Adult spawners: A critical period for subarctic Chinook salmon in a changing climate

Concurrent, distribution-wide abundance declines of some Pacific salmon species, including Chinook salmon (Oncorhynchus tshawytscha), highlights the need to understand how vulnerability at different life stages to climate stressors affects population dynamics and fisheries sustainability. Yukon River Chinook salmon stocks are among the largest subarctic populations, near the northernmost extent of the species range. Existing research suggests that Yukon River Chinook salmon population dynamics are largely driven by factors occurring between the adult spawner life stage and their offspring's first summer at sea (second year post-hatching). However, specific mechanisms sustaining chronic poor productivity are unknown, and there is a tremendous sense of urgency to understand causes, as declines of these stocks have taken a serious toll on commercial, recreational, and indigenous subsistence fisheries. Therefore, we leveraged multiple existing datasets spanning parent and juvenile stages of life history in freshwater and marine habitats. We analyzed environmental data in association with the production of offspring that survive to the marine juvenile stage (juveniles per spawner). These analyses suggest more than 45% of the variability in the production of juvenile Chinook salmon is associated with river temperatures or water discharge levels during the parent spawning migration. Over the past two decades, parents that experienced warmer water temperatures and lower discharge in the mainstem Yukon River produced fewer juveniles per spawning adult. We propose the adult spawner life stage as a critical period regulating population dynamics. We also propose a conceptual model that can explain associations between population dynamics and climate stressors using independent data focused on marine nutrition and freshwater heat stress. It is sobering to consider that some of the northernmost Pacific salmon habitats may already be unfavorable to these cold-water species. Our findings have immediate implications, given the common assumption that northern ranges of Pacific salmon offer refugia from climate stressors.     

Asynchrony in population dynamics of sockeye salmon in southwest Alaska

Ecologists have examined the synchronization of population dynamics across space as a means to understand how populations respond to climate variation. However, response diversity may reflect important variation among local population dynamics driven by population‐specific responses to regional environmental change. We used long‐term data on sockeye salmon Oncorhynchus nerka from pristine watersheds of southwestern Alaska to show that populations spawning in close proximity (<40 km) to one another have a limited degree of synchrony in their dynamics, even after accounting for density‐dependent processes. In fact, the dynamics of local populations of stream‐spawning sockeye salmon were no more coherent than those of stocks at a much coarser resolution across this region of Alaska. We examined four hypotheses to explain the observed patterns of asynchrony among stream‐spawning populations, and found that populations spawning in dissimilar habitats, and using different nursery lakes were less synchronized in their productivity. Similarity in the age structure of spawning adults was less correlated with synchrony in productivity. These results emphasize the importance of maintaining diverse spawning and rearing habitat for the conservation of Pacific salmon, and should guide conservation planning for Pacific salmon populations in regions where natural dynamics have been altered by habitat loss, hatchery practices, and over‐fishing.     

Signals of large scale climate drivers,hatchery enhancement,and marine factors in Yukon River Chinook salmon survival revealed with a Bayesian life history model

Understanding how species might respond to climate change involves disentangling the influence of co‐occurring environmental factors on population dynamics, and is especially problematic for migratory species like Pacific salmon that move between ecosystems. To date, debate surrounding the causes of recent declines in Yukon River Chinook salmon (Oncorhynchus tshawytscha) abundance has centered on whether factors in freshwater or marine environments control variation in survival, and how these populations at the northern extremity of the species range will respond to climate change. To estimate the effect of factors in marine and freshwater environments on Chinook salmon survival, we constructed a stage‐structured assessment model that incorporates the best available data, estimates incidental marine bycatch mortality in trawl fisheries, and uses Bayesian model selection methods to quantify support for alternative hypotheses. Models fitted to two index populations of Yukon River Chinook salmon indicate that processes in the nearshore and marine environments are the most important determinants of survival. Specifically, survival declines when ice leaves the Yukon River later in the spring, increases with wintertime temperature in the Bering Sea, and declines with the abundance of globally enhanced salmon species consistent with competition at sea. In addition, we found support for density‐dependent survival limitations in freshwater but not marine portions of the life cycle, increasing average survival with ocean age, and age‐specific selectivity of bycatch mortality in the Bering Sea. This study underscores the utility of flexible estimation models capable of fitting multiple data types and evaluating mortality from both natural and anthropogenic sources in multiple habitats. Overall, these analyses suggest that mortality at sea is the primary driver of population dynamics, yet under warming climate Chinook salmon populations at the northern extent of the species’ range may be expected to fare better than southern populations, but are influenced by foreign salmon production.     

Balancing competing life‐stage requirements in salmon habitat rehabilitation: between a rock and a hard place

Gravel augmentation is often applied to rivers and streams to rehabilitate salmonid spawning and incubation habitat. However, the effect of gravel size on salmon spawning utilization and embryo survival during incubation is not well understood. We conducted an experiment on a regulated and previously mined Northern California salmonid‐bearing stream in which different sized gravel (small, medium, and large) patches were placed into the stream's degraded spawning reach. We documented Oncorhynchus tshawytscha (Chinook salmon) spawning activity within the three gravel sizes for two seasons. In addition, we deployed Chinook salmon embryos into each gravel size patch and allowed them to incubate until estimated emergence time. Although all experimental gravel sizes were predicted to be within the spawning population's mobilization capabilities, model results indicated the probability of salmon building redds decreased as substrate size increased. Conversely, embryo survival increased as gravel size increased. A possible mechanism of disparate Chinook salmon embryo survival is provided by an observed decrease in embryo survival correlating with greater presence of embryo predators (leeches), which are associated with smaller gravel. Our results indicate a parent‐offspring conflict in optimal spawning gravel size for Chinook salmon, and suggest that an intermediate gravel size would maximize overall reproductive success across both spawning and incubation life stages.     

Main stem and off‐channel habitat use by juvenile Chinook salmon in a sub‐Arctic riverscape

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Carcass analogues provide marine subsidies for macroinvertebrates and juvenile Atlantic salmon in temperate oligotrophic streams

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Alaskan brown bears (Ursus arctos) aggregate and display fidelity to foraging neighborhoods while preying on Pacific salmon along small streams

The interaction between brown bears (Ursus arctos) and Pacific salmon (Oncorhynchus spp.) is important to the population dynamics of both species and a celebrated example of consumer‐mediated nutrient transport. Yet, much of the site‐specific information we have about the bears in this relationship comes from observations at a few highly visible but unrepresentative locations and a small number of radio‐telemetry studies. Consequently, our understanding of brown bear abundance and behavior at more cryptic locations where they commonly feed on salmon, including small spawning streams, remains limited. We employed a noninvasive genetic approach (barbed wire hair snares) over four summers (2012–2015) to document patterns of brown bear abundance and movement among six spawning streams for sockeye salmon, O. nerka, in southwestern Alaska. The streams were grouped into two trios on opposite sides of Lake Aleknagik. Thus, we predicted that most bears would forage within only one trio during the spawning season because of the energetic costs associated with swimming between them or traveling around the lake and show fidelity to particular trios across years because of the benefits of familiarity with local salmon dynamics and stream characteristics. Huggins closed‐capture models based on encounter histories from genotyped hair samples revealed that as many as 41 individuals visited single streams during the annual 6‐week sampling season. Bears also moved freely among trios of streams but rarely moved between these putative foraging neighborhoods, either during or between years. By implication, even small salmon spawning streams can serve as important resources for brown bears, and consistent use of stream neighborhoods by certain bears may play an important role in spatially structuring coastal bear populations. Our findings also underscore the efficacy of noninvasive hair snagging and genetic analysis for examining bear abundance and movements at relatively fine spatial and temporal scales.     

Straying of hatchery salmon in Prince William Sound,Alaska

The straying of hatchery salmon may harm wild salmon populations through a variety of ecological and genetic mechanisms. Surveys of pink (Oncorhynchus gorbuscha), chum (O. keta) and sockeye (O. nerka) salmon in wild salmon spawning locations in Prince William Sound (PWS), Alaska since 1997 show a wide range of hatchery straying. The analysis of thermally marked otoliths collected from carcasses indicate that 0–98% of pink salmon, 0–63% of chum salmon and 0–93% of sockeye salmon in spawning areas are hatchery fish, producing an unknown number of hatchery-wild hybrids. Most spawning locations sampled (77%) had hatchery pink salmon from three or more hatcheries, and 51% had annual escapements consisting of more than 10% hatchery pink salmon during at least one of the years surveyed. An exponential decay model of the percentage of hatchery pink salmon strays with distance from hatcheries indicated that streams throughout PWS contain more than 10% hatchery pink salmon. The prevalence of hatchery pink salmon strays in streams increased throughout the spawning season, while the prevalence of hatchery chum salmon decreased. The level of hatchery salmon strays in many areas of PWS are beyond all proposed thresholds (2–10%), which confounds wild salmon escapement goals and may harm the productivity, genetic diversity and fitness of wild salmon in this region     

Overview of salmon stock enhancement in southeast Alaska and compatibility with maintenance of hatchery and wild stocks

Modern salmon hatcheries in Southeast Alaska were established in the 1970s when wild runs were at record low levels. Enhancement programs were designed to help rehabilitate depressed fisheries and to protect wild salmon stocks through detailed planning and permitting processes that included focused policies on genetics, pathology, and management. Hatcheries were located away from significant wild stocks, local sources were used to develop hatchery broodstocks, and juveniles are marked so management can target fisheries on hatchery fish. Initially conceived as a state-run system, the Southeast Alaska (SEAK) program has evolved into a private, non-profit concept centered around regional aquaculture associations run by fishermen and other stakeholders that pay for hatchery operations through landing fees and sale of fish. Today there are 15 production hatcheries and 2 research hatcheries in SEAK that between 2005 and 2009 released from 474 to 580 million (average 517 million) juvenile salmon per year. During this same period commercial harvest of salmon in the region ranged from 28 to 71 million salmon per year (average 49 million). Contributions of hatchery-origin fish to this harvest respectively averaged 2%, 9%, 19%, 20%, and 78% for pink, sockeye, Chinook, coho, and chum salmon. Both hatchery and wild salmon stocks throughout much of Alaska have experienced high marine survivals since the 1980s and 1990s resulting in record harvests over the past two decades. Although some interactions between hatchery salmon and wild salmon are unavoidable including increasing concerns over straying of hatchery fish into wild salmon streams, obvious adverse impacts from hatcheries on production of wild salmon populations in this region are not readily evident.     

Linking landscape-level change to habitat quality: an evaluation of restoration actions on the freshwater habitat of spring-run Chinook salmon

1. Conservation planning is often hampered by the lack of causal quantitative links between landscape characteristics, restoration actions and habitat conditions that impact the status of imperilled species. Here we present a first step toward linking actions on the landscape to the population status of endangered stream‐type Chinook salmon (Oncorhynchus tshawytscha). 2. We developed relationships between land use, landscape characteristics and freshwater habitat of spring Chinook salmon in the Wenatchee River basin. Available data allowed us to find relationships that described water temperatures at several life stages (prespawning, egg incubation and summer rearing) and substratum characteristics, including fine sediments, cobble and embeddedness. Predictors included altitude, gradient, mean annual precipitation, total and riparian forest cover, road density, impervious surface and alluvium. We used a model averaging approach to account for parameter and model selection uncertainty. Key predictors were total forest cover and impervious surface area for prespawning and summer rearing temperatures; precipitation and stream gradients were important predictors of the percent of fine sediments in stream substrata. 3. We estimated habitat conditions using these relationships in three alternative landscape scenarios: historical, no restoration and one that included a set of restoration actions from local conservation planning. We found that prespawning and summer temperatures were estimated to be slightly higher historically relative to current conditions in dry sparsely forested areas, but lower in some important Chinook salmon spawning and rearing areas and lower in those locations under the restoration scenario. Fine sediments were lower in the historical scenario and were reduced as a consequence of restoration actions in two areas currently unoccupied by Chinook salmon that contain reaches with some potential for high quality spawning and rearing. Cobble and embeddedness in general were predicted to be higher historically and changed little as a result of restoration actions relative to current conditions. 4. This modelling framework converts suites of restoration actions into changes in habitat condition, thereby enabling restoration planners to evaluate alternative combinations of proposed actions. It also provides inputs to models linking habitat conditions to population status. This approach represents a first step in estimating impacts of restoration strategies, and can provide key information for conservation managers and planners.     

Climate variation is filtered differently among lakes to influence growth of juvenile sockeye salmon in an Alaskan watershed

The physical landscape filters regional climate variation such that the environmental conditions an organism experiences are unique to the characteristics of local habitat features. While it has become apparent that populations may show watershed‐specific responses to changing climate within a geographic region, the population dynamics of Pacific salmon Oncorhynchus spp. suggest that within watershed climate filtering is also important. Growth provides an integrated measure of habitat quality capturing the overall response of individuals to climate as filtered by their habitat and their response to the biological interactions in the ecosystem. We used two different long‐term datasets, scales from returning adults and juvenile length measurements, to assess the response of sockeye salmon O. nerka growth during their juvenile life phase to single and integrated measures of climate within a watershed between 1950 and 2010. Scale growth showed evidence for differences among stocks rearing in different lake habitats within the same Alaska Peninsula watershed. These lakes have substantially different morphometry and showed opposite responses to changes in spring and fall air temperatures. Juvenile length data were also available for one of these stocks and indicated that density effects were relatively weak in contrast to the effects of temperature. While direct measures of juvenile length and measures derived from adult scales showed some similarity in their trends over time, they indicated opposite effects of air temperature. Throughout the range of Pacific salmon, climate change is altering freshwater ecosystems through changes to temperature, precipitation and associated variables. These data suggest that sockeye salmon populations are experiencing climate filtering at a sub‐watershed scale. Maintaining connected, heterogeneous landscapes will therefore likely be important for providing productive habitat for sockeye salmon across a range of climate conditions that they are going to experience under new climate regimes.     

Changes in Size and Age of Chinook Salmon Oncorhynchus tshawytscha Returning to Alaska

The average sizes of Pacific salmon have declined in some areas in the Northeast Pacific over the past few decades, but the extent and geographic distribution of these declines in Alaska is uncertain. Here, we used regression analyses to quantify decadal trends in length and age at maturity in ten datasets from commercial harvests, weirs, and spawner abundance surveys of Chinook salmon Oncorhynchus tshawytscha throughout Alaska. We found that on average these fish have become smaller over the past 30 years (~6 generations), because of a decline in the predominant age at maturity and because of a decrease in age-specific length. The proportion of older and larger 4-ocean age fish in the population declined significantly (P < 0.05) in all stocks examined by return year or brood year. Our analyses also indicated that the age-specific lengths of 4-ocean fish (9 of 10 stocks) and of 3-ocean fish (5 of 10 stocks) have declined significantly (P < 0.05). Size-selective harvest may be driving earlier maturation and declines in size, but the evidence is not conclusive, and additional factors, such as ocean conditions or competitive interactions with other species of salmon, may also be responsible. Regardless of the cause, these wide-spread phenotypic shifts influence fecundity and population abundance, and ultimately may put populations and associated fisheries at risk of decline.     

Stream network geomorphology mediates predicted vulnerability of anadromous fish habitat to hydrologic change in southeast Alaska

In rivers supporting Pacific salmon in southeast Alaska, USA, regional trends toward a warmer, wetter climate are predicted to increase mid‐ and late‐21st‐century mean annual flood size by 17% and 28%, respectively. Increased flood size could alter stream habitats used by Pacific salmon for reproduction, with negative consequences for the substantial economic, cultural, and ecosystem services these fish provide. We combined field measurements and model simulations to estimate the potential influence of future flood disturbance on geomorphic processes controlling the quality and extent of coho, chum, and pink salmon spawning habitat in over 800 southeast Alaska watersheds. Spawning habitat responses varied widely across watersheds and among salmon species. Little variation among watersheds in potential spawning habitat change was explained by predicted increases in mean annual flood size. Watershed response diversity was mediated primarily by topographic controls on stream channel confinement, reach‐scale geomorphic associations with spawning habitat preferences, and complexity in the pace and mode of geomorphic channel responses to altered flood size. Potential spawning habitat loss was highest for coho salmon, which spawn over a wide range of geomorphic settings, including steeper, confined stream reaches that are more susceptible to streambed scour during high flows. We estimated that 9–10% and 13–16% of the spawning habitat for coho salmon could be lost by the 2040s and 2080s, respectively, with losses occurring primarily in confined, higher‐gradient streams that provide only moderate‐quality habitat. Estimated effects were lower for pink and chum salmon, which primarily spawn in unconfined floodplain streams. Our results illustrate the importance of accounting for valley and reach‐scale geomorphic features in watershed assessments of climate vulnerability, especially in topographically complex regions. Failure to consider the geomorphic context of stream networks will hamper efforts to understand and mitigate the vulnerability of anadromous fish habitat to climate‐induced hydrologic change.     

Maternal and environmental influences on egg size and juvenile life‐history traits in Pacific salmon

Life‐history traits such as fecundity and offspring size are shaped by investment trade‐offs faced by mothers and mediated by environmental conditions. We use a 21‐year time series for three populations of wild sockeye salmon (Oncorhynchus nerka) to test predictions for such trade‐offs and responses to conditions faced by females during migration, and offspring during incubation. In years when their 1100 km upstream migration was challenged by high water discharges, females that reached spawning streams had invested less in gonads by producing smaller but not fewer eggs. These smaller eggs produced lighter juveniles, and this effect was further amplified in years when the incubation water was warm. This latter result suggests that there should be selection for larger eggs to compensate in populations that consistently experience warm incubation temperatures. A comparison among 16 populations, with matching migration and rearing environments but different incubation environments (i.e., separate spawning streams), confirmed this prediction; smaller females produced larger eggs for their size in warmer creeks. Taken together, these results reveal how maternal phenotype and environmental conditions can shape patterns of reproductive investment and consequently juvenile fitness‐related traits within and among populations.     

Stream geomorphology regulates the effects on periphyton of ecosystem engineering and nutrient enrichment by Pacific salmon

1. Pacific salmon (Oncorhynchus spp.) returning to streams deliver substantial quantities of nutrients (nitrogen and phosphorus) that may stimulate primary production. Salmon can also affect the phytobenthos negatively via physical disturbance during nest excavation, a process that may counteract the positive effects of salmon‐derived nutrients on benthic algae. The ability of salmon to disturb benthic habitats may be a function of substratum particle size, and therefore, the geomorphology of streams could determine the net effect of salmon on benthic communities. 2. Based on surveys of 17 streams in southwest Alaska before the salmon run and during peak salmon density, we identified size thresholds for the disturbance of substratum particles by salmon and classified particles as vulnerable (<60 mm B‐axis), invulnerable (>110 mm) or transitional (61–110 mm). At the scale of individual rocks, algal biomass on vulnerable substrata decreased at peak spawning (relative to values before the run) as a power function of salmon density; transitional and invulnerable substrata showed no quantifiable pattern. However, invulnerable substrata in streams with more than 0.11 salmon m^?2 showed net algal accrual, or relatively smaller declines in algal biomass, than vulnerable substrata, indicating that large rocks provide refuge for benthic algae from salmon disturbance. 3. We expected that streams with proportionally larger rocks would respond positively to salmon at the whole‐stream scale, after accounting for the relative abundance of rocks of different sizes within streams. Invulnerable rocks made up only 0–12% of the total substratum particle size distribution in salmon‐bearing streams, however, and algal accrual on invulnerable substrata did not outweigh the strong disturbance effects on the more spatially extensive vulnerable substrata. The change in whole‐stream benthic algal biomass among streams was negatively related to salmon density. 4. Stable isotopes of nitrogen (δ¹⁵N) were used to track nutrients from salmon into benthic biota. Periphyton δ¹⁵N on rocks of all size classes was higher at peak salmon spawning than before the salmon run, indicating the uptake of salmon‐derived nitrogen. Peak δ¹⁵N values were positively related to salmon abundance and followed a two‐isotope mixing relationship. The per cent of N from salmon in periphyton was also related to salmon density and was best explained by a saturating relationship. Spring δ¹⁵N was unrelated to salmon returns in the previous year, suggesting little annual carryover of salmon nutrients.     

Self-sustaining populations, population sinks or aggregates of strays: chum (Oncorhynchus keta) and Chinook salmon (Oncorhynchus tshawytscha) in the Wood River system, Alaska

Small populations can provide insights into ecological and evolutionary aspects of species distributions over space and time. In the Wood River system in Alaska, USA, small aggregates of Chinook (Oncorhynchus tshawytscha) and chum salmon (O. keta) spawn in an area dominated by sockeye salmon (O. nerka). Our objective was to determine whether these Chinook and chum salmon are reproductively isolated, self-sustaining populations, population sinks that produce returning adults but receive immigration, or strays from other systems that do not produce returning adults. DNA samples collected from adult chum salmon from 16 streams and Chinook salmon from four streams in the Wood River system over 3 years were compared to samples from large populations in the nearby Nushagak River system, a likely source of strays. For both species, microsatellite markers indicated no significant genetic differentiation between the two systems. Simulations of microsatellite data in a large source and a smaller sink population suggested that considerable immigration would be required to counteract the diverging effects of genetic drift and produce genetic distances as small as those observed, considering the small census sizes of the two species in the Wood River system. Thus, the Wood River system likely receives substantial immigration from neighbouring watersheds, such as the Nushagak River system, which supports highly productive runs. Although no data on population productivity in the Wood River system exist, our results suggest source-sink dynamics for the two species, a finding relevant to other systems where salmonid population sizes are limited by habitat factors.     

Population diversity in salmon: linkages among response,genetic and life history diversity

Response diversity and asynchrony are important for stability and resilience of meta‐populations, however little is known about the mechanisms that might drive such processes. In salmon populations, response diversity and asynchrony have been linked to the stability of their meta‐populations and the fisheries that integrate across them. We examined how population diversity influenced response diversity and asynchrony in 42 populations of Chinook salmon from the Fraser River, British Columbia. We examined diversity in the survival responses to large‐scale ocean climate variables for populations that differed in life history. Different life‐histories responded differently to ocean environmental conditions. For instance, an increase of offshore temperature was associated with decreased survival for a population with ocean rearing juveniles but increased survival for a population with stream rearing juveniles. In a second analysis, we examined asynchrony in abundance between populations, which we then correlated with life history, spatial, and genetic diversity. Populations that were more genetically distant had the most different population dynamics. Collectively, these results suggest that fine‐scale population diversity can contribute to the asynchrony and response diversity that underpins the stability of fisheries or metapopulation dynamics, and emphasize the need to manage and conserve this scale of population diversity.     

Juvenile Salmon Usage of the Skeena River Estuary

Migratory salmon transit estuary habitats on their way out to the ocean but this phase of their life cycle is more poorly understood than other phases. The estuaries of large river systems in particular may support many populations and several species of salmon that originate from throughout the upstream river. The Skeena River of British Columbia, Canada, is a large river system with high salmon population- and species-level diversity. The estuary of the Skeena River is under pressure from industrial development, with two gas liquefaction terminals and a potash loading facility in various stages of environmental review processes, providing motivation for understanding the usage of the estuary by juvenile salmon. We conducted a juvenile salmonid sampling program throughout the Skeena River estuary in 2007 and 2013 to investigate the spatial and temporal distribution of different species and populations of salmon. We captured six species of juvenile anadromous salmonids throughout the estuary in both years, and found that areas proposed for development support some of the highest abundances of some species of salmon. Specifically, the highest abundances of sockeye (both years), Chinook in 2007, and coho salmon in 2013 were captured in areas proposed for development. For example, juvenile sockeye salmon were 2–8 times more abundant in the proposed development areas. Genetic stock assignment demonstrated that the Chinook salmon and most of the sockeye salmon that were captured originated from throughout the Skeena watershed, while some sockeye salmon came from the Nass, Stikine, Southeast Alaska, and coastal systems on the northern and central coasts of British Columbia. These fish support extensive commercial, recreational, and First Nations fisheries throughout the Skeena River and beyond. Our results demonstrate that estuary habitats integrate species and population diversity of salmon, and that if proposed development negatively affects the salmon populations that use the estuary, then numerous fisheries would also be negatively affected.     

Understanding the behavior and ecology of Chinook salmon (Oncorhynchus tshawytscha) on an important feeding ground in the Gulf of Alaska

Population declines and demographic changes of Chinook salmon (Oncorhynchus tshawytscha), have been documented throughout this species’ range, though information on natural and anthropogenic mechanisms related to these changes are not fully understood. To provide insights into marine behaviors and survival of Chinook salmon, 40 pop-up satellite archival tags (PSATs), that collected environmental data, were attached to large (69–100 cm FL) Chinook salmon caught in the marine waters of Cook Inlet, Alaska. PSATs provided evidence of predation on tagged Chinook salmon by ectothermic and unconfirmed predators, and provided valuable information about the migratory characteristics and occupied depths and temperatures of this species while occupying Cook Inlet and the Gulf of Alaska. The results from this study suggest that late-marine mortality of Chinook salmon of a variety of stock-origins by apex predators is more common in Cook Inlet than previously thought, and may be used to improve our understanding this species’ population dynamics. Furthermore, results from this study adds to the existing knowledge of marine habitat use by Chinook Salmon and may be useful in assessing the vulnerability and interactions between this species and anthropogenic activities.

     

Interspecific habitat associations of juvenile salmonids in Lake Ontario tributaries: implications for Atlantic salmon restoration

Diel variation in habitat use of subyearling Chinook salmon (Oncorhynchus tshawytscha), subyearling coho salmon (O. kisutch), yearling steelhead (O. mykiss), and yearling Atlantic salmon (Salmo salar) was examined during the spring in two tributaries of Lake Ontario. A total of 1318 habitat observations were made on juvenile salmonids including 367 on steelhead, 351 on Chinook salmon, 333 on Atlantic salmon, and 261 on coho salmon. Steelhead exhibited the most diel variation in habitat use and Chinook the least. Juvenile salmonids were generally associated with more cover and larger substrate during the day in both streams. Interspecific differences in habitat use in both streams occurred with Atlantic salmon (fast velocities) and coho salmon (pools) using the least similar habitat. Chinook salmon and Atlantic salmon used similar habitat in both streams. These findings should help guide future management actions specific to habitat protection and restoration of Atlantic salmon in Lake Ontario tributaries.