Chub Gila Research Articles

Field Assessment of Colorado pikeminnow Exposure to Mercury Within Its Designated Critical Habitat in Colorado, Utah, and New Mexico

Mercury contamination in freshwater fish is widespread across North America, including the western United States. Atmospheric mercury from both natural and manmade emissions deposits into watersheds and, through methylation and biomagnification, accumulates in aquatic food webs. Highest mercury concentrations are found in predatory fish. The endangered Colorado pikeminnow (Ptychocheilus lucius) is a long-lived, top-level piscivore endemic to the Colorado River basin. Mercury exposure to Colorado pikeminnow and another native fish species, the roundtail chub (Gila robusta), was assessed by analyzing muscle tissues collected using a nonlethal technique. Mercury concentrations in Colorado pikeminnow > 400-mm long, captured from critical habitat throughout the species’ present range, exceeded the tissue threshold-effect level of 0.2 µg/g wet weight (WW) for whole body fish (0.31 µg/g WW in muscle) recommended to protect fish from injury. Mercury is a neurotoxin and endocrine disruptor, and impacts to fish may include reduced ability to avoid predators, secure food, and reproduce. The highest mercury concentrations were found in both Colorado pikeminnow and roundtail chub collected from the White River, a tributary to the Green River. Colorado pikeminnow from the White and Green rivers had the highest mean mercury concentrations and the lowest mean relative body conditions. Exposure to high mercury concentrations may act in concert with other threatening factors to compromise Colorado pikeminnow population viability and eventual recovery.

Food Web Structure Informs Potential Causes of Bimodal Size Structure in a Top Predator

Background: Assemblages of fishes in lakes and reservoirs in the western USA are dominated by non-native, large-bodied, piscivorous fishes that lack a shared evolutionary history. Top predators in these crowded systems are often characterized by unstable population dynamics and poor somatic growth rates. One such assemblage is in Fish Lake, located in southern Utah, USA, in which introduced lake trout (Salvelinus namaycush, Walbaum) exhibit a bimodal growth pattern. A few lake trout in Fish Lake grow rapidly to large size typical of the species; whereas, most never grow beyond 600 mm total length. Objective: To inform competitive interactions in this evolutionarily novel fish assemblage that might cause the low recruitment to large body size in lake trout, we characterized trophic niche (from stable isotope analysis of C and N) of all fishes in the lake. Methods: We used a Bayesian mixing model to describe the trophic niche and infer diet of lake trout and their potential prey, and we used Bayesian ellipse analysis to identify potential areas of high competition within the food web. Large lake trout feed mostly on small lake trout and splake (Salvelinus namaycush, Walbaum x Salvelinus fontinalis, Mitchill) despite availability of abundant yellow perch. (Perca flavescens, Mitchill). Small lake trout and splake feed mostly on zooplankton and exhibit substantial overlap of their trophic niche implying competition for food. Yellow perch and Utah chub (Gila atraria, Girard; formerly an important food item for lake trout in Fish Lake) exhibit extreme overlap of their trophic niche implying strong competitive interactions. Results: Our data suggest that lack of recruitment to large body size in lake trout may result from a reduction in availability of Utah chub resulting from competitive interactions with yellow perch, and increased competition from introduced splake for available prey. Conclusion: Management actions that may help ameliorate the poor somatic growth rates of most lake trout include efforts to reduce perch populations or increase vulnerability of perch to predation by lake trout, and removal of splake as a competitor of small lake trout.

Are Hatchery‐Reared Rainbow Trout and Brown Trout Effective Predators on Juvenile Native Fish?

AbstractHatchery‐reared Rainbow Trout Oncorhynchus mykiss and Brown Trout Salmo trutta are typically fed exclusively on commercially prepared pelleted feeds and have no experience catching or consuming live fish at the time of stocking. Despite this lack of predation experience, it is commonly assumed that stocked Rainbow Trout and Brown Trout will adversely impact native fish populations by preying upon juvenile native fish. We evaluated the relative predation effectiveness of wild‐caught Rainbow Trout (210–389 mm TL) and Brown Trout (185–313 mm TL) compared with hatchery‐reared Rainbow Trout (198–321 mm TL) and Brown Trout (196–290 mm TL). We used Bonytail Gila elegans (60–85 mm TL), Humpback Chub Gila cypha (24–59 mm TL), and Roundtail Chub Gila robusta (40–65 mm TL) as prey in overnight predation trials conducted in the laboratory from 2013 to 2016. After 14 d in a captive setting, wild Rainbow Trout and Brown Trout consumed >70% of prey in trials with no cover, while hatchery‐reared fish consumed <30% of prey. In addition, we evaluated if the predation ability of hatchery fish would improve over time by feeding them Fathead Minnows Pimephales promelas, rather than pelleted feed, for up to 30 d. Predation success of Rainbow Trout and Brown Trout increased by an average of 28% and 21%, respectively, after 14 d of eating exclusively fish. Rainbow Trout tested after 30 d of eating fish increased in their ability to catch and eat small prey by an average of 29%. The predation effectiveness of hatchery‐reared fish appears to improve with experience eating live fish. Although stocking Rainbow Trout and Brown Trout does increase the number of predators present in natural systems, the relative predation threat posed by hatchery‐reared fish may be less than that of wild fish, especially in locations where stocked fish do not persist. Lack of experience in catching fish and the effects of captive rearing practices and environments on both physiology and behavior all likely contribute to reduced predation effectiveness of hatchery Rainbow Trout and Brown Trout.

Effectiveness of Ultrasonic Imaging for Evaluating Presence and Maturity of Eggs in Fishes in Remote Field Locations

AbstractUltrasonic imaging is an effective, nonlethal method used to determine sex and maturity in a variety of fishes. However, many previous studies of this technique have been performed in laboratory environments. Using Common Carp Cyprinus carpio, we developed a standardized method for ultrasonically scanning cyprinids, and we accurately sexed 78% (N = 58) of those individuals. We then applied this method to endangered Humpback Chub Gila cypha and obtained 751 scans of Humpback Chub in remote field locations within the Grand Canyon, Arizona. Ultrasonic scanning took less than 1 min per fish to perform, and we identified females containing eggs based on two jpeg images and one 10‐s video clip. Using ImageJ software to evaluate ultrasonic scans of ovulatory, captive Humpback Chub, we determined that female fish were potentially ovulatory when scanned eggs exhibited an ImageJ brightness value within the range of 32–44. Although we could successfully evaluate egg maturity, we were unable to estimate egg mass. Fisheries managers can use this noninvasive scanning method or a similar technique in remote field locations to collect ultrasonic scans from cyprinids to determine their reproductive status.

Humpback Chub (Gila cypha) Range Expansion in the Western Grand Canyon

The Colorado River in the Grand Canyon holds the largest remaining population of Humpback Chub Gila cypha, an endangered fish endemic to the Colorado River basin. Early surveys in the 1990s found that most Humpback Chub occupied the Little Colorado River and nearby areas in the mainstem Colorado River and were uncommon in the western Grand Canyon (below Kanab Creek, at river kilometer [rkm] 257.2). From 1939 to 2002, the Colorado River was typically inundated by Lake Mead to rkm 407 (at full pool). Since 2000, Lake Mead water levels have declined, and the current inflow is located almost 100 km downstream at rkm 503.1. Thus, the Arizona Game and Fish Department (AGFD) extended its system-wide monitoring downstream to Pearce Ferry rapid (rkm 478.6) in 2011. Electrofishing is relatively inefficient at capturing Humpback Chub, but a small number of captures from 2011 to 2015 in the western Grand Canyon suggested that the Humpback Chub may have expanded its range downstream. Subsequently, hoop nets were added to AGFD system-wide monitoring in 2016 to better describe Humpback Chub distribution. The distribution of Humpback Chub observed in 2016 and 2017 differed from previous descriptions; we documented relatively large numbers of Humpback Chub, including age-0 fish, in the western Grand Canyon and observed the highest catches downstream of Diamond Creek (rkm 389.1). This suggests that a recent range expansion has occurred and that Humpback Chub in the western Grand Canyon do not utilize the Little Colorado River for reproduction and might therefore constitute a new subpopulation.

Does Bioelectrical Impedance Analysis Accurately Estimate the Physiological Condition of Threatened and Endangered Desert Fish Species?

AbstractBioelectrical impedance analysis (BIA) is a nonlethal tool with which to estimate the physiological condition of animals that has potential value in research on endangered species. However, the effectiveness of BIA varies by species, the methodology continues to be refined, and incidental mortality rates are unknown. Under laboratory conditions we tested the value of using BIA in addition to morphological measurements such as total length and wet mass to estimate proximate composition (lipid, protein, ash, water, dry mass, energy density) in the endangered Humpback Chub Gila cypha and Bonytail G. elegans and the species of concern Roundtail Chub G. robusta and conducted separate trials to estimate the mortality rates of these sensitive species. Although Humpback and Roundtail Chub exhibited no or low mortality in response to taking BIA measurements versus handling for length and wet‐mass measurements, Bonytails exhibited 14% and 47% mortality in the BIA and handling experiments, respectively, indicating that survival following stress is species specific. Derived BIA measurements were included in the best models for most proximate components; however, the added value of BIA as a predictor was marginal except in the absence of accurate wet‐mass data. Bioelectrical impedance analysis improved the R2 of the best percentage‐based models by no more than 4% relative to models based on morphology. Simulated field conditions indicated that BIA models became increasingly better than morphometric models at estimating proximate composition as the observation error around wet‐mass measurements increased. However, since the overall proportion of variance explained by percentage‐based models was low and BIA was mostly a redundant predictor, we caution against the use of BIA in field applications for these sensitive fish species.Received November 7, 2016; accepted March 1, 2017 Published online July 5, 2017

Estimating Ages of Utah Chubs by Use of Pectoral Fin Rays, Otoliths, and Scales

Abstract. Utah chub Gila atraria is native to the Upper Snake River system in Wyoming and Idaho and to the Lake Bonneville Basin in Utah and southeastern Idaho. However, the Utah chub has been introduced into many other waterbodies in the western United States, where it competes with ecologically and economically important species. The objectives of this study were to evaluate between-reader precision and reader confidence in age estimates obtained from pectoral fin rays, lapilli (otoliths), asterisci (otoliths), and scales for Utah chubs collected from Henrys Lake, Idaho. Lapilli have been previously shown to provide accurate age estimates for Utah chubs; therefore, we sought to compare age estimates from fin rays, asterisci, and scales to those from lapilli. The between-reader coefficient of variation (CV) in age estimates was lowest and the percent of exact reader agreement (PA-0) was highest for pectoral fin rays (CV = 4.7, PA-0 = 74%), followed by scales (CV = 10.3, PA-0 = 52.3%), lapilli (CV = 11.6,...

Earning their Stripes: The Potential of Tiger Trout and Other Salmonids as Biological Controls of Forage Fishes in a Western Reservoir

AbstractMaintaining a balance between predator and prey populations can be an ongoing challenge for fisheries managers, especially in managing artificial ecosystems such as reservoirs. In a high‐elevation Utah reservoir, the unintentional introduction of the Utah Chub Gila atraria and its subsequent population expansion prompted managers to experimentally shift from exclusively stocking Rainbow Trout Oncorhynchus mykiss to also stocking tiger trout (female Brown Trout Salmo trutta × male Brook Trout Salvelinus fontinalis) and Bonneville Cutthroat Trout O. clarkii utah (hereafter, Cutthroat Trout) as potential biological control agents. We measured a combination of diet, growth, temperature, and abundance and used bioenergetic simulations to quantify predator demand versus prey supply. Utah Chub were the predominant prey type for tiger trout, contributing up to 80% of the diet depending on the season. Utah Chub represented up to 70% of the total diet consumed by Cutthroat Trout. Although Utah Chub dominated the fish biomass in the reservoir, we still estimated abundances of 238,000 tiger trout, 214,000 Cutthroat Trout, and 55,000 Rainbow Trout. Consequently, when expanded to the population level of each predator, tiger trout and Cutthroat Trout consumed large quantities of Utah Chub on an annual basis: tiger trout consumed 508,000 kg (2,660 g/predator) of the standing prey population, and Cutthroat Trout consumed an estimated 322,000 kg (1,820 g/predator). The estimated combined consumption by Cutthroat Trout and tiger trout exceeded the estimate of Utah Chub annual production. As such, our results suggest that the high rates of piscivory exhibited by Cutthroat Trout and tiger trout in artificial lentic ecosystems are likely sufficient to effectively reduce the overall abundance of forage fishes and to prevent forage fishes from dominating fish assemblages. Collectively, this research provides the first documented findings on tiger trout ecology and performance, which will aid managers in designing and implementing the best stocking strategy to optimize sport fish performance, control undesirable forage fish, and enhance and maintain angler satisfaction.Received April 1, 2016; accepted November 8, 2016 Published online March 8, 2017

Growth of Endangered Humpback Chub in Relation to Temperature and Discharge in the Lower Colorado River

Abstract Assessments of growth can provide information needed to understand how fish populations respond to changing environmental conditions and management actions, including ecosystem experimentation. We estimated growth rates and parameter uncertainty from otoliths of endangered Humpback Chub Gila cypha from the Colorado River in Grand Canyon, Arizona. We then compared growth of Humpback Chub < age 2 that were 1) occupying the mainstem Colorado River during a period of variable discharge and cooler water temperatures (1980–1998; epoch 1), 2) occupying the Colorado River during a period of moderate discharge variability and warmer water (2001–2011; epoch 2), and 3) occupying the unregulated Little Colorado River. Because growth rates of juvenile Humpback Chub (< age 2) may be more sensitive to changes in environmental conditions than adult fish, we used analysis of covariance and linear models to compare growth of juvenile fish (slopes) between epochs and capture sites (mainstem Colorado River vs. Little Colorado River). Our analysis of covariance results were ambiguous (age × epoch × site interaction; P = 0.06). However, individual linear regressions of size and age by epoch and site suggest biologically important differences in growth, as evidenced by slower growth in the Colorado River in epoch 1 than in epoch 2, and slower growth in the Colorado River compared with the Little Colorado River for all time periods. Overall our results 1) provide information on growth and growth variability useful for parameterizing models to assess population viability and 2) provide empirical information on how growth of juvenile and adult Humpback Chub growth may respond to changing environmental conditions.

Body Condition of Endangered Humpback Chub in Relation to Temperature and Discharge in the Lower Colorado River

Abstract Determining the population status of endangered Humpback Chub Gila cypha is a major component of the adaptive management program designed to inform operation of Glen Canyon Dam upstream from Grand Canyon, Arizona. In recent decades, resource managers have identified a portfolio of management actions (with intermittent implementation) to promote population recovery of Humpback Chub, including nonnative fish removal, changes in water release volumes and discharge ramping schedules, and reductions in hydropower peaking operations. The Humpback Chub population in Grand Canyon has increased over this same period, causal factors for which are unclear. We took advantage of unusual hydrology in the Colorado River basin in 2011 to assess trends in juvenile Humpback Chub length–weight relationships and condition in the Colorado River below Glen Canyon Dam as well as in the unregulated Little Colorado River. Within each river, we observed higher length–weight b-parameter estimates (exponent of the standard power equation) at higher water temperatures. We also found higher slope estimates for the length–weight relationship at higher temperatures in the Little Colorado River. Slope estimates were more variable in the Colorado River, where mean water temperatures were more uniform. The next step is to examine whether Humpback Chub length–weight relationships influence population metrics such as abundance or survival. If these relationships exist, then monitoring condition in juvenile Humpback Chub would provide a quick and low-cost technique for assessing population response to planned management experiments or changing environmental conditions.

Molecular Genetics Informs Spatial Segregation of Two Desert Stream Gila Species

AbstractHeadwater Chub Gila nigra and Roundtail Chub G. robusta are two distinct but morphologically similar cyprinid species that historically were parapatric in Fossil Creek (Gila River basin) in central Arizona. The creek was chemically treated in 2005 to eradicate nonnative fishes, and chubs salvaged before the treatment were repatriated afterward. Annual samples from multiple sites along a transect throughout the treated reach were characterized for mtDNA and microsatellites from 2008 to 2014. Genetic variation was compared within and among Fossil Creek samples and with reference material for Headwater Chub from Fossil Springs at the headwaters of the creek and for Roundtail Chub from the Verde River and West Clear Creek. Measures of mtDNA and microsatellite diversity identified significant differences among Headwater Chub from Fossil Springs, Roundtail Chub from the Verde River, and samples above and below Irving Falls, a natural fish barrier within the treatment reach. The frequency of Headwater Chub mtDNA was low at the most downstream sampling sites, progressively increasing upstream. There was also temporal variation, with Headwater Chub mtDNA generally becoming rarer in later samples. F‐statistic analysis of microsatellite data and Bayesian clustering of microsatellite genotypes with STRUCTURE were generally consistent and identified three groups: Headwater Chub and two forms of Roundtail Chub. Using the species assignments provided by STRUCTURE and mitotypes, we found 153 “pure” Headwater Chub and only a single pure Roundtail Chub above Irving Falls and 27 pure Headwater Chub and 398 pure Roundtail Chub below the falls. Tests of cytonuclear disequilibrium indicated nonrandom mating (with the rare species more likely to mate than the more common one) and/or selection against hybrids. These data support the recognition of the Headwater Chub as a distinct full species and demonstrate that conservation actions within the genus Gila should typically be carried out at the population level.Received April 8, 2016; accepted August 16, 2016 Published online December 2, 2016

Spawning and Hatching of Endangered Gila Chub in Captivity

AbstractInformation on reproductive characteristics of the endangered Gila Chub Gila intermedia is largely limited and qualitative, and culture techniques and requirements are virtually unknown. Here we provide the first published data on spawning and selected reproductive and developmental characteristics of Gila Chub. Fish were brought to the laboratory in March 2003 from Sabino Creek, Arizona (12.3°C). Fish were then warmed slowly and spawned at 14.9°C, 10 d after collection. Following this initial spawning, Gila Chub spawned consistently in the laboratory without hormonal, chemical, photoperiod, temperature, or substrate manipulation during all times of the year. Spawns were noted at temperatures ranging from about 15°C to 26°C; however, we noted that Gila Chub spawned less frequently at temperatures above 24°C. Multiple spawning attempts per year per individual are probable. There was a strong, inverse relationship between time to hatch and incubation temperature. The hatch rate of eggs was high (mean = 99.43%), and larval Gila Chub accepted a variety of natural and formulated diets at first feeding. The future of Gila Chub may someday depend in part on hatchery propagation to provide specimens for restocking formerly occupied habitats and establishing refuge populations. Information from our study can aid future efforts to successfully spawn and rear Gila Chub and related species.

Morphometric and meristic characterization of the endemic Desert chub Gila eremica (Teleostei: Cyprinidae), and its related congeners in Sonora, Mexico

The Desert chub, Gila eremica DeMarais, 1991 is a freshwater fish endemic to Northwest Mexico, being described from the Sonora, Matape and Yaqui River basins in Sonora, Mexico. The recent discovery of 2 isolated small populations from the known distribution for this taxon makes necessary an evaluation to determine their specific taxonomical identities (herein designated as G. cf. eremica). Thirty-three morphometric and 6 meristic characters were evaluated in 219 specimens of several populations of the genus Gila in Sonora, including all the known populations of G. eremica. Morphometric measures based on box-truss protocol were standardized by regression models, and analyzed by means of a discriminant function analysis (DFA). The canonical variables 1 and 2 explained 72.4% of the total variation among analyzed populations. Tree diagram based on squared Mahalanobis distances, as well as scatterplots of centroids, showed G. cf. eremica to be morphologically divergent with respect to known populations of G. eremica as well as the other species analyzed. Box plots indicated a slimmer body and a smaller number of pectoral fin rays, and a higher number of lateral line scales as the most distinguishable characters in G. cf. eremica with respect to nominal G. eremica.

Geographic distribution of genetic diversity in populations of Rio Grande Chub Gila pandora

In the southwestern United States (US), the Rio Grande chub (Gila pandora) is state-listed as a fish species of greatest conservation need and federally listed as sensitive due to habitat alterations and competition with non-native fishes. Characterizing genetic diversity, genetic population structure, and effective number of breeders will assist with conservation efforts by providing a baseline of genetic metrics. Genetic relatedness within and among G. pandora populations throughout New Mexico was characterized using 11 microsatellite loci among 15 populations in three drainage basins (Rio Grande, Pecos, Canadian). Observed heterozygosity (HO) ranged from 0.71–0.87 and was similar to expected heterozygosity (0.75–0.87). Rio Ojo Caliente (Rio Grande) had the highest allelic richness (AR = 15.09), while Upper Rio Bonito (Pecos) had the lowest allelic richness (AR = 6.75). Genetic differentiation existed among all populations with the lowest genetic variation occurring within the Pecos drainage. STRUCTURE analysis revealed seven genetic clusters. Populations of G. pandora within the upper Rio Grande drainage (Rio Ojo Caliente, Rio Vallecitos, Rio Pueblo de Taos) had high levels of admixture with Q-values ranging from 0.30–0.50. In contrast, populations within the Pecos drainage (Pecos River and Upper Rio Bonito) had low levels of admixture (Q = 0.94 and 0.87, respectively). Estimates of effective number of breeders (N b ) varied from 6.1 (Pecos: Upper Rio Bonito) to 109.7 (Rio Grande: Rio Penasco) indicating that populations in the Pecos drainage are at risk of extirpation. In the event that management actions are deemed necessary to preserve or increase genetic diversity of G. pandora, consideration must be given as to which populations are selected for translocation.

Effects of Turbidity on Predation Vulnerability of Juvenile Humpback Chub to Rainbow Trout and Brown Trout

Abstract Predation on juvenile native fish by introduced rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta is considered a significant threat to the persistence of endangered humpback chub Gila cypha in the Colorado River in Grand Canyon. Diet studies of rainbow trout and brown trout in Glen and Grand canyons indicate that these species eat native fish, but impacts are difficult to assess because predation vulnerability is highly variable depending on the physical conditions under which the predation interactions take place. We conducted laboratory experiments to evaluate how short-term predation vulnerability of juvenile humpback chub changes in response to changes in turbidity. In overnight laboratory trials, we exposed hatchery-reared juvenile humpback chub and bonytail Gila elegans (a surrogate for humpback chub) to adult rainbow trout and brown trout at turbidities ranging from 0 to 1,000 formazin nephlometric units. We found that turbidity as low as 25 formazin nephlometric units significantly reduced predation vulnerability of bonytail to rainbow trout and led to a 36% mean increase in survival (24–60%, 95% CI) compared to trials conducted in clear water. Predation vulnerability of bonytail to brown trout at 25 formazin nephlometric units also decreased with increasing turbidity and resulted in a 25% increase in survival on average (17–32%, 95% CI). Understanding the effects of predation by trout on endangered humpback chub is important when evaluating management options aimed at preservation of native fishes in Grand Canyon National Park. This research suggests that relatively small changes in turbidity may be sufficient to alter predation dynamics of trout on humpback chub in the mainstem Colorado River and that turbidity manipulation may warrant further investigation as a fisheries management tool.

Conservation Genetics of an Urban Desert Fish, the Arroyo Chub

AbstractUrbanization, habitat degradation, fragmentation, and invasive species have led to the severe decline or extirpation of many endemic southern California freshwater fish species, including the Arroyo Chub Gila orcuttii, which has declined precipitously in recent years. Classified by the California Department of Fish and Wildlife as a species of high concern, the Arroyo Chub is native to the Los Angeles, San Gabriel, San Luis Rey, Santa Ana, and Santa Margarita rivers and Malibu and San Juan creeks. To examine Arroyo Chub population structure and genetic diversity within the species' native range, we used 10 microsatellite markers to genotype 259 individuals. We observed moderate to high genetic diversity and population differentiation both between and within drainages; Bayesian clustering supported eight distinct clusters of Arroyo Chub corresponding to eight isolated populations. Of these populations, the Big Tujunga Creek population (Los Angeles River) was the least genetically differentiated (genetic differentiation index FST = 0.048–0.208) and also had the highest genetic diversity (observed heterozygosity Ho = 0.890). Populations in Malibu Creek, Pacoima Canyon (Los Angeles River), and the Santa Margarita River were the most genetically differentiated (FST = 0.163–0.400), had the lowest genetic diversity (Ho = 0.556–0.680), and showed evidence of past bottlenecks. Arroyo Chub at these localities are at risk for continued loss of genetic diversity due to drift and small population sizes; therefore, we suggest that in the event of extirpation, translocations from the most closely related source populations should be considered. However, we recommend that management efforts focus on improving habitat quality and habitat area for Arroyo Chub in order to maximize population genetic diversity and adaptive potential over time.Received July 10, 2015; accepted November 16, 2015

Assessing Predation Risks for Small Fish in a Large River Ecosystem between Contrasting Habitats and Turbidity Conditions

Abstract This study examined predation risk for juvenile native fish between two riverine shoreline habitats, backwater and debris fan, across three discrete turbidity levels (low, intermediate, high) to understand environmental risks associated with habitat use in a section of the Colorado River in Grand Canyon, AZ. Inferences are particularly important to juvenile native fish, including the federally endangered humpback chub Gila cypha. This species uses a variety of habitats including backwaters which are often considered important rearing areas. Densities of two likely predators, adult rainbow trout Oncorhynchus mykiss and adult humpback chub, were estimated between habitats using binomial mixture models to examine whether higher predator density was associated with patterns of predation risk. Tethering experiments were used to quantify relative predation risk between habitats and turbidity conditions. Under low and intermediate turbidity conditions, debris fan habitat showed higher relative predation...

Effects of Water Temperature and Fish Size on Predation Vulnerability of Juvenile Humpback Chub to Rainbow Trout and Brown Trout

AbstractPredation on juvenile native fish by introduced Rainbow Trout and Brown Trout is considered a significant threat to the persistence of endangered Humpback Chub Gila cypha in the Colorado River in the Grand Canyon. Diet studies of Rainbow Trout and Brown Trout in Glen and Grand canyons indicate that these species do eat native fish, but impacts are difficult to assess because predation vulnerability is highly variable, depending on prey size, predator size, and the water temperatures under which the predation interactions take place. We conducted laboratory experiments to evaluate how short‐term predation vulnerability of juvenile native fish changes in response to fish size and water temperature using captivity‐reared Humpback Chub, Bonytail, and Roundtail Chub. Juvenile chub 45–90 mm total length (TL) were exposed to adult Rainbow and Brown trouts at 10, 15, and 20°C to measure predation vulnerability as a function of water temperature and fish size. A 1°C increase in water temperature decreased short‐term predation vulnerability of Humpback Chub to Rainbow Trout by about 5%, although the relationship is not linear. Brown Trout were highly piscivorous in the laboratory at any size > 220 mm TL and at all water temperatures we tested. Understanding the effects of predation by trout on endangered Humpback Chub is critical in evaluating management options aimed at preserving native fishes in Grand Canyon National Park.

Population Structure in the Roundtail Chub (Gila robusta Complex) of the Gila River Basin as Determined by Microsatellites: Evolutionary and Conservation Implications

Ten microsatellite loci were characterized for 34 locations from roundtail chub (Gila robusta complex) to better resolve patterns of genetic variation among local populations in the lower Colorado River basin. This group has had a complex taxonomic history and previous molecular analyses failed to identify species diagnostic molecular markers. Our results supported previous molecular studies based on allozymes and DNA sequences, which found that most genetic variance was explained by differences among local populations. Samples from most localities were so divergent species-level diagnostic markers were not found. Some geographic samples were discordant with current taxonomy due to admixture or misidentification; therefore, additional morphological studies are necessary. Differences in spatial genetic structure were consistent with differences in connectivity of stream habitats, with the typically mainstem species, G. robusta, exhibiting greater genetic connectedness within the Gila River drainage. No species exhibited strong isolation by distance over the entire stream network, but the two species typically found in headwaters, G. nigra and G. intermedia, exhibited greater than expected genetic similarity between geographically proximate populations, and usually clustered with individuals from the same geographic location and/or sub-basin. These results highlight the significance of microevolutionary processes and importance of maintaining local populations to maximize evolutionary potential for this complex. Augmentation stocking as a conservation management strategy should only occur under extreme circumstances, and potential source populations should be geographically proximate stocks of the same species, especially for the headwater forms.

Exploring Crowded Trophic Niche Space in a Novel Reservoir Fish Assemblage: How Many is Too Many?

AbstractIn highly managed reservoir systems, species interactions within novel fish assemblages can be difficult to predict. In high‐elevation Scofield Reservoir in Utah the unintentional introduction of Utah Chub Gila atraria and subsequent population expansion prompted a shift from stocking exclusively Rainbow Trout Oncorhynchus mykiss to include tiger trout (female Brown Trout Salmo trutta × male Brook Trout Salvelinus fontinalis) and Bonneville Cutthroat Trout O. clarkii utah, which composed a novel suite of top predators and potential competitors. We examined the interspecific interactions among Scofield Reservoir piscivores using a multifaceted approach including gut analyses, stable isotopes, and gape limitation. Large Cutthroat Trout consumed 50–100% Utah Chub and tiger trout consumed 45–80%. In contrast, small and large Rainbow Trout consumed primarily invertebrate prey and exhibited significant overlap with small tiger trout, Cutthroat Trout, and Utah Chub. Large Cutthroat Trout and tiger trout occupy a top piscivore trophic niche and are more littoral, while Rainbow Trout occupy an omnivore niche space and are more pelagic. Both Cutthroat and tiger trout varied in niche space with respect to size‐class, demonstrating an ontogenetic shift to piscivory at approximately 350 mm TL. Cutthroat Trout and tiger trout are capable of consuming prey up to 50% of their own size, which is larger than predicted based on their theoretical gape limit. Because it appears food resources (Utah Chub) are not limited, and performance metrics are high, competition is unlikely between Cutthroat Trout and tiger trout. In contrast, apparent survival of Rainbow Trout has recently declined significantly, potentially due to shared food resources with Utah Chub or negative behavioral interactions with other members of the community. Collectively, this research aids in understanding biotic interactions within a top‐heavy and novel fish community and assists towards developing and implementing suitable management strategies to control nuisance species.