Nonnative Rainbow Trout Research Articles

Hybridization rapidly reduces fitness of a native trout in the wild

Human-mediated hybridization is a leading cause of biodiversity loss worldwide. How hybridization affects fitness and what level of hybridization is permissible pose difficult conservation questions with little empirical information to guide policy and management decisions. This is particularly true for salmonids, where widespread introgression among non-native and native taxa has often created hybrid swarms over extensive geographical areas resulting in genomic extinction. Here, we used parentage analysis with multilocus microsatellite markers to measure how varying levels of genetic introgression with non-native rainbow trout (Oncorhynchus mykiss) affect reproductive success (number of offspring per adult) of native westslope cutthroat trout (Oncorhynchus clarkii lewisi) in the wild. Small amounts of hybridization markedly reduced fitness of male and female trout, with reproductive success sharply declining by approximately 50 per cent, with only 20 per cent admixture. Despite apparent fitness costs, our data suggest that hybridization may spread due to relatively high reproductive success of first-generation hybrids and high reproductive success of a few males with high levels of admixture. This outbreeding depression suggests that even low levels of admixture may have negative effects on fitness in the wild and that policies protecting hybridized populations may need reconsideration.

Competition between native and introduced salmonid fishes: cutthroat trout have lower growth rate in the presence of cutthroat–rainbow trout hybrids

When nonnative species become established within new communities, competition may play a role in determining the persistence of ecologically similar native species. In western North America, many native cutthroat trout ( Oncorhynchus clarkii ) populations have been replaced by nonnative rainbow trout ( Oncorhynchus mykiss ). Superior competitive ability of rainbow trout and cutthroat–rainbow trout hybrids is often cited for this replacement; however, few studies have tested for mechanisms that might allow introduced rainbow trout to out-compete native trout species. Our previous work found individual-based differences in swimming and foraging ability among cutthroat trout, rainbow trout, and their hybrids. In this study, we tested for the presence and strength of competition between cohorts of cutthroat trout, rainbow trout, and their reciprocal hybrids. We assayed the growth rate of juvenile cutthroat trout in allopatry versus cutthroat trout when sympatric with rainbow trout and each hybrid cross. After controlling for size and density of trout, cutthroat trout cohorts in stream channels that contained hybrid genotypes experienced lower growth than cutthroat trout in allopatry. Averaged across heterospecific treatments, cutthroat trout growth was also lower than that of cutthroat trout cohorts in allopatry. Our study suggests that juvenile cutthroat trout experience a growth disadvantage when competing against cutthroat–rainbow hybrids.

Evaluating an ecosystem service provided by Atlantic salmon, sea trout and other fish species in the River Spey, Scotland: The economic impact of recreational rod fisheries

In 2003 a study was undertaken in the River Spey catchment, north-east Scotland, to estimate the economic impact of recreational rod fisheries for Atlantic salmon ( Salmo salar), brown and sea trout ( S. trutta), pike ( Esox lucius), and non-native rainbow trout ( Oncorhynchus mykiss). Thirty-one fishery owners and 372 anglers completed questionnaire surveys on average catches and angler effort in 1998–2002. Anglers reported their daily expenditure, and the CogentSI model was used to derive multiplier effects. Total annual angler days were 54,746, of which 74% were from salmon and sea trout anglers. Angler expenditure was estimated to be £11.8 million annum −1, of which £10.8 million was generated by salmon and sea trout anglers. Accounting for multiplier effects, fisheries contributed £12.6 million annum −1 to household incomes and 420 full time equivalent (FTE) jobs in the catchment. Of this, salmon and sea trout fisheries contributed £11.6 million annum −1 and 401 FTEs. On average rod caught salmon and sea trout contributed approximately £970 fish −1 to household incomes, equating to £26 smolt −1 and £1 m −2 annum −1 for riverine nursery habitat. The capital value of the salmon and sea trout rod fishery was £56.7 million. Comparison with a national survey of angler expenditure in 2003 suggests that the relative impact of salmon and sea trout in the Spey catchment's economy is one of the highest in the country. The application of angler expenditure as an evaluation of utilitarian ecosystem services provided by fish species and freshwater habitat is discussed.

Wildland Fire: An Opportunistic Event for Reintroducing a Native Salmonid

The Missionary Ridge Fire (summer 2002) burned approximately 28 520 ha in the San Juan Basin, Colorado. Prior to the fire, no native Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) had been observed in the San Juan Basin for over 100 yr due to over-fishing, introduction of non-native fishes, and habitat alteration. Mud and ash flows into the Florida River in fall 2002 and spring 2003 resulted in a complete fish kill of non-native rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta). Between 2003 and 2007, the Colorado Division of Wildlife released 138728 Colorado River cutthroat trout ranging from 5 cm to 35 cm in length within a 22.5 km reach of the Florida River. Two barriers, a reservoir dam and an irrigation diversion dam, prevented non-native fish from entering this reach. Each fall from 2003 through 2007, we surveyed two locations on the Florida River and estimated population size. Heavy silt deposits and extremely high river flows (3.8 m3 s−1) precluded us from making population estimates during 2003 to 2005. The catch of Colorado River cutthroat trout decreased from 76 % in 2006 to only 44 % in 2007. Interspecific competition and predation by rainbow and brown trout, whirling disease (Myxobolus cerebralis), and extreme fluctuations in regulated river flows in 2007 may have contributed to the observed decline in cutthroat trout. It is still too early to conclude whether reintroduction efforts of Colorado River cutthroat trout have been successful, but early indications suggest non-native trout are beginning to re-colonize this section of the Florida River at the expense of the cutthroat trout.

Use of an Annular Chamber for Testing Thermal Preference of Westslope Cutthroat Trout and Rainbow Trout

ABSTRACT Remaining populations of westslope cutthroat trout (Oncorhynchus clarkii iewisi) in western North America are primarily confined to cold headwaters whereas nonnative rainbow trout (Oncorhynchus mykiss) predominate in warmer, lower elevation stream sections historically occupied by westslope cutthroat trout. We tested whether differing thermal preferences could account for the spatial segregation observed in the field. Thermal preferences of age-1 westslope cutthroat trout and rainbow trout (125 to 150 mm total length) were assessed in the laboratory using a modified annular preference chamber at acclimation temperatures of 10, 12, 14, and 16°C. Final preferred temperature of westslope cutthroat trout (14.9°C) was similar to that of rainbow trout (143°C) when tested in a thermal gradient of 11–17°C. The high degree of overlap in thermal preference indicates the two species have similar thermal niches and a high potential for competition. We suggest several modifications to the annular preference chamber to improve performance in future studies.

A paradox of trout invasions in North America

A paradox of invasion biology is that even though native species are locally adapted to environmental conditions, nonnative species without this advantage often invade. Ecologists have advanced four main theories to explain why invaders are successful in some places and not others: biotic resistance, environmental resistance, human disturbance, and natural enemies. However, none of these theories alone can account for invasions by two trout species outside their native ranges in North America. Brook trout (Salvelinus fontinalis) are able to displace native cutthroat trout (Oncorhynchus clarkii) in the inland western US, but are themselves displaced by nonnative rainbow trout (O. mykiss) in the southeastern US. An alternative hypothesis is that an interaction among zoogeography, evolutionary history, and environmental resistance from the natural flow regimes can account for this paradox. The nonnative species invade successfully at the southern edge of the ranges of the native species, which are farthest from their ancestral origins. Due to their evolutionary history, the native species are poorly adapted to the natural disturbance regime at the southern limit of their ranges, but the nonnative species are preadapted by chance due to theirs. This alternative hypothesis about the interaction between the historical contingency of evolution and environmental resistance should be more widely tested, to inform both invasion biology and the conservation of native trout.

Invading rainbow trout usurp a terrestrial prey subsidy from native charr and reduce their growth and abundance

Movements of prey organisms across ecosystem boundaries often subsidize consumer populations in adjacent habitats. Human disturbances such as habitat degradation or non-native species invasions may alter the characteristics or fate of these prey subsidies, but few studies have measured the direct effects of this disruption on the growth and local abundance of predators in recipient habitats. Here we present evidence, obtained from a combined experimental and comparative study in northern Japan, that an invading stream fish usurped the flux of allochthonous prey to a native fish, consequently altering the diet and reducing the growth and abundance of the native species. A large-scale field experiment showed that excluding terrestrial invertebrates that fell into the stream with a mesh greenhouse reduced terrestrial prey in diets of native Dolly Varden charr (Salvelinus malma) by 46-70%, and reduced their growth by 25% over six weeks. However, when nonnative rainbow trout (Oncorhynchus mykiss) were introduced, they monopolized these prey and caused an even greater reduction of terrestrial prey in charr diets of 82-93%, and reduced charr growth by 31% over the same period. Adding both greenhouse and rainbow trout treatments together produced similar results to adding either alone. Results from a comparative field study of six other stream sites in the region corroborated the experimental findings, showing that at invaded sites rainbow trout usurped the terrestrial prey subsidy, causing a more than 75% decrease in the biomass of terrestrial invertebrates in Dolly Varden diets and forcing them to shift their foraging to insects on the stream bottom. Moreover, at sites with even low densities of rainbow trout, biomass of Dolly Varden was more than 75% lower than at sites without rainbow trout. Disruption of resource fluxes between habitats may be a common, but unidentified, consequence of invasions, and an additional mechanism contributing to the loss of native species.

Morphological and swimming stamina differences between Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri), rainbow trout (Oncorhynchus mykiss), and their hybrids

We hypothesized that body shape differences between Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri), rainbow trout (Oncorhynchus mykiss), and their hybrids may influence swimming ability and thus play an important role in the invasion of nonnative rainbow trout and hybrid trout into native cutthroat trout populations. We reared Yellowstone cutthroat trout, rainbow trout, and reciprocal hybrid crosses in a common environment and conducted sustained swimming trials in order to test for genetically based morphological and swimming stamina differences. Linear and geometric morphometric analyses identified differences in body shape, with cutthroat trout having slender bodies and small caudal peduncles and rainbow trout having deep bodies and long caudal peduncles. Hybrid crosses were morphologically intermediate to the parental genotypes, with a considerable maternal effect. Consistent with morphological differences, cutthroat trout had the lowest sustained swimming velocity and rainbow trout had the highest sustained swimming velocity. Sustained swimming ability of hybrid genotypes was not different from that of rainbow trout. Our results suggest that introduced rainbow trout and cutthroat-rainbow trout hybrids potentially out-compete native Yellowstone cutthroat trout through higher sustained swimming ability.

Hybridization and Cytonuclear Associations among Native Westslope Cutthroat Trout, Introduced Rainbow Trout, and Their Hybrids within the Stehekin River Drainage, North Cascades National Park

AbstractHistoric introductions of nonnative rainbow trout Oncorhynchus mykiss into the native habitats of cutthroat trout O. clarkii have impacted cutthroat trout populations through introgressive hybridization, creating challenges and concerns for cutthroat trout conservation. We examined the effects of rainbow trout introductions on the native westslope cutthroat trout O. c. lewisii within the Stehekin River drainage, North Cascades National Park, Washington, by analyzing 1,763 salmonid DNA samples from 18 locations with nine diagnostic nuclear DNA markers and one diagnostic mitochondrial DNA (mtDNA) marker. Pure westslope cutthroat trout populations only occurred above upstream migration barriers in the Stehekin River and Park Creek. Two categories of rainbow trout admixture were observed: (1) less than 10% within the Stehekin River drainage above the Bridge Creek confluence and the middle and upper Bridge Creek drainage and (2) greater than 30% within the Stehekin River below the Bridge Creek confluence and in lower Bridge Creek. Hybrid indices and multilocus genotypes revealed an absence of rainbow trout and reduced hybrid diversity within the Stehekin River above the Bridge Creek confluence relative to hybrid diversity in the Stehekin River below the confluence and within lower Bridge Creek. Cytonuclear disequilibrium statistics revealed assortative mating between westslope cutthroat and rainbow trout but not among hybrids within the same locations. This suggests that a randomly mating hybrid swarm does not currently exist. However, continual migration of parental genotypes into the study location could also create significant cytonuclear disequilibria. The Stehekin River represents a novel and unique example of a dynamic hybridization zone where the invasion of rainbow trout alleles into the Stehekin River westslope cutthroat trout population above the Bridge Creek confluence appears to be impeded, suggesting that divergent ecological or evolutionary mechanisms promote the population structure within the Stehekin River drainage, depending upon location.

Juvenile Rainbow Trout Production in New York Tributaries of Lake Ontario: Implications for Atlantic Salmon Restoration

AbstractThree Pacific salmonid species Onchorynchus spp. have replaced the extirpated Atlantic salmon Salmo salar as the main migratory salmonid in the Lake Ontario drainage. One of those species, the nonnative rainbow trout O. mykiss, has become widely distributed within the historical Atlantic salmon habitat, occupying an ecological niche similar to that of juvenile Atlantic salmon. Consequently, both a tributary's carrying capacity for Atlantic salmon and competition from established nonnative species are important when considering the feasibility of Atlantic salmon restoration. Estimation of juvenile rainbow trout production will help evaluate the capacity of tributaries to produce salmonids that occupy similar niches. Geostatistical methods were applied to standardized and efficiency‐corrected electrofishing data from three of New York's best salmonid‐producing streams to precisely estimate juvenile rainbow trout populations. Results indicated that each study stream could produce 20,000–40,000 age‐0 and 4,000–10,000 age‐1 and older rainbow trout per year. Statistical interpolation indicated areas of significantly different production potential and points of significant changes in productivity. Closer examination of the niche similarity and competitive potential of these two species is needed to properly interpret these estimates with regard to Atlantic salmon restoration.

FISH INVASION RESTRUCTURES STREAM AND FOREST FOOD WEBS BY INTERRUPTING RECIPROCAL PREY SUBSIDIES

Habitat alteration and biotic invasions are the two leading causes of global environmental change and biodiversity loss. Recent innovative experiments have shown that habitat disturbance can have drastic effects that cascade to adjacent ecosystems by altering the flow of resource subsidies from donor systems. Likewise, exotic species in- vasions could alter subsidies and affect distant food webs, but very few studies have tested this experimentally. Here we report evidence from a large-scale field experiment in northern Japan that invasion of nonnative rainbow trout (Oncorhynchus mykiss) interrupted reciprocal flows of invertebrate prey that drove stream and adjacent riparian forest food webs. Rainbow trout usurped terrestrial prey that fell into the stream, causing native Dolly Varden charr (Salvelinus malma) to shift their foraging to insects that graze algae from the stream bottom. This indirectly increased algal biomass, but also decreased biomass of adult aquatic insects emerging from the stream to the forest. In turn, this led to a 65% reduction in the density of riparian-specialist spiders in the forest. Thus, species invasions can interrupt flows of resources between interconnected ecosystems and have effects that propagate across their boundaries, effects that may be difficult to anticipate without in-depth understanding of food web relationships.

Spread of hybridization between native westslope cutthroat trout, Oncorhynchus clarki lewisi, and nonnative rainbow trout, Oncorhynchus mykiss

We examined spatial and temporal patterns of hybridization between native westslope cutthroat trout, Oncorhynchus clarki lewisi, and nonnative rainbow trout, O. mykiss, in streams of the Flathead River system in Montana, U.S.A. We detected hybridization in 24 of 42 sites sampled from 1998 to 2001. We found new Oncorhynchus mykiss introgression in seven of 11 sample populations that were determined to be nonhybridized in 1984. Patterns of spatial autocorrelation and linkage disequilibrium indicated that hybridization is spreading among sites and is advancing primarily via post-F1 hybrids. Although hybridized populations were distributed widely throughout the study area, the genetic contribution from O. mykiss decreased with increasing upstream distance from the Flathead River mainstem, suggesting that O. mykiss introgression is spreading in an upstream direction. The spread of hybridization may be constrained more by demographic than by environmental factors, given that (i) hybridized populations generally encompassed the range of environmental variability in nonhybridized populations, and (ii) hybridization status was more strongly associated with neighborhood statistics than measured environmental gradients.

Habitat use by Nonnative Rainbow Trout, Oncorhynchus mykiss, and Native Little Colorado spinedace, Lepidomeda vittata

We evaluated overlap in microhabitat use between nonnative rainbow trout, Oncorhynchus mykiss, and native Little Colorado spinedace, Lepidomeda vittata, a federally threatened cyprinid, in natural and experimental settings. In natural settings, we also examined occurrence and microhabitat use of two other native fishes, speckled dace, Rhinichthys osculus, and bluehead sucker, Catostomus discobolus. Native species co-occurred, as did rainbow trout and bluehead sucker. However, occurrences of Little Colorado spinedace and speckled dace were not significantly correlated with occurrence of rainbow trout. Total lengths of all three native species were significantly smaller at allopatric sites than at sites sympatric with rainbow trout. Microhabitat characteristics at sites with rainbow trout did not differ from those where the other three species were found, but did differ among the native species. In laboratory experiments with Little Colorado spinedace and rainbow trout, rainbow trout used the lower depth zone most, and spinedace increased use of the lower depth zone upon addition of rainbow trout. In addition, species tended to co-occur in zones, but used cover independently of one-another, suggesting a low level of agonistic interactions. However, after addition of a high density of rainbow trout, spinedace tended to use cover less than before. We suggest that the species can coexist at low rainbow trout densities. Potential negative effects of rainbow trout on Little Colorado spinedace likely increase with increasing densities of rainbow trout, and rainbow trout likely affect smaller size classes of Little Colorado spinedace more than larger ones.

Effects of a Test Flood on Fishes of the Colorado River in Grand Canyon, Arizona

A beach/habitat-building flow (i.e., test flood) of 1274 m3/s, released from Glen Canyon Dam down the Colorado River through Grand Canyon, had little effect on distribution, abundance, or movement of native fishes, and only short-term effects on densities of some nonnative species. Shoreline and backwater catch rates of native fishes, including juvenile humpback chub (Gila cypha), flannelmouth suckers (Catostomus latipinnis), and bluehead suckers (C. discobolus), and all ages of speckled dace (Rhinichthys osculus), were not significantly different before and after the flood. Annual spring spawning migrations of flannelmouth suckers into the Paria River and endangered humpback chub into the Little Colorado River (LCR) took place during and after the flood, indicating no impediment to fish migrations. Pre-spawning adults staged in large slack water pools formed at the mouths of these tributaries during the flood. Net movement and habitat used by nine radio-tagged adult humpback chub during the flood were not significantly different from prior observations. Diet composition of adult humpback chub varied, but total biomass did not differ significantly before, during, and after the flood, indicating opportunistic feeding for a larger array of available food items displaced by the flood. Numbers of nonnative rainbow trout (Oncorhynchus mykiss) <152 mm total length decreased by ∼8% in electrofishing samples from the dam tailwaters (0–25 km downstream of the dam) during the flood. Increased catch rates in the vicinity of the LCR (125 km downstream of the dam) and Hell's Hollow (314 km downstream of the dam) suggest that these young trout were displaced downstream by the flood, although displacement distance was unknown since some fish could have originated from local populations associated with intervening tributaries. Abundance, catch rate, body condition, and diet of adult rainbow trout in the dam tailwaters were not significantly affected by the flood, and the flood did not detrimentally affect spawning success; catch of young-of-year increased by 20% in summer following the flood. Post-flood catch rates of nonnative fathead minnows (Pimephales promelas) in shorelines and backwaters, and plains killifish (Fundulus zebrinus) in backwaters decreased in the vicinity of the LCR, and fathead minnows increased near Hell's Hollow, suggesting that the flood displaced this nonnative species. Densities of rainbow trout and fathead minnows recovered to pre-flood levels eight months after the flood by reinvasion from tributaries and reproduction in backwaters. We concluded that the flood was of insufficient magnitude to substantially reduce populations of nonnative fishes, but that similar managed floods can disadvantage alien predators and competitors and enhance survival of native fishes.

Ebb and Flow of Encroachment by Nonnative Rainbow Trout in a Small Stream in the Southern Appalachian Mountains

Abstract Brook trout Salvelinus fontinalis is the native salmonid species of streams in the southern Appalachian Mountains. The present distribution of this species, once widespread from headwaters to lower reaches of large streams, is restricted to mostly headwater areas. Changes in the distribution of native brook trout in the presence of' nonnative rainbow trout Oncorhynchus mykiss have been documented in Great Smoky Mountains National Park. When rainbow trout were first found in a tributary (Rock Creek) in the park in 1979, a study was begun to assess changes through time in distribution and abundance of rainbow trout in Rock Creek and to compare the brook trout and rainbow trout associations in Rock Creek with associations found in other park streams. Abundance of brook trout was low in the downstream sections of Rock Creek in 1979–1993. Brook trout abundance was highest in the steep-gradient, pool-dominated headwater section which was only 2 km from the confluence of Rock Creek and Cosby Creek. Rain...

Angling and Electrofishing for Removing Nonnative Rainbow Trout from a Stream in a National Park

Abstract An experimental 9-week fishery was conducted in a Great Smoky Mountains National Park stream (Lynn Camp Prong) to evaluate the use of angling and electro fishing as techniques for reducing the density of a non-native rainbow trout (Salmo gairdneri) population. Most anglers were local residents in Tennessee. Angler visits were highest during the first 2 weeks and declined steadily thereafter. Anglers harvested 5,655 rainbow trout whose total lengths fell within the 80-and 280-mm size classes. The electrofishing crew caught fish in length classes between 30 and 280 mm. The adjusted preseason electrofishing catch per unit effort (CPUE) was 3.71 fish/h based on the time taken for daily preparation, travel, and electrofishing. When based on electrofishing time only, the CPUE was 5.90. The CPUE for one clerk at a creel station was projected to be substantially greater (6.70) than that for the electrofishing crew. In addition, total costs were far greater for the electrofishing crew because of equipment...

Electrophoretic Evidence for a Genetic Admixture of Native and Nonnative Rainbow Trout in the Yakima River, Washington

Abstract Allele frequencies for wild rainbow trout Salmo gairdneri from the upper Yakima River and two tributary creeks were intermediate between those for introduced hatchery populations and those for inland populations native to other areas of the Columbia River basin. Previously published results had demonstrated a widespread geographic consistency in allele frequencies at two loci among both anadromous and nonanadromous populations in the Columbia River drainage east of the Cascade Mountains. The intermediate allele frequencies at these two loci for rainbow trout from the upper Yakima River, therefore, suggest that these populations represent genetic admixtures of native and nonnative stocks. Allele frequencies at several other loci plus field surveys of spawning fish further suggest that nonanadromous rainbow trout from domesticated hatchery strains, rather than hatchery-reared steelhead (anadromous S. gairdneri), have been responsible for the suspected introgression of nonnative genes into the Yakim...

Patterns of Variation and Distribution in Apache Trout (Salmo apache) Relative to Co-Occurrence with Introduced Salmonids

Examination of Apache trout (Salmo apache) populations in the White Mountains, Arizona, indicated wider ranges of morphological and meristic variation than in the type description, greater genetic purity of stocks on Fort Apache Indian Reservation than on Apache-Sitgreaves National Forest, and distributional patterns that primarily reflect stocking of nonnative rainbow trout (S. gairdneri). APACHE trout (Salmo apache Miller) occur naturally in streams of the White Mountains of east-central Arizona. Although recognized for several decades as a distinctive taxon, this species was not described (Miller, 1972) until after drastic reduction of its historic range through habitat alteration and interspecific interactions, including apparent hybridization with nonnative trouts. Salmo apache was first list