Yukon River Drainage Research Articles

Behaviors of the Yukon River Sediment Plume in the Bering Sea: Relations to Glacier-Melt Discharge and Sediment Load

Sediment plumes, released to the Bering Sea from the delta front of the Yukon River, Alaska, are initiated mainly by glacier-melt sediment runoffs in the glacierized regions of the Yukon River drainage basin. The surface sediment plumes are extended around the fan-shaped Yukon River delta, which is followed by the northwestward dispersion. During continuous measurements of the Yukon River discharge and sediment load, behaviors of the sediment plumes were explored by shipboard and coastal observations in the Bering Sea. At the high river sediment load of ca. 2500 kg/s, the plume partially plunged into the sea bottom layer. The plunging probably originated in the nepheloid-layer formation from the flocculation of river-suspended sediment, of which more than 90% wt. is silt and clay (grain size d < 63 μm). In order to numerically obtain the area of the surface sediment plumes, a satellite image analysis was performed by using three near-infrared bands in MODIS/Aqua or MODIS/Terra. The plume area was significantly correlated (R2 = 0.735, p < 0.01) to the sediment load averaged for the two days with time lags of 20 days and 21 days to the date of a certain satellite image. Hence, the dispersion of plume-suspended sediment appears to be controlled by the sediment runoff events in the Yukon River rather than the northward “Alaskan Coastal Water”.

Can late stage marine mortality explain observed shifts in age structure of Chinook salmon?

Chinook salmon (Oncorhynchus tshawytscha) populations have experienced widespread declines in abundance and abrupt shifts toward younger and smaller adults returning to spawn in rivers. The causal agents underpinning these shifts are largely unknown. Here we investigate the potential role of late-stage marine mortality, defined as occurring after the first winter at sea, in driving this species’ changing age structure. Simulations using a stage-based life cycle model that included additional mortality during after the first winter at sea better reflected observed changes in the age structure of a well-studied and representative population of Chinook salmon from the Yukon River drainage, compared with a model estimating environmentally-driven variation in age-specific survival alone. Although the specific agents of late-stage mortality are not known, our finding is consistent with work reporting predation by salmon sharks (Lamna ditropis) and marine mammals including killer whales (Orcinus orca). Taken as a whole, this work suggests that Pacific salmon mortality after the first winter at sea is likely to be higher than previously thought and highlights the need to investigate selective sources of mortality, such as predation, as major contributors to rapidly changing age structure of spawning adult Chinook salmon.

Genetic variation and population structure among larvalLethenteronspp. within the Yukon River drainage, Alaska

The absence of information on genetic variation and population structure of lampreys Lethenteron spp. in the eastern part of their distribution limits our understanding of the migration ecology and spatial population genetic structure of the species. We examined genetic variation within and among three aggregations of Lethenteron spp. larvae in the Yukon River drainage, Alaska, using microsatellite genotypes. A total of 120 larval lampreys were genotyped at eight microsatellite loci. Global FST was 0.053 (95% CI 0.021-0.086), while pairwise FST values ranged from 0.048-0.057. Model-based Bayesian clustering analyses with sample locality priors (LOCPRIOR) identified three distinct, but admixed, genetic clusters that corresponded with the three aggregations. Estimates of contemporary gene flow indicate substantial reciprocal migration among sites consistent with no or low-fidelity natal homing. These results are largely in agreement with previous reports of historic and contemporary gene flow among Lethenteron spp. in other parts of their geographic distribution.

Effect of Ichthyophonus on blood plasma chemistry of spawning Chinook salmon and their resulting offspring in a Yukon River tributary.

Ichthyophonus is a protozoan parasite of Alaska Chinook salmon Oncorhynchus tshawytscha. In this study, we determined whether spawning Chinook salmon in the Yukon River drainage exhibited a measurable stress response (i.e. elevated plasma cortisol concentrations) and detectable changes in selected blood plasma chemistry parameters when infected with Ichthyophonus. The resulting alevin were also analyzed for any differences in blood plasma chemistry caused by parental infection with Ichthyophonus. In 2010, 2011, and 2012, spawning adult Chinook salmon were collected from the Salcha River, Alaska, USA, and the prevalence of Ichthyophonus in these fish was 7.8, 6.3, and 8.3%, respectively. Fish with no clinical signs of Ichthyophonus and Ichthyophonus-positive parents were cross-fertilized to investigate potential second-generation effects as a result of Ichthyophonus infection. We found no significant difference in cortisol concentrations in blood plasma between Ichthyophonus-positive and -negative adults or between alevin from Ichthyophonus-positive and -negative parents. There were no significant differences in blood plasma parameters (e.g. alanine aminotransferase, creatine kinase, glucose) of Ichthyophonus-negative and -positive adults, with the exception of aspartate aminotransferase, which was significantly higher in plasma of Ichthyophonus-negative adults. All clinical chemistry parameters for alevin resulting from both Ichthyophonus-negative and -positive parents were not significantly different. Based on this study, which has a limited sample size and low prevalence of Ichthyophonus, offspring of Chinook salmon appear to suffer no disadvantage as a result of Ichthyophonus infection in their parents on the Salcha River.

Statistical arrival models to estimate missed passage counts at fish weirs

Missed counts are commonplace when enumerating fish passing a weir. Typically “connect-the-dots” linear interpolation is used to impute missed passage; however, this method fails to characterize uncertainty about estimates and cannot be implemented when the tails of a run are missed. Here, we present a statistical approach to imputing missing passage at weirs that addresses these shortcomings, consisting of a parametric run curve model to describe the smoothed arrival dynamics of a fish population and a process variation model to describe the likelihood of observed data. Statistical arrival models are fit in a Bayesian framework and tested with a suite of missing data simulation trials and against a selection of Pacific salmon (Oncorhynchus spp.) case studies from the Yukon River drainage, Alaska, USA. When compared against linear interpolation, statistical arrival models produced equivalent or better expected accuracy and a narrower range of bias outcomes. Statistical arrival models also successfully imputed missing passage counts for scenarios where the tails of a run were missed.

Low productivity of Chinook salmon strongly correlates with high summer stream discharge in two Alaskan rivers in the Yukon drainage

Yukon River Chinook salmon (Oncorhynchus tshawytscha) populations are declining for unknown reasons, creating hardship for thousands of stakeholders in subsistence and commercial fisheries. An informed response to this crisis requires understanding the major sources of variation in Chinook salmon productivity. However, simple stock–recruitment models leave much of the variation in this system’s productivity unexplained. We tested adding environmental predictors to stock–recruitment models for two Yukon drainage spawning streams in interior Alaska — the Chena and Salcha rivers. Low productivity was strongly associated with high stream discharge during the summer of freshwater residency for young-of-the-year Chinook salmon. This association was more consistent with the hypothesis that sustained high discharge negatively affects foraging conditions than with acute mortality during floods. Productivity may have also been reduced in years when incubating eggs experienced major floods or cold summers and falls. These freshwater effects — especially density dependence and high discharge — helped explain population declines in both rivers. They are plausible as contributors to the decline of Chinook salmon throughout the Yukon River drainage.

NEW ICHNOLOGICAL, PALEOBOTANICAL, AND DETRITAL ZIRCON DATA FROM AN UNNAMED ROCK UNIT IN YUKON-CHARLEY RIVERS NATIONAL PRESERVE (CRETACEOUS: ALASKA): STRATIGRAPHIC IMPLICATIONS FOR THE REGION

A paleontological reconnaissance survey on Cretaceous and Paleogene terrestrial units along the Yukon River drainage through much of east-central Alaska has provided new chronostratigraphic constraints, paleoclimatological data, and the first information on local biodiversity within an ancient, high-latitude ecosystem. The studied unnamed rock unit is most notable for its historic economic gold placer deposits, but our survey documents its relevance as a source rock for Mesozoic terrestrial vertebrates, invertebrates, and associated flora. Specifically, new U-Pb ages from detrital zircons combined with ichnological data are indicative of a Late Cretaceous age for at least the lower section of the studied rock unit, previously considered to be representative of nearly exclusively Paleogene deposition. Further, the results of our survey show that this sedimentary rock unit preserves the first record of dinosaurs in the vast east-central Alaska region. Lastly, paleobotanical data, when compared to correlative rock units, support previous interpretations that the Late Cretaceous continental ecosystem of Alaska was heterogeneous in nature and seasonal.

Measuring perceptions of climate change in northern Alaska: pairing ethnography with cultural consensus analysis

Given current and projected warming trends in the Arctic and the important role played by subsistence hunting and fishing in the life of northern rural communities, it is increasingly important to document local observations of climate change and its impacts on livelihood practices. We describe ethnographic research exploring local observations of climate changes and related impacts on subsistence fisheries in three Inupiat communities in northwest Alaska and six Athabascan communities in the Yukon River drainage. We found consistent agreement among perceptions concerning a broad range of environmental changes affecting subsistence practices in these communities. These observations of environmental changes are not experienced in isolation but within the context of accompanying social changes that are continually reshaping rural Alaskan communities and subsistence economies. In this paper we reflect on our research approach combining multiple methods of inquiry. Participant observation and semidirected interviews provided the conceptual framework for broadening our focus from climate and environmental change to community residents’ understanding of climate change in the context of their holistic human-environment worldview. Cultural consensus analysis allowed us to assess the extent to which perceptions of change are shared among hunters and fishers within and between villages and regions and to identify those phenomena occurring or experienced at smaller scales. Reflecting on this multimethods approach, we highlight important questions that have emerged about how we understand, synthesize, and represent local knowledge, especially as it is used in regulatory or management arenas.

Documentation of Annual Spawning Migrations of Anadromous Coregonid Fishes in a Large River using Maturity Indices, Length and Age Analyses, and CPUE

Coregonid fi shes contribute to major food fi sheries throughout the Yukon River drainage in northwest North America. Research and management activities related to these coregonid fi shes, however, have been minimal because of the commercial and international focus on Pacifi c salmon Oncorhynchus spp. populations that share the drainage. We studied fi ve coregonid species at a fi sh- wheel sampling site 1,200 km from the Bering Sea. They were inconnu Stenodus leucichthys, broad whitefi sh Coregonus nasus, humpback whitefi sh C. clupeaformis, least cisco C. sardinella, and Bering cisco C. laurettae. Otolith chemistry studies have shown that anadromy is a common or prevailing life history strategy for all fi ve species at our fi sh-wheel sampling site. Radio telemetry studies revealed major spawning habitats for four species in the Yukon Flats, an extensive braided region of the river 1,600 to 1,700 km upstream from the Bering Sea. The objectives of this study were to document the demographic qualities of migrating coregonids at the fi sh-wheel sampling site and to defi ne seasonal periods of relative abundance based on daily catch rates. Maturity indices indicated that nearly all fi sh were mature and preparing to spawn. Minimum lengths and ages of maturity ranged from low values of 23 cm and 2 years for least cisco, to high values of 58 cm and 7 years for inconnu. A video system on the sampling fi sh wheel provided seven years of species-specifi c catch rate data that we used to identify the timing of seasonal spawning migrations for all species except least cisco.

Coho Salmon Population Structure in North America Determined from Microsatellites

AbstractPopulation structure of coho salmon Oncorhynchus kisutch from Russia to California was examined with a survey of microsatellite variation to describe the distribution of genetic variation. Variation at 17 microsatellite loci was surveyed for approximately 50,000 coho salmon sampled from 318 localities. The genetic differentiation index (FST) over all populations and loci was 0.058, with individual locus FST values ranging from 0.027 to 0.143. The least genetically diverse coho salmon were observed from Russia, the Porcupine River in the Yukon River drainage, and the middle Fraser and Thompson rivers in southern British Columbia. Coho salmon from Vancouver Island, British Columbia; Puget Sound, Hood Canal, and Juan de Fuca Strait, northern Washington; and Oregon displayed the greatest number of alleles compared with coho salmon in other regions. Differentiation in coho salmon allele frequencies among regions and among populations within regions was approximately seven times greater than that of annual variation within populations. A regional structuring of populations was the general pattern observed; coho salmon spawning in different tributaries within a major river drainage or spawning in smaller rivers in a geographic area were generally more similar to each other than to populations in different major river drainages or geographic areas. The distribution of microsatellite variation in coho salmon likely reflects the origins of salmon radiating from refuges after the last glaciation period.

Trends in streamflow in the Yukon River Basin from 1944 to 2005 and the influence of the Pacific Decadal Oscillation

Summary Streamflow characteristics in the Yukon River Basin of Alaska and Canada have changed from 1944 to 2005, and some of the change can be attributed to the two most recent modes of the Pacific Decadal Oscillation (PDO). Seasonal, monthly, and annual stream discharge data from 21 stations in the Yukon River Basin were analyzed for trends over the entire period of record, generally spanning 4–6 decades, and examined for differences between the two most recent modes of the PDO: cold-PDO (1944–1975) and warm-PDO (1976–2005) subsets. Between 1944 and 2005, average winter and April flow increased at 15 sites. Observed winter flow increases during the cold-PDO phase were generally limited to sites in the Upper Yukon River Basin. Positive trends in winter flow during the warm-PDO phase broadened to include stations in the Middle and Lower Yukon River drainage basins. Increases in winter streamflow most likely result from groundwater input enhanced by permafrost thawing that promotes infiltration and deeper subsurface flow paths. Increased April flow may be attributed to a combination of greater baseflow (from groundwater increases), earlier spring snowmelt and runoff, and increased winter precipitation, depending on location. Calculated deviations from long-term mean monthly discharges indicate below-average flow in the winter months during the cold PDO and above-average flow in the winter months during the warm PDO. Although not as strong a signal, results also support the reverse response during the summer months: above-average flow during the cold PDO and below-average flow during the warm PDO. Changes in the summer flows are likely an indirect consequence of the PDO, resulting from earlier spring snowmelt runoff and also perhaps increased summer infiltration and storage in a deeper active layer. Annual discharge has remained relatively unchanged in the Yukon River Basin, but a few glacier-fed rivers demonstrate positive trends, which can be attributed to enhanced glacier melting. A positive trend in annual flow during the warm PDO near the mouth of the Yukon River suggests that small increases in flow throughout the Yukon River Basin have resulted in an additive effect manifested in the downstream-most streamflow station. Many of the identified changes in streamflow patterns in the Yukon River Basin show a correlation to the PDO regime shift. This work highlights the importance of considering proximate climate forcings as well as global climate change when assessing hydrologic changes in the Arctic.

Otolith Trace Element Chemistry as an Indicator of Anadromy in Yukon River Drainage Coregonine Fishes

AbstractEight coregonine species have been documented in the Yukon River drainage. They include inconnuStenodus leucichthys, broad whitefishCoregonus nasus, humpback whitefishC. pidschian, Alaska whitefishC. nelsonii, least ciscoC. sardinella, Bering ciscoC. laurettae, round whitefishProsopium cylindracium, and pygmy whitefishP. coulterii. Personal use, sport, and commercial fisheries within the drainage target several of these species. Some species are capable of anadromous life histories, as evidenced by their presence in estuaries, yet few studies have investigated the upstream migrations of anadromous components of these populations. Only inconnu migrations have been previously examined in the Yukon River drainage. We investigated the distribution of anadromous coregonine fish in the Yukon River drainage in Alaska using sampling and otolith chemistry procedures. Six species were identified in sample collections from eight regions of the drainage between 1,200 and 2,000 km upstream from the Bering Sea. Gonadosomatic indices indicated that most sampled fish of all species were mature and preparing to spawn. Anadromous inconnu, broad whitefish, and humpback whitefish were distributed in the Yukon River and its tributaries to a maximum distance of 1,700 km from the sea. Anadromous least cisco were distributed in the Yukon River and its tributaries up to 1,600 km from the sea, whereas anadromous Bering cisco were present in the Yukon River main stem as far upstream as 2,000 km. No anadromous round whitefish were detected. Few coregonine spawning areas have been identified, so the actual migration distances of anadromous species may be greater than presented here.

The Gwich'in Traditional Caribou Skin Clothing Project: Repatriating Traditional Knowledge and Skills

... The Gwich'in are the most northerly of the Athapaskan peoples occupying parts of the Yukon River drainage in Alaska and the Yukon Territory, and the northern Mackenzie Basin of the Northwest Territories. The project described here was sponsored by the Gwich'in who reside in the Northwest Territories. Traditionally their lands extended from the interior of the Yukon into the Mackenzie Basin and included the watersheds of the Peel, Mackenzie, and Arctic Red Rivers. Today, most NWT Gwich'in live in the four communities of Aklavik, Fort McPherson, Inuvik, and Tsiigehtchic (formerly called Arctic Red River). These communities all fall within the Gwich'in Settlement Area that was established by the Gwich'in Comprehensive Land Claim Agreement signed in 1992 with the Government of Canada. People in the Gwich'in Settlement Area are greatly interested in materials that were collected in earlier times and are now housed throughout the world in museums, archives, and private collections. These items represent a bygone era and have great historical, cultural, and sometimes spiritual meaning. Of particular interest is traditional Gwich'in summer clothing made of white caribou hides, sewn with sinew, and decorated with porcupine quills, trade beads, silverberry seeds, finges, and ochre. Distinctively styled and striking to look at, these garments are a testament to Gwich'in women's great skill and artistic expression. ... It has been well over 100 years since Gwich'in traditional caribou skin clothing was made, and there are no examples of this clothing in either the Gwich'in communities or the Northwest Territories today. It has been over 50 years since porcupine quillwork was used as the primary decorative motif on Gwich'in jackets, slippers, and gloves. For the past two years, the Gwich'in Social and Cultural Institute (GSCI) has worked in partnership with the Prince of Wales Northern Heritage Centre (PWNHC) to create five replicas of a multipiece 19th-century Gwich'in traditional summer outfit that is housed at the Canadian Museum of Civilization (CMC). The project has given us an opportunity to document, understand, and appreciate how this clothing was manufactured and the extraordinary amount of time, knowledge, and skill that Gwich'in women needed to clothe their families and protect them from the elements. It has also helped to repatriate skills and knowledge no longer practiced in the Gwich'in Settlement Area.

Historical contamination of Yukon Lake sediments by PCBs and organochlorine pesticides: influence of local sources and watershed characteristics

PCBs and other persistent organochlorine (OC) pesticides were analyzed in sediment cores collected from six lakes in Yukon Territory and one in northern British Columbia, Canada, with the objective of establishing sources and historical trends of these contaminants. DDT was found to be the most prominent OC in the sediment profiles of most of the lakes. Maximum ΣDDT levels (3.47–2680 ng g −1 dw) were observed in sediment slices dated to the 1950s from lakes near populated areas. In contrast, in more remote lakes (Hanson, Kusawa and Lindeman), the maximum ΣDDT concentrations were observed in the sediments dated to the 1970s. Highest ΣPCB and ΣDDT concentrations were measured in sediments from Watson Lake, near a suspected PCB waste disposal site and in a region where DDT was heavily applied in the 1950s and 1960s. Elevated ΣPCB concentrations [16.1–93.6 ng g −1 dry weight (dw)] were also observed in sediments from lakes situated near populated areas, relative to Kusawa and Lindeman (11.1 and 12.7 ng g −1 dw, respectively). Recent ΣPCB fluxes ranged from 621 ng m −2 y −1 in Kusawa Lake to 16 400 ng m −2 y −1 in Little Atlin Lake. The extremely high sedimentation rate (2050 g m −2 y −1) in glacial fed Lindeman Lake gave rise to elevated fluxes of ΣPCB (2410 ng m −2 y −1) and other OCs, despite much lower concentrations in the sediment. Levels of hexachlorocyclohexanes (ΣHCH), chlordane-related compounds (ΣCHL), and chlorobenzenes (ΣCBz) were in the low ng g −1 (dw) range in all lake sediments, similar to concentrations previously reported for Arctic lakes in Canada, indicating that their major source was long range atmospheric transport. Contamination of the lakes with PCBs and DDT near populated areas of the Yukon Territory appears to be a result of regional activities rather than long range transport and deposition. The results also point to glacial runoff as a significant source of OCs to small, high elevation lakes (Lindeman), but not to larger lakes within the Yukon River drainage basin that are also affected by glacial sources (Kusawa, Laberge).

Ecology of juvenile chinook salmon in a small non-natal stream of the Yukon River drainage and the role of ice conditions on their distribution and survival

We investigated the ecology of juvenile stream-type chinook salmon (Oncorhynchus tshawytscha) in Croucher Creek, a small non-natal tributary of the upper Yukon River, in 1998 and 1999. Underyearling (age 0+) salmon enter Croucher Creek from the Yukon River in June, and by midsummer reached an average density of >0.5/m2. Fish were most commonly found in small pools. Their mean size increased until the end of August, but growth virtually ceased after that, when water temperatures fell. Juveniles remained in the stream through winter, and their distribution and survival were strongly influenced by aufeis, a thick layer of ice that develops from the freezing of groundwater. Over-winter survival was not dependent on fish size. Those fish that survived the winter grew rapidly and doubled in body mass in the spring. About 900 yearling fish emigrated from Croucher Creek in late June and early July at a mean length of 89 mm and mass of 7.2 g. Most of the migrants overwintered in a 700 m long reach of the creek that was downstream from groundwater sources and did not experience severe icing conditions. We suggest that small streams may be important habitats for juvenile salmon in the Yukon drainage, especially if there is a year-round source of groundwater flow that creates conditions suitable for overwintering.

Geologic evolution of the Yukon River: implications for placer gold

The greatest placer potential in northwest Canada lies in preglacial drainage systems. The largest placer gold deposits are associated with Pliocene pre-glacial fluvial gravels of the Dawson Range (in particular Klondike Plateau) of west-central Yukon. The abundance and/or the lack of gold concentration in this area were controlled by the ability of the streams to aggrade in response to changes caused by extensional faulting in the Tintina Trench, differential uplift, and regional denudation. Gold bearing gravels in the northern and eastern slopes of the Dawson Range are found in low order tributaries of the pre-glacial south-flowing Yukon River. Glaciation played an important role in masking and changing the direction of drainage, and creating new channels in this region. The earliest known drainage changes relate to Late Miocene formation of the Tintina Trench, and Early Pliocene uplift of the St. Elias Mountains. Prior to these events the Yukon River drained south into the Gulf of Alaska. Initial regional glaciation (2.6–2.9 Ma) diverted the southward drainage toward the northwest, becoming part of the Kwikhpak River in Alaska. The present Yukon River drainage basin is about 27.5% larger than the ancestral Kwikhpak River basin. The evolution of the river is described here in three stages (1) drainage in pre-Late Miocene, (2) drainage in Early Pliocene and (3) drainage after earliest Pliocene glaciation.

Ecology of juvenile chinook salmon in a small non-natal stream of the Yukon River drainage and the role of ice conditions on their distribution and survival

Ecology of juvenile chinook salmon in a small non-natal stream of the Yukon River drainage and the role of ice conditions on their distribution and survival

A Dendrochronological Study of the Origin of Driftwood in Frobisher Bay, Baffin Island, N.W.T., Canada

Archaeological sites in Frobisher Bay, located far above the treeline, yielded large amounts of wooden remains. Questions were raised about the source of that wood. Forty driftwood samples were collected from the shores and analyzed by dendrochronological and wood-anatomical methods to determine their origins and ages. Four Picea logs are cross-dated via tree-ring chronologies from the Yukon River drainage in Alaska and two Pinus samples are cross-dated with a tree-ring chronology from the Yenisey River in Siberia. The Picea driftwood and some Larix driftwood were eroded naturally from river terraces, while most Pinus and Larix wood escaped logging and floating operations along Russian rivers. We propose that wood on the shores of Frobisher Bay drifted from the Arctic Ocean via Fram Strait, between Greenland and Svalbard, and around the southern tip of Greenland rather than through the Canadian Arctic Archipelago.

Genetic Relationships of Arctic Grayling in the Koyukuk and Tanana Rivers, Alaska

Abstract Electrophoretic data were used to examine the stock structure of Arctic grayling Thymallus arcticus collected from the Yukon River drainage, Alaska. Arctic grayling were sampled from Grayling Lake in the Koyukuk River drainage and from three tributaries (Caribou Creek, Goodpaster River, and Volkmar River) entering a 60-km stretch of the Tanana River. Of 30 protein-coding loci examined, three (Mdh-2, Pgm-1, and Sod-2, coding for malate dehydrogenase, phosphoglucomutase, and superoxide dismutase, respectively) were polymorphic. Differences in allele frequency between the Koyukuk and Tanana drainages accounted for about 5% of the observed variability, whereas differences among the collections from the Tanana drainage accounted for less than 1%. The difference observed between collections from the Koyukuk and Tanana rivers, major tributaries to the Yukon River, suggests that genetic comparisons are useful for biogeographical studies of Alaskan Arctic grayling. The small divergence exhibited among the...

Age, Morphology, and Biochemical Genetic Variation of Yukon River Chinook Salmon

Abstract We examined Chinook salmon Oncorhynchus tshawytscha populations in the Yukon Territory for variation in age and size of spawning adults, juvenile morphology, and biochemical genetics during 1985–1987. Yukon River Chinook salmon spent at least one winter in fresh water as juveniles; about 6% of the returning adults had spent two winters in fresh water as juveniles. Males matured at younger ages and generally at smaller mean lengths at age than did females. We observed differences in juvenile morphology among populations: juveniles in habitats with faster water velocities had larger fins and thicker bodies than those in habitats with slower water velocity. We also observed genetic differentiation among populations; the greatest difference was between those populations in the Yukon River drainage and an Alsek River population. Within the Yukon River drainage, Chinook salmon from the Whitehorse fishway and Takhini River were distinct from other populations surveyed. We noted some variation in allelic...