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The maintenance or restoration of natural flow regimes has been proposed as one means of conserving native fishes. Native fish conservation is enhanced either through the restoration of natural fluvial geomorphic processes (and thus the maintenance of essential habitats) or by the suppression of nonnative fishes. The San Juan River of Colorado, New Mexico, and Utah was dammed in 1962 and its natural flow regime was lost. Beginning in 1993, the river was regulated to mimic a natural flow regime by increasing reservoir releases to mimic timing, but only partially to mimic amplitude, volume, and duration of spring snowmelt discharge. We evaluated the responses of native and nonnative fishes to this natural flow regime...
We used 10 microsatellite loci to examine the genetic population structure of cutthroat trout Oncorhynchus clarki within the Lahontan Basin complex. Genetic diversity was analyzed for trout from Nevada, California, and Utah representing three putative subspecies: Lahontan O. c. henshawi, Paiute O. c. seleniris, and Humboldt (an unnamed subspecies) cutthroat trout. We found significant differences in microsatellite diversity among the three putative subspecies found in this area. Analysis of molecular variance partitioned microsatellite variation as 9.8% among subspecies, 27.7% among populations, and 62.5% within populations of Lahontan Basin cutthroat trout. Genetic distance analyses (Cavalli-Sforza–Edwards and...
Kokanees Oncorhynchus nerka (lacustrine sockeye salmon) and Utah chub Gila atraria feed extensively on similar sizes and species of zooplankton in Flaming Gorge Reservoir. We measured the effects of Utah chub on the growth of kokanees in six 80-m3 enclosures containing ambient zooplankton populations. Three kokanees and 0, 3, 6, 12, or 24 Utah chub were weighed and placed in each enclosure. After 21 d, fish were removed and kokanee growth was compared among treatments. Zooplankton biomass, chlorophyll-a concentrations, and temperature profiles were monitored during each test. In May 1992, no competitive interactions were observed; in June 1992, however, zooplankton biomass and kokanee growth declined significantly...
We determined prolonged swimming ability of larval and juvenile Colorado Squawfish Ptychocheilas lucius at 10, 14, and 20°C. Fatigue velocity (velocity at which 50% of the fish failed to sustain swimming in 30-min tests) increased significantly with water temperature and fish length. Differences in swimming ability between 10 and 14°C became less pronounced as fish length increased, but differences remained substantial between the two cooler and the 20°C swimming experiments for all size-classes tested. We conclude that cold hypolimnetic releases from hydroelectric dams can significantly reduce prolonged swimming ability of young-of-year Colorado Squawfish, and may help explain the decline of this species in...
Capture?recapture sampling of 819 km of streams in the Green River subbasin, Utah and Colorado, of the Colorado River basin was conducted in 2000?2003 to estimate population demographic parameters and recovery status for endangered Colorado pikeminnow Ptychocheilus lucius. Parameter estimates from a Huggins robust-design, multistratum model suggested a basinwide decline in the abundance of adult (total length, ?450 mm) Colorado pikeminnow. Reductions were most severe in the middle Green River (59%) and White River (63%), the two largest population segments. Reductions were less severe in the Yampa River (29%), Desolation?Gray Canyon (11%), and lower Green River reaches (36%). In 2001, the first year the entire subbasin...
he razorback sucker Xyrauchen texanus is an endangered fish endemic to the Colorado River basin in the western United States. Once widely distributed and common throughout the basin, the species has been eliminated from most of its former range by establishment of nonnative fishes and water development, and remaining numbers have dwindled precipitously from historical levels. Although Lake Mohave, Arizona and Nevada, supports the largest and genetically most diverse remaining population, razorback sucker abundance in the lake plummeted from historical numbers in the hundreds of thousands to only 44,000 in 1991 and fewer than 3,000 in 2001. This population is limited primarily to large, old adults because predation...
We measured signatures of naturally occurring stable isotopes of carbon (13C) and nitrogen (15N) in important invertebrate and fish taxa in two coldwater reservoirs in Colorado that had different food webs. One reservoir, Lake Granby, contained a large population of an opossum shrimp, Mysis relicta, and the other, Blue Mesa Reservoir, did not. We compared temporal dynamics of isotopic signatures of all taxa between lakes to quantify sources of variability in consumer signatures and to assess potential turnover rates of isotopic signatures in the top predator. Stable isotope signatures varied little across season or body size within most taxa, but large differences across reservoirs were observed. Nitrogen signatures...
Gonadosomatic indices (100 � gonad weight/whole-body weight) and the occurrence of expressible gametes indicated a temporal overlap in the spawning of humpback chub Gila cypha and roundtail chub G. robusta at Black Rocks, a turbulent, deepwater reach of the upper Colorado River, Colorado (39�N, 109�W). Radiotelemetry showed that roundtail chub moved throughout the upper Colorado River during the spawning season, whereas humpback chub were restricted almost entirely to the 1.8-km Black Rocks reach; however, spatial isolation of the species during spawning was not demonstrated. A principal component analysis of morphologic measurements taken from live fish supported taxonomic assignments made in the field and suggested...
We used tag?recapture data to estimate apparent survival and capture probability for 119,129 hatchery-reared, federally endangered razorback suckers Xyrauchen texanus stocked into upper Colorado River basin streams during 1995?2005. Effects investigated included reach, year, and season of stocking; fish total length (TL) at time of stocking; survival in the first year after stocking versus in subsequent years; and sampling effort. Recapture data were also used to describe poststocking movement. First-year survival rate for stocked razorback suckers of average TL (252.5 mm) was low: 0.05 (95% confidence interval [CI], 0.042?0.071). Total length at stocking and first-year survival were positively correlated; survival...