Nonnative Rainbow Trout Research Articles

Local environments, not invasive hybridization, influence cardiac performance of native trout under acute thermal stress.

Climate-induced expansion of invasive hybridization (breeding between invasive and native species) poses a significant threat to the persistence of many native species worldwide. In the northern U.S. Rocky Mountains, hybridization between native cutthroat trout and non-native rainbow trout has increased in recent decades due, in part, to climate-driven increases in water temperature. It has been postulated that invasive hybridization may enhance physiological tolerance to climate-induced thermal stress because laboratory studies indicate that rainbow trout have a higher thermal tolerance than cutthroat trout. Here, we assessed whether invasive hybridization improves cardiac performance response to acute water temperature stress of native wild trout populations. We collected trout from four streams with a wide range of non-native admixture among individuals and with different temperature and streamflow regimes in the upper Flathead River drainage, USA. We measured individual cardiac performance (maximum heart rate, "MaxHR", and temperature at arrhythmia, "ArrTemp") during laboratory trials with increasing water temperatures (10-28°C). Across the study populations, we observed substantial variation in cardiac performance of individual trout when exposed to thermal stress. Notably, we found significant differences in the cardiac response to thermal regimes among native cutthroat trout populations, suggesting the importance of genotype-by-environment interactions in shaping the physiological performance of native cutthroat trout. However, rainbow trout admixture had no significant effect on cardiac performance (MaxHR and ArrTemp) within any of the three populations. Our results indicate that invasive hybridization with a warmer-adapted species does not enhance the cardiac performance of native trout under warming conditions. Maintaining numerous populations across thermally and hydrologically diverse stream environments will be crucial for native trout to adapt and persist in a warming climate.

Evaluating the effects of selective passage of migratory Westslope Cutthroat Trout on nonnative admixture

AbstractObjectiveHybridization with nonnative Rainbow Trout Oncorhynchus mykiss is a primary threat to the persistence of Westslope Cutthroat Trout O. clarkii lewisi. Managers concerned with conserving migratory populations of Westslope Cutthroat Trout in the presence of Rainbow Trout often face the predicament of tolerating the spread of hybridization, intentionally isolating Westslope Cutthroat Trout populations, or suppressing Rainbow Trout source populations.MethodsSelective passage of migratory Westslope Cutthroat Trout above existing barriers is a management approach with the potential to limit hybridization, while minimizing the population impacts of barriers in populations with preexisting low levels of hybridization. We took a before‐after–control‐impact approach to evaluate a phenotype‐based selective passage management program for migratory Westslope Cutthroat Trout in the Jocko River, Montana.ResultOf the 364 genotyped individuals selectively passed upstream of a barrier, 82% had a proportion of Rainbow Trout admixture < 0.01 and 98% had < 0.10. Over 14 years (2005–2019), there was no significant increase in hybridization at sites (n = 12) upstream of the barrier, but metrics increased within some of our control (no barrier, n = 8) sites. This increase was greatest at a site just downstream from the barrier, suggesting that blocked fish might have spawned opportunistically in this adjacent tributary.ConclusionWe demonstrate that phenotypic traits were useful in implementing a selective Westslope Cutthroat Trout passage program that allowed managers to promote the migratory life history without increasing nonnative admixture in this watershed.

Behavioural response of Dryophytes plicatus tadpoles to the cues of non-native rainbow trout and a native snake

Abstract Non-native species have numerous and significant, often negative, effects on amphibians, but that threat may be ameliorated if the native species is able to respond behaviourally to the non-native predators. We experimentally compared the behavioural response of tadpoles of the Ridged Tree Frog, Dryophytes plicatus, to cues from non-native Rainbow Trout, Oncorhynchus mykiss, to those to a native predator, the Short-tail Alpine Garter Snake, Thamnophis scaliger. Dryophytes plicatus tadpoles did not alter activity in response to cues from O. mykiss but increased activity in the presence of cues from T. scaliger, although it is possible this increase is related, at least in part, to the order of the presentation of the cues. The activity of D. plicatus tadpoles was higher in the absence of the vegetation than in its presence when exposed to predator cues, both T. scaliger and O. mykiss, but not in the control. In conclusion, our results show D. plicatus tadpoles alter their overall activity when exposed to cues from a native predator, T. scaliger, but not the non-native O. mykiss. These results may explain, in part, why D. plicatus can co-occur with T. scaliger but not O. mykiss.

Diet composition and resource overlap of sympatric native and introduced salmonids across neighboring streams during a peak discharge event.

Species assemblages composed of non-native and native fishes are found in freshwater systems throughout the world, and interactions such as interspecific competition that may negatively affect native species are expected when non-native species are present. In the Smith River watershed, Montana, rainbow trout were introduced by 1930. Native mountain whitefish and non-native rainbow trout have presumably occurred in sympatry since the introduction of rainbow trout; however, knowledge about how these two species compete with one another for food resources is sparse. We quantified diet compositions of rainbow trout and mountain whitefish in the mainstem Smith River and in a tributary to the Smith River-Sheep Creek-to determine the degree of overlap in the diets of mountain whitefish and rainbow trout in the Smith River and between the mainstem Smith River and a tributary stream. Rainbow trout and mountain whitefish had generalist feeding strategies, which probably contribute to the amicable coexistence of these species. Diet overlap between rainbow trout and mountain whitefish was high (Pianka's index value = 0.85) in the Smith River and moderate in Sheep Creek (Pianka's index value = 0.57). Despite overlap in diets, some resource partitioning may alleviate resource competition (e.g., rainbow trout consumed far more Oligochaeta than mountain whitefish but fewer Brachycentridae and Chironomidae). Diet composition of rainbow trout and mountain whitefish did not differ greatly between the Smith River and Sheep Creek. Prey categories most commonly used by mountain whitefish at the population and individual levels (i.e., Ephemeroptera and Trichoptera) are sensitive taxa and many species within these orders have experienced extinctions and population declines. Therefore, future changes in resource availability or competition could be of concern.

High dispersal rates in hybrids drive expansion of maladaptive hybridization

Hybridization between native and invasive species, a major cause of biodiversity loss, can spread rapidly even when hybrids have reduced fitness. This paradox suggests that hybrids have greater dispersal rates than non-hybridized individuals, yet this mechanism has not been empirically tested in animal populations. Here, we test if non-native genetic introgression increases reproductive dispersal using a human-mediated hybrid zone between native cutthroat trout (<i>Oncorhynchus clarkii</i>) and invasive rainbow trout (<i>Oncorhynchus mykiss</i>) in a large and connected river system. We quantified the propensity for individuals to migrate from natal rearing habitats (migrate), reproduce in non-natal habitats (stray), and the joint probability of dispersal as a function of genetic ancestry. Hybrid trout with predominantly non-native rainbow trout ancestry were more likely to migrate as juveniles and to stray as adults. Overall, hybrids with greater than 50% rainbow trout ancestry were 5.7 times more likely to disperse than native or hybrid trout with small amounts of rainbow trout ancestry. Our results show a genetic basis for increased dispersal in hybrids that is likely contributing to the rapid expansion of invasive hybridization between these species. Management actions that decrease the probability of hybrid dispersal may mitigate the harmful effects of invasive hybridization on native biodiversity.

Genomics facilitates evaluation and monitoring of McCloud River Redband Trout (Oncorhynchus mykiss stonei)

The McCloud River Redband Trout (MRRT; Oncorhynchus mykiss stonei) is a unique subspecies of rainbow trout that inhabits the isolated Upper McCloud River of Northern California. A major threat to MRRT is introgressive hybridization with non-native rainbow trout from historical stocking and contemporary unauthorized introductions. To help address this concern, we collected RAD-sequencing data on 308 total individuals from MRRT and other California O. mykiss populations and examined population structure using Principal Component and admixture analyses. Our results are consistent with previous studies; we found that populations of MRRT in Sheepheaven, Swamp, Edson, and Moosehead creeks are nonintrogressed. Additionally, we saw no evidence of introgression in Dry Creek, and suggest further investigation to determine if it can be considered a core MRRT conservation population. Sheepheaven Creek was previously thought to be the sole historical lineage of MRRT, but our analysis identified three: Sheepheaven, Edson, and Dry creeks, all of which should be preserved. Finally, we discovered diagnostic and polymorphic SNP markers for monitoring introgression and genetic diversity in MRRT. Collectively, our results provide a valuable resource for the conservation and management of MRRT.

Modeling the freshwater ecological response to changes in flow and thermal regimes influenced by reservoir dynamics

Capturing the ecological effects of climate-induced shifts in hydrologic and thermal regimes in regulated river systems remains a challenge in regional-scale studies. In this study, we used a well-established species distribution model to analyze the results of a process-based hydrologic modeling approach that accounts explicitly for regulation impacts on river flow and temperatures. We also accounted for the stream network fragmentation resulting from dam blockage. This combined model framework can be used to predict historical environmental suitability of river reaches for specific fish species and estimate changes in suitability in response to changes in climate and reservoir operations. As a case study in the highly regulated and fragmented Tennessee River system, we examined the environmental suitability for nonnative rainbow trout (Oncorhynchus mykiss), a species of high recreational value, and native blackfin darter (Etheostoma nigripinne), an endemic species of unique ecological importance. To quantify the impacts of reservoir regulation, we examined the historical environmental suitability, climate change signals, and the environmental drivers of change between the regulated and unregulated model setups. By the end of the 21st century (2080s), historically suitable streams for rainbow trout will disappear due to higher river temperatures. Only 8% of historically suitable streams for blackfin darter will remain but over half of them will be unreachable by current populations due to dam blockage. For river reaches influenced by reservoir regulations, the regulated model projects decreasing environmental suitability for blackfin darter while the unregulated model projects the opposite because of higher river temperatures. The contradictory climate signals between regulated and unregulated models highlight the necessity of considering reservoir regulation.

Metabolic rate and critical thermal maximum CTmax estimates for westslope cutthroat trout, Oncorhynchus clarkii lewisi.

Global warming is changing the thermal habitat of cold-water freshwater fishes, which can lead to decreased fitness and survival and cause shifts in species distributions. The Alberta population of westslope cutthroat trout (Oncorhynchus clarkii lewisi) is listed as 'Threatened' under the Canadian Species at Risk Act. The major threats to the species are the alteration in habitat and water flow, competition and hybridization with non-native trout species and climate change. Here, we conducted (i) intermittent-flow respirometry experiments with adult native westslope cutthroat trout and non-native rainbow trout (Oncorhynchus mykiss) and (ii) critical thermal maximum experiments (CTmax ) with adult westslope cutthroat trout to obtain valuable input data for species distribution models. For both species, standard metabolic rate (SMR) was lower at 10°C compared to 15°C and westslope cutthroat trout had higher SMR than rainbow trout. Although there were inter-specific differences in SMR, forced aerobic scope (using a standardized chase protocol) was different at 10°C, but no significant differences were observed at 15°C because of relative smaller differences in maximum metabolic rate between the species. CTmax of westslope cutthroat trout acclimated to 10°C was 27.0 ± 0.8°C and agitation temperature was 25.2 ± 1.0°C. The results from this study will inform and parametrize cumulative effects assessments and bioenergetics habitat modelling for the recovery planning of the species.

Local factors affecting winter habitat use of non-native rainbow trout in a boreal stream in northern Japan

Local factors affecting winter habitat use of non-native rainbow trout in a boreal stream in northern Japan

Intra- and inter-specific variability of IGF-I expression in farmed cold and warm-water species in Iran

Changes in the expression of liver insulin-like growth factor IGF-I mRNA in two growing cold-water fish species Oncorhynchusmykiss(one from locally produced and one from imported broodstock) and in two warm-water species, Hypophthalmichthysnobilis and Hypophthalmichthysmolitrix were analyzed. Gene expression analysis in liver tissues by using real-time PCR revealed a significant difference in expression level of IGF-I between native and non-native rainbow trout (P bighead carp> silver carp> native rainbow trout. Local strains of rainbow trout, bighead carp and silver carp exhibited stable IGF-I mRNA levels, at different stages of growth. The imported broodstock of rainbow trout exhibited increased IGF-I levels during the entire period studied, with a peak in the IGF-I mRNA levels of fingerlings. The results indicate intra- and inter-specific variability of IGF-I expression in farmed fish.

Movement of non-native rainbow trout in an estuary with periodic summer hypoxia

Movement of non-native rainbow trout in an estuary with periodic summer hypoxia

Population genomics and conservation of Gila Trout (Oncorhynchus gilae)

Many salmonid species exist in highly structured and isolated populations, and are susceptible to habitat fragmentation and disturbances. Gila Trout (Oncorhynchus gilae) is a threatened species found in the Southwestern United States, and is managed to preserve relict populations (i.e., lineages) distributed across a fragmented landscape. We evaluated genomic variation within and among remaining lineages of Gila Trout using RADseq to assess how drift and selection have structured populations using neutral and outlier loci. We also examined whether a signature of hybridization was evident in relict populations. Gila Trout lineages were significantly differentiated from one another and were characterized by low effective population sizes. However, most lineages maintained genomic diversity and are potentially adapted to local conditions. Hybridization with non-native Rainbow Trout (O. mykiss) was not detected in any lineage. All lineages have experienced population bottlenecks associated with mortality from drought and severe wildfires. Current management strategies should be reevaluated and adapted to better account for long-term effects of climate change. Specifically, we suggest reconnecting some populations via dendritic stream networks to facilitate natural dispersal in a metapopulation context. This would allow natural genetic mixing on the landscape and potentially increase adaptive potential. Furthermore, genetic rescue should be implemented to preserve integrity of the unique Spruce Creek lineage that is currently compromised by extremely low diversity.

Predation By the Nonnative Rainbow Trout, Oncorhynchus Mykiss (Walbaum, 1792), on the Native Biota from Freshwater Environment of the Central Andes (Argentina)

Abstract This study aimed to examine the impact of the nonnative rainbow trout Oncorhynchus mykiss on the native biota that inhabits freshwater environments of the central Andes, Argentina. To assess the predation pressure on native Andean vertebrates and invertebrates, the stomach contents were taken from rainbow trout caught at three Andean sites. Results showed that the major prey items were the native torrent catfish Hatcheria macraei (100% in diet), Elmidae coleopterans (23%), and the native crab Aegla affinis (38%) in the rainbow trout stomachs collected from Cipolletti Dam (North zone), Yaucha River (Middle zone), and Vaina Stream (South zone), respectively. This preliminary study exhibits the predation pressure of the rainbow trout on native Andean species and suggests that policies are necessary to control the release of fry of this exotic salmonid in the Andes region.

Controls on Somatic Growth and Population Dynamics of Rainbow Trout

Photo 1: A rainbow trout holding immediately downstream of Glen Canyon Dam in the clearwater Glen Canyon reach of the Colorado River. Photo credit: David Herasimtschuk, Freshwaters Illustrated. Photo 2: Rainbow trout holding over a cobble bar in the clearwater Glen Canyon reach of the Colorado River and feeding on drifting aquatic invertebrates. Photo credit: David Herasimtschuk, Freshwaters Illustrated. Photo 3: Nighttime boat electrofishing was used to capture rainbow trout for tagging. Time-lapse photography captures the path of electrofishing boats on left and right shores. Captured fish were brought to a centrally located processing boat (foreground) to be measured, weighed, tagged, and scanned, and were then returned to their original capture site. Photo credit: Dustin Patar. Photo 4: The Marble Canyon reach of the Colorado River can be very turbid during the summer monsoon season. High turbidity reduced feeding efficiency and growth rates of rainbow trout, which, combined with higher water temperatures and reduced prey production, led to a severe collapse of the population in this reach. Photo credit: Dustin Patar. Photo 5: The confluence of the Little Colorado River (LCR, entering from top of photograph) and the Colorado River. The LCR is an important spawning and rearing area for endangered Humpback Chub. Juvenile Humpback Chub disperse from the LCR into the main stem Colorado River where their survival and growth rates are reduced due to cooler water temperature and competition and predation from non-native rainbow trout. Our sampling reaches IVa and IVb are located in the Colorado River upstream (left side of photograph) and downstream (right side of photograph) of the confluence. Occasional floods from the LCR result in high levels of turbidity downstream of the confluence which reduces growth rates and densities of rainbow trout. Photo credit: Ted Grussing, tedgrussing photography. These photographs illustrate the article “Changes in prey, turbidity, and competition reduce somatic growth and cause the collapse of a fish population” by J. Korman, M.D. Yard, M.C. Dzul, C.B. Yackulic, M.J. Dodrill, B.R. Deemer, and T.A. Kennedy published in Ecological Monographs. https://doi:10.1002/ecm.1427

Hybridization alters growth and migratory life-history expression of native trout.

Human‐mediated hybridization threatens many native species, but the effects of introgressive hybridization on life‐history expression are rarely quantified, especially in vertebrates. We quantified the effects of non‐native rainbow trout admixture on important life‐history traits including growth and partial migration behavior in three populations of westslope cutthroat trout over five years. Rainbow trout admixture was associated with increased summer growth rates in all populations and decreased spring growth rates in two populations with cooler spring temperatures. These results indicate that non‐native admixture may increase growth under warmer conditions, but cutthroat trout have higher growth rates during cooler periods. Non‐native admixture consistently increased expression of migratory behavior, suggesting that there is a genomic basis for life‐history differences between these species. Our results show that effects of interspecific hybridization on fitness traits can be the product of genotype‐by‐environment interactions even when there are minor differences in environmental optima between hybridizing species. These results also indicate that while environmentally mediated traits like growth may play a role in population‐level consequences of admixture, strong genetic influences on migratory life‐history differences between these species likely explains the continued spread of non‐native hybridization at the landscape‐level, despite selection against hybrids at the population‐level.

Otolith microchemistry and acoustic telemetry reveal anadromy in non-native rainbow trout (Oncorhynchus mykiss) in Prince Edward Island, Canada

This study examined the migratory patterns of introduced rainbow trout (Oncorhynchus mykiss) in three rivers in Prince Edward Island, Canada, using acoustic telemetry and otolith microchemistry. Only 6% of acoustically tagged fish in three river systems left coastal embayments. A cohort of rainbow trout in all three rivers entered saline waters. Habitat use differed among migrants in the three rivers, as Montague River fish occupied estuary habitat (mean 20.79 PSU) more often than West River and Dunk River fish that tended to occupy both riverine tidal (mean 1.27 and 4.29 PSU, respectively) and freshwater habitats (<0.5 PSU), particularly during summer months (July and August). A second cohort of rainbow trout remained exclusively in fresh water. Migratory individuals were more likely to arise from anadromous mothers, but freshwater mothers produced migratory offspring in all sites. Migratory individuals were significantly larger than nonmigratory freshwater residents. This study suggests that partial residency was the primary strategy, with prominent tidal occupation, while secondary marine and freshwater contingents were included in the full range of successful migratory strategies.

Report on the Short-Term Scavenging of Decomposing Native and Non-Native Trout in Appalachian Streams

Trout fishing is one of the primary recreational activities in the southern Appalachians, with large amounts of fish stocked on a regular basis. However, very little is known regarding the fate of hatchery reared carcasses not captured by anglers, representing a likely important ecological resource to local communities. We tested the efficacy of underwater video to characterize short term decomposition and consumption by aquatic scavengers of native brook and non-native rainbow trout, Salvelinus fontinalis and Oncorhychus mykiss. This study took place on the Cherokee Qualla Boundary in North Carolina, a location with one of the highest riverine stocked trout densities in the eastern United States. During May 2017, 10 waterproof cameras were deployed for 1-hour intervals on each carcass twice daily for a period of 5 days. We observed that 75.3% of recorded video contained river chub, Nocomis micropogon, with only 24.7% visited by crayfish, with a maximum of 9 and a mean of 1.93 for N. micropogon. Half of the carcasses were removed within 2 days. Based on natural history evidence and some trail cameras, we believe that otters were removing carcasses. Otters showed no preference for either trout species. Underwater video allowed us to characterize initial decomposition within stream diurnal scavengers in a short period using a visual, non-destructive low-cost method. Future studies should monitor large mammalian scavengers to further elucidate the role of fish stocking on aquatic communities.

Preliminary evaluation of non-native rainbow trout (Oncorhynchus mykiss) impact on the Cederberg ghost frog (Heleophryne depressa) in South Africa’s Cape Fold Ecoregion

We evaluated the impact of non-native rainbow trout Oncorhynchus mykiss on a population of endemic Cedarberg ghost frog Heleophryne depressa in the upper Krom River (Olifants-Doring River Catchment, Cape Fold Ecoregion). We compared H. depressa abundance (using kick-sampling and underwater video analysis) and environmental conditions between sites above and below a waterfall that marks the upper distribution limit of O. mykiss. Heleophryne depressa abundance was significantly greater above the waterfall than that below it, and, because there was no significant difference in measured environmental variables, O. mykiss presence is identified as the most likely explanation for the observed decrease in H. depressa abundance.

Effects of nonnative Rainbow Trout on two species of endemic Mexican amphibians

Introduction of nonnative salmonids into fishless aquatic ecosystems often can have negative effects on these ecosystems. Amphibians appear to be particularly susceptible to the introduction of trout. Nonnative Rainbow Trout (Oncorhynchus mykiss) have been introduced to Mexico, including to streams in the mountains around Mexico City that are home to several endemic Mexican amphibians. We examined the effect of Rainbow Trout on the distributions of 2 endemic Mexican amphibians, Ambystoma altamirani and Hyla plicata, in the Sierra de Las Cruces. Ambystoma altamirani was never found at sites with Rainbow Trout, and H. plicata was observed only once at sites with Rainbow Trout. Stream site characteristics, for the most part, did not vary among streams occupied by different species, suggesting environmental variables were not primarily responsible for the species distributions. Our results provide strong circumstantial evidence that the introduction of Rainbow Trout in streams in the Sierra de Las Cruces has a negative effect on the distribution of native Mexican amphibians.

Temperature mediates the impact of non-native rainbow trout on native freshwater fishes in South Africa’s Cape Fold Ecoregion

Introduced rainbow trout Oncorhynchus mykiss have invaded many headwater streams in South Africa’s Cape Fold Ecoregion (CFE) and pose arguably the greatest threat to several species of threatened native fishes. Trout impacts in these systems appear to be density-dependent; we hypothesized that temperature is a key factor determining trout density and corresponding impacts on native fishes. We took advantage of natural spatial and temporal thermal heterogeneity in two CFE headwater streams to investigate the influence of temperature and other environmental factors on trout density and impacts on native fish assemblages. Temperature limited trout density (negative relationship) during summer surveys (hottest months), but not during spring and autumn (cooler months). During summer, the highest trout densities were recorded at sites that remained relatively cool (7-day moving average of the daily maximum temperature (Max_7) 27 °C). Native fish density was best explained by trout density (inverse relationship) during summer, but by other environmental variables such as habitat complexity and current flow velocity during autumn and spring. Trout distributions expand during cooler seasons when thermal heterogeneity and maximum temperatures are relatively low, but contract into thermal refugia (habitat patches that remain relatively cool) as temperatures and thermal heterogeneity increase over summer, leaving warmer habitat patches vacant and able to function as predation refugia for native fish. We hypothesize that climate warming could reduce trout density and distribution, and associated impacts on native fish, in CFE headwater streams, but may increase the potential for invasions by other warm-adapted non-native predatory fish already established in downstream river reaches in the region.