Rainbow Trout Abundance Research Articles

Declines in prey production during the collapse of a tailwater Rainbow Trout population are associated with changing reservoir conditions

AbstractObjectiveUnderstanding how energy moves through food webs and limits productivity at various trophic levels is a central question in aquatic ecology and can provide insight into drivers of fish population dynamics since many fish populations are food limited. In this study, we seek to better understand what factors drove a decline of >85% in the number of Rainbow TroutOncorhynchus mykiss found in the tailwater portion of the Colorado River below Glen Canyon Dam during 2012–2016.MethodsWe estimate the production of dominant prey using data from previously published studies of Rainbow Trout abundance and growth alongside drift and diet samples. We test how prey production correlates to both proximate (e.g., nutrients) and distal (e.g., limnological conditions in the upriver reservoir) drivers.ResultResults suggest that gross consumption of invertebrate prey by the Rainbow Trout population declined from an annual mean of 423 to 69 kg/d. Daily production rates of dominant prey in aggregate declined from a high of 0.173 to 0.018 g·m−2·d−1. Chironomids accounted for 70% of the decline in prey production. Foraging efficiency by Rainbow Trout (range, 0.99–0.67) was high across the range of prey production rates. After the Rainbow Trout population had declined by ~90%, prey consumption saturated at higher rates of prey production and the gross quantity of daily drift exported from the reach increased from 8.9 to 12.7 kg/d.ConclusionRainbow Trout population dynamics are largely influenced by changes in prey production, which is itself driven by soluble reactive phosphorus (SRP) concentrations in the reservoir. The SRP model predicted that prey production would increase by 32 kg/d (SE, 9) for each 1 μg/L increase in SRP. These concentrations were indirectly influenced by reservoir hydrology and biogeochemistry, linkages that may extend far beyond the confines of this tailwater fishery and into the downstream reaches of the Grand Canyon's Colorado River ecosystem.

Hydroacoustic Studies of the Migration of Anadromous Rainbow Trout Parasalmo mykiss (Salmonidae) in the Kvachina River (Western Kamchatka)

The NetCor research hydroacoustic system has been used to study the anadromous migration of the rainbow trout Parasalmo mykiss (Walbaum, 1792) in the Kvachina River (Western Kamchatka) during its autumn entry from the sea. The intensity of the fish movement increases in similar periods of the neighboring years (2010–2011), which indicates the cyclical migration of the species associated with changes in temperature and water level in the river. The daily dynamics of the movement is characterized by two periods of increased intensity of migration: the first is timed to dawn, while the second begins in the evening hours. The extrapolation of the obtained data provides the estimated numbers of the rainbow trout abundance in the river: 9832 and 2855 individuals in 2010 and 2011, respectively. The difference in the abundance of anadromous rainbow trout in the neighboring years probably results from the peculiarities of the geomorphology and hydrological regime of the river.

Seasonal changes in abundance and feeding ecology of an endemic, keystone predator following the introduction of large rainbow trout Oncorhynchus mykiss

Despite documented negative effects on aquatic ecosystems, management agencies commonly receive requests to stock non-native rainbow trout Oncorhynchus mykiss (Walbaum). We used a multinomial N-mixture model and a suite of diet analyses to evaluate effects of large (265–530 mm) rainbow trout on reach-scale abundance and feeding ecology of an important endemic, keystone species, Neosho bass Micropterus velox (Hubbs and Bailey). We evaluated potential changes by seasonally sampling multiple reaches of an Ozark Highlands stream before and after the introduction of 10,800 rainbow trout. Results suggest that rainbow trout altered the distribution of Neosho bass, as indicated by a negative relationship between rainbow trout density and Neosho bass abundance. Trophically, rainbow trout and Neosho bass <101 mm were equivalent, but rainbow trout diets significantly overlapped with larger Neosho bass. High rainbow trout abundance in the spring post-stocking period changed Neosho bass diets by altering predator–prey dynamics with crayfish. Controlling for fish size revealed Neosho bass consumed larger crayfish than rainbow trout, although rainbow trout and Neosho bass between 200 and 299 mm consumed equal-sized crayfish. Outcomes of rainbow trout stockings are likely density- and size-dependent, but caution is warranted for introductions in warmwater streams.

Evaluation of different packaging methods and storage temperature on MPs abundance and fillet quality of rainbow trout

There are many studies on microplastics (MPs) about the aquatic ecosystems and its components. However, there is limited study on the MPs abundance, identification and sources in processed seafood products which are manufactured for direct human consumption. In this study, rainbow trout (Oncorhynchus mykiss) fillets were packed with different packaging techniques and stored at two different temperatures (+4 and −20°C) for 21 days. The presence, shape, size and polymer type of MPs were determined by ATR-FTIR on certain days (7, 14 and 21 days) in fillets during storage. The chemical quality changes in fillets [with pH, thiobarbituric acid reactive substrate (TBARS), and total volatile basic nitrogen (TVB-N) data] were monitored and the effect of MPs presence was evaluated. At the last step, the estimated MPs intake level in humans was determined with considering the presence of MPs (determined in fillets). The presence of MPs was determined the most in the Polystyrene plate + wrapped film (S) group and the least in the Chitosan film + Polystyrene plate + wrapped film (C) group. When evaluated in terms of chemical parameters, although good results were obtained in all samples stored at − 20°C, the presence of MPs was determined at a high level in fillets which stored at this temperature. As a result of the study, it was determined that the packaging type and storage temperature have significant effects on the presence of MPs and fillet quality.

Population estimate of wild rainbow trout in a remote stream of southern California

Recreational fishing for Rainbow Trout (Oncorhynchus mykiss) is important economically in California. We determined the upstream and downstream distribution of Rainbow Trout in a southern California stream, and classified all available habitat within that area as riffle, pool, cascade-pool-complex, and flatwater. Approximately 10% (based on length) of each habitat type was sampled using depletion electrofishing. We estimated Rainbow Trout abundance, both &lt;100 mm and ≥100 mm, by extrapolating average number of fish per m² in each habitat type sampled to the total m² of each habitat type. A total of 854 fish were captured, with the greatest proportion coming from the cascade-pool-complex habitat type, followed by pool, flatwater, and riffle. The population estimate for Rainbow Trout &lt;100 mm was 1,763 fish (95% CI ±442), and for Rainbow Trout ≥100 mm was 5,383 fish (95% CI ±1,688).

Surprising salmonid response to water diversion at four run-of-river hydroelectric projects in British Columbia

Run-of-river (RoR) hydroelectric power provides renewable energy with potentially less impact on fish habitat productivity than large, reservoir-storage hydroelectric projects. This is the first empirical study of resident fish response to water diversion at RoR projects in BC, where the abundance of rainbow trout (Oncorhynchus mykiss) was monitored in four small streams using a before–after control–impact design. Although flows in the diversion reaches of these projects were reduced by 63% to 70%, the total biomass of rainbow trout increased from 35% to 157% across the streams compared with the “before period” and “control reaches”. The response to water diversion differed among age classes: adult biomass increased with reduced flows in the growing season; juvenile biomass increased with reduced flows in the winter, higher stream conductivity, and increased flows in late summer; and fry biomass increased with higher stream conductivity and temperatures. One novel way to contextualize these shifts in resident salmonid productivity and differential growth and (or) survival by age class is via a size–density approach, which we use here to demonstrate how the carrying capacity of a stream for rainbow trout is affected by water diversion.

Effects of Wintertime Stocking of Rainbow Trout on the Forage Community of an Oklahoma Impoundment

AbstractRainbow Trout Oncorhynchus mykiss are sometimes stocked to create put‐and‐take fisheries in small impoundments. Information about the effects of these stockings on existing piscivores is poorly understood. Therefore, we performed a study to evaluate the forage consumption by Rainbow Trout stocked in Lake Carl Etling, Oklahoma. A creel survey during the 2014–2015 trout season determined that angling pressure was low, which left the majority of stocked trout in the lake until their thermal maximum was exceeded in late May–June. Rainbow Trout stomach contents were collected monthly from November through April annually (2015–2018) via gastric lavage. Gizzard Shad Dorosoma cepedianum were consumed during most months, but their occurrence in Rainbow Trout stomachs was highest in January and December. A sharp decline in Gizzard Shad consumption was followed by an increase in detritus consumption each year. Using either Gizzard Shad numbers observed in monthly Rainbow Trout diets or the estimated monthly abundance of Rainbow Trout, we postulated that Rainbow Trout substantially affected age‐0 Gizzard Shad biomass (reducing it by 66.7 or 54.6 kg/ha, respectively), potentially negatively affecting growth rates of other piscivores in the system. This study demonstrates the unintended consequences of stocking an additional predatory fish into small impoundments and the importance of evaluating angler harvest of these stocking programs to ensure the stocked species receives sufficient angler harvest to match stocking rates and prevent negative impacts on the fish community (both forage and other piscivorous species).

Inferring species interactions through joint mark-recapture analysis.

Introduced species are frequently implicated in declines of native species. In many cases, however, evidence linking introduced species to native declines is weak. Failure to make strong inferences regarding the role of introduced species can hamper attempts to predict population viability and delay effective management responses. For many species, mark-recapture analysis is the more rigorous form of demographic analysis. However, to our knowledge, there are no mark-recapture models that allow for joint modeling of interacting species. Here, we introduce a two-species mark-recapture population model in which the vital rates (and capture probabilities) of one species are allowed to vary in response to the abundance of the other species. We use a simulation study to explore bias and choose an approach to model selection. We then use the model to investigate species interactions between endangered humpback chub (Gila cypha) and introduced rainbow trout (Oncorhynchus mykiss) in the Colorado River between 2009 and 2016. In particular, we test hypotheses about how two environmental factors (turbidity and temperature), intraspecific density dependence, and rainbow trout abundance are related to survival, growth, and capture of juvenile humpback chub. We also project the long-term effects of different rainbow trout abundances on adult humpback chub abundances. Our simulation study suggests this approach has minimal bias under potentially challenging circumstances (i.e., low capture probabilities) that characterized our application and that model selection using indicator variables could reliably identify the true generating model even when process error was high. When the model was applied to rainbow trout and humpback chub, we identified negative relationships between rainbow trout abundance and the survival, growth, and capture probability of juvenile humpback chub. Effects on interspecific interactions on survival and capture probability were strongly supported, whereas support for the growth effect was weaker. Environmental factors were also identified to be important and in many cases stronger than interspecific interactions, and there was still substantial unexplained variation in growth and survival rates. The general approach presented here for combining mark-recapture data for two species is applicable in many other systems and could be modified to model abundance of the invader via other modeling approaches.

Identifying cost-effective invasive species control to enhance endangered species populations in the Grand Canyon, USA

Recovering endangered species populations when confronted with the threat of invasive species is an ongoing natural resource management challenge. While eradication of the invasive species is often the optimal economic solution, it may not be a feasible nor desirable management action in other cases. For example, when invasive species are desired in one area, but disperse into areas managed for endangered species, managers may be interested in persistent, but cost-effective means of managing dispersers rather than eradicating the source. In the Colorado River, a nonnative rainbow trout (Oncorhynchus mykiss) sport fishery is desired within Glen Canyon National Recreation Area, however, dispersal downriver into the Grand Canyon National Park is not desired as rainbow trout negatively affect endangered humpback chub (Gila cypha). Here, we developed a bioeconomic model incorporating population abundance goals and cost-effectiveness analyses to approximate the optimal control strategies for invasive rainbow trout conditional on achieving endangered humpback chub adult population abundance goals. Model results indicated that the most cost-effective approach to achieve target adult humpback chub abundance was a high level of rainbow trout control over moderately high rainbow trout population abundance. Adult humpback chub abundance goals were achieved at relatively low rainbow trout abundance and control measures were not cost-effective at relatively high rainbow trout abundance. Our model considered population level dynamics, species interaction and economic costs in a multi-objective decision framework to provide a preferred solution to long-run management of invasive and native species.

The fishing and natural mortality of large, piscivorous Bull Trout and Rainbow Trout in Kootenay Lake, British Columbia (2008-2013).

BackgroundEstimates of fishing and natural mortality are important for understanding, and ultimately managing, commercial and recreational fisheries. High reward tags with fixed station acoustic telemetry provides a promising approach to monitoring mortality rates in large lake recreational fisheries. Kootenay Lake is a large lake which supports an important recreational fishery for large Bull Trout and Rainbow Trout.MethodsBetween 2008 and 2013, 88 large (≥500 mm) Bull Trout and 149 large (≥500 mm) Rainbow Trout were marked with an acoustic transmitter and/or high reward ($100) anchor tags in Kootenay Lake. The subsequent detections and angler recaptures were analysed using a Bayesian individual state-space Cormack–Jolly–Seber (CJS) survival model with indicator variable selection.ResultsThe final CJS survival model estimated that the annual interval probability of being recaptured by an angler was 0.17 (95% CRI [0.11–0.23]) for Bull Trout and 0.14 (95% CRI [0.09–0.19]) for Rainbow Trout. The annual interval survival probability for Bull Trout was estimated to have declined from 0.91 (95% CRI [0.76–0.97]) in 2009 to just 0.46 (95% CRI [0.24–0.76]) in 2013. Rainbow Trout survival was most strongly affected by spawning. The annual interval survival probability was 0.77 (95% CRI [0.68–0.85]) for a non-spawning Rainbow Trout compared to 0.41 (95% CRI [0.30–0.53]) for a spawner. The probability of spawning increased with the fork length for both species and decreased over the course of the study for Rainbow Trout.DiscussionFishing mortality was relatively low and constant while natural mortality was relatively high and variable. The results indicate that angler effort is not the primary driver of short-term population fluctations in the Rainbow Trout abundance. Variation in the probability of Rainbow Trout spawning suggests that the spring escapement at the outflow of Trout Lake may be a less reliable index of abundance than previously assumed. Multi-species stock assessment models need to account for the fact that large Bull Trout are more abundant than large Rainbow Trout in Kootenay Lake.

Evaluation of Rainbow Trout Abundance, Biomass, and Condition Following Coho Salmon Reintroduction in Taneum Creek, Washington

Abstract Reintroducing fish to previously occupied habitats appears promising for recovery of extirpated fish populations in cold water systems. Uncertainty still exists surrounding the ecological effects of reintroductions however, particularly when they involve historically sympatric taxa. We initiated a study to estimate any potential impacts to rainbow/steelhead trout (Oncorhynchus mykiss) that may arise from reintroducing coho salmon (O. kisutch) in Taneum Creek, Washington, following their extirpation approximately 100 years ago. Prior to reintroducing coho salmon into Taneum Creek, we conducted a formal risk assessment to predict potential impacts to rainbow trout that might result from restoring coho salmon natural production in this stream. Following the assessment, adult coho salmon were released to spawn naturally in experimental reaches in Taneum Creek during a five year period, 2008–2012. Rainbow trout abundance, average size, condition, and growth were not reduced in our experimental reaches...

Estimating disperser abundance using open population models that incorporate data from continuous detection passive integrated transponder arrays

Autonomous passive integrated transponder (PIT) tag antenna systems continuously detect individually marked organisms at one or more fixed points over long time periods. Estimating abundance using data from autonomous antennas can be challenging because these systems do not detect unmarked individuals. Here we pair PIT antenna data from a tributary with mark–recapture sampling data in a mainstem river to estimate the number of fish moving from the mainstem to the tributary. We then use our model to estimate abundance of non-native rainbow trout (Oncorhynchus mykiss) that move from the Colorado River to the Little Colorado River, the latter of which is important spawning and rearing habitat for federally endangered humpback chub (Gila cypha). We estimate that 226 rainbow trout (95% confidence interval: 127–370) entered the Little Colorado River from October 2013 to April 2014. We discuss the challenges of incorporating detections from autonomous PIT antenna systems into mark–recapture population models, particularly in regards to using information about spatial location to estimate movement and detection probabilities.

Impacts of Northern Pike on Stocked Rainbow Trout in Pactola Reservoir, South Dakota

AbstractEstablishment of nonnative Northern Pike Esox lucius in Pactola Reservoir, South Dakota, has prompted concern among biologists about the influence of this species on the lake's intensively managed salmonid fisheries. Ancedotal information suggests that catch rates of Rainbow Trout Oncorhynchus mykiss have declined while mean size and abundance of Northern Pike has increased, although quantitative information on diet and growth of the Northern Pike population is lacking. To address potential interactions between Northern Pike and Rainbow Trout, we assessed size‐dependent predation by Northern Pike on Rainbow Trout and determined the relative energetic contribution of stocked Rainbow Trout to Northern Pike growth using bioenergetics modeling. Stable isotopes combined with traditional diet analyses revealed that smaller Northern Pike (&lt;600 mm TL) consumed primarily centrarchids and Rainbow Smelt Osmerus mordax, and Rainbow Trout contributed less than 10% to their annual energy consumption. In contrast, larger Northern Pike (≥600 mm TL) consumed primarily Rainbow Trout, which accounted for 56% of their annual energy consumption. Combining estimates of Northern Pike predation with production costs of catchable‐size Rainbow Trout revealed that annual economic losses ranged from US$15,259 to $24,801 per year. Over its lifespan, an age‐10 Northern Pike was estimated to consume ~117 Rainbow Trout worth approximately $340. Thus, Northern Pike predation substantially influences salmonid management initiatives and is likely a primary factor contributing to reduced Rainbow Trout abundance and return to anglers in Pactola Reservoir. Strategies for reducing Northern Pike predation on Rainbow Trout include increasing the size of stocked fish or altering the timing and spatial distribution of stocking events.Received June 5, 2015; accepted November 2, 2015 Published online March 8, 2016

Factors controlling the abundance of rainbow trout in the Colorado River in Grand Canyon in a reach utilized by endangered humpback chub

We estimated the abundance, survival, movement, and recruitment of non-native rainbow trout (Oncorhynchus mykiss) in the Colorado River in Grand Canyon to determine what controls their abundance near the Little Colorado River (LCR) confluence where endangered humpback chub (Gila cypha) rear. Over a 3-year period, we tagged more than 70 000 trout and recovered over 8200 tagged fish. Trout density was highest (10 000–25 000 fish·km–1) in the reach closest to Glen Canyon Dam, where the majority of trout recruitment occurs, and was 30- to 50-fold lower (200–800 fish·km–1) in reaches near the LCR confluence ∼100 km downstream. The extent of rainbow trout movement was limited with less than 1% of recaptures making movements greater than 20 km. However, because of high trout densities in upstream source areas, this small dispersal rate was sufficient to explain the threefold increase in the relatively small population near the LCR. Reducing dispersal rates of trout from upstream sources is the most feasible solution to maintain low densities near the LCR to minimize negative effects of competition and predation on humpback chub.

Seasonal and spatial patterns of growth of rainbow trout in the Colorado River in Grand Canyon, Arizona

Rainbow trout (Oncorhynchus mykiss) have been purposely introduced in many regulated rivers, with inadvertent consequences on native fishes. We describe how trout growth rates and condition could be influencing trout population dynamics in a 130 km section of the Colorado River below Glen Canyon Dam based on a large-scale mark–recapture program where ∼8000 rainbow trout were recaptured over a 3-year period (2012–2014). There were strong temporal and spatial variations in growth in both length and weight as predicted from von Bertalanffy and bioenergetic models, respectively. There was more evidence for seasonal variation in the growth coefficient and annual variation in the asymptotic length. Bioenergetic models showed more variability for growth in weight across seasons and years than across reaches. These patterns were consistent with strong seasonal variation in invertebrate drift and effects of turbidity on foraging efficiency. Highest growth rates and relative condition occurred in downstream reaches with lower trout densities. Results indicate that reduction in rainbow trout abundance in Glen Canyon will likely increase trout size in the tailwater fishery and may reduce downstream dispersal into Grand Canyon.

Effects of benthic substrate, nutrient enrichment and predatory fish on freshwater crayfish (kōura, Paranephrops planifrons) population characteristics in seven Te Arawa (Rotorua) lakes, North Island, New Zealand

Populations of kōura, or freshwater crayfish (Paranephrops planifrons White, 1842), were quantified along a eutrophication gradient in seven Te Arawa lakes using the tau kōura, a traditional Māori harvesting method, in April, July and November 2009. Six of the seven lakes contained kōura. No kōura were captured in hypertrophic Lake Ōkaro. Kōura catches were highly variable and mean catch per unit effort (CPUE) varied from 1.5 to 99.3kōura per whakaweku (fern bundle) in the six lakes with kōura present. The highest abundance of kōura was recorded in eutrophic Lake Rotorua and the highest biomass in oligotrophic Lake Rotomā. Kōura orbit carapace length (OCL) ranged from 6 to 52mm, with the largest kōura recorded in lakes Rotoiti and Rotomā. Our results suggest that benthic substrate is more important in determining kōura population abundance than nutrient enrichment (using chlorophyll-a as a proxy) or predatory fish (rainbow trout abundance). Nevertheless, lake trophic status in conjunction with lake morphology appeared to indirectly affect kōura distribution in the sheltered, steep-sided lakes, through hypolimnetic deoxygenation.

Rainbow trout performance in food-limited environments: implications for future assessment and management

We evaluated the performance of rainbow trout in food-limited lake and hatchery environments using whole-body water content as a proxy for fish energy reserves and lipid content. Relative abundance of rainbow trout stocked in an oligotrophic lake from 2002 to 2006 decreased by 88% in 145 days. Whole-body water content of rainbow trout increased following stocking in the lake and similar increases in water content were observed in fish from a food-deprived hatchery treatment. Water content in the fed hatchery fish was significantly lower than water content observed in stocked lake fish. Traditional metrics of body condition (i.e., Fulton's K and relative weight) based on length–weight relationships were insufficient to detect the observed changes in whole-body water content for all lake and hatchery treatments. We conclude that depletion of energy reserves contributed to poor survival and low angling returns of stocked rainbow trout in the study lake.

Effects of Stocking Catchable‐Sized Hatchery Rainbow Trout on Wild Rainbow Trout Abundance, Survival, Growth, and Recruitment

AbstractThe Idaho Department of Fish and Game has proactively dealt with the potential adverse genetic effects of stocking catchable‐sized hatchery trout in waters that support native salmonids by adopting a policy in 2001 whereby only sterile rainbow trout Oncorhynchus mykiss are stocked in flowing waters; however, concerns regarding the competitive effects of introducing hatchery trout into streams and rivers supporting wild trout have not been addressed. We stocked fish in the middle 3 years of a 5‐year study to assess whether stocking hatchery rainbow trout of catchable size (hereafter, catchables) reduced the abundance, survival, growth, or recruitment of wild rainbow trout in streams. Catchables averaging 249 mm total length (TL) were stocked from 2006 to 2008 at an annual density of 4.2 fish/100 m2 into 12 treatment reaches of stream that were paired with control reaches at least 3 km apart in the same stream in which no stocking occurred. Wild rainbow trout abundance (including all fish ≥75 mm TL), recruitment, survival, and growth were determined from population estimates and recaptures of fish tagged with passive integrated transponder (PIT) tags during mark–recapture electrofishing sampling. The abundance of wild rainbow trout averaged 13.2 fish/100 m2 but varied substantially across sites and years, ranging from a low of 0.5 to a high of 131.3 fish/100 m2; similar variability was observed in recruitment to age 1. Estimates of total annual survival averaged 0.53 based on the population abundance estimates (which allowed for emigration and immigration) and 0.26 based on the PIT‐tag recaptures (which allowed for emigration but not immigration). Our paired study design demonstrated that the abundance, survival, growth, and recruitment to age 1 of wild rainbow trout were all unaffected by stocking catchables. The lack of population‐level effects from stocking catchables was not surprising considering the high short‐term mortality and the socially and physiologically naive behavior typically exhibited by hatchery catchables stocked in lotic systems.

ABIOTIC &amp; BIOTIC RESPONSES OF THE COLORADO RIVER TO CONTROLLED FLOODS AT GLEN CANYON DAM, ARIZONA, USA

ABSTRACTClosure of Glen Canyon Dam reduced sand supply to the Colorado River in Grand Canyon National Park by about 94% while its operation has also eroded the park's sandbar habitats. Three controlled floods released from the dam since 1995 suggest that sandbars might be rebuilt and maintained, but only if repeated floods are timed to follow tributary sand deliveries below the dam. Monitoring data show that sandbars are dynamic and that their erosion after bar building is positively related with mean daily discharge and negatively related with tributary sand production after controlled floods. The March 2008 flood affected non‐native rainbow trout abundance in the Lees Ferry tailwater, which supports a blue ribbon fishery. Downstream trout dispersal from the tailwater results in negative competitive interactions and predation on endangered humpback chub. Early survival rates of age‐0 trout increased more than fourfold following the 2008 flood, and twofold in 2009, relative to prior years (2006–2007). Hatch‐date analysis indicated that early survival rates were much higher for cohorts that emerged about 2 months after the 2008 flood relative to cohorts that emerged earlier that year. The 2009 survival data suggest that tailwater habitat improvements persisted for at least a year, but apparently decreased in 2010. Increased early survival rates for trout coincided with the increased availability of higher quality drifting food items after the 2008 flood owing to an increase in midges and black flies, preferred food items of rainbow trout. Repeated floods from the dam might sustainably rebuild and maintain sandbars if released when new tributary sand is available below the tailwater. Spring flooding might also sustain increased trout abundance and benefit the tailwater fishery, but also be a potential risk to humpback chub in Grand Canyon. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Nonnative Fish Control in the Colorado River in Grand Canyon, Arizona: An Effective Program or Serendipitous Timing?

AbstractThe federally endangered humpback chub Gila cypha in the Colorado River within Grand Canyon is currently the focus of a multiyear program of ecosystem‐level experimentation designed to improve native fish survival and promote population recovery as part of the Glen Canyon Dam Adaptive Management Program. A key element of this experiment was a 4‐year effort to remove nonnative fishes from critical humpback chub habitat, thereby reducing potentially negative interactions between native and nonnative fishes. Over 36,500 fish from 15 species were captured in the mechanical removal reach during 2003–2006. The majority (64%) of the catch consisted of nonnative fish, including rainbow trout Oncorhynchus mykiss (19,020), fathead minnow Pimephales promelas (2,569), common carp Cyprinus carpio (802), and brown trout Salmo trutta (479). Native fish (13,268) constituted 36% of the total catch and included flannelmouth suckers Catostomus latipinnis (7,347), humpback chub (2,606), bluehead suckers Catostomus discobolus (2,243), and speckled dace Rhinichthys osculus (1,072). The contribution of rainbow trout to the overall species composition fell steadily throughout the study period from a high of approximately 90% in January 2003 to less than 10% in August 2006. Overall, the catch of nonnative fish exceeded 95% in January 2003 and fell to less than 50% after July 2005. Our results suggest that removal efforts were successful in rapidly shifting the fish community from one dominated numerically by nonnative species to one dominated by native species. Additionally, increases in juvenile native fish abundance within the removal reach suggest that removal efforts may have promoted greater survival and recruitment. However, drought‐induced increases in river water temperature and a systemwide decrease in rainbow trout abundance concurrent with our experiment made it difficult to determine the cause of the apparent increase in juvenile native fish survival and recruitment. Experimental efforts continue and may be able to distinguish among these factors and to better inform future management actions.