Bluehead Sucker Research Articles

Anthropogenic Impacts Facilitate Native Fish Hybridization in the Bonneville Basin of Western North America

AbstractHabitat modification and introduced species are impacting native fishes in western North America, a situation now exacerbated by drought and anthropogenic water acquisition. One species of concern, the Bluehead Sucker Catostomus discobolus in the Bonneville basin of Utah, Idaho, and Wyoming, has experienced sharp population declines potentially augmented by hybridization with native Utah Sucker C. ardens. To evaluate this situation, we sequenced three diagnostic nuclear DNA loci and two mitochondrial genes across 81 Bluehead Suckers, 74 of which were collected in a basinwide genetic assessment and 7 from a targeted morphological evaluation. Combined, we detected 14 hybrids from the single site in the Bear River and the three sites in the Weber River that hosts the three largest remaining populations in the basin. All individuals sampled as putative hybrids were confirmed as F1 hybrids, ascertaining efficacy of field‐based morphological identifications. Hybridization may be especially problematic for the numerically reduced Bluehead Sucker because it can further depress recruitment. However, no evidence of post‐F1 backcrosses was found, suggesting in turn that hybrids are not reproducing, although the reasons for this are currently not understood. These results underscore the need to quantify the proportion of Bluehead Sucker recruitment lost to hybridization.Received January 6, 2016; accepted August 31, 2016 Published online November 28, 2016

Integrating Nonnative Species in Niche Models to Prioritize Native Fish Restoration Activity Locations along a Desert River Corridor

AbstractThe efficient allocation of restoration resources is critical for the effective conservation of species. Here, we developed an ecological niche model to predict the response of three imperiled desert fishes to potential restoration actions along the longitudinal gradient of a desert river. The San Rafael River, Utah, is home to a complex of three endemic fishes (Flannelmouth Sucker Catostomus latipinnis, Bluehead Sucker C. discobolus, and Roundtail Chub Gila robusta; referred to as the “three species”). Like many Colorado River basin tributaries, the river is overallocated for human use, has experienced extensive physical degradation, and is now home to several nonnative fish species. To determine the factors most limiting to the three species, we first fit random forest models to fish CPUE and habitat data. We next combined these models with a longitudinal habitat survey and modeled nonnative species abundance to predict the continuous distribution of the three species in the lower San Rafael River, under current conditions and following simulated restoration. Nonnative fishes were important negative predictors of Flannelmouth Sucker and Bluehead Sucker relative abundance, and broadscale habitat variables were important positive predictors for all of the three species. Each of the three species was predicted to experience a significant increase in abundance following simulated eradication of nonnative fishes. Responses to simulated reach‐specific habitat restoration were dependent on the reach restored, demonstrating that the choice of restoration location is critical. Nonnative species removal and restoring degraded reaches near already suitable habitat were predicted to be the most beneficial to the three species. Ecological niche models developed at the river scale, and incorporating both physical and biotic predictor variables, can provide spatially explicit information that appropriately parallels the spatial distribution of the needs of endemic fishes and can facilitate effective management and conservation decisions.

An occupancy-based quantification of the highly imperiled status of desert fishes of the southwestern United States.

Desert fishes are some of the most imperiled vertebrates worldwide due to their low economic worth and because they compete with humans for water. An ecological complex of fishes, 2 suckers (Catostomus latipinnis, Catostomus discobolus) and a chub (Gila robusta) (collectively managed as the so-called three species) are endemic to the U.S. Colorado River Basin, are affected by multiple stressors, and have allegedly declined dramatically. We built a series of occupancy models to determine relationships between trends in occupancy, local extinction, and local colonization rates, identify potential limiting factors, and evaluate the suitability of managing the 3 species collectively. For a historical period (1889-2011), top performing models (AICc) included a positive time trend in local extinction probability and a negative trend in local colonization probability. As flood frequency decreased post-development local extinction probability increased. By the end of the time series, 47% (95% CI 34-61) and 15% (95% CI 6-33) of sites remained occupied by the suckers and the chub, respectively, and models with the 2 species of sucker as one group and the chub as the other performed best. For a contemporary period (2001-2011), top performing (based on AICc ) models included peak annual discharge. As peak discharge increased, local extinction probability decreased and local colonization probability increased. For the contemporary period, results of models that split all 3 species into separate groups were similar to results of models that combined the 2 suckers but not the chub. Collectively, these results confirmed that declines in these fishes were strongly associated with water development and that relative to their historic distribution all 3 species have declined dramatically. Further, the chub was distinct in that it declined the most dramatically and therefore may need to be managed separately. Our modeling approach may be useful in other situations in which targeted data are sparse and conservation status and best management approach for multiple species are uncertain.

Do Management Actions to Restore Rare Habitat Benefit Native Fish Conservation? Distribution of Juvenile Native Fish Among Shoreline Habitats of the Colorado River

AbstractMany management actions in aquatic ecosystems are directed at restoring or improving specific habitats to benefit fish populations. In the Grand Canyon reach of the Colorado River, experimental flow operations as part of the Glen Canyon Dam Adaptive Management Program have been designed to restore sandbars and associated backwater habitats. Backwaters can have warmer water temperatures than other habitats, and native fish, including the federally endangered humpback chub Gila cypha, are frequently observed in backwaters, leading to a common perception that this habitat is critical for juvenile native fish conservation. However, it is unknown how fish densities in backwaters compare with that in other habitats or what proportion of juvenile fish populations reside in backwaters. Here, we develop and fit multi‐species hierarchical models to estimate habitat‐specific abundances and densities of juvenile humpback chub, bluehead sucker Catostomus discobolus, flannelmouth sucker Catostomus latipinnis and speckled dace Rhinichthys osculus in a portion of the Colorado River. Densities of all four native fish were greatest in backwater habitats in 2009 and 2010. However, backwaters are rare and ephemeral habitats, so they contain only a small portion of the overall population. For example, the total abundance of juvenile humpback chub in this study was much higher in talus than in backwater habitats. Moreover, when we extrapolated relative densities based on estimates of backwater prevalence directly after a controlled flood, the majority of juvenile humpback chub were still found outside of backwaters. This suggests that the role of controlled floods in influencing native fish population trends may be limited in this section of the Colorado River. Copyright © 2014 John Wiley & Sons, Ltd.

Can translocated native fishes retain their trophic niche when confronted with a resident invasive?

AbstractDiet interactions between native and non‐native fishes may influence the establishment of native species within their historical range (i.e., reintroduction). Therefore, we illustrated the food web structure of and followed the transition of the federally endangered humpback chub Gila cypha into a novel food web following translocation and determined the potential for a non‐native species, rainbow trout Oncorhynchus mykiss, to influence translocation success. Humpback chub and rainbow trout used resources high in the food web and assimilated similar proportions of native fishes, suggesting non‐native rainbow trout may occupy an ecological role similar to humpback chub. Subsequently, humpback chub may be well suited to colonise tributaries because of their ability to consume resources high in the food web. Additionally, diet partitioning may occur between all members of the fish community as indicated by separation in trophic niche space and little trophic overlap; although all species, particularly bluehead sucker Catostomus discobolus, used a broad range of food resources. Rainbow trout stomach content analysis corroborated stable isotope analysis and suggested rainbow trout diet consisted of aquatic and terrestrial macroinvertebrates, while larger rainbow trout (>120 mm total length) consumed a greater proportion of fish (incidence of piscivory = 5.3%). Trophic interactions may reveal an underutilized niche space or biotic resistance to the establishment of translocated native fishes. Continued translocation of humpback chub into tributaries appears to be one option for conservation. However, successful establishment of humpback chub may depend on continued removal of non‐native trout, increasing availability of diet sources at higher trophic levels.

High Diet Overlap between Native Small‐Bodied Fishes and Nonnative Fathead Minnow in the Colorado River, Grand Canyon, Arizona

AbstractRiver regulation may mediate the interactions among native and nonnative species, potentially favoring nonnative species and contributing to the decline of native populations. We examined food resource use and diet overlap among small‐bodied fishes in the Grand Canyon section of the Colorado River as a first step in evaluating potential resource competition. We compared the diets of the predominant small‐bodied fishes (native Speckled Dace Rhinichthys osculus, juvenile Flannelmouth Sucker Catostomus latipinnis, and juvenile Bluehead Sucker C. discobolus, and nonnative Fathead Minnow Pimephales promelas) across seasons at four sites downstream of Glen Canyon Dam using nonmetric multidimensional scaling and Schoener's similarity index. The diets of these fishes included diatoms, amorphous detritus, aquatic invertebrates (especially simuliid and chironomid larvae), terrestrial invertebrates, and terrestrial vegetation. Diets varied with season and were affected by high turbidity. Fish consumed more amorphous detritus and terrestrial vegetation during the summer monsoon season (July–September), when turbidity was higher. The diets of all species overlapped, but there was large variation in the degree of overlap. The diets of juvenile suckers and Fathead Minnows were most similar, while Speckled Dace had relatively distinct diets. The differences took the form of higher proportions of diatoms and amorphous detritus in the diets of Bluehead Suckers and Fathead Minnows and higher proportions of simuliids and chironomids in those of Speckled Dace. If food resources are or become limiting, diet overlap suggests that competition may occur among native and nonnative species, which could have implications for the population dynamics of these fishes and for the management of the Colorado River ecosystem in Grand Canyon.Received September 4, 2013; accepted February 11, 2014

Comparative Swimming Performance of Five Catostomus Species and Roundtail Chub

Abstract.—Decreased habitat connectivity and competition with nonnative species have led to declines of many freshwater fishes. An understanding of swimming performance can aid in the conservation of these fishes; however, acquiring sufficient numbers of rare and threatened species to perform swimming studies can be logistically challenging and ecologically costly. In order to determine whether swimming data for common sucker species may be substituted for that of similar but rare sucker species, we compared the swimming abilities of two rare western catostomids, Bluehead Sucker Catostomus discobolus and Flannelmouth Sucker C. latipinnis, as well as one catostomid with a less well understood status, Mountain Sucker C. platyrhynchus, with those of the common White Sucker C. commersonii and Longnose Sucker C. catostomus. We also examined Roundtail Chub Gila robusta because they are often included in conservation efforts involving Bluehead Sucker and Flannelmouth Sucker. The critical swimming velocities (Ucrit), standardized by body length, of Bluehead Sucker and Longnose Sucker differed significantly from those of White Sucker. However, there was no significant difference between the Ucrit of Mountain Sucker, Flannelmouth Sucker, and White Sucker. During constant acceleration trials, Bluehead Sucker exhibited the greatest swimming ability, reaching a mean maximum velocity of 4.56 ± 1.28 body lengths per second (BL/s; mean ± SD), followed by Mountain Sucker (3.56 ± 0.57 BL/s), White Sucker (3.28 ± 0.90 BL/s), Longnose Sucker (2.97 ± 0.31 BL/s), and Flannelmouth Sucker (2.22 ± 0.42 BL/s). Additionally, key behavioral differences in the swimming behaviors of the fishes studied were observed. We conclude that swimming performance data for common White Sucker should not be used in place of data for rarer species. Comprehensive swimming studies should be conducted on individual sucker species before implementing conservation strategies involving fish passageways or barriers.Received November 6, 2013; accepted February 28, 2014

Detecting Razorback Suckers Using Passive Integrated Transponder Tag Antennas in the Green River, Utah

Abstract In order to increase detections of razorback suckers Xyrauchen texanus tagged with passive integrated transponders in the upper Colorado River basin, we deployed two passive instream flat-plate antennas (33 × 68 cm) at a razorback sucker spawning location in the Green River, Utah, during spring of 2012 and 2013. Over the course of 29 d in 2012 and 90 d in 2013, the antennas detected 569 razorback suckers, 19 Colorado pikeminnow Ptychocheilus lucius, 16 flannelmouth suckers Catostomus latipinnis, and 1 bluehead sucker Catostomus discobolus. Despite extensive sampling via boat electrofishing (rafts and hard-bottom boats) and netting (fyke, trammel, and gill) in wetlands that occurred from the 1990s to present in the upper Colorado River basin, a large number of tagged razorback suckers and Colorado pikeminnow, including a fish released in 1996, were detected for the first time by our antennas. Our data indicate that the detectability of razorback suckers, and precision and accuracy of survival and population estimates might be increased significantly with the addition of data gathered by passive instream flat-plate antennas in the Green River.

Long-Term Monitoring of an Endangered Desert Fish and Factors Influencing Population Dynamics

AbstractThe lower perennial corridor of the Little Colorado River in Grand Canyon, Arizona, is numerically dominated by endemic desert fishes and therefore significant for conservation of these species. From 2000 to 2012, the U.S. Fish and Wildlife Service conducted monitoring of native fishes in the Little Colorado River near its confluence with the Colorado River. The primary focus of these efforts was to estimate the spring and fall abundance of native fishes, especially the federally endangered humpback chub Gila cypha. Because humpback chub in Grand Canyon are influenced by operations of Glen Canyon Dam, our efforts provide managers of the Glen Canyon Dam Adaptive Management Program with abundance estimates and trends of humpback chub in the Little Colorado River, the most important tributary in Grand Canyon for spawning and production of this species. From 2001 to 2006, the spring abundance estimates of humpback chub ≥150 and ≥200 mm remained relatively low (≤3,419 and ≤2,002 fish, respectively), thereafter significantly increasing to highs of 8,083 and 6,250, respectively, by spring 2010. Also from 2000 to 2006, the fall abundance estimates of humpback chub were substantially below those abundances estimated after 2006. In addition, flannelmouth sucker Catostomus latipinnis and bluehead sucker Catostomus discobolus showed post-2006 increases in relative abundance, suggesting a systemwide event occurred that was beneficial to native fishes. Most of the increases of humpback chub occurred during the spring season in the reaches of the Little Colorado River between 5 and 13.57 km upstream from the confluence. Successful production of age 0 year classes of humpback chub may be partially driven by hydrograph dynamics of the Little Colorado River, whereas water temperatures and predation pressures in the mainstem Colorado River likely influence survivorship of native fishes into subadult and adult life stages.

Longer food chains and crowded niche space: effects of multiple invaders on desert stream food web structure

AbstractTributaries of the Colorado River Basin, historically home to a complex of endemic omnivores collectively referred to as the ‘three species’; flannelmouth sucker (Catostomus latipinnis), bluehead sucker (C. discobolus) and roundtail chub (Gila robusta), have experienced the establishment of numerous non‐native fish species. In this study, we examine the impacts of the trophic ecology of non‐native fishes on the ‘three species’ in the San Rafael River, Utah, USA. We employ a suite of abundance comparisons, stable isotope techniques and size‐at‐age back‐calculation analyses to compare food web structure and growth rates of the ‘three species’ in study areas with and without established populations of non‐native species. We found that the ‘three species’ are more abundant in areas with few non‐native fishes present, regardless of habitat complexity. Stable isotope analyses indicate non‐native fishes lengthen the food chain by 0.5 trophic positions. Further, the trophic niche spaces of the native fishes shift and are narrower in the presence of non‐native fishes, as several non‐native species’ trophic niche spaces overlap almost entirely with each of the ‘three species’ (bluehead sucker and flannelmouth sucker 100%, roundtail chub 98.5%) indicating strong potential for competition. However, the ‘three species’ demonstrated no evidence of reduced growth in the presence of these non‐native fishes. Collectively, these results suggest that while non‐native fishes alter the food web structure presenting novel sources of predation and competition, mechanisms other than competition are controlling the size‐structure of ‘three species’ populations in the San Rafael River.

Stream hierarchy defines riverscape genetics of a North American desert fish

Global climate change is apparent within the Arctic and the south-western deserts of North America, with record drought in the latter reflected within 640,000 km(2) of the Colorado River Basin. To discern the manner by which natural and anthropogenic drivers have compressed Basin-wide fish biodiversity, and to establish a baseline for future climate effects, the Stream Hierarchy Model (SHM) was employed to juxtapose fluvial topography against molecular diversities of 1092 Bluehead Sucker (Catostomus discobolus). MtDNA revealed three geomorphically defined evolutionarily significant units (ESUs): Bonneville Basin, upper Little Colorado River and the remaining Colorado River Basin. Microsatellite analyses (16 loci) reinforced distinctiveness of the Bonneville Basin and upper Little Colorado River, but subdivided the Colorado River Basin into seven management units (MUs). One represents a cline of three admixed gene pools comprising the mainstem and its lower-gradient tributaries. Six others are not only distinct genetically but also demographically (i.e. migrants/generation <9.7%). Two of these (i.e. Grand Canyon and Canyon de Chelly) are defined by geomorphology, two others (i.e. Fremont-Muddy and San Raphael rivers) are isolated by sharp declivities as they drop precipitously from the west slope into the mainstem Colorado/Green rivers, another represents an isolated impoundment (i.e. Ringdahl Reservoir), while the last corresponds to a recognized subspecies (i.e. Zuni River, NM). Historical legacies of endemic fishes (ESUs) and their evolutionary potential (MUs) are clearly represented in our data, yet their arbiter will be the unrelenting natural and anthropogenic water depletions that will precipitate yet another conservation conflict within this unique but arid region.

Status and Structure of Two Populations of the Bluehead Sucker (Catostomus discobolus) in the Weber River, Utah

Abstract We compared two populations of the bluehead sucker (Catostomus discobolus) during 2007–2009 in the Weber River, Davis, Summit, and Weber counties, Utah. We estimated 225 and 546 individuals in these populations. Based on recaptured, PIT-tagged fish, annual survival of adults (202–575 mm total length) was high (77%); however, our top model indicated mortality increased with size (i.e., senescence). We documented movements ≤15 km downstream and 5 km upstream and 88% of detections from a stationary antenna occurred at night. Despite high rates of survival of adults, recruitment appeared minimal in one of the populations because it was composed primarily of mature adults. Recruitment potentially was limited by interactions with a high density of brown trout (Salmo trutta) and combined effects of an altered hydrograph (magnitude, duration, and timing) and thermal regime. If conservation of these populations is a priority, recruitment must be increased immediately in one of the populations to avoid ext...

Long‐Term Dynamics of Native and Nonnative Fishes in the San Juan River, New Mexico and Utah, under a Partially Managed Flow Regime

AbstractNonnative fishes and flow alteration are primary threats to native fish persistence in lotic systems. We used several flow regime attributes and fish sampling data obtained from the San Juan River, New Mexico and Utah, during 1993–2010 to evaluate the potential use of flow manipulations to increase recruitment of native fishes that must cope with nonnative species. During this period, discharge in the river was partially manipulated by reservoir releases that augmented naturally high spring flows in this snowmelt‐driven system. An information theoretic approach was used to rank candidate models that predicted species densities based upon selected combinations of flow attributes and abundances of nonnative species. Autumn density of age‐0 fishes in secondary channels was the main response variable. The main predictor variables included flow attributes associated with interannual variation in daily discharge and water temperature; densities of nonnative competitors; and catch rates of a numerically dominant nonnative predator (the channel catfish Ictalurus punctatus). Top‐ranked models for native species included positive associations with small‐bodied nonnative fishes and negative associations with the abundance of channel catfish adults. Densities of native speckled dace Rhinichthys osculus and flannelmouth suckers Catostomus latipinnis increased with mean spring discharge, but the density of native bluehead suckers C. discobolus did not. With the exception of juvenile channel catfish, the top candidate models predicting densities of nonnative fishes all included the duration of low summer flows. These results confirmed findings from a previous study that demonstrated different responses of native and nonnative fishes to seasonal flows; the present study also revealed that densities of all fishes were generally lower in years with greater abundance of adult channel catfish. Regression analysis indicated that seasonal flow manipulations and suppression of nonnative predator populations could be effective management tools to restore and maintain the native fish community.

Bioenergetics and Population Dynamics of Flannelmouth Sucker and Bluehead Sucker in Grand Canyon as Evidenced by Tag Recapture Observations

AbstractFlannelmouth suckers Catostomus latipinnus and bluehead suckers C. discolobus in the Grand Canyon of the Colorado River are among the few native species to persist after river conditions changed with the completion of Glen Canyon Dam in 1963. To evaluate the effectiveness of adaptive management manipulations to the system for recovering native species, it is necessary to estimate basic demographic and population dynamic parameters for the species of interest, such as growth, mortality, and recruitment. With this in mind, we present a two‐stage analysis in which we first estimated growth and individual ages using a field‐based bioenergetics model and then input the age estimates into age‐structured mark–recapture (ASMR) models to estimate mortality and recruitment. The analysis was based on 18,500 flannelmouth suckers and 13,975 bluehead suckers that had been tagged with passive integrated transponder tags in the main‐stem Colorado River in Grand Canyon and its tributaries since 1989. The bioenergetics estimates of growth suggest that the growth of flannelmouth suckers is faster and that of bluehead suckers slower than in the tributaries reported in other studies. The results from the bioenergetics model provide necessary parameters for input into future ecosystem models. The ASMR models that include the effects of age‐specific gear vulnerability indicate that annual natural mortality rates are about 0.2 for flannelmouth suckers and 0.4 for bluehead suckers, which is in agreement with independent estimates from growth parameters and longevity. The estimates of age‐1 sucker recruitment and adult abundance correspond well with independent electrofishing catch rate data. The estimated recruitment and abundance estimates provide insights into the efficacy of adaptive management actions targeted to benefit native fishes over the past two decades.

Identification and Implementation of Native Fish Conservation Areas in the Upper Colorado River Basin

AbstractFreshwater fishes continue to decline at a rapid rate despite substantial conservation efforts. Native fish conservation areas (NFCAs) are a management approach emphasizing persistent native fish communities and healthy watersheds while simultaneously allowing for compatible human uses. We identified potential NFCAs in the Upper Colorado River Basin in Wyoming—focusing on Colorado River cutthroat trout (Oncorhynchus clarkii pleuriticus), flannelmouth sucker (Catostomus latipinnis), bluehead sucker (Catostomus discobolus), and roundtail chub (Gila robusta)—through a process that combined known and modeled species distributions, spatial prioritization analysis, and stakeholder discussions. The network of potential NFCAs is intended to serve as a funding framework for a National Fish and Wildlife Foundation (NFWF) Keystone Initiative focused on Colorado River Basin native fishes. We discuss current opportunities for and impediments to implementing the potential NFCAs we identified for the NFWF Initiative over the long term. NFCAs represent a promising approach to fisheries management that complements existing approaches by focusing on persistent native fish communities.

Land Use Associations with Distributions of Declining Native Fishes in the Upper Colorado River Basin

AbstractThe upper Colorado River basin contains one of the most imperiled fish faunas in North America. Anthropogenic land use and nonnative species impacts are considered among the top reasons for imperilment. We determined the association of anthropogenic land use intensity (road density, percentage of converted land, and oil and gas well density), relative abundance of nonnative white suckers Catostomus commersonii and their hybrids, and natural landscape features with the occurrences of three native fishes in the Colorado River basin, Wyoming, that have declined throughout their ranges: flannelmouth sucker C. latipinnis, bluehead sucker C. discobolus, and roundtail chub Gila robusta. We found that flannelmouth suckers occurred more frequently in large, low‐gradient stream reaches, but their occurrence was not associated with land use intensity or the relative abundance of white suckers and hybrid suckers. In contrast, bluehead sucker occurrence decreased with increasing intensity of each land use type within a 0.5‐km stream buffer, which suggests that localized land use disturbances have negatively affected the species’ distribution. Roundtail chub occurred more frequently in low‐gradient stream segments with more riparian vegetation, but associations with land uses were more complex. Roundtail chub were negatively associated with road density in a 0.5‐km buffer, but in contrast to our expectations they were positively associated with road density in the contributing watershed and with local oil and gas well density. We think that rocky substrates and pools—habitats preferred by roundtail chub and sometimes associated with oil and gas infrastructure—have concentrated the remaining individuals within their currently reduced distribution. The associations we identified highlight the potential role of land use in distributional declines and can help managers to determine whether proposed land use changes may affect existing populations. Spatial predictions of occurrence have already been used in regional fish conservation planning efforts in Wyoming.

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.

Seasonal Movements of Native and Introduced Catostomids in the Big Sandy River, Wyoming

During 2006–2007, we studied movements of native bluehead suckers (Catostomus discobolus) and flannelmouth suckers (C. latipinnis), and non-native white suckers (C. commersonii) and longnose suckers (C. catostomus) in the Big Sandy River, Wyoming. Radiotransmitters were surgically implanted into 20–22 fish of each species in September and October 2006. Movements of all four species in autumn and winter were short with most fish remaining ≤2 km from locations where they were captured initially. Bluehead suckers and flannelmouth suckers began movements downstream in mid-to-late April and returned upstream to locations ≤2 km from their original locations by early summer. White suckers did not make substantial movements during spring, whereas longnose suckers made movements upstream during mid-June and returned downstream by early July. During their estimated spawning periods, longnose suckers used upstream portions, bluehead suckers and white suckers used middle portions, and flannelmouth suckers used downstream portions of the study area.

Overriding Effects of Species‐Specific Turbidity Thresholds on Hoop‐Net Catch Rates of Native Fishes in the Little Colorado River, Arizona

AbstractI examined the effects of turbidity, discharge, and temperature on hoop‐net catch rates of native humpback chub Gila cypha, speckled dace Rhinichthys osculus, flannelmouth suckers Catostomus latipinnis, and bluehead suckers C. discobolus in the Little Colorado River, Arizona. Results indicated that native fish catch rates were primarily influenced by whether turbidity levels were below or above species‐specific thresholds of approximately 545 nephelometric turbidity units (NTU) for humpback chub, 221 NTU for speckled dace, 846 NTU for flannelmouth suckers, and 70 NTU for bluehead suckers. The effects of discharge were negligible, but discharge did dictate the turbidity level. Turbidity levels at hoop‐net deployment relative to the turbidity thresholds predetermined much of the catchability of fish. Catch rates were highest in a high‐catch zone consisting of the lowermost turbidities, which ranged up to the start of a transition zone for each species (humpback chub, ≤54 NTU; speckled dace, ≤29 NTU; flannelmouth sucker, ≤81 NTU; bluehead sucker, ≤26 NTU), and the secondary effects of temperature were only detectable in this zone. Catch rates within the transition zone decreased at higher turbidities up to the thresholds and thereafter remained consistently low at all higher turbidities within this low‐catch zone. The effects of the turbidity thresholds on catch rates decreased for larger fish of all species. Theoretically, the effects of turbidity reflect a behavioral switch by native fishes from relying primarily on structural cover (e.g., hoop nets) to using turbidity as cover to reduce the predation risk as turbidity increases from below to above the thresholds. Moreover, turbidities within the low‐catch zone could be totally incapacitating the fishes' visual capabilities such that fish perceive these turbidity levels as unlimited visual cover. The turbidity threshold concept probably applies to the hoop‐net catch rates of other fishes in other riverine systems.

Complex influences of low‐head dams and artificial wetlands on fishes in a Colorado River tributary system

Abstract Low‐head dams in arid regions restrict fish movement and create novel habitats that have complex effects on fish assemblages. The influence of low‐head dams and artificial wetlands on fishes in Muddy Creek, a tributary of the Colorado River system in the USA was examined. Upstream, fish assemblages were dominated by native species including two species of conservation concern, bluehead sucker, Catostomus discobolus Cope, and roundtail chub, Gila robusta Baird and Girard. The artificial wetlands contained almost exclusively non‐native fathead minnow, Pimephales promelas Rafinesque, and white sucker, Catostomus commersonii (Lacepède). Downstream, fish assemblages were dominated by non‐native species. Upstream spawning migrations by non‐native white suckers were blocked by dams associated with the wetlands. However, the wetlands do not provide habitat for native fishes and likely inhibit fish movement. The wetlands appear to be a source habitat for non‐native fishes and a sink habitat for native fishes. Two non‐native species, sand shiner, Notropis stramineus (Cope), and redside shiner, Richardsonius balteatus (Richardson), were present only downstream of the wetlands, suggesting a beneficial role of the wetlands in preventing upstream colonisation by non‐native fishes.