Cannibalism in non‐native brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss stream‐dwelling populations

Introduced and allopatric populations of brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss were sampled in Slovenia for stable isotope analysis to assess dietary niche shifts through ontogeny and estimate the propensity for cannibalism. Both S. trutta and O. mykiss are cannibals, with higher average relative contribution of conspecific assimilated energy for S. trutta (27·9%) compared with O. mykiss (7·7%). The smallest cannibal was 166 mm in the S. trutta population and 247 mm in the O. mykiss population.     

The complete history of salmonid introductions in the Kerguelen Islands, Southern Ocean

Since the early 1950s, several species of salmonids have been introduced more or less successfully in the Kerguelen Islands, a 7,215 km² archipelago located in the Southern Ocean (49°S, 70°E) and previously devoid of any freshwater fish. The aim of this work was to establish a documented chronicle of these events from available archives, to better understand the causes of the colonization failure or success for the different species. The history that emerged from the analysis of the archives appeared much more complex than previously published. Stocks of various origins were used, and numerous attempts were made at different sites involving variable numbers of fish released at different life stages. Between 1951 and 1991, 22 importation attempts took place, involving about 2 million individuals. Of the 8 species introduced (Salmo trutta, S. salar, Oncorhynchus mykiss, O. tshawytscha, O. kisutch, Salvelinus namaycush, S. fontinalis and S. alpinus), only 3 failed to establish local populations (O. mykiss, O. tshawytscha and S. namaycush). Overall, 23 watersheds were stocked. At present, 45 watersheds are colonized by one or several species. S. trutta, S. fontinalis, S. alpinus and O. kisutch were capable of migrating toward new habitats. The brown trout (S. trutta) was the only species to colonize a large number of watersheds (32 in about 10 generations). Its success can be explained by the diversity of origins, the number and importance of introduction and transfer attempts, the diversity of release sites and the peculiarities of its life cycle.     

Towards using thermal stress thresholds to predict salmonid invasion potential

The salmonids rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta are amongst the world’s worst invaders with devastating impacts where introduced. Information on their invasive potential is key to predicting range expansion in recipient environments. Salmonids were introduced into the Keiskamma River system, South Africa in the late 1890s, established and spread throughout its headwaters. This provided an opportunity to document their distribution in relation to stream thermal regimes. Salmonids conformed to documented thermal tolerances predominantly occupying cool-water sites (13.7 ± 3.5 °C). Salmonid probability of occurrence was assessed using seven-day moving average of daily mean water temperatures (MWAT) and exceedances of 100 and 59 days reduce the probability of occurrence for S. trutta and O. mykiss to 0.5 respectively, with probability of occurrence falling to zero at 150 days for both species. Greater tolerance of MWAT exceedance was observed in impoundment (233 days) rather than riverine (146 days) habitats. Epilimnetic releases from impoundments increased temporal heat loads and average temperatures were 3 °C higher below than immediately above impoundments. Salmonids did not occupy all sites within their thermal preferendum, indicating invasion debt and further potential to spread. These models highlight subtle ecological niche differences between both salmonids with O. mykiss more likely to survive warmer water temperatures than S. trutta, but at the expense of a lower altitudinal range and with a greater susceptibility to extinction with an increase in water temperatures. Aquaculture and fisheries enhancements using salmonids are increasing, and these results may contribute to risk assessments for salmonid invasions globally.     

The most variable vertebrate on Earth

In the search for the most variable non-human vertebrate on Earth, intraspecific variation of ten variable traits was compared among ten highly variable species. Mammals, birds and many reptiles, amphibians and fishes were excluded because most of the variation is among, and not within species. The focus was on northern fishes, where high intraspecific variation is well documented. The ten selected species were European whitefish Coregonus lavaretus, chinook salmon Oncorhyncus tshawytscha, sockeye salmon O. nerka, rainbow trout O. mykiss, atlantic salmon Salmo salar, brown trout S. trutta, arctic charr Salvelinus alpinus, brook charr S. fontinalis, dolly varden charr S. malma and threespine stickleback Gasterosteus aculeatus. Variation included not only size and phenotype, but also ecology, behaviour and life history. The traits were geographic range, migration, habitat, adult size, colour, body form, polymorphism, diet, reproduction and genetics. Arctic charr came on top in the final ranking, followed by dolly varden charr and rainbow trout. The two least variable were chinook salmon and threespine stickleback. It is proposed that arctic charr, which is also the northernmost fish on Earth, has evolved its unique variability in range, size, phenotype, ecology and life history by adapting to the extreme and highly unpredictable ecological conditions of arctic and other northern lakes for many glacial periods.     

Selenium Species and Their Distribution in Freshwater Fish from Argentina

The distribution and speciation of selenium (Se) in freshwater fish (muscle and liver tissue) from lakes in Argentina was investigated. Three introduced species, brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis), and one native species, creole perch (Percichthys trucha), were investigated. Values for total selenium in muscle ranged from 0.66 to 1.61 μg/g, while in the liver, concentrations were much higher, from 4.46 to 73.71 μg/g on a dry matter basis. Separation of soluble Se species (SeCys₂, selenomethionine (SeMet), SeMeSeCys, selenite and selenate) was achieved by ion exchange chromatography and detection was performed by inductively coupled plasma–mass spectrometry. The results showed that in fish muscle, from 47 to 55 % of selenium was soluble and the only Se species identified was SeMet, which represented around 80 % of soluble Se, while in the liver, the amount of soluble Se ranged from 61 to 76 % and the percentage of species identified (SeMet and SeCys₂) was much lower and ranged from 8 to 17 % of soluble Se.     

Density‐dependent effects of non‐native brown trout Salmo trutta on the species–area relationship in stream fish assemblages

The spatial scale and density‐dependent effects of non‐native brown trout Salmo trutta on species richness of fish assemblages were examined at 48 study sites in Mamachi Stream, a tributary of Chitose River, Hokkaido, Japan. The density of age ≥1 year S. trutta was high in the upstream side of the main stem of Mamachi Stream. Fish species richness increased with increasing area of study sites (habitat size), but the increasing magnitude of the species richness with area decreased with increasing age of ≥1 year S. trutta density. The relationships between age ≥1 year S. trutta, however, and presence–absence of each species seemed to be different among species. Species richness was also determined by location and physical environmental variables, i.e. it was high on the downstream side and in structurally complex environments.     

Sympatric relationship between redband trout and non-native brook trout in the Southeastern Oregon Great Basin

Brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) have been widely introduced outside their respective ranges within North America causing declines and displacement of native trout. Yet, successful coexistence of native and non-native trout has received little attention. Here we evaluated the effect of introduced brook trout on the size and density of native redband trout in two invaded sub-basins in southeastern Oregon. In a multi-year study, we investigated whether habitat and fish communities differed between streams and stream reaches where redband trout were allopatric versus where redband trout were sympatric with brook trout. We hypothesized that redband trout would be less dense and have smaller total length in sympatry with brook trout than in allopatry, but that total trout density would not differ. We investigated whether differences in habitat existed between sympatric and allopatric locations that would indicate differentiation in site level habitat preferences for each trout species. We found that sympatric locations had more wood but similar fish community structure. Mean length and densities of redband trout were higher at allopatric locations. However, in most years at sympatric locations total trout density was twice that of allopatric redband trout sites. Using comparable data from an eastern United States system where brook trout are native, sympatric sites had lower densities of brook trout; however, total trout density did not differ. We conclude that invading trout negatively impact native trout densities; but in southeastern Oregon system the negative impact is minimized.     

Life strategies of mykiss <Emphasis Type="Italic">Parasalmo mykiss</Emphasis> and trout <Emphasis Type="Italic">Salmo trutta</Emphasis> introduced in water bodies of Chili

Materials of preliminary investigations of the life strategy of mykiss Parasalmo mykiss and trout Salmo trutta introduced to water bodies of Chili are considered. Analysis of the structure of scales demonstrated that, in mykiss and trout, the estuarine and riverine-estuarine types of life strategy prevail. This seems to be related to good feeding conditions in fjords. The hypothesis is suggested that, in the area of Chilean fjords, the successful introduction to rivers and artificial rearing in cages in brackish water resulted in the formation of a peculiar and highly dynamic population system. Elements of this system are from both fish of natural self-reproducing populations and fish grown in cages. There is a continuous exchange with genetic information between them as the fish that have escaped from cages attain maturity, arrive to rivers for spawning, and reproduce together with wild fish.     

Comparison of competitive ability between native and introduced salmonids: evidence from pairwise contests

Brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, have been introduced to freshwaters in Hokkaido, Japan. Today, it is recognized that these introduced salmonids have negative impacts on native salmonids such as white-spotted charr, Salvelinus leucomaenis, and masu salmon, O. masou. In particular, interspecific competition may be an important mechanism that could contribute to the exclusion for native salmonids. In this study, experimental pairwise contests were conducted to compare interference competitive ability between native and introduced salmonids. We demonstrated that brown trout were competitively superior to white-spotted charr and masu salmon whereas rainbow trout were superior to white-spotted charr. We suggest that introduced brown trout negatively impact both white-spotted charr and masu salmon, and introduced rainbow trout negatively impact white-spotted charr.     

Population genetic structure and ancestry of steelhead/rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) at the extreme southern edge of their range in North America

Steelhead (Oncorhynchus mykiss) populations have declined dramatically in many parts of their range in North America, most critically in Southern California, where these anadromous trout are now classified as ‘Endangered’ under the United States Endangered Species Act. The widespread introduction of hatchery rainbow trout, the domesticated freshwater resident form of the species O. mykiss, is one factor threatening the long-term persistence of native steelhead and other trout populations. To identify where native fish of coastal steelhead lineage remained, we performed a population genetic analysis of microsatellite and SNP genotypes from O. mykiss populations at the extreme southern end of their range in Southern California, USA and Baja California, Mexico. In the northern part of this region, nearly all populations appeared to be primarily descendants of native coastal steelhead. However, in the southern, more urbanized part of this region, the majority of the sampled populations were derived primarily from hatchery trout, indicating either complete replacement of native fish or a strong signal of introgression overlaying native ancestry. Nevertheless, these genetically introgressed populations represent potentially critical genetic resources for the continued persistence of viable networks of O. mykiss populations, given the limited native ancestry uncovered in this region and the importance of genetic variation in adaptation. This study elucidates the geographic distribution of native trout populations in this region, and serves as a baseline for evaluating the impacts of hatchery trout on native O. mykiss populations and the success of steelhead conservation and recovery efforts.     

The importance of substrate size and interstitial space in the microhabitat selection by the stream-dwelling catfish Hatcheria macraei (Actinopterygii, Trichomycteridae)

The study tests whether diurnal microhabitat use by Hatcheria macraei depends upon specific environmental parameters and/or the abundance of other fish. We carried out a 1-year field study in a low-order river of northern Patagonia, Pichileufu River, and used experimental trials to determine substrate preferences. Fishes were captured during daylight and physicochemical environmental variables were recorded. Headwater zones were dominated by rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta), while native fishes, H. macraei and the creole perch (Percichthys trucha) were more abundant downstream. H. macraei inhabited mostly shallow microhabitats with fast water velocity and substrates having significant interstitial spaces, independently of the abundance of other fishes. Experimental trials pointed out that H. macraei preferred mostly coarser substrates (>6 cm), avoiding fine ones. This study highlights the importance of erosional zones with high water velocity, large substrates, and suitable interstitial space in the microhabitat selection of H. macraei. The knowledge of microhabitat use by native fish populations is critical for management and conservation strategies and should be taken into account before any river modification.     

Reciprocal trophic niche shifts in native and invasive fish: salmonids and galaxiids in Patagonian lakes

1. Rainbow (Oncorhynchus mykiss) and brown trout (Salmo trutta) are widespread and invasive salmonids with important lethal effects as predators, although indirect effects are also possible. We used stable isotope analyses (δ¹⁵N, δ¹³C) to explore how the density of invasive trout in 25 Patagonian lakes alters the trophic niche (TN) of a widespread native fish, Galaxias platei (Galaxiidae). We also explored how the density of the galaxiid influences the TN of invasive trout. 2. We quantified two aspects of the TN: (i) the proportion of littoral carbon (PL) and (ii) trophic height (TH) (i.e. the ‘height’ at which the fish feeds in the food web). We related these measures of TN in a given species to the density of other species (as estimated by catch‐per‐unit‐effort). 3. As G. platei body size increased, their PL increased (increasing littoral feeding) in several lakes. However, none of the fish species investigated showed changes in PL with increasing density of the other fish species. TH increased with body size in all three species. In addition, the TH of large G. platei declined with increasing trout density and, reciprocally, the TH of large S. trutta decreased with decreasing G. platei density. 4. The reciprocal effects of native and the invasive fish on TH were as large as a shift of one trophic level. This pattern is consistent with an exhaustion of galaxiid prey for both piscivorous G. platei and S. trutta in lakes with high trout density. 5. These finding support the suggested management strategy of culling trout from overpopulated lakes, which should simultaneously protect native fish and enhance a lucrative sport fishery for large trout.     

Migratory patterns of lower Feather River natural and hatchery-origin Oncorhynchus mykiss

Oncorhynchus mykiss are known to exhibit variable life histories expressed under genetic control or through the influence of the environment. Altered environmental conditions and the presence of large numbers of non-indigenous hatchery fish can affect the rate of anadromy observed in California’s Central Valley O. mykiss. The lower Feather River, a large regulated river in Northern California, supports one of the largest populations of O. mykiss in the Central Valley, heavily subsidized by hatchery production. I examined downstream migratory behavior, movement rate, and success of natural-origin juveniles, natural-origin adults, and hatchery-origin fish using acoustic telemetry. I found that all three groups displayed a mix of migratory and non-migratory life histories. Hatchery-origin adults exhibited more migratory behavior (anadromy) than natural-origin juveniles and natural-origin adults, 41 %, 24 %, and 13 % respectively. Downstream migrants were also significantly larger than non-migratory fish. Migration timing was similar among the three groups, but natural-origin juveniles displayed the most rapid rate of movement. The relative success of each group was highest for fish migrating through the Sacramento-San Joaquin Delta to the Pacific Ocean. Hatchery-origin adults migrating through the lower Feather River showed the lowest relative success (33 %). The dominance of non-migratory life histories pattern among lower Feather River O. mykiss may be in response to the suitable habitat provided by hypolimnetic releases below dams and the current hydrological regime within the highly altered Sacramento-San Joaquin watershed.     

Surface feeding and aggressive behaviour of diploid and triploid brown trout Salmo trutta during allopatric pair‐wise matchings

Diploid and triploid brown trout Salmo trutta were acclimated for 6 weeks on two feeding regimes (floating and sinking). Thereafter, aggression and surface feeding response were compared between pairs of all diploid, all triploid and diploid and triploid S. trutta in an experimental stream. In each pair‐wise matching, fish of similar size were placed in allopatry and rank was determined by the total number of aggressive interactions recorded. Dominant individuals initiated more aggression than subordinates, spent more time defending a territory and positioned themselves closer to the surface food source (Gammarus pulex), whereas subordinates occupied the peripheries. In cross ploidy trials, diploid S. trutta were more aggressive than triploid, and dominated their sibling when placed in pair‐wise matchings. Surface feeding, however, did not differ statistically between ploidy irrespective of feeding regime. Triploids adopted a sneak feeding strategy while diploids expended more time defending a territory. In addition, we also tested whether triploids exhibit a similar social dominance to diploids when placed in allopatry. Although aggression was lower in triploid pairs than in the diploid and triploid pairs, a dominance hierarchy was also observed between individuals of the same ploidy. Dominant triploid fish were more aggressive and consumed more feed items than subordinate individuals. Subordinate fish displayed a darker colour index than dominant fish suggesting increased stress levels. Dominant triploid fish, however, appeared to be more tolerant of subordinate individuals and did not display the same degree of invasive aggression as seen in the diploid and diploid or diploid and triploid matchings. These novel findings suggest that sterile triploid S. trutta feed similarly but are less aggressive than diploid trout. Future studies should determine the habitat choice of triploid S. trutta after release and the interaction between wild fish and triploids during the breeding season prior to utilization of triploids as an alternative management strategy within freshwater fisheries.     

Using stable isotope analysis to assess the effects of hypolimnetic oxygenation on diet in a mixed cold- and warmwater fish community

Line-diffuser hypolimnetic oxygenation was initiated in North Twin Lake, Washington, in 2009 to mitigate reductions in Rainbow Trout (Oncorhynchus mykiss) and Brook Trout (Salvelinus fontinalis) habitat due to temperature-dissolved oxygen “habitat squeeze”. Previous studies demonstrated that trout populations rapidly expanded into increased hypolimnetic habitat within the first few years of oxygenation and previous short-term diet analyses indicated an effect on fish diet; however, the long-term effects on fish ecology have yet to be established. In this study, stable isotope analysis of fish liver tissue suggests relatively few differences in feeding ecology of principal coldwater fish species in North Twin Lake compared to unoxygenated South Twin Lake. When compared between lakes, Rainbow Trout and Brook Trout diets contained similar proportions of Daphnia, Chironomidae, and Chaoboridae. Littoral and epilimnetic-focused Golden Shiner (Notemigonus crysoleucas) diets were also similar between lakes. Observed similarities between Golden Shiner and trout diets suggest the effects of interspecific competition between salmonids and non-salmonids may be limiting trout growth and survival. Fisheries managers should therefore consider both habitat limitations and interspecific competition when managing for coldwater fish species in mesotrophic, dimictic lakes.     

Differential invasion success of salmonids in southern Chile: patterns and hypotheses

Biological invasions create complex ecological and societal issues worldwide. Most of the knowledge about invasions comes only from successful invaders, but less is known about which processes determine the differential success of invasions. In this review, we develop a framework to identify the main dimensions driving the success and failure of invaders, including human influences, characteristics of the invader, and biotic interactions. We apply this framework by contrasting hypotheses and available evidence to explain variability in invasion success for 12 salmonids introduced to Chile. The success of Oncorhynchus mykiss and Salmo trutta seems to be influenced by a context-specific combination of their phenotypic plasticity, low ecosystem resistance, and propagule pressure. These well-established invaders may limit the success of subsequently introduced salmonids, with the possible exception of O. tshawytscha, which has a short freshwater residency and limited spatial overlap with trout. Although propagule pressure is high for O. kisutch and S. salar due to their intensive use in aquaculture, their lack of success in Chile may be explained by environmental resistance, including earlier spawning times than in their native ranges, and interactions with previously established and resident Rainbow Trout. Other salmonids have also failed to establish, and they exhibit a suite of ecological traits, environmental resistance, and limited propagule pressure that are variably associated with their lack of success. Collectively, understanding how the various drivers of invasion success interact may explain the differential success of invaders and provide key guidance for managing both positive and negative outcomes associated with their presence.     

Environmental and anthropogenic correlates of hybridization between westslope cutthroat trout (Oncorhynchus clarkii lewisi) and introduced rainbow trout (O. mykiss)

Hybridization with introduced taxa is one of the major threats to the persistence of native biodiversity. The westslope cutthroat trout (Oncorhynchus clarkii lewisi) is found in southeastern British Columbia and southwestern Alberta, Canada, and adjacent areas of Montana, Idaho, and Washington State, USA. Through much of this area, native populations are threatened by hybridization with introduced rainbow trout (O. mykiss). We surveyed 159 samples comprising over 5,000 fish at 10 microsatellite DNA loci to assess the level of admixture between native westslope cutthroat trout (wsct) and introduced rainbow trout in southwestern Alberta. Admixture levels (q_wsct of 0 = pure rainbow trout, q_wsct of 1.0 = pure westslope cutthroat trout) ranged from <0.01 to 0.99 and averaged from 0.72 to 0.99 across seven drainage areas. Regression tree analyses indicated that water temperature, elevation, distance to the nearest stocking site, and distance to the nearest railway line were significant components of a model that explained 34 % of the variation across sites in q_wsct across 58 localities for which habitat variables were available. Partial dependence plots indicated that admixture with rainbow trout increased with increasing water temperature and distance to the nearest railway line, but decreased with increasing elevation and distance from stocking site to sample site. Our results support the hypothesis that westslope cutthroat trout may be less susceptible to hybridization with rainbow trout in colder, higher elevation streams, and illustrate the interaction between abiotic and anthropogenic factors in influencing hybridization between native and introduced taxa.     

A preliminary assessment of the impact of alien rainbow trout (Oncorhynchus mykiss) on indigenous fishes of the upper Berg River,Western Cape Province,South Africa

Impacts of alien rainbow trout (Oncorhynchus mykiss) on critically endangered Berg River redfin (Pseudobarbus burgi), Cape kurper (Sandelia capensis) and Cape galaxias (Galaxias zebratus) in the upper Berg River were investigated in terms of predation and spatial interactions. Trout stomach contents revealed that invertebrates dominate trout diet within the study area, whilst only six fish were recovered from 45 stomachs. The apparent low fish predation success of O. mykiss within the stream suggests a smaller impact compared to that of other alien piscivores such as bass (Micropterus spp.). Galaxias zebratus was the only fish species identified as prey, and its conservation status in the river requires further investigation. Snorkelling surveys revealed that rainbow trout co-exist with S. capensis and adult P. burgi within pools on this river. Galaxias zebratus was absent from the pools, while P. burgi juveniles were segregated from rainbow trout along a depth gradient, possibly indicating avoidance behaviour. Sandelia capensis juveniles may avoid predation by hiding under rocks. Rainbow trout probably compete with indigenous fish for food and space in the pools, though this could not be quantified. The impacts of O. mykiss on all indigenous fauna within the river are likely to be density-dependent.     

The impact of an introduced predator on a threatened galaxiid fish is reduced by the availability of complex habitats

1. The availability of complex habitats such as macrophytes may be vital in determining the outcomes of interactions between introduced predators and native prey. Introduced brown trout (Salmo trutta) have impacted numerous small native freshwater fishes in the southern hemisphere, but the potential role of complex habitats in determining the direct outcomes of brown trout – native fish interactions has not been experimentally evaluated. 2. An in‐lake enclosure experiment was used to evaluate the importance of structurally complex habitats in affecting the direct impacts of brown trout on a threatened galaxiid fish. Five Galaxias auratus and a single brown trout were added to enclosures containing one of three different habitat types (artificial macrophytes, rocks and bare silt substrate). The experiment also had control enclosures without brown trout. Habitat‐dependence of predation risk was assessed by analysis of G. auratus losses to predation, and stomach contents of remaining fish were analysed to determine if brown trout directly affect the feeding of G. auratus and whether this is also habitat‐dependent. 3. Predation risk of G. auratus differed significantly between habitat types, with the highest mortality in enclosures with only bare silt substrate and the lowest in enclosures containing artificial macrophytes. This result highlights the importance of availability of complex habitats for trout – native fish interactions and suggests that increasing habitat degradation and loss in fresh waters may exacerbate the direct impacts of introduced predators. 4. Stomach contents analyses were restricted to fish in enclosures with artificial macrophytes and rocks, as most fish were consumed in enclosures with brown trout and only bare silt substrate. These analyses suggest that brown trout do not directly affect the feeding of G. auratus in complex habitats, but it is still unknown whether its feeding is reduced if complex habitats are unavailable.     

A Comparison of Genetic Variation Between an Anadromous Steelhead, Oncorhynchus mykiss, Population and Seven Derived Populations Sequestered in Freshwater for 70 Years

In 1926 cannery workers from the Wakefield Fisheries Plant at Little Port Walter in Southeast Alaska captured small trout, Oncorhynchus mykiss, from a portion of Sashin Creek populated with a wild steelhead (anadromous O. mykiss) run. They planted them into Sashin Lake which had been fishless to that time and separated from the lower stream by two large waterfalls that prevented upstream migration of any fish. In 1996 we sampled adult steelhead from the lower creek and juvenile O. mykiss from an intermediate portion of the creek, Sashin Lake, and five lakes that had been stocked with fish from Sashin Lake in 1938. Tissue samples from these eight populations were compared for variation in: microsatellite DNA at 10 loci; D-loop sequences in mitochondrial DNA; and allozymes at 73 loci known to be variable in steelhead. Genetic variability was consistently less in the Sashin Lake population and all derived populations than in the source anadromous population. The cause of this reduction is unknown but it is likely that very few fish survived to reproduce from the initial transplant in 1926. Stockings of 50–85 fish into five other fishless lakes in 1938 from Sashin Lake did not result in a similar dramatic reduction in variability. We discuss potential explanations for the observed patterns of genetic diversity in relation to the maintenance of endangered anadromous O. mykiss populations in freshwater refugia.