Current status of cutthroat trout subspecies in the western Bonneville Basin

Recent discoveries of native cutthroat trout populations in desert mountain ranges on the western fringe of the Bonneville Basin have prompted intensified management efforts by state and federal agencies. Analysis of Snake Valley cutthroat specimens in Trout Creek, Deep Creek Mountain Range, Utah, indicate this is a pure strain of the trout which once inhabited Pleistocene Lake Bonneville and which was thought to be extinct in Utah. The Snake Valley cutthroat is similar to Salmo clarki utah of the eastern Bonneville Basin; however, electrophoretic and morphomeristic analysis show unique genetic differences brought about by long - term isolation (8,000 years) from the remainder of the Bonneville Basin cutthroat. This cutthroat is a common ancestor to several other limited cutthroat populations within the basin in Nevada. In May 1977 the BLM withdrew from mineral entry about 27,000 acres within the Deep Creek Mountains for protection of this salmonid cutthroat and other unique resources on the range. Results of 1977 stream surveys on the Pilot Peak Mountain Range, Utah, indicate the presence of the threatened Lahontan cutthroat, Salmo clarki henshawi, in one isolated stream.     

Electrophoretic study of cutthroat trout populations in Utah

Thirty - nine Utah streams were sampled for cutthroat trout. Of these, 31 contain cutthroat or cutthroat / rainbow hybrid populations. By using starch gel electrophoresis, these populations were segregated into three groups. One group consisted predominately of fish from the Sevier River (of the Bonneville Basin) and Colorado drainages. A second was primarily populations from the Bear River Drainage (Bonneville Basin) as well as some scattered populations along the Wasatch Front (Bonneville Basin). The third consisted of Wasatch Front populations and populations that have hybridized with rainbow trout. Since different subspecies of cutthroat trout are native to the Colorado and Bonneville drainages, one would expect the populations from within the Bonneville Basin to be more similar to one another and less similar to the Colorado River populations. That this did not occur raises questions concerning the evolutionary relationships of the subspecies and the populations. It is clear that at least a northern (Bear River) and southern (Sevier River) form of the Bonneville cutthroat exists. The Wasatch Front may represent an intermediate zone where these two forms intergrade.         

Phylogenetic divergence in a desert fish: differentiation of speckled dace within the Bonneville, Lahontan, and upper Snake river basins

Historical events have had a great impact on the biogeography of fishes of western North America. We examined the genetic variation of the speckled dace ( Rhinichthys osculus ) to determine the effects on this species of extensive hydrological changes during the last 10 million years in the Bonneville and Lahontan basins of the Great Basin and the upper Snake River Basin. Eight hundred sixty-nine base pairs of the mitochondrial gene cytochrome b were sequenced from 97 individuals representing 22 populations within these 3 basins, as well as from 2 individuals of longnose dace ( Rhinichthys cataractae ) that served as outgroups. Additionally, 13 speckled dace sequences representing 3 Bonneville populations were used from GenBank. Phylogenetic relationships were reconstructed using maximum parsimony and maximum likelihood criteria. Analysis of molecular variance was used to determine population structure and to estimate the amount of gene flow across the community boundaries. Three distinct clades were reconstructed representing the Lahontan Basin, the northern Bonneville and upper Snake River basins, and the southern Bonneville Basin. Additionally, most of the population structuring was explained by variation among basins (65.33%). Speckled dace demonstrated high genetic variation. As hypothesized, the northern and southern Bonneville specimens formed separate clades; however, the southern Bonneville clade was basal to a sister clade formed by the northern Bonneville/upper Snake River and Lahontan clades. These relationships indicate that Pliocene connections between the Snake, Lahontan, and Bonneville drainages, rather than more recent Pleistocene connections, best explain population structuring in speckled dace.     

Status and conservation of salmonids in relation to hydrologic integrity in the Greater Yellowstone Ecosystem

Native salmonid status was evaluated with an index quantifying distribution and abundance of cutthroat trout ( Oncorhynchus clarki ) and grayling ( Thymallus arcticus ) in 41 watersheds comprising the Greater Yellowstone Ecosystem. We assessed hydrologic integrity with a percentile-based index measuring cumulative effects of reservoirs, surface water withdrawals, and consumptive water use. Status of native salmonids was poor in 70% of the watersheds; exceptions occurred in a north-south core extending from the Upper Yellowstone southward through the national parks to Bear Lake. Hydrologic integrity was highest in headwater areas and lowest in lower-elevation watersheds. Status of native and nonnative salmonid populations currently existing in the ecosystem was positively correlated with hydrologic integrity ( r = 0.58), indicating that the hydrologic index performed well on a watershed scale in quantifying suitability of stream environments for salmonids. However, native trout status and hydrologic integrity were similarly correlated ( r = 0.63) only when watersheds receiving the lowest possible native salmonid index score were removed from analysis because these watersheds were uniformly distributed across hydrologic integrity. We infer that nonphysical factors such as interactions with introduced fish species have played an important role in the disappearance of native salmonids. The highest priority for conservation is preservation of core watersheds, where both hydrologic integrity and native trout status are high. Restoration opportunities exist in the Teton, Idaho Falls, Willow Creek, Central Bear, and Bear Lake watersheds, where viable cutthroat trout populations remain but are threatened by habitat degradation.     

Diplostomiasis in native and introduced fishes from Yellowstone Lake, Wyoming

Totals of 101 native Yellowstone cutthroat ( Oncorhynchus clarki bouvieri ), 27 introduced lake trout ( Salvelinus namaycush ), and 40 introduced longnose sucker ( Catostomus catostomus ) from Yellowstone Lake, Wyoming, USA, were examined for eye flukes. Metacercariae of the trematode fluke Diplostomum were in vitreous humor and/or lens of 94% of Yellowstone cutthroat trout, 92% of lake trout, and 78% of longnose sucker. Longnose sucker had 7% prevalence of infection in both lens and vitreous humor of metacercariae, while Yellowstone cutthroat trout had 3% and lake trout 8%. Diplostomum spathaceum was in lens tissue of 5% of infected Yellowstone cutthroat trout and 93% of longnose sucker; Diplostomum baeri was in vitreous humor of 92% each of infected Yellowstone cutthroat trout and lake trout. Morphological characteristics indicate that a single species infected the lens of Yellowstone cutthroat trout and longnose sucker, while another species infected lake trout. Impacts of the parasite interchange between native and introduced fishes of Yellowstone Lake, Wyoming, are unknown but should be monitored each year.     

The scotopic visual sensitivity of four species of trout: a comparative study

We compared the maximum scotopic visual sensitivity of 4 species of trout from twilight (mesotopic) to fully dark-adapted vision. Scotopic vision is the minimum number of photons to which a fully dark-adapted animal will show a behavioral response. A comparison of visual sensitivity under controlled laboratory conditions showed that brown trout ( Salmo trutta ) and brook trout ( Salvelinus fontinalis ) had maximum scotopic thresholds (1.1 × 10 –4 μmol ? m –2 s –1 , ～0.005 lux) 2 times lower than rainbow trout ( Oncorhyncus mykiss ) and Snake River cutthroat trout ( Oncorhynchus clarki bouvieri ), which did not differ from each other (2.1 × 10 –4 μmol ? m –2 s –1 , ～0.01 lux). A literature review tended to corroborate these results in that brown trout and brook trout were reported to be more active during the night and at twilight than cutthroat trout and rainbow trout. We also measured light intensity within open versus shaded reaches during the day, dusk, and night in 3 Rocky Mountain streams. The scotopic sensitivity of brown trout and brook trout was sufficient to allow foraging during all twilight periods and under average nighttime light intensities in open and shaded reaches, whereas the scotopic sensitivity of rainbow trout and cutthroat trout may restrict their foraging to relatively bright nocturnal conditions (twilight or a moonlit night). Native cutthroat trout restoration efforts may have greater success in open versus shaded stream reaches in the Rocky Mountains and elsewhere.     

Aquatic habitats, life history observations, and zoogeographic considerations of the spotted frog ( Rana pretiosa ) in Tule Valley, Utah

Four populations of the spotted frog, Rana pretiosa , occur in western Bonneville Basin. Only the Tule Valley populations occupy aquatic habitats associated with warm (28°C) and slightly saline (1700-2700 μmhos/cm) springs. The spotted frog in Tule Valley breeds in cold-water portions of the peripheral wetlands, which exhibit maximum temperature variations (1-25°C), maximum conductivity up to 3200 μmhos/cm, and maximum pH values up to 9.7. Adult frogs are found in habitats with temperatures of 29°C, conductivity of 4700 μmhos/cm, and pH above 9.0 in the summer. The increased summer salinity and pH in frog habitats returns to lower values by the next breeding season due to underground recharge. Breeding in Tule Valley occurs earlier than in other Bonneville locations because of the warm-water sources. Spatial and temporal distribution of the spotted frog since the regression of Lake Bonneville 15,000 years ago and threats to present habitats are discussed.     

Optimal temperatures for growth and upper thermal tolerance of juvenile northern leatherside chub

We examined optimal temperatures for growth and the upper thermal tolerance of juvenile northern leatherside chub ( Lepidomeda copei ). We conducted 2 experiments using the acclimated chronic-exposure method to estimate optimal temperature for growth of age-0 northern leatherside chub (range 12.8–28.3 °C). Upper thermal tolerance was estimated using the critical thermal maximum (CTM) and upper incipient lethal temperature (UILT) methods for fish acclimated at 15, 18, 23, and 28 °C. We also measured stream temperatures in Yellow Creek, Summit County, Utah, during July–August 2006 to compare our results to actual summer stream temperatures. Survival in growth tests was not significantly different between treatment temperatures in either experiment (P > 0.098). The optimal temperature for growth in the 1st trial estimated from the 2nd-order polynomial regression was 23.0 °C, falling outside the range of experimental temperatures (12.8–22.2 °C). The estimated optimal temperature for growth in the 2nd trial was 23.2 °C. In the upper thermal tolerance tests, juvenile northern leatherside chub had CTM values between 29.6 and 35.0 °C; CTM values increased as acclimation temperature increased. Upper incipient lethal temperatures (LT50) ranged from 26.5 to 30.2 °C, increasing with acclimation temperature. Summer stream temperatures in Yellow Creek had a lower mean (14.0–18.1 °C) than did the optimal temperature for growth determined in these studies, but these temperatures exhibited diel fluctuations as large as 15.7 °C.     

Occurrence of native Colorado River cutthroat trout ( Oncorhynchus clarki pleuriticus ) in the Escalante River drainage, Utah

Field surveys were conducted during 1997 and 1998 documenting the distribution and abundance of Colorado River cutthroat trout ( Oncorhynchus clarki pleuriticus ) in Escalante River tributaries. This documented occurrence of native trout in the Escalante River drainage of southern Utah represents an expansion of the known historic range of this subspecies as reported before the 1990s. We found 5 populations of native trout ranging in biomass from 3.0 to 104.2 kg ha -1 and occupying 13.2 km of stream of 130 km of estimated historic habitat. Current distribution and abundance of Colorado River cutthroat trout were compared to early introductions of nonnative trout stocked for sport fishing purposes. Native cutthroat trout have been displaced by nonnative cutthroat trout ( O. c. bouveri ), rainbow trout ( O. mykiss ), brook trout ( Salvelinus fontinalis ), and brown trout ( Salmo trutta ) except where they were isolated by physical or biological barriers. Displacement may have been more extensive except for the remoteness of the drainage and relatively recent introductions of nonnative trout. These conditions limited the overall amount of the drainage stocked, numbers of nonnative trout stocked, and time over which stocking occurred. Discoveries of native trout populations within the Escalante River drainage have allowed expanded conservation of this subspecies by adding new populations to what was known to exist and by increasing the known natural range of this fish.     

Parasites of the cutthroat trout, Salmo clarki, and longnose suckers, Catostomus catostomus, from Yellowstone Lake, Wyoming

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Twenty-five cutthroat trout ( Salmo clarki ) and eight longnose suckers ( Catostomus catostomus ) from Yellowstone Lake, Wyoming, were collected and examined for parasites in 1985. Cutthroat trout had at least six different species of parasites that included both protozoans and helminths. The greatest number of parasite species on one fish was nine. Parasites added to the known list for cutthroat trout from Yellowstone Lake, Wyoming, were: Myxosoma sp., Diphyllobothrium ditremum, Diphyllobothrium dendriticum, Diplostomum baeri, and Posthodiplostomum minimum. These data were compared with a previous survey (1971) and a checklist of parasites of cutthroat trout in North America. There are 17 species of parasites and two fungal species reported for cutthroat trout from Yellowstone Lake. Trichophrya catostomi, Diplostomum spathaceum, and Ligula sp. were observed in the small sample of longnose suckers.        

Host-parasite studies of Trichophrya infesting cutthroat trout ( Salmo clarki ) and longnose suckers ( Catostomus catostomus ) from Yellowstone Lake, Wyoming

Trichophrya sp. (Protozoa) on the gills of cutthroat trout ( Salmo clarki ) and longnose suckers ( Catostomus catostomus ) was studied using light and electron microscopy and tracer techniques. All cutthroat trout, 14 cm in total length and above, from Yellowstone Lake, Yellowstone National Park, Wyoming, were infested with the suctorian. No trichophryans were found on fry or fingerling cutthroat trout. Sixty percent of the examined longnose suckers from the same location were infested. Light microscopy disclosed extensive pathology of gill epithelium in longnose suckers infested with Trichophrya that was not observed for infested cutthroat trout. Electron micrographs show damage to immediate host gill cells by both parasites, depicted by a reduction and lack of mitochondria. Both parasites form attachment helices (0.52 × 0.04 μ m), which may originate in the protozoan cell membrane and function for maintenance of parasite position on the host cell. There was no uptake of 14 C, injected into host fish, via the attachment helices by the parasite that further substantiated the mechanical function for the spiral structure. Protozoan feeding on host tissue may be accomplished by use of necrotic gill tissue and mucus.        

Sources of variation in counts of meristic features of Yellowstone cutthroat trout ( Oncorhynchus clarki bouvieri )

We determined variability in counts of meristic features (pyloric caecae, vertebrae, pelvic fin rays, gillrakers, basibranchial teeth, scales above the lateral line, and scales in the lateral series) of Yellowstone cutthroat trout ( Oncorhynchus clarki bouvieri ) by 3 independent readers, by the same reader on 3 different occasions, and among fish from 12 sampling sites within a 650-km 2 watershed. Genetic purity of the cutthroat trout was determined by electrophoretic analysis. Significant differences in meristic counts were observed among 3 readers and among sampling sites, but not among 3 occasions by a single reader. Scale counts were within the reported range for Yellowstone cutthroat trout, but counts of other structures (pyloric caecae, gillrakers, vertebrae) were as similar to rainbow trout as to Yellowstone cutthroat trout. Meristic counts identified the fish as cutthroat trout; however, variation among readers and sampling sites, as well as within the species, limits their use when identifying genetically pure cutthroat trout or assessing possible integration with rainbow trout.     

Life history of the Lahontan cutthroat trout, Salmo clarki henshawi, in Pyramid Lake, Nevada

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} The Pyramid Lake Lahontan cutthroat trout ( Salmo clarki henshawi ) population was sampled on a monthly basis from November 1975 through December 1977. A subsample of 676 trout, stratified by fish size and lake habitat, provided biological data. The entire population is presently derived from hatchery production, stocked at lengths of approximately 75 to 300 mm. Peak annulus formation occurs in March and April, followed by the period of maximum growth. Scale patterns illustrate a variable growing season. Maximum growth in length is in the first three years of life; after that males begin to grow faster than females. Males attained a greater age in our sample; i.e., the oldest male was seven years old compared to six years for females. The Pyramid Lake Lahontan cutthroat trout exhibit nearly isometric growth. The legal sport fishery removed 380 mm); other decimating factors are poorly understood. No evidence of the following diseases or pathogens was found in the Pyramid Lake population, presuming a carrier incidence of 2 percent at the 95 percent confidence level: infectious pancreatic necrosis, infectious hematopoietic necrosis, viral hemorrhagic septicema, bacterial kidney disease, enteric redmouth, furunculosis, whirling disease, blood fluke; however, 7 of 235 (≈3 percent) adults sampled at the Marble Bluff fishway were positive for furunculosis. Small trout feed primarily on zooplankton and benthic invertebrates; cutthroat trout >300 mm are piscivorous, feeding almost exclusively on tui chub ( Gila bicolor ). The spawning migration of Pyramid Lake cutthroat trout to the Marble Bluff egg taking facility in spring 1976 and 1977 peaked in April and May. Females mature at three or four years (352–484 mm), and males mature at two or three years (299–445 mm). Mean diameter of mature eggs is 4.51 mm; both ovum size and fecundity are a function of fish size. Fecundity ranges from 1241 to 7963 eggs, with a mean of 3815. Lahontan cutthroat trout comprise     

Modeling global warming scenarios in greenback cutthroat trout ( Oncorhynchus clarki stomias ) streams: implications for species recovery

Changes in global climate may exacerbate other anthropogenic stressors, accelerating the decline in distribution and abundance of rare species throughout the world. We examined the potential effects of a warming climate on the greenback cutthroat trout ( Oncorhynchus clarki stomias ), a resident salmonid that inhabits headwater streams of the central Rocky Mountains. Greenbacks are outcompeted at lower elevations by nonnative species of trout and currently are restricted to upper-elevation habitats where barriers to upstream migration by nonnatives are or have been established. We used likelihood-based techniques and information theoretics to select models predicting stream temperature changes for 10 streams where greenback cutthroat trout have been translocated. These models showed high variability among responses by different streams, indicating the usefulness of a stream-specific approach. We used these models to project changes in stream temperatures based on 2° C and 4° C warming of average air temperatures. In these warming scenarios, spawning is predicted to begin from 2 to 3.3 weeks earlier than would be expected under baseline conditions. Of the 10 streams used in this assessment, 5 currently have less than a 50% chance of translocation success. Warming increased the probability of translocation success in these 5 streams by 11.2% and 21.8% in the 2 scenarios, respectively. Assuming barriers to upstream migration by nonnative competitors maintain their integrity, we conclude that an overall habitat improvement results because greenbacks have been restricted through competition with nonnatives to suboptimal habitats, which are generally too cold to be highly productive.     

Growth rate and thermal tolerance of two endangered Snake River snails

We investigated the temperature tolerance and growth of 2 federally protected freshwater gastropods from southern Idaho: Valvata utahensis and Pyrgulopsis idahoensis . Snails were collected in the Snake River and transported to the laboratory where they were kept under highly controlled conditions. In varying-temperature, short-duration experiments, the temperatures tolerated by both species were between 7°C and 34°C. In constant-temperature, longduration experiments, growth rates were between 0.004 mm ? d -1 and 0.016 mm ? d -1 , and we created growth curves for both species that were previously lacking. Our results are among the first to report temperature tolerances and growth rates of native Snake River snails, and we discuss implications for the biology and management of both species.     

Distribution and abundance of native Bonneville cutthroat trout ( Oncorhynchus clarki utah ) in southwestern Utah

The Bonneville cutthroat trout ( Oncorhynchus clarki utah ; BCT) was once abundant throughout the Bonneville Basin. In southwestern Utah, however, only 3 local populations of the subspecies were known to exist in 1977, when conservation efforts to protect and replicate them began. By 1995 remnant populations were known in 6 streams, and replicate populations had been established in an additional 16. Populations of BCT in southwestern Utah streams were survey by electrofishing in 1994 and 1995 to describe the subspecies' status. Estimated densities of age-1 and older BCT ranged from 118 to 546 fish per km. Biomass estimates ranged from 8 to 64 kg per ha. Several age groups of BCT were collected at most locations. Six populations were classified as self-sustaining, 9 as expanding through natural recruitment, 6 as new, and 1 as hybridized. Overall status of BCT in southwestern Utah has improved since 1977, but conservation measures must continue to maintain a positive trend.     

Effects of exotic species on Yellowstone's grizzly bears

Humans have affected grizzly bears ( Ursus arctos horribilis ) by direct mortality, competition for space and resources, and introduction of exotic species. Exotic organisms that have affected grizzly bears in the Greater Yellowstone Area include common dandelion ( Taraxacum officinale ), nonnative clovers ( Trifolium spp.), domesticated livestock, bovine brucellosis ( Brucella abortus ), lake trout ( Salvelinus namaycush ), and white pine blister rust ( Cronartium ribicola ). Some bears consume substantial amounts of dandelion and clover. However, these exotic foods provide little digested energy compared to higher-quality bear foods. Domestic livestock are of greater energetic value, but use of this food by bears often leads to conflicts with humans and subsequent increases in bear mortality. Lake trout, blister rust, and brucellosis diminish grizzly bears foods. Lake trout prey on native cutthroat trout ( Oncorhynchus clarkii ) in Yellowstone Lake; white pine blister rust has the potential to destroy native whitebark pine ( Pinus albicaulis ) stands; and management response to bovine brucellosis, a disease found in the Yellowstone bison ( Bison bison ) and elk ( Cervus elaphus ), could reduce populations of these 2 species. Exotic species will likely cause more harm than good for Yellowstone grizzly bears. Managers have few options to mitigate or contain the impacts of exotics on Yellowstones grizzly bears. Moreover, their potential negative impacts have only begun to unfold. Exotic species may lead to the loss of substantial highquality grizzly bear foods, including much of the bison, trout, and pine seeds that Yellowstone grizzly bears currently depend upon.     

Distribution and abundance of pelagic fish in Lake Powell, Utah, and Lake Mead, Arizona-Nevada

Pelagic fish communities (waters with depths > 20 m) of Lakes Powell and Mead were examined quarterly from 1995 to1998 using vertical gill nets and a scientific echosounder. Nets captured a total of 449 fish consisting of striped bass (57%/45% [Lake Powell/Lake Mead]), threadfin shad (24%/50%), common carp (15%/4%), walleye (3%), channel catfish (2%), and rainbow trout ( -1 ). Reservoirs experienced dramatic seasonal and annual fluctuations in pelagic biomass. Lake Powell's biomass peaked at the Colorado River at 709.7 (± 46.5) kg · ha -1 and Lake Mead's reached 291.9 (± 58.2) kg · ha -1 at Las Vegas Wash. These locations supported estimated fish densities of 124,668 fish · ha -1 and 15,131 fish · ha -1 , respectively. Maximum reservoir biomass peaked in August 1996, with Lake Powell supporting 10,852,738 ± 5,195,556 kg (27.6 × 10 7 fish) and Lake Mead 1,926,697 ± 892.994 kg (10.8 × 10 7 fish). Biomass ebbed in May (1996 and 1997), when Lake Mead supported 65% (296,736 kg vs. 453,097 kg) and 62% (101,016 kg vs. 162,262 kg) of biomass levels found in Lake Powell.     

Lahontan cutthroat trout ( Oncorhynchus clarki henshawi ) spawning and downstream migration of juveniles into Summit Lake, Nevada

The only remaining self-sustaining native population of lacustrine Lahontan cutthroat trout ( Oncorhynchus clarki henshawi ) not affected by nonnative salmonids is in Summit Lake, Humboldt County, Nevada. Annual spawning runs in 1993 and 1994 were monitored at a fish trap on Mahogany Creek, the only spawning tributary for Summit Lake. Number of spawners was similar in both years, with 1290 upstream migrants observed in 1993 and 1255 in 1994. In 1993, 137 postspawners (10.6% of upstream migrants) returned to the lake, and in 1994, 434 postspawners (34.6% of upstream migrants) returned downstream through the fish trap. Two distinct groups of subadult Lahontan cutthroat trout were observed moving downstream in 1994. The first group passed downstream between 27 April and 29 July and included 1188 fish (average fork length = 90 mm). Between 1 August and 31 October, 1160 fish (average fork length = 42 mm) moved downstream. Size differences of these 2 groups suggest that the 1st group comprised fish that had overwintered in Mahogany Creek, while the 2nd group were probably young-of-the-year.     

Spawning ecology of finespotted Snake River cutthroat trout in spring streams of the Salt River valley, Wyoming

We studied spawning ecology of cutthroat trout ( Oncorhynchus clarki ) in streams that originate as springs along the Salt River, a Snake River tributary in western Wyoming. We assessed (1) relative numbers of upstream-migrant and resident adults present during the spawning period in spring streams, (2) influence of habitat modification on use of spring streams for spawning, and (3) habitat features used for spawning in spring streams. Four spring streams were studied, 2 with substantial modification to enhance trout habitat and 2 with little or no modification. Modifications consisted primarily of constructing alternating pools and gravel-cobble riffles. Only a small portion of adult fish in spring streams during the spawning period had migrated upstream from the Salt River between March and the middle of June. Larger numbers of adult fish and more redds were observed in the 2 modified streams compared with the 2 streams with little or no modification. Most spawning occurred on constructed riffles with small gravel and over a narrow range of depths and velocities. Cutthroat trout, rainbow trout ( Oncorhynchus mykiss ), and their hybrids were observed in 1 stream with habitat modifications, indicating that measures to halt invasion by rainbow trout, as well as habitat improvement, are needed to preserve this native trout within the Salt River valley.