeared only at the end of the twentieth century. In the article, the author analyses controversial events in the history of Great Britain and Canada in the nineteenth century. The purpose of the work is to consider the reasons for resettlement, Rye’s migration plan, the attitude of the British public towards it, its practical implementation in the Dominion, and the living conditions of minors in Canada. The author notes that the pauperisation of the British population was a major factor in the juvenile emigration in the context of industrialisation and demographic growth of the cities. Rye’s plan was to take children from slums and place them in Canadian families. The author pays special attention to Andrew Doyle’s mission. He was commissioned by the British government to investigate the activities of Rye, her colleagues, and the living conditions of the migrants. The author demonstrates that Doyle exaggerated the abuse and lack of supervision of children in order to discredit Rye’s project and impose the workhouse system on Canada. She concludes that the informal control of migrants practised by Rye should have been supplemented by legislative protection of the rights and interests of British minors in Canada.

A video monitoring and computational system for estimating migratory juvenile fish abundance in river systems

AbstractDiadromous fishes migrate between marine and fresh waters for reproduction. For anadromous species, which spawn in freshwater, improved access to freshwater spawning and nursery habitats and ability of juveniles to emigrate to the ocean may support population recovery. Despite the potentially enormous influence of early life stage survival on adult population size, managers and scientists have limited capacity to assess numbers of juvenile anadromous fishes leaving freshwater ecosystems. Such data are critical for evaluating reproductive success and habitat suitability and have been identified as a top priority in anadromous fish research and management. We developed a state‐of‐the‐art underwater video and computational system to collect videos to estimate abundances and migration timing for juvenile river herring (Alosa pseudoharengus; Alosa aestivalis). We collected continuous video in the Monument River (Bourne, Massachusetts, USA) from June to November 2017. We trained three types of neural network models to detect and count fish in video frames and evaluated model performance by comparing human counts to model outputs. Our top model assessed presence and absence (F1 = 87%) and counted fish (counting error 9.4%) with an accuracy comparable to human counters (F1 = 88%). Our system's capability to collect accurate counts of emigrating juveniles will provide critical information that could be related to the numbers of spawning adults, system‐specific productivity, and spawning and nursery habitat suitability. Both the video collection system and computational model may be transferrable to other sites and for other species where tracking juvenile emigration may inform management efforts.

Applying parentage methods to detect gravel augmentation effects on juvenile Chinook Salmon recruitment rates

AbstractQuantifiable measurement of habitat restoration effects on salmonid reproductive performance is limited, although it is necessary for evaluating whether population responses follow management actions. We investigated using close‐kin mark‐recapture methods to partition restoration effects within standard monitoring metrics of juvenile emigrants leaving the natal reach. This approach allowed for statistical comparison of relative juvenile capture rates (recruitment) from Chinook Salmon (Oncorhynchus tshawytscha) naturally reproducing in gravel augmented (restored) and nonaugmented (unrestored) reaches of a highly managed California River. Genetic parentage techniques linked adult females that had spawned in augmented and nonaugmented spawning habitats with juvenile Chinook Salmon sampled the following spring at a trap located below the natal spawning area. Successful recruitment was documented from both augmented and nonaugmented habitats, with no statistical difference between the two habitat types. The capture rate per adult female was low (0.19–0.24 juveniles/female) compared to rates observed in other systems. Within the juvenile collections, most females in the study had 0 or 1 offspring observed; however, one female that spawned in a restored habitat had 25 offspring recovered at the trap. We modeled juvenile capture rates in relation to a range of biological and environmental variables including spawning habitat site, spawning habitat treatment (augmented and nonaugmented), annual spawner abundance, year spawned, female fork length, spawning and hatch day, and flow maximum and variance. There was an inverse relationship between annual adult abundance (escapement obtained from carcass surveys) and recruitment (juvenile recoveries at trap), suggesting habitat limitation may be creating density‐dependent effects. Additionally, female body size was positively associated with recruitment, while spawning day and mean daily temperature were inversely associated with recruitment, suggesting that both biological and environmental factors independent of habitat influenced reproduction potential. This study provides evidence that habitat restoration could have an additive (incremental) positive effect on recruitment rate, informing hatchery management and restoration activities related to population recovery.

Six decades of ecohydrological research connecting landscapes and riverscapes in the Girnock Burn, Scotland: Atlantic salmon population and habitat dynamics in a changing world

AbstractLong‐term data are crucial for understanding ecological responses to climate and land use change; they are also vital evidence for informing management. As a migratory fish, Atlantic salmon are sentinels of both global and local environmental change. This paper reviews the main insights from six decades of research in an upland Scottish stream (Girnock Burn) inhabited by a spring Atlantic salmon population dominated by multi‐sea‐winter fish. Research began in the 1960s providing a census of returning adults, juvenile emigrants and in‐stream production of Atlantic salmon. Early research pioneered new monitoring techniques providing new insights into salmon ecology and population dynamics. These studies underlined the need for interdisciplinary approaches for understanding salmon interactions with physical, chemical and biological components of in‐stream habitats at different life‐stages. This highlighted variations in catchment‐scale hydroclimate, hydrology, geomorphology and hydrochemistry as essential to understanding freshwater habitats in the wider landscape context. Evolution of research has resulted in a remarkable catalogue of novel findings underlining the value of long‐term data that increases with time as modelling tools advance to leverage more insights from “big data”. Data are available on fish numbers, sizes and ages across multiple life stages, extending over many decades and covering a wide range of stock levels. Combined with an unusually detailed characterization of the environment, these data have enabled a unique process‐based understanding of the controls and bottlenecks on salmon population dynamics across the entire lifecycle and the consequences of declining marine survival and ova deposition. Such powerful datasets, methodological enhancements and the resulting process understanding have informed and supported the development of fish population assessment tools which have been applied to aid management of threatened salmon stocks at large‐catchment, regional and national scales. Many pioneering monitoring and modelling approaches developed have been applied internationally. This history shows the importance of integrating discovery science with monitoring for informing policy development and assessing efficacy of management options. It also demonstrates the need to continue to resource long‐term sites, which act as a focus for inter‐disciplinary research and innovation, and where the overall value of the research greatly exceeds the costs of individual component parts.

Incorporating life history diversity in an integrated population model to inform viability analysis

Life history diversity can significantly affect population dynamics and effects of management actions. For instance, variation in individual responses to environmental variability can reduce extirpation risk to populations, as the portfolio effect dampens temporal variability in abundance. Moreover, differences in habitat use may cause individuals to respond differently to habitat management and climate variability. To explore the role of life history diversity in population trajectories, population models need to incorporate within-population variation. Integrated population modeling (IPM) is a population modeling approach that offers several advantages for sharing information and propagating uncertainty across datasets. In this study, we developed an IPM for an endangered population of Chinook salmon ( Oncorhynchus tshawytscha) in the Wenatchee River, Washington, USA, that accounts for diversity in juvenile life histories, spawning location, and return age. Our analysis revealed that diversity in the age of juvenile emigration from natal streams had a portfolio effect, resulting in a 20% reduction in year-to-year variability in adult abundance in population projections. Our population viability analysis suggests that management interventions may be necessary to meet recovery goals, and our model should be useful for simulating the outcomes of proposed actions.

State‐dependent estuary stopover boosts juvenile salmon growth: Implications for marine survival

AbstractStopover habitats allow migratory animals to bolster energy stores prior to the resumption of migration, which can increase an individual's likelihood of survival. Carryover effects from prior habitats may mediate stopover behavior: evidence suggests that low‐condition migrants exhibit longer stopovers and attain more growth during stopover than high‐condition individuals. For Pacific salmon (Oncorhynchus spp.), estuaries may be important stopover habitats during juvenile emigration from natal freshwaters to the ocean. Favorable foraging conditions in high‐productivity estuaries could boost growth, and body size at marine entry is generally positively correlated with survival to adulthood. However, the contribution of the estuarine life stage to individual fitness across the salmon life cycle remains poorly understood. Here, we show that body size at freshwater exit mediates estuary stopover behavior by juvenile coho salmon (O. kisutch) and that estuary growth can increase the probability of marine survival. We performed a multiyear mark–recapture study of juvenile coho salmon in the Koeye River and its estuary, located on the Central Coast of British Columbia. We found that stopover duration decreased with increasing body size: small juveniles spent more than three times longer in the estuary than their larger conspecifics. Mean annual estuary growth rates ranged from 0.58 to 0.95 mm/day, and the smallest juveniles grew by nearly 30% in fork length during estuary stopover. Consequently, each out‐migrating juvenile cohort entered the ocean larger and less variable in body size than it was at freshwater exit. Analysis of returning spawners from two juvenile cohorts (2017, 2018) indicated that larger individuals had higher ocean survival rates. Using a simplistic analytical extrapolation, we demonstrate that estuary stopover could increase marine survival: For median‐sized juveniles, growth achieved in the estuary was associated with 39.8% (34.0%–46.2%) higher survival for the 2017 cohort and 39.1% (31.6%–47.0%) higher survival for the 2018 cohort. These results, from a watershed unaltered by intensive logging or development, suggest that growth achieved during estuary stopover can mitigate freshwater carryover effects and buffer size‐selective survival in a changing ocean. More broadly, these findings highlight the importance of state‐dependent use of stopover habitats in the study of migration ecology and conservation of migratory species.

Effects of population density and environmental conditions on life-history prevalence in a migratory fish.

Individual variation in life-history traits can have important implications for the ability of populations to respond to environmental variability and change. In migratory animals, flexibility in the timing of life-history events, such as juvenile emigration from natal areas, can influence the effects of population density and environmental conditions on habitat use and population dynamics. We evaluated the functional relationships between population density and environmental covariates and the abundance of juveniles expressing different life-history pathways in a migratory fish, Chinook salmon (Oncorhynchus tshawytscha), in the Wenatchee River basin in Washington State, USA. We found that the abundance of younger emigrants from natal streams was best described by an accelerating or near-linear function of spawners, whereas the abundance of older emigrants was best described by a decelerating function of spawners. This supports the hypothesis that emigration timing varies in response to density in natal areas, with younger-emigrating life-history pathways comprising a larger proportion of emigrants when densities of conspecifics are high. We also observed positive relationships between winter stream discharge and abundance of younger emigrants, supporting the hypothesis that habitat conditions can also influence the prevalence of different life-history pathways. Our results suggest that early emigration, and a resultant increase in the use of downstream rearing habitats, may increase at higher population densities and with greater winter precipitation. Winter precipitation is projected to increase in this system due to climate warming. Characterizing relationships between life-history prevalence and environmental conditions may improve our understanding of species habitat requirements and is a first step in understanding the dynamics of species with diverse life-history strategies. As environmental conditions change-due to climate change, management, or other factors-resultant life-history changes are likely to have important demographic implications that will be challenging to predict when life-history diversity is not accounted for in population models.

Using multiple lines of evidence to assess recovery potential of a warm water fish population in a cold water impacted river

IntroductionHumans have substantially altered landscapes across the globe, generating novel ecosystems with varying states of modification. The principles of reconciliation ecology emphasise that such novel ecosystems must also be considered for conservation outcomes. This requires an understanding of how anthropogenic habitat alterations in a novel ecosystem may disrupt life cycle processes of key biota, thereby enabling the development of management strategies that may bypass or ameliorate potential bottlenecks in life history stages. We explore this by assessing Murray cod (Maccullochella peelii), a native fish species of considerable cultural, recreational and conservation significance in a severely altered river reach in south-eastern Australia. This species was considered to have disappeared in the reach following alterations to the river’s thermal profile (cold water pollution - CWP), but instead has persisted.MethodsWe examined the life cycle of Murray cod and use multiple lines of evidence to assess the impacts of CWP pollution on key processes such as adult migration, recruitment and juvenile survival. We also evaluated the potential for recovery and persistence of the species in this novel system using mitigation measures such as flow management and stocking and opportunistically tested their effectiveness.ResultsWe found the key impact pathway of CWP on the Murray cod population is via its effects on the key processes influencing natural recruitment. Conversely, the effects of CWP on emigration and survival of larger juveniles and adults did not appear to be a major bottleneck for the population. Using a stochastic population model, we showed that the population could be sustained by stocking and opportunistic water management while still meeting irrigation demands. Monitoring to test our recommendations showed natural recruitment and improved survival of stocked fish during years with limited flow releases, thereby corroborating our predictions.DiscussionOur use of multiple lines of evidence to assess a species’ responses to impacts of habitat alteration in a novel ecosystem enabled the formulation of recommendations for management that could then be tested. The success of such actions add to the growing body of literature that shows species of conservation significance can be maintained in novel ecosystems which should be specifically considered within the context of conservation planning across the landscape.

Long-term satellite tracking reveals patterns of long-distance dispersal in juvenile and adult Arctic foxes (Vulpes lagopus).

Long-distance dispersal plays a key role in species distribution and persistence. However, its movement metrics and ecological implications may differ whether it is undertaken by juveniles (natal dispersal) or adults (breeding dispersal). We investigated the influence of life stage on long-distance dispersal in the Arctic fox, an important tundra predator. We fitted 170 individuals with satellite collars during a 13-year study on Bylot Island (Nunavut, Canada), and analysed the tracks of 10 juveniles and 27 adults engaging in long-distance dispersal across the Canadian High Arctic. This behaviour was much more common than expected, especially in juveniles (62.5%, adults: 19.4%). Emigration of juveniles occurred mainly at the end of summer while departure of adults was not synchronized. Juveniles travelled for longer periods and over longer cumulative distances than adults, but spent similar proportions of their time travelling on sea ice versus land. Successful immigration occurred mostly in late spring and was similar for juveniles and adults (30% versus 37%). Our results reveal how life stage influences key aspects of long-distance dispersal in a highly mobile canid. This new knowledge is critical to understand the circumpolar genetic structure of the species, and how Arctic foxes can spread zoonoses across vast geographical areas.

Water availability drives instream conditions and life-history of an imperiled desert fish: A case study to inform water management

In arid ecosystems, available water is a critical, yet limited resource for human consumption, agricultural use, and ecosystem processes—highlighting the importance of developing management strategies to meet the needs of multiple users. Here, we evaluated how water availability influences stream thermal regimes and life-history expressions of Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) in the arid Truckee River basin in the western United States. We integrated air temperature and stream discharge data to quantify how water availability drives stream temperature during annual spawning and rearing of Lahontan cutthroat trout. We then determined how in situ stream discharge and temperature affected adult spawning migrations, juvenile growth opportunities, and duration of suitable thermal conditions. Air temperatures had significant, large effects (+) on stream temperature across months; the effects of discharge varied across months, with significant effects (−) during May through August, suggesting increased discharge can help mitigate temperatures during seasonally warm months. Two models explained adult Lahontan cutthroat trout migration, and both models indicated that adult Lahontan cutthroat trout avoid migration when temperatures are warmer (~ > 12 °C) and discharge is higher (~ > 50 m3*s−1). Juvenile size was best explained by a quadratic relationship with cumulative degree days (CDD; days>4 °C) as size increased with increasing CDDs but decreased at higher CDDs. We also found an interaction between CDDs and discharge explaining juvenile size: when CDDs were low, higher discharge was associated with larger size, but when CDDs were high, higher discharge was associated with smaller size. Stream temperatures also determined the duration of juvenile rearing, as all juvenile emigration ceased at temperatures >24.4 °C. Together, our results illustrated how stream discharge and temperature shape the life-history of Lahontan cutthroat trout at multiple stages and can inform management actions to offset warming temperatures and facilitate life-history diversity and population resilience.

Local population collapse of Ross's and lesser snow geese driven by failing recruitment and diminished philopatry

Dynamics of free‐ranging animal populations can result from complex interplays of survival, recruitment and movement. Yet incomplete understanding of demography impedes conservation strategies intended to modify population dynamics of focal species. We estimated survival and per capita production of young, as well as emigration and immigration, from 1997 to 2017 in Ross's goose Anser rossii and lesser snow goose Anser caerulescens caerulescens, which are sympatric species of migratory birds that nest in the central Canadian Arctic at one of the largest breeding colonies in North America. We formed age‐structured integrated population models (IPMs) for each species that jointly analyzed live and dead encounter data as well as breeding adult population size and fecundity data to understand drivers of population dynamics. We compared the demography between species because both species increased during the 1990s and early 2000s yet thereafter snow geese declined, while Ross's geese continued to increase, then stabilized and similarly declined. Declines in Ross's and snow goose populations were caused by reduced per capita production of young, and juvenile survival, as well as increased adult and juvenile emigration. Stronger declines in juvenile survival in snow geese explain their earlier population decline compared to Ross's geese. Despite the divergence in population trends in Ross's and snow geese, we found strong synchrony in demographic rates which suggested substantial emigration from this colony and similar responses to environmental conditions. Direct estimation of demographic patterns in the IPM framework permitted hypothesis testing and inference about the role of immigration, even though immigrant sources were unsampled. We provide a novel m‐array implementation specific to a multi‐state Burnham model which greatly improved computational efficiency and convergence of posterior estimates. Our findings are an important reminder of the role that permanent movements can play in animal demography and metapopulation structure.

Orientation and emigration of larval and juvenile amphibians: selected topics and hypotheses.

Most amphibians have a complex life cycle with an aquatic larval and an adult (semi-) terrestrial stage. However, studies concerning spatial behaviour and orientation mainly focus on either the aquatic larvae or the adult animals on land. Consequently, behavioural changes that happen during metamorphosis and the consequences for emigration and population distribution are less understood. This paper aims to summarize the knowledge concerning specific topics of early amphibian life history stages and proposes several testable hypotheses within the following fields of research: larval and juvenile orientation, influences of environmental and genetic factors on juvenile emigration, their habitat choice later in life as well as population biology. I argue that studying larval and juvenile amphibian spatial behaviour is an understudied field of research, however, could considerably improve our understanding of amphibian ecology.

A Synthesis of Juvenile Coastal Cutthroat Trout Emigration Studies: Considerations for Researchers and Resource Managers

AbstractFor the management of Coastal Cutthroat Trout Oncorhynchus clarkii clarkii, resource managers require abundance estimates and demographic structure of juvenile emigrants. However, Coastal Cutthroat Trout research and monitoring have been confounded by the variability in assessment approaches and ecological plasticity of the subspecies. The primary purpose of this study is to help address this current Coastal Cutthroat Trout fisheries management issue by conducting a synthesis of juvenile emigration studies and proposing solutions that should be considered by resource managers when designing juvenile emigration studies. A total of 100 studies conducted in British Columbia and Washington since the 1970s, representing 1,100 annual estimates, were reviewed. The results of this review highlighted inconsistencies in study timing, biological data collection, juvenile emigrant life stage assignments, species identification, and ecotype assumptions. Solutions to address these inconsistencies include increased recognition of juvenile emigrant studies targeting Coastal Cutthroat Trout, strengthened partnerships and data sharing between management agencies, implementation of a life stage assignment standard for juvenile emigration studies, implementation of a field‐based and genetic classification model to quantify study‐specific identification error rates of juvenile emigrants, and genetic research to determine ecotype expression at the population and individual scales. We highlight the practical issues related to juvenile emigration studies and provide recommendations for management actions to account for the great plasticity in Coastal Cutthroat Trout behavior across the subspecies’ range.

Demographic effects of phenological variation in natural populations of two pond-breeding salamanders

Phenology is a key driver of population and community dynamics. Phenological metrics (e.g., first date that an event occurred) often simplify information from the full phenological distribution, which may undermine efforts to determine the importance of life history events. Data regarding full phenological distributions are especially needed as many species are shifting phenology with climatic change which can alter life-history patterns and species dynamics. We tested whether skewness, kurtosis or maximum duration of breeding phenology affected juvenile emigration phenology and survival in natural populations of ringed (Ambystoma annulatum) and spotted salamanders (A. maculatum) spanning a 7-year period at two study locations. We evaluated the relative importance of different phenological metrics in breeding phenology and larval density dependence on emigration phenology and survival. We found that variability in emigration phenology differed by species, with ringed salamanders having a shorter duration and distributions that were more often right-skewed and leptokurtic compared to spotted salamanders. Emigration phenology was not linked to any measure of variability in breeding phenology, indicating phenological variability operates independently across life stages and may be subject to stage-specific influences. Emigration duration and skewness were partially explained by larval density, which demonstrates how phenological distributions may change with species interactions. Further tests that use the full phenological distribution to link variability in timing of life history events to demographic traits such as survival are needed to determine if and how phenological shifts will impact species persistence.

Outmigration survival of a threatened steelhead population through a tidal estuary

Juvenile steelhead (Oncorhynchus mykiss) are exposed to numerous threats in heterogeneous, estuarine environments, yet understanding of survival patterns and processes during this migratory stage is often limited by studies that use surrogate species or are restricted in duration and spatial specificity. Lack of detailed survival information in this critical migratory stage limits the effectiveness of management to maintain juvenile life history diversity in threatened populations. We used acoustic telemetry with multistate mark–recapture models to investigate survival patterns during a key stage of the juvenile emigration of anadromous steelhead through the Sacramento–San Joaquin River Delta of California, United States, over multiple years, including three drought years. Survival was highly variable both within and among the six years of the study; estimated total survival through the Delta ranged from 0.06 (May 2014) to 0.69 (March 2011). Survival in the upstream reaches was associated with river discharge into the Delta, while survival through the lower reaches was associated with migration route. The lack of a single factor associated with survival in all reaches counteracts preconceived ideas of survival processes. Hydrodynamic manipulation and habitat improvements are recommended to support this anadromous population in a changing climate.

Assessing spillover from marine protected areas and its drivers: A meta‐analytical approach

AbstractOverfishing may seriously impact fish populations and ecosystems. Marine protected areas (MPAs) are key tools for biodiversity conservation and fisheries management, yet the fisheries benefits remain debateable. Many MPAs include a fully protected area (FPA), restricting all activities, within a partially protected area (PPA) where potentially sustainable activities are permitted. An effective tool for biodiversity conservation, FPAs, can sustain local fisheries via spillover, that is the outward export of individuals from FPAs. Spillover refers to both: “ecological spillover”: outward net emigration of juveniles, subadults and/or adults from the FPA; and “fishery spillover”: the fraction of ecological spillover that directly benefits fishery yields and revenues through fishable biomass. Yet, how common is spillover remains controversial. We present a meta‐analysis of a unique global database covering 23 FPAs worldwide, using published literature and purposely collected field data, to assess the capacity of FPAs to export biomass and whether this response was mediated by specific FPA features (e.g. size, age) or species characteristics (e.g. mobility, economic value). Results show fish biomass and abundance outside FPAs was higher: (a) in locations close to FPA borders (<200 m) than further away (>200 m); (b) for species with a high commercial value; and (c) in the presence of PPA surrounding the FPA. Spillover was slightly higher in FPAs that were larger and older and for more mobile species. Based on the broadest data set compiled to date on marine species ecological spillover beyond FPAs' borders, our work highlights elements that could guide strategies to enhance local fishery management using MPAs.

Migrations of Young Fish in Regulated Rivers: Effects of Ecological Filters (Review)

Unlike in nonregulated rivers, new ecological filters are formed in artificial reservoirs: structures (gradient zones between biotopes) which affect the behavior and distribution of juvenile fish and therefore change parameters of the downstream migration. One distinct feature of such filters is their selective effect on different species and age groups of migrants when they pass boundaries between biotopes. The differential transparency of boundaries considerably affects the composition of migrants and dispersal of juvenile fish over biotopes. These suggestions are confirmed by the data of long-term studies on the downstream migration of juvenile Cyprinidae and Percidae in several reservoirs in Eurasia: Sheksna (Sheksna River), Ivankovo (Volga River), Kapchagay (Ili River), and Sarez (Murghab River) Reservoirs. The formation of ecological filters is shown in the zone of wedging-out backwaters (lotic–limnetic transformation) and in the zone between the littoral (resident) and pelagic (migratory) biotopes in the reservoir and in the near-dam broads. Emigration of juveniles from the reservoir depends on the location of the water intake (deep-water or surface) and overlapping of water intake zones with those of the spatial distribution of juvenile fish. The different selectivity of water intake sites of the hydroelectric power station located not far from each other and of a navigation lock forms different (in species and age composition) complexes of emigrants. The mechanisms and consequences of functioning ecological filters and ecological barriers are considered.

Unnatural selection of salmon life histories in a modified riverscape.

Altered river flows and fragmented habitats often simplify riverine communities and favor non‐native fishes, but their influence on life‐history expression and survival is less clear. Here, we quantified the expression and ultimate success of diverse salmon emigration behaviors in an anthropogenically altered California river system. We analyzed two decades of Chinook salmon monitoring data to explore the influence of regulated flows on juvenile emigration phenology, abundance, and recruitment. We then followed seven cohorts into adulthood using otolith (ear stone) chemical archives to identify patterns in time‐ and size‐selective mortality along the migratory corridor. Suppressed winter flow cues were associated with delayed emigration timing, particularly in warm, dry years, which was also when selection against late migrants was the most extreme. Lower, less variable flows were also associated with reduced juvenile and adult production, highlighting the importance of streamflow for cohort success in these southernmost populations. While most juveniles emigrated from the natal stream as fry or smolts, the survivors were dominated by the rare few that left at intermediate sizes and times, coinciding with managed flows released before extreme summer temperatures. The consistent selection against early (small) and late (large) migrants counters prevailing ecological theory that predicts different traits to be favored under varying environmental conditions. Yet, even with this weakened portfolio, maintaining a broad distribution in migration traits still increased adult production and reduced variance. In years exhibiting large fry pulses, even marginal increases in their survival would have significantly boosted recruitment. However, management actions favoring any single phenotype could have negative evolutionary and demographic consequences, potentially reducing adaptability and population stability. To recover fish populations and support viable fisheries in a warming and increasingly unpredictable climate, coordinating flow and habitat management within and among watersheds will be critical to balance trait optimization versus diversification.

Migrations of Fish Juveniles in Dammed Rivers: the Role of Ecological Barriers

Analysis of previously published characteristics of the downstream migration of juvenile cyprinids (Cyprinidae) and percids (Percidae) from the Volga and Ivankovskoe, Volgogradskoe, and Tsimlayanskoe reservoirs is presented. In case of river damming, migrations of river fish are modified first of all due to transformation of the hydrophysical and morphological structure of the water course creating a different biotopic pattern influencing life activities of fish, including migratory behavior. In contrast to the natural river, where the conditions controlling the characteristics of the downstream migration change gradually from the upper reaches to the lower reaches, ecological barriers are formed in the dammed river—the water reservoir and the dam—which essentially change these conditions. A leading role in formation of these barriers and in regulation of the downstream migration is performed by the morphological complexity of a water body and by the intensity of water exchange. These factors act both on the scale of the whole water reservoir and on the scale of local biotopes where the downstream migration takes place and behavioral mechanisms act efficiently. The synergistic effect of these factors (a high ramification index of the reservoir at a low water exchange) may decrease the intensity of emigration of fish juveniles from a reservoir by several orders of magnitude.