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     

The occurrence and run timing of naturally spawning chum salmon in northern Japan

Since the late 20th century, the biomass of Pacific salmon Oncorhynchus spp. has increased. Hokkaido, northern Japan, is one of the main areas of chum salmon O. keta production in the North Pacific and intensive hatchery programs support the recent high abundance. However, proper management of naturally spawning populations is necessary to conserve healthy stocks of this species. In 2008, we started a program to assess the naturally spawning chum salmon populations in Hokkaido. Of the total of approximately 1,500 rivers in Hokkaido, 238 rivers with lengths of longer than 8 km (excluding those rivers used for hatchery broodstock collection) were surveyed in 2008 and 2009. The number of non-enhanced rivers found to contain naturally reproducing chum salmon was 59 (31.4% of surveyed rivers) and 50 (37.6% of surveyed rivers) rivers in 2008 and 2009, respectively. Including the rivers where hatchery broodstock were collected and rivers shorter than 8 km that contain naturally spawning chum salmon, chum salmon ascended at least 191 and 175 rivers in Hokkaido in 2008 and 2009, respectively. Repeated foot surveys indicated that the run timings of naturally spawning chum salmon may be affected by coastal commercial fisheries. This study showed that naturally spawning chum salmon remain in many rivers in Hokkaido where hatchery programs have been intensively conducted.     

Rapid expansion of an enhanced stock of chum salmon and its impacts on wild population components

A harvested stock of chum salmon homing to Kurilskiy Bay, Iturup Island, consists of two genetically distinct river populations that reproduce in two rivers that drain into the bay and are characterized by limited gene flow. One of these is small and can be regarded as wild, whereas the other is much larger and, until recently, was composed of naturally reproducing components spawning in the river??s mainstem and tributaries, with almost no hatchery reproduction during the past two decades. The only human impact on reproduction of the chum salmon stock was regulation of the escapement, with officially accepted limits to avoid ??over-escapement??. Recently the hatchery began to release a large amount of chum salmon juveniles. As confirmed by data on variation in both age composition and microsatellite DNA, first-generation hatchery-origin fish that returned from the first large releases occupied spawning grounds and presumably competed directly with, and potentially displaced wild fish. The most dramatic example is a genetically distinct beach-spawning form of chum salmon that was swamped by much more numerous hatchery-origin fish of the river-spawning form. In order to restore and support naturally reproduced population components, careful estimation of the carrying capacity of natural spawning grounds is necessary with efforts to increase escapement to these habitats. We also recommend concerted efforts to restore and conserve a unique beach-spawning population of chum salmon. We further recommend development of a marking program for direct estimation of straying and evaluation of ecological and genetic impacts of hatchery fish on neighboring wild and natural populations.     

Wild and hatchery reproduction of pink and chum salmon and their catches in the Sakhalin-Kuril region,Russia

In the Sakhalin-Kuril region hatchery culture of pink and chum salmon is of great importance compared to other regions of the Russian Far East. During the last 30 years the number of hatcheries increased two-fold, and significant advances were made in hatchery technologies. As a result, chum salmon capture in regions where hatcheries operate (southwestern and eastern Sakhalin coasts, and Iturup Island) was 9 times as high during 2006–2010 than during 1986–1990, whereas wild chum salmon harvest markedly declined. Recent dynamics in pink salmon catch appear to track trends in natural spawning in monitored index rivers, suggesting natural-origin pink salmon play a dominant role in supporting the commercial fishery. It remains uncertain as to whether hatcheries have substantially supplemented commercial catch of pink salmon in this region, and I recommend continued research (including implementing mass marking and recovery programs) before decisions are made regarding increasing pink salmon hatchery production. Location of hatcheries in spawning river basins poses problems for structuring a management system that treats hatchery and wild populations separately. Debate continues regarding the existence and importance of density-dependent processes operating in the ocean environment and the role hatcheries play in these processes. Loss of critical spawning habitat for chum salmon in the Sakhalin-Kuril region has lead to significant declines in their abundance. I conclude by recommending increases in releases of hatchery chum salmon numbers in the region to help recover depressed wild populations and provide greater commercial fishing benefits in the region.     

Evidence for competition at sea between Norton Sound chum salmon and Asian hatchery chum salmon

Increasing production of hatchery salmon over the past four decades has led to concerns about possible density-dependent effects on wild Pacific salmon populations in the North Pacific Ocean. The concern arises because salmon from distant regions overlap in the ocean, and wild salmon populations having low productivity may compete for food with abundant hatchery populations. We tested the hypothesis that adult length-at-age, age-at-maturation, productivity, and abundance of a Norton Sound, Alaska, chum salmon population were influenced by Asian hatchery chum salmon, which have become exceptionally abundant and surpassed the abundance of wild chum salmon in the North Pacific beginning in the early 1980s. We found that smaller adult length-at-age, delayed age-at-maturation, and reduced productivity and abundance of the Norton Sound salmon population were associated with greater production of Asian hatchery chum salmon since 1965. Modeling of the density-dependent relationship, while controlling for other influential variables, indicated that an increase in adult hatchery chum salmon abundance from 10 million to 80 million adult fish led to a 72% reduction in the abundance of the wild chum salmon population. These findings indicate that competition with hatchery chum salmon contributed to the low productivity and abundance of Norton Sound chum salmon, which includes several stocks that are classified as Stocks of Concern by the State of Alaska. This study provides new evidence indicating that large-scale hatchery production may influence body size, age-at-maturation, productivity and abundance of a distant wild salmon population.     

Interaction between hatchery and wild Pacific salmon in the Far East of Russia: A review

We review studies of interactions between hatchery and wild Pacific salmon in the Russian Far East. This includes the role of hatchery practices that result in premature migration to the sea and increased mortality, and data on feeding and territorial competition between juveniles of hatchery and wild origin. In the course of downstream migration many juvenile hatchery salmon are eliminated by wild salmon predation. During the marine period, Japanese hatchery chum salmon (Oncorhynchus keta) distribution overlaps the distribution of Russian wild salmon. Consequently, replacement of wild populations by hatchery fishes, as a result of abundant juvenile hatchery releases combined with extensive poaching in spawning grounds, is apparent in some Russian rivers. Interactions between the populations occur in all habitats. The importance of conservation of wild salmon populations requires a more detailed study of the consequences of interactions between natural and artificially reared fishes.     

Occurrence and genetic typing of infectious hematopoietic necrosis virus in Kamchatka, Russia

Infectious hematopoietic necrosis virus (IHNV) is a well known rhabdoviral pathogen of salmonid fish in North America that has become established in Asia and Europe. On the Pacific coast of Russia, IHNV was first detected in hatchery sockeye from the Kamchatka Peninsula in 2001. Results of virological examinations of over 10,000 wild and cultured salmonid fish from Kamchatka during 1996 to 2005 revealed IHNV in several sockeye salmon Oncorhynchus nerka populations. The virus was isolated from spawning adults and from juveniles undergoing epidemics in both hatchery and wild sockeye populations from the Bolshaya watershed. No virus was detected in 2 other watersheds, or in species other than sockeye salmon. Genetic typing of 8 virus isolates by sequence analysis of partial glycoprotein and nucleocapsid genes revealed that they were genetically homogeneous and fell within the U genogroup of IHNV. In phylogenetic analyses, the Russian IHNV sequences were indistinguishable from the sequences of North American U genogroup isolates that occur throughout Alaska, British Columbia, Washington, and Oregon. The high similarity, and in some cases identity, between Russian and North American IHNV isolates suggests virus transmission or exposure to a common viral reservoir in the North Pacific Ocean.     

Source-Sink Estimates of Genetic Introgression Show Influence of Hatchery Strays on Wild Chum Salmon Populations in Prince William Sound,Alaska

The extent to which stray, hatchery-reared salmon affect wild populations is much debated. Although experiments show that artificial breeding and culture influence the genetics of hatchery salmon, little is known about the interaction between hatchery and wild salmon in a natural setting. Here, we estimated historical and contemporary genetic population structures of chum salmon (Oncorhynchus keta) in Prince William Sound (PWS), Alaska, with 135 single nucleotide polymorphism (SNP) markers. Historical population structure was inferred from the analysis of DNA from fish scales, which had been archived since the late 1960’s for several populations in PWS. Parallel analyses with microsatellites and a test based on Hardy-Weinberg proportions showed that about 50% of the fish-scale DNA was cross-contaminated with DNA from other fish. These samples were removed from the analysis. We used a novel application of the classical source-sink model to compare SNP allele frequencies in these archived fish-scales (1964–1982) with frequencies in contemporary samples (2008–2010) and found a temporal shift toward hatchery allele frequencies in some wild populations. Other populations showed markedly less introgression, despite moderate amounts of hatchery straying. The extent of introgression may reflect similarities in spawning time and life-history traits between hatchery and wild fish, or the degree that hybrids return to a natal spawning area. The source-sink model is a powerful means of detecting low levels of introgression over several generations.     

Migration of Pacific Rim Chum Salmon on the High Seas: Insights from Genetic Data

Wild stocks of chum salmon, Oncorhynchus keta, have experienced recent declines in some areas of their range. Also, the release of hatchery chum salmon has escalated to nearly three billion fish annually. The decline of wild stocks and the unknown effects of hatchery fish combined with the uncertainty of future production caused by global climate change have renewed interest in the migratory patterns of chum salmon on the high seas. We studied the composition of high-seas mixtures of maturing and immature individuals using baseline data for 20 allozyme loci from 356 populations from throughout the Pacific Rim. Composition estimates were made from three time series. Two of these time series were from important coastal migratory corridors: the Shumagin Islands south of the Alaska Peninsula and the east coast of the Kamchatka Peninsula. The third was from chum salmon captured incidentally in the Bering Sea trawl fishery for walleye pollock. We also analyzed geographically dispersed collections of chum salmon captured in the month of July. The time series show dynamic changes in stock composition. The Shumagin Island corridor was used primarily by Northwest Alaskan and Asian populations in June; by the end of July stocks from the Alaska Peninsula and southern North America dominated the composition. The composition along the Kamchatka coast changed dramatically from primarily Russian stocks in May to primarily Japanese stocks in August; the previously undocumented presence of stocks from the Alaska Peninsula and Gulf of Alaska was also demonstrated. Immature chum salmon from throughout the Pacific Rim, including large proportions of southern North American stocks, contributed to the Bering Sea bycatch during the months of September and October. The migration routes of North American stocks is far more widespread than previously observed, and the Bering Sea is an important rearing area for maturing and immature chum salmon from throughout the species' range.     

Managed metapopulations: do salmon hatchery 'sources' lead to in-river 'sinks' in conservation?

Maintaining viable populations of salmon in the wild is a primary goal for many conservation and recovery programs. The frequency and extent of connectivity among natal sources defines the demographic and genetic boundaries of a population. Yet, the role that immigration of hatchery-produced adults may play in altering population dynamics and fitness of natural populations remains largely unquantified. Quantifying, whether natural populations are self-sustaining, functions as sources (population growth rate in the absence of dispersal, λ>1), or as sinks (λ<1) can be obscured by an inability to identify immigrants. In this study we use a new isotopic approach to demonstrate that a natural spawning population of Chinook salmon, (Oncorhynchus tshawytscha) considered relatively healthy, represents a sink population when the contribution of hatchery immigrants is taken into consideration. We retrieved sulfur isotopes ((34)S/(32)S, referred to as δ(34)S) in adult Chinook salmon otoliths (ear bones) that were deposited during their early life history as juveniles to determine whether individuals were produced in hatcheries or naturally in rivers. Our results show that only 10.3% (CI = 5.5 to 18.1%) of adults spawning in the river had otolith δ(34)S values less than 8.5‰, which is characteristic of naturally produced salmon. When considering the total return to the watershed (total fish in river and hatchery), we estimate that 90.7 to 99.3% (CI) of returning adults were produced in a hatchery (best estimate = 95.9%). When population growth rate of the natural population was modeled to account for the contribution of previously unidentified hatchery immigrants, we found that hatchery-produced fish caused the false appearance of positive population growth. These findings highlight the potential dangers in ignoring source-sink dynamics in recovering natural populations, and question the extent to which declines in natural salmon populations are undetected by monitoring programs.     

Have Native Coho Salmon (Oncorhynchus Kisutch) Persisted in the Nooksack and Samish rivers Despite Continuous Hatchery Production Throughout the Past Century?

For over a century, Washington State Department of Fish and Wildlife has implemented hatchery programs as a means to boost salmon abundance. Concerns have developed that native populations may be replaced by hatchery strains, decreasing the genetic diversity required to respond to environmental changes. We report a comparison of microsatellite DNA variation in wild-spawning and hatchery-strain coho salmon from the Nooksack and Samish rivers in northern Puget Sound. Significant heterogeneity in genotype frequencies was detected between wild-spawning coho salmon from the upper North Fork (NF) Nooksack River and hatchery-strain coho salmon from the Nooksack River (descendants of primarily Nooksack River broodstock). Little difference in genotype frequencies was detected between wild-spawning coho salmon from the Samish River and hatchery-strain coho salmon from the Nooksack River. The 13-locus suite provided high resolution: in assignment tests over 85% of wild-spawning coho salmon from the upper NF Nooksack River were assigned to source. Wild-spawning coho salmon collected below hatcheries in the Nooksack River and 50% of wild-spawning Samish River coho salmon were assigned to hatchery collections. The genetic divergence of wild-spawning coho salmon in the upper NF Nooksack River is remarkable given the extensive stocking history and proximity of a hatchery. We suggest that these upper river fish are native coho salmon and that wild spawners in the lower Nooksack and Samish River are descendants of hatchery productions. We attribute divergence to earlier run timing in upper NF Nooksack River wild spawners, availability of extensive spawning and rearing habitat upstream of a hatchery in the upper NF Nooksack River, and a longer stocking history in the Samish River.     

An overview of salmon enhancement and the need to manage and monitor natural spawning in Hokkaido, Japan

The chum and pink salmon catches in Hokkaido, Japan have increased dramatically since the 1970s and the 1990s, respectively. In contrast, masu salmon catches have been steadily decreasing. Despite intensive hatchery development in Hokkaido, naturally spawning salmon populations persist based on results from a recent river survey. This paper focuses on the challenges of maintaining hatchery salmon populations while protecting natural chum, pink and masu salmon populations in Hokkaido. Two important initiatives related to meeting this ambitious goal are managing hatcheries in a way that minimizes negative interactions between natural and hatchery salmon populations, and initiating new efforts at restoring and rehabilitating degraded freshwater habitats. In addition, in order to maintain a balance of demand and supply in the domestic market through the exportation of extra salmon, Hokkaido has decided to enter full assessment to gain Marine Stewardship Council (MSC) certification of the Hokkaido chum salmon trap net fishery. This would involve a fundamental shift in fisheries management as practiced in Japan, specifically elevating the importance of managing the fishery in a way that conserves natural salmon populations. A key component of a new salmon management strategy is the establishment of a zone management framework based on the designation of stream units to spatially separate natural salmon from hatchery salmon to minimize negative effects of hatchery fish and to utilize effectively hatchery salmon for commercial fisheries. This effort is allied with similar initiatives in other Pacific Rim countries that are focusing on management reform to restore natural ecosystem function and maintain the coexistence of wild and hatchery salmon.     

Interregional differentiation of chum salmon from Sakhalin and South Kurils inferred from microsatellite markers

Variability at ten microsatellite loci was examined in wild and hatchery populations of chum salmon (Oncorhynchus keta Walbaum) from the Sakhalin Island and Southern Kuril Islands, Iturup and Kunashir. Substantial genetic differences between Sakhalin and South Kurils chum salmon (the differentiation reached 6.0%) were revealed. Statistically significant differences between chum salmon from Iturup and those from Kunashir were demonstrated, as well as between the chum salmon populations from different rivers within the islands. It was shown that in different types of population comparisons, different marker sets were most informative.     

Interregional differentiation of chum salmon from Sakhalin and South Kurils infered from microsatellite markers

Variability at ten microsatellite loci was examined in wild and hatchery populations of chum salmon from the Sakhalin Island and Southern Kuril Islands, Iturup and Kunashir. Substantial genetic differences between Sakhalin and South Kurils chum salmon (the differentiation theta reached 6.0%) were revealed. Statistically significant differences between chum salmon from Iturup and that from Kunashir were demonstrated, as well as between the chum salmon populations from different rivers within the islands. It was shown that in different types of population comparisons, required different marker sets most informative were.     

Understanding the adaptive consequences of hatchery-wild interactions in Alaska salmon

About 31% of salmon harvested in Alaska comes from the hatchery production of hundreds of millions of pink and chum salmon and smaller numbers of sockeye, Chinook, and coho salmon. The numbers of hatchery-reared juveniles released in some areas are greater than the numbers of juveniles from wild populations. However, virtually nothing is known about the effects of hatchery fish on wild populations in Alaska. Possible effects of these interactions can be inferred from studies of salmonids in other areas, from studies of other animals, and from theory. Numerous studies show a complex relationship between the genetic architecture of a population and its environment. Adaptive responses to nature and anthropogenic selection can be influenced by variation at a single gene, or more often, by the additive effects of several genes. Studies of salmonids in other areas show that hatchery practices can lead to the loss of genetic diversity, to shifts in adult run timing and earlier maturity, to increases in parasite load, to increases in straying, to altered levels of boldness and dominance, to shifts in juvenile out-migration timing, and to changes in growth. Controlled experiments across generations show, and theory predicts, that the loss of adaptive fitness in hatchery salmon, relative to fitness in wild salmon, can occur on a remarkably short time scale. All of these changes can influence survival and impose selective regimes that influence genetically based adaptive traits. The preservation of adaptive potential in wild populations is an important buffer against diseases and climate variability and, hence, should be considered in planning hatchery production levels and release locations. The protection of wild populations is the foundation for achieving sustained harvests of salmon in Alaska.     

Genetic Structure of Chum Salmon (Oncorhynchus keta) Populations in the Lower Columbia River: Are Chum Salmon in Cascade Tributaries Remnant Populations?

The lower Columbia River drainage once supported a run of over a million chum salmon. By the late 1950s, the run had decreased to often a few hundred fish. With the exception of Grays River near the coast and an aggregation of chum salmon spawning in creeks and the main stem near Bonneville Dam in the Columbia Gorge, most populations were thought to be extinct. However, chum salmon consistently return in low numbers to tributaries originating in the Cascade Range: the Cowlitz, Lewis, and Washougal rivers. To assess whether Cascade spawners were strays or remnants of former populations, chum salmon from the Coastal, Cascade and Gorge ecoregional zones were characterized at 17 microsatellite loci. Significant heterogeneity in genotype distributions was detected between zones and collections formed regional groups in a neighbor-joining tree. Cascade collections had higher allelic richness and private alleles, and the Cowlitz River supported genetically divergent fall and summer runs, the only summer chum salmon run extant in the Columbia River drainage. We propose that chum salmon in the Cascade zone are remnants of original populations. We attribute the divergence between zonal groups to diverse ecological conditions in each zone, which promoted regional genetic adaptation, and to genetic drift experienced in small populations.     

Lack of trophic competition among wild and hatchery juvenile chum salmon during early marine residence in Taku Inlet,Southeast Alaska

Early marine trophic interactions of wild and hatchery chum salmon (Oncorhynchus keta) were examined as a potential cause for the decline in harvests of adult wild chum salmon in Taku Inlet, Southeast Alaska. In 2004 and 2005, outmigrating juvenile chum salmon were sampled in nearshore habitats of the inlet (spring) and in epipelagic habitat at Icy Strait (summer) as they approached the Gulf of Alaska. Fish were frozen for energy density determination or preserved for diet analyses, and hatchery stocks were identified from the presence of thermal marks on otoliths. We compared feeding intensity, diets, energy density, and size relationships of wild and hatchery stocks (n = 3123) across locations and weeks. Only hatchery fish feeding intensity was negatively correlated with fish abundance. In both years, hatchery chum salmon were initially larger and had greater energy density than wild fish, but lost condition in early weeks after release as they adapted to feeding on wild prey assemblages. Diets differed between the stocks at all inlet locations, but did not differ for hatchery salmon between littoral and neritic habitats in the outer inlet, where the stocks overlapped most. Both diets and energy density converged by late June. Therefore, if density-dependent interactions affect wild chum salmon, these effects must be very rapid because survivors in Icy Strait showed few differences. Our study also demonstrates that hatchery release strategies used near Taku Inlet successfully promote early spatial segregation and prey partitioning, which reduce the probability of competition between wild and hatchery chum salmon stocks.     

Fecundity and egg size variation in North American Pacific salmon (Oncorhynchus)

We examined regional and latitudinal variation in fecundity and egg weight for five species of Pacific salmon ( Oncorhynchus ) along the Pacific coast of North America. Data were examined for 24 chum salmon, 15 pink salmon, 34 sockeye salmon, 44 chinook salmon, and 40 coho salmon populations from published sources, unpublished Canadian hatchery records, our own laboratory investigations, and other unpublished sources. Substantial regional variation in fecundity and egg weight was observed, with salmon on the Queen Charlotte Islands and Vancouver Island in British Columbia generally having lower fecundity and larger egg size than nearby mainland populations. The relative distance of freshwater migration to the spawning grounds generally had a marked effect on both fecundity and egg size, with populations spawning in the upper portions in the drainages of large rivers like the Fraser River in British Columbia having reduced fecundity and egg size compared with coastal spawning populations. Fecundity was generally higher and egg size generally lower in more northern populations of sockeye, chinook, and coho salmon compared with southern ones. We suggest that egg size tends to be lower in northern populations of some species as a result of increased fecundity due to their older ages at maturity and a limited amount of energy that can be expended on egg production.     

Atlantic salmon straying from the River Imsa

Mean estimated straying rate for Atlantic salmon Salmo salar L. leaving the River Imsa as smolts during 1976–1999 was 15% for hatchery fish and 6% for wild conspecifics. Hatchery Atlantic salmon selected for production traits during four or more generations strayed >50%. The straying rate was higher for Atlantic salmon staying 2 rather than 1 year at sea before attaining maturity. For spawning, 96% of the strays entered streams within 420 km from the River Imsa, and c . 80% entered streams within 60 km of the mouth of the River Imsa, whether the fish were wild or hatchery released. Within the 60 km zone, the number of strays caught in a river increased with the Atlantic salmon catch in that river, but there was no significant relationship between straying rate and water discharge or distance from the river to the River Imsa. The observed straying rate of hatchery Atlantic salmon decreased with increasing number of fish entering the River Imsa. Sexual maturation as parr did not influence the tendency to stray. The results suggest that the establishment of temporary zones, free of fish farms, outside important Atlantic salmon rivers by the fisheries authorities in Norway should be large, whole fjords, to be effective.     

Divergence of the seasonal races of chum salmon, <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> Walbaum, 1792, in the Amur and Poronai rivers: Ecology,genetics, and morphology

The chum salmon of the Amur River (the mainland part of the Far East) and the Poronai River (Terpeniya Bay, Sakhalin Island) are historically related to one another, as the drainage basins of these rivers are the remnants of a formerly single river system, the Paleoamur, which existed when Sakhalin Island was a part of the continent. Both river populations of chum salmon consist of the early-run and late-run ecological forms (seasonal races), which are also referred to as the summer and autumn races. They are reproductively isolated from each other due to their spawning at different times and in different types of spawning grounds. To assess the direction, pattern, and degree of divergence between these chum salmon races in the both river fragments since the Paleoamur, it is necessary to compare them using two types of traits: selectively neutral DNA markers and morphological and physiological traits, variations in which may have an adaptive value. For this, we have studied chum salmon from both rivers in terms of microsatellite DNA markers, body counts and measurements, body weight, and fecundity. Both in the Amur River and in the Poronai River, the autumn race of chum salmon prevails over the summer race in body length and weight, fecundity, number of pyloric caeca, and several other meristic traits. The intra-basin differences between the races are much more pronounced in the Amur chum salmon. The inter-race differences in microsatellites are also greater in the Amur chum salmon compared to the Poronai chum salmon. Using microsatellites, three levels of differentiation have been revealed: (1) between the basins of the Amur and Poronai rivers, (2) between the races within each of the river basins, (3) and between population samples within each race of each basin. A hypothesis is proposed that the currently existing races of chum salmon in the Amur and Poronai rivers have evolved since the breakup of the Paleoamur, and the intra-basin divergence of the races started in the Amur River earlier than in the Poronai River. An analysis of our own data and the published data suggests that the adaptation of the seasonal races of chum salmon to the conditions of their spawning grounds is determined by a complex of morphological and physiological traits, including the number of pyloric caeca, which is an adaptive and highly heritable trait associated with the incubation temperature of the water.