The genetic population structure of lacustrine sockeye salmon, <Emphasis Type="Italic">Oncorhynchus nerka,</Emphasis> in Japan as the endangered species

Lacustrine sockeye salmon (Oncorhynchus nerka) are listed as an endangered species in Japan despite little genetic information on their population structure. In order to clarify the genetic diversity and structure of Japanese populations for evaluating on the bottleneck effect and an endangered species, we analyzed the ND5 region of mitochondrial DNA (mtDNA) and 45 single nucleotide polymorphisms (SNPs) in 640 lacustrine sockeye salmon in Japan and 80 anadromous sockeye salmon in Iliamna Lake of Alaska. The genetic diversity of the Japanese population in both mtDNA and SNPs was significantly less than that of the Iliamna Lake population. Moreover, all Japanese populations had SNP loci deviating from the HWE. In spite of low genetic diversity, the SNP analyses resulted that the Japanese population was significantly divided into three groups. These suggest that Japanese sockeye salmon populations should be protected as an endangered species and genetically disturbed by the hatchery program and transplantations.     

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.     

The impact of supplementation in winter-run chinook salmon on effective population size

Supplementation of young raised at a protected site, such as a hatchery, may influence the effective population size of an endangered species. A supplementation program for the endangered winter-run chinook salmon from the Sacramento River, California, has been releasing fish since 1991. A breeding protocol, instituted in 1992, seeks to maximize the effective population size from the captive spawners by equaling their contributions to the released progeny. As a result, the releases in 1994 and 1995 appear not to have decreased the overall effective population size and may have increased it somewhat. However, mistaken use of non-winter-run chinook spawners resulted in artificial crosses between runs with a potential reduction in effective population size, and imprinting of the released fish on Battle Creek, the site of the hatchery, resulted in limiting the contribution of the released fish to the target mainstem population. Rapid genetic analysis of captured spawners and a new rearing facility on the Sacramento River should alleviate these problems and their negative effect on the effective population size in future years.     

Use of genetic tags to identify captive-bred pallid sturgeon (<Emphasis Type="Italic">Scaphirhynchus albus</Emphasis>) in the wild: improving abundance estimates for an endangered species

The ability to distinguish captive-bred and natural-origin individuals in the wild is critical for evaluating the impact of captive breeding programs on natural populations. Continued persistence of endangered pallid sturgeon (Scaphirhynchus albus) in the Missouri River is largely dependent on captive breeding efforts that spawn natural-origin adults in fish hatcheries and release their progeny into the wild. Prior to release, hatchery-origin individuals are physically marked so they can be distinguished from natural-origin individuals when recaptured. During the years 2004–2006, 24 unmarked juvenile pallid sturgeon tissue samples were collected in the Missouri River downstream of Gavins Point Dam, South Dakota, USA that were presumed natural-origin. However, these individuals were similar in size to hatchery-origin fish released in this area raising concerns that these individuals were actually hatchery-origin fish with lost or malfunctioning tags. We used microsatellite based parentage analysis to determine if the unmarked fish were members of hatchery families that had been released in this area. This retrospective genetic tagging approach revealed that 23 of 24 unmarked fish were indeed hatchery-origin. The origin of the remaining individual remains unknown because genetic samples were not available from all of the families released below the dam and the unassigned fish may have originated from one of these un-sampled families. These results provide important insight into the conservation status of endangered pallid sturgeon as well as provide data important for guiding management decisions. Our results also demonstrate the efficacy of using genetic tags as an alternative or complimentary approach to physically marking individuals.     

Reduced reproductive success of hatchery coho salmon in the wild: insights into most likely mechanisms

Supplementation of wild salmonids with captive-bred fish is a common practice for both commercial and conservation purposes. However, evidence for lower fitness of captive-reared fish relative to wild fish has accumulated in recent years, diminishing the apparent effectiveness of supplementation as a management tool. To date, the mechanism(s) responsible for these fitness declines remain unknown. In this study, we showed with molecular parentage analysis that hatchery coho salmon (Oncorhynchus kisutch) had lower reproductive success than wild fish once they reproduced in the wild. This effect was more pronounced in males than in same-aged females. Hatchery spawned fish that were released as unfed fry (age 0), as well as hatchery fish raised for one year in the hatchery (released as smolts, age 1), both experienced lower lifetime reproductive success (RS) than wild fish. However, the subset of hatchery males that returned as 2-year olds (jacks) did not exhibit the same fitness decrease as males that returned as 3-year olds. Thus, we report three lines of evidence pointing to the absence of sexual selection in the hatchery as a contributing mechanism for fitness declines of hatchery fish in the wild: (i) hatchery fish released as unfed fry that survived to adulthood still had low RS relative to wild fish, (ii) age-3 male hatchery fish consistently showed a lower relative RS than female hatchery fish (suggesting a role for sexual selection), and (iii) age-2 jacks, which use a sneaker mating strategy, did not show the same declines as 3-year olds, which compete differently for females (again, implicating sexual selection).     

Comparison of maturation timing,egg size and fecundity between hatchery lines of chinook salmon and their wild donor stocks

Increasing concern has been expressed about the genetic effects of cultured salmonid fishes on natural populations. Avoidance of extreme negative outcomes was one reason for the establishment of a genetic management policy for the State of Alaska. However, domestication within the hatchery may still cause divergence from the wild donor population. This divergence could potentially lead to adverse impacts on wild stocks through straying and introgression. This study examines potential domestication in two Alaskan chinook salmon stocks. The Little Port Walter (LPW) Hatchery Chickamin River stock resulted from a small collection of wild broodstock in 1976. The LPW Unuk stock was founded with a larger number of individuals in 1976 and has had subsequent infusion of wild gametes. These lines have been maintained at LPW through ocean ranching of tagged smolts. Comparisons are made between the hatchery lines, progeny of wild chinook collected from the Chickamin and Unuk Rivers, and hybrids between the hatchery and wild groups. Mature ocean‐ranched female chinook salmon returning to the facility were periodically graded for ripeness and spawned. Body size and meristic measurements were collected from these mature spawners. Maturation timing, fecundity, and individual egg size of these fourth generation hatchery fish are compared with that of offspring of wild fish from the same donor stock. Stock of origin is confirmed for all spawners and offspring using microsatellite DNA analysis.     

Consequences to fitness‐related traits of hybridization between farmed and wild Atlantic salmon, Salmo salar

Increasing concern has been expressed about the genetic effects of cultured salmonid fishes on natural populations. Avoidance of extreme negative outcomes was one reason for the establishment of a genetic management policy for the State of Alaska. However, domestication within the hatchery may still cause divergence from the wild donor population. This divergence could potentially lead to adverse impacts on wild stocks through straying and introgression. This study examines potential domestication in two Alaskan chinook salmon stocks. The Little Port Walter (LPW) Hatchery Chickamin River stock resulted from a small collection of wild broodstock in 1976. The LPW Unuk stock was founded with a larger number of individuals in 1976 and has had subsequent infusion of wild gametes. These lines have been maintained at LPW through ocean ranching of tagged smolts. Comparisons are made between the hatchery lines, progeny of wild chinook collected from the Chickamin and Unuk Rivers, and hybrids between the hatchery and wild groups. Mature ocean‐ranched female chinook salmon returning to the facility were periodically graded for ripeness and spawned. Body size and meristic measurements were collected from these mature spawners. Maturation timing, fecundity, and individual egg size of these fourth generation hatchery fish are compared with that of offspring of wild fish from the same donor stock. Stock of origin is confirmed for all spawners and offspring using microsatellite DNA analysis.     

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     

Founding events influence genetic population structure of sockeye salmon (Oncorhynchus nerka) in Lake Clark, Alaska

Bottlenecks can have lasting effects on genetic population structure that obscure patterns of contemporary gene flow and drift. Sockeye salmon are vulnerable to bottleneck effects because they are a highly structured species with excellent colonizing abilities and often occupy geologically young habitats. We describe genetic divergence among and genetic variation within spawning populations of sockeye salmon throughout the Lake Clark area of Alaska. Fin tissue was collected from sockeye salmon representing 15 spawning populations of Lake Clark, Six-mile Lake, and Lake Iliamna. Allele frequencies differed significantly at 11 microsatellite loci in 96 of 105 pairwise population comparisons. Pairwise estimates of FST ranged from zero to 0.089. Six-mile Lake and Lake Clark populations have historically been grouped together for management purposes and are geographically proximate. However, Six-mile Lake populations are genetically similar to Lake Iliamna populations and are divergent from Lake Clark populations. The reduced allelic diversity and strong divergence of Lake Clark populations relative to Six-mile Lake and Lake Iliamna populations suggest a bottleneck associated with the colonization of Lake Clark by sockeye salmon. Geographic distance and spawning habitat differences apparently do not contribute to isolation and divergence among populations. However, temporal isolation based on spawning time and founder effects associated with ongoing glacial retreat and colonization of new spawning habitats contribute to the genetic population structure of Lake Clark sockeye salmon. Nonequilibrium conditions and the strong influence of genetic drift caution against using estimates of divergence to estimate gene flow among populations of Lake Clark sockeye salmon.     

Long-term captive breeding does not necessarily prevent reestablishment: lessons learned from Eagle Lake rainbow trout

Captive breeding of animals is often cited as an important tool in conservation, especially for fishes, but there are few reports of long-term (<50 years) success of captive breeding programs, even in salmonid fishes. Here we describe the captive breeding program for Eagle Lake rainbow trout, Oncorhynchus mykiss aquilarum, which is endemic to the Eagle Lake watershed of northeastern California. The population in Eagle Lake has been dependent on captive breeding for more than 60 years and supports a trophy fishery in the lake. Nevertheless, the basic life history, ecological, and genetic traits of the subspecies still seem to be mostly intact. Although management has apparently minimized negative effects of hatchery rearing, reestablishing a wild population would ensure maintenance of its distinctive life history and its value for future use as a hatchery fish. An important factor that makes reestablishment possible is that the habitat in Eagle Lake is still intact and that Pine Creek, its major spawning stream, is recovering as habitat. With the exception of an abundant alien brook trout (Salvelinus fontinalis) population in Pine Creek, the habitat factors that led to the presumed near-extinction of Eagle Lake rainbow trout in the early twentieth century have been ameliorated, although the final stages of reestablishment (eradication of brook trout, unequivocal demonstration of successful spawning migration) have still not been completed. The Eagle Lake rainbow trout story shows that long-term captive breeding of migratory salmonid fishes does not necessarily prevent reestablishment of wild populations, provided effort is made to counter the effects of hatchery selection and that natural habitats are restored for reintroduction. Long-term success, however, ultimately depends upon eliminating hatchery influences on wild-spawning populations. Extinction of Eagle Lake rainbow trout as a wild species becomes increasingly likely if we fail to act boldly to protect it and the Eagle Lake watershed.     

Lake‐specific variation in growth,migration timing and survival of juvenile sockeye salmon Oncorhynchus nerka: separating environmental from genetic influences

Time series on juvenile life‐history traits obtained from sockeye salmon Oncorhynchus nerka were analysed to assess lake‐specific environmental influences on juvenile migration timing, size and survival of fish from a common gene pool. Every year for the past two decades, O. nerka have been spawned at a hatchery facility, and the progeny released into two lakes that differ in average summer temperatures, limnological attributes and growth opportunities. Juveniles reared in the warmer, more productive Crosswind Lake were larger and heavier as smolts compared to those from the cooler, less productive Summit Lake and had higher in‐lake and subsequent marine survival. Crosswind Lake smolts migrated from the lake to sea slightly earlier in the season but the migration timing distributions overlapped considerably across years. Fry stocking density had a negative effect on smolt length for both lakes, and a negative effect on in‐lake survival in Summit Lake. Taken together, the results revealed a strong effect of lake‐rearing environment on the expression of life‐history variation in O. nerka. The stocking of these lakes each year with juveniles from a single mixed‐source population provided a large‐scale reverse common‐garden experiment, where the same gene pool was exposed to different environments, rather than the different gene pools in the same environment approach typical of evolutionary ecology studies. Other researchers are encouraged to seek and exploit similar serendipitous situations, which might allow environmental and genetic influences on ecologically important traits to be distinguished in natural or semi‐natural settings.     

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.     

Efficacy of releasing captive reared broodstock into an imperilled wild Atlantic salmon population as a recovery strategy

The strategy of releasing captive reared adult Atlantic salmon Salmo salar into the Magaguadavic River, New Brunswick, Canada, to spawn, was not an effective tool for rebuilding a seriously depressed wild population. The fish were first generation progeny from wild parents, and had spent their entire lives in captivity in either sea or fresh water. No differences in movement or behaviour patterns were observed between freshwater and seawater reared groups. Fish released in the lower river early (35 to 80 days prior to the natural spawning period) moved into a lake low in the system, and most stayed there near the commercial hatchery where they had been reared from egg to smolt. During the spawning season, none moved to the upper river reaches where most spawning habitat exists. Most broodstock released in the upper river reaches near the time of spawning stayed there during the spawning period. The following year few to no Atlantic salmon fry were found, and most appeared not to be offspring of released adults.     

Conservation of population structure and genetic diversity under captive breeding of remnant coaster brook trout (<Emphasis Type="Italic">Salvelinus fontinalis</Emphasis>) populations

Rebuilding wild populations often involves captive broodstocks derived from small, remnant populations. We measured a hatchery program’s ability to conserve genetic diversity when founding captive broodstocks from such populations. Migratory coaster brook trout were extirpated from most of their historic range in US waters of Lake Superior and were proposed for listing under the Endangered Species Act. Two captive broodstocks, one with 19 founders and another with 99 founders, were established to rebuild US populations. We used microsatellite markers to examine genetic variation in source populations and early hatchery generations. Broodstocks retained the strong differentiation found between source populations; however, one founder, with a low probability of belonging to either source population, sired 5.7% of F₁ progeny. We found small changes in within-population genetic variation across successive wild and hatchery generations of broodstocks. Evaluation of stage-specific survivorship indicated that equalizing family sizes of embryos produced modest gains in the effective number of breeders, and that survival in the hatchery was nearly random across families. Our study demonstrates the value of genetic monitoring during initial stages of hatchery programs for small and declining populations.     

Genetic monitoring of the Amazonian fish matrinchã (Brycon cephalus) using RAPD markers: insights into supportive breeding and conservation programmes

The importance of genetic evaluations in aquaculture programmes has been increased significantly not only to improve effectiveness of hatchery production but also to maintain genetic diversity. In the present study, wild and captive populations of a commercially important neotropical freshwater fish, Brycon cephalus (Amazonian matrinchã), were analyzed in order to evaluate the levels of genetic diversity in a breeding programme at a Brazilian research institute of tropical fish. Random Amplified Polymorphic DNA fingerprinting was used to access the genetic variability of a wild stock from the Amazon River and of three captive stocks that correspond to consecutive generations from the fishery culture. Although farmed stocks showed considerably lower genetic variation than the wild population, a significantly higher level of polymorphism was detected in the third hatchery generation. The results seem to reflect a common breeding practice on several hatchery fish programmes that use a small number of parents as broodstocks, obtaining reproductive success with few non‐identified mating couples. The obtained data were useful for discussing suitable strategies for the genetic management and biodiversity conservation of this species.     

Captive rearing for Chinook salmon (Oncorhynchus tshawytscha) and Atlantic salmon (Salmo salar): the Idaho and Maine experiences

Captive rearing is a conservation strategy where juveniles are collected from the natural environment, reared to maturity in a hatchery environment, and then released back into the natural environment at maturity for volitional spawning. This strategy has been used to produce adult outplants for stock enhancement where natural escapement is poor or capture of adults is difficult. In both Idaho (Chinook salmon, Oncorhynchus tshawytscha) and Maine (Atlantic salmon, Salmo salar), captive rearing programs have been initiated as an experimental strategy to prevent cohort collapse and conserve genetic integrity of select depressed populations. In this paper, we provide an overview of these programs and describe some of the methods used to evaluate the effectiveness of this approach. Behaviors such as habitat selection, courting, and spawn timing were monitored. Data collected for both programs indicate that the captive fish display similar behaviors as their wild conspecifics in terms of habitat selection and spawning, although there were some differences in spawn timing. Evaluations of egg and fry production also indicate that captive-reared adults are successfully spawning and producing offspring. Each program is still waiting on final evaluations of reproductive success through genetic analyses of returning adults, but results so far indicate that this could be an additional captive propagation strategy for depressed populations.     

Some consequences of Pacific salmon hatchery production in Kamchatka: changes in age structure and contributions to natural spawning populations

Aggregate hatchery production of Pacific salmon in the Kamchatka region of the Russian Federation is very low (< 0.5% of total harvest, with five hatcheries releasing approximately 41 M juvenile salmon annually), but contributions in certain rivers can be substantial. Enhancement programs in these rivers may strongly influence fitness and production of wild salmon. In this paper we document significant divergence in demographic traits in hatchery salmon populations in the Bolshaya River and we estimate the proportion of hatchery chum salmon in the total run in the Paratunka River to demonstrate the magnitude of enhancement in this system. We observed a reduction in the expression of life history types in hatchery populations (ranging from 1 to 9 types) compared to wild populations (17 types) of sockeye salmon in the Bolshaya River. We found similar trends in Chinook salmon in the same river system. This reduced life history diversity may make these fish less resilient to changes in habitat and climate. We estimate hatchery chum salmon currently contribute 17-45% to the natural spawning population in the Paratunka River. As hatchery fish increase in numbers at natural spawning sites, this hatchery production may affect wild salmon production. It is important to investigate the risk of introgression between hatchery and wild salmon that can lead to reduction in salmon fitness in Kamchatka rivers, as well as the potential of ecological interactions that can have consequences on status of wild salmon and overall salmon production in this region.     

Environmental rearing conditions produce forebrain differences in wild Chinook salmon Oncorhynchus tshawytscha

Recent studies suggest that hatchery-reared fish can have smaller brain-to-body size ratios than wild fish. It is unclear, however, whether these differences are due to artificial selection or instead reflect differences in rearing environment during development. Here we explore how rearing conditions influence the development of two forebrain structures, the olfactory bulb and the telencephalon, in juvenile Chinook salmon (Oncorhynchus tshawytscha) spawned from wild-caught adults. First, we compared the sizes of the olfactory bulb and telencephalon between salmon reared in a wild stream vs. a conventional hatchery. We next compared the sizes of forebrain structures between fish reared in an enriched NATURES hatchery and fish reared in a conventional hatchery. All fish were size-matched and from the same genetic cohort. We found that olfactory bulb and telencephalon volumes relative to body size were significantly larger in wild fish compared to hatchery-reared fish. However, we found no differences between fish reared in enriched and conventional hatchery treatments. Our results suggest that significant differences in the volume of the olfactory bulb and telencephalon between hatchery and wild-reared fish can occur within a single generation.     

Effective size of a wild salmonid population is greatly reduced by hatchery supplementation

Many declining and commercially important populations are supplemented with captive-born individuals that are intentionally released into the wild. These supplementation programs often create large numbers of offspring from relatively few breeding adults, which can have substantial population-level effects. We examined the genetic effects of supplementation on a wild population of steelhead (Oncorhynchus mykiss) from the Hood River, Oregon, by matching 12 run-years of hatchery steelhead back to their broodstock parents. We show that the effective number of breeders producing the hatchery fish (broodstock parents; N(b)) was quite small (harmonic mean N(b)=25 fish per brood-year vs 373 for wild fish), and was exacerbated by a high variance in broodstock reproductive success among individuals within years. The low N(b) caused hatchery fish to have decreased allelic richness, increased average relatedness, more loci in linkage disequilibrium and substantial levels of genetic drift in comparison with their wild-born counterparts. We also documented a substantial Ryman-Laikre effect whereby the additional hatchery fish doubled the total number of adult fish on the spawning grounds each year, but cut the effective population size of the total population (wild and hatchery fish combined) by nearly two-thirds. We further demonstrate that the Ryman-Laikre effect is most severe in this population when (1) >10% of fish allowed onto spawning grounds are from hatcheries and (2) the hatchery fish have high reproductive success in the wild. These results emphasize the trade-offs that arise when supplementation programs attempt to balance disparate goals (increasing production while maintaining genetic diversity and fitness).     

Factors affecting the outcome of territorial contests between hatchery and naturally reared coho salmon parr in the laboratory

In aquarium experiments using coho salmon as a model species, prior residents dominated intruders of the same size but intruders with a 6% length advantage were equally matched against prior residents. Prior winning experience (distinct from individual recognition) also strongly influenced competitive success and overcame a prior residence effect. Coho salmon reared in a hatchery dominated size-matched fish from the same parental population reared in a stream. Hatchery-reared salmon also dominated naturally spawned salmon, even when the wild salmon were prior residents. Thus the combined effects of greater size and rearing experience of hatchery-produced salmon were sufficient to overcome a wild salmon's advantage of prior residence. Efforts to rehabilitate salmonid populations must consider such behavioural interactions if displacement of wild fish is to be prevented.