Paul C. Marsh

A Conservation Plan for Native Fishes of the Lower Colorado River

Abstract The native fish fauna of the lower Colorado River, in the western United States, includes four “big-river” fishes that are federally listed as endangered. Existing recovery implementation plans are inadequate for these critically imperiled species. We describe a realistic, proactive management program founded on demographic and genetic principles and crafted to avoid potential conflicts with nonnative sport fisheries. In this program, native species would breed and their progeny grow in isolated, protected, off-channel habitats in the absence of nonnative fishes. Panmictic adult populations would reside in the main channel and connected waters, exchanging reproductive adults and repatriated subadults with populations occupying isolated habitats. Implementation of the plan would greatly enhance recovery potential of the four listed fishes.

Indigenous fishes of western North America and the hypothesis of competitive displacement: Meda fulgida (Cyprinidae) as a case study

ern North America by the nonnative red shiner (Cyprinella lutrensis) seems linked with the dramatic decline of a threatened cyprinid (spikedace, Medafulgida) native to the Gila River subbasin. The mechanism by which red shiner impacts spikedace is unknown. Two hypotheses have been offered: displacement of the native through competitive interaction with invader, and replacement of native by nonnative as a result of environmental perturbation. To ascertain whether spikedace was being actively displaced by red shiner, we compared niche requirements of each in syntopy, allotopy, and disjunct allopatry. Fishes were collected by seining one to three sites in each of six different stream reaches; and current velocity, substrate particle size, and water depth were measured at each site. Red shiner occupied similar microhabitat whether allopatric, allotopic, or syntopic with spikedace. Spikedace occupied the same microhabitat when allopatric or allotopic to red shiner. However, spikedace syntopic with red shiner displayed a niche shift into currents significantly swifter than those selected when in isolation. Displacement of spikedace by red shiner suggests negative interspecific interactions potentially detrimental to the indigenous species.

Predation by Introduced Fishes on Endangered Humpback Chub and other Native Species in the Little Colorado River, Arizona

Abstract Fishes in the Little Colorado River in the Grand Canyon, Arizona, were sampled monthly from July 1991 to June 1995 as part of a study of the ecology of endangered humpback chub Gila cypha. Diets of five introduced predatory fish species were examined. Stomach contents varied among species and were low in diversity and dominated by algae (primarily Cladophora), aquatic insects, and fishes. Humpback chub plus other native species were a significant component of the diet (13.7% frequency of occurrence among 219 of 408 stomachs that contained food). Predation mortality from introduced fishes may significantly affect the native species by depleting numbers and reducing recruitment.

Conflicts between Native Fish and Nonnative Sport Fish Management in the Southwestern United States

Abstract The ubiquitous presence of nonnative fishes, both sport and nongame, within waters of the southwestern United States is the foremost factor preventing immediate conservation and recovery of imperiled native fishes. We present evidence that the two fishery types cannot be co-managed in sympatry if natives are to persist. A dual responsibility of federal and state fish and wildlife agencies to manage both fishery types creates internal conflicts that typically are resolved in favor of nonnative sport fisheries, despite existence of the Endangered Species Act. We advocate designation of watersheds to be managed exclusively for one fishery type or the other, and implementation of an aggressive program to eliminate nonnatives in native-designated waters and protect against their reinvasion. To mitigate institutional conflicts, agency infrastructures should be segregated to promote independent management of native fisheries and introduced sport fisheries. This approach can fulfill mandates of both the ...

Population estimates/population movements of Gila cypha,an endangered cyprinid fish in the Grand Canyon Region of Arizona.

within that river is restricted to the reproductive period. During 1991-1992, adult G. cypha were captured and tagged during 19 6-14 day sampling periods in three separate reaches of the LCR. From these data, population estimates were derived for each reach on a monthly basis, by month for the entire LCR, and over the entire study period. Results indicate an upriver migration by some individuals in early spring, followed by a slow, protracted postreproductive movement downstream. Localized stasis by adults in the LCR, particularly summer through winter, is also strongly supported by the data. Movements by G. cypha in the LCR thus appears to be an amalgam of two processes: upriver movement in spring coupled with localized movements by overwintering adults. The latter suggests a possible alteration in life-history strategy for the species and is discussed in the context of Glen Canyon Dam, built in 1963 to impound Lake Powell at the northern extent of Grand Canyon.

Feeding and fate of wild larval razorback sucker

SynopsisThe razorback sucker (Xyrauchen texanus) is disappearing throughout its native range in the Colorado River basin of western North America. The largest remaining wild population in Lake Mohave, Arizona-Nevada, has shown no recruitment since the 1950s. Although annual spawning is successful and larvae are seasonally abundant, no juveniles have been collected in recent decades. To evaluate the potential role of food availability in determining fate of larvae, fish and zooplankton samples were taken in 1985 from the reservoir and an adjacent, isolated backwater in which larvae were naturally produced. Food availability and primary dietary constituents were similar in both habitats. Reservoir larvae selectedBosmina spp. (Cladocera) and apparently avoided Copepoda, while larvae from the backwater selectedBosmina, but avoided Rotifera. Larvae from both places showed evidence of selection for certain sizes of zooplankters, but preferred sizes differed between habitats. These differences were neither attributable to larval size nor zooplankton community structure. Nutritional factors such as type, number, or size of available foods do not explain disappearance of larval razorback suckers from Lake Mohave, since larvae survive to far greater ages and size in the backwater. Predation by introduced fishes appears a significant cause of larval mortality.

Detection of Larval Remains after Consumption by Fishes

Abstract In southwestern North America, consumption of native fish larvae by nonnative predators has imperiled native populations. Field-acquired dietary analyses have provided little evidence of this cause-effect relationship. In this study, small, nonnative green sunfish Lepomis cyanellus, bluegills L. macrochirus, red shiners Cyprinella lutrensis, fathead minnow Pimephales promelas, and yellow bullheads Ameiurus natalis were each fed a single larva of the native razorback sucker Xyrauchen texanus. Gut content analysis revealed that prey detection generally became increasingly difficult over a short postconsumption time period under laboratory conditions. For green sunfish, bluegills, and yellow bullheads, significant relationships between prey detection and time were revealed; the probability of prey identification was initially 50% or greater for about 30 min postconsumption, whereas few prey (3%) were identifiable at 60 min postconsumption. For red shiners and fathead minnow (pooled for analysis), no...

Long‐term effective population size of three endangered Colorado River fishes

The extant genetic variation of a population is the legacy of both long-term and recent population dynamics. Most practical methods for estimating effective population size are only able to detect recent effects on genetic variation and do not account for long-term fluctuations in species abundance. The utility of a maximum likelihood estimator of long-term effective population size based upon the coalescent theory of gene genealogies is examined for three endangered Colorado River fishes: humpback chub (Gila cypha), bonytail chub (Gila elegans) and razorback sucker (Xyrauchen texanus). Extant mitochondrial DNA (mtDNA) variation in humpback chub suggests this species has retained its historical equilibrium genetic variation despite recent declines in abundance. The mtDNA variation in razorback suckers indicates the population was quite large and expanding prior to recent declines and that rare alleles still survive in the remnant populations. The remaining mtDNA variation in bonytail chub indicates that dramatic, recent declines may have already obliterated a substantial portion of any historical variation. The results from long-term effective population size analyses are consistent with known natural history and illustrate the utility of the analysis for endangered species management.

Repatriation as a Management Strategy to Conserve a Critically Imperiled Fish Species

Abstract A repatriation program to conserve critically imperiled razorback sucker Xyrauchen texanus, an endemic fish of the Colorado River basin in western North America, was initiated in 1990. The species, once widespread and abundant, now is extirpated from most of its range because of human-induced factors. Natural recruitment to wild populations rangewide is largely precluded by nonnative predation. The largest remaining wild population occurs in Lake Mohave, Arizona and Nevada, but its numbers have declined dramatically over the past decade, such that the genetic legacy of the species may soon be lost. As part of a cooperative repatriation program, more than 440,000 naturally produced razorback sucker larvae were harvested and grown in protective custody; from these, nearly 58,000 marked juveniles were released into the lake between 1993 and 2002. Annual repatriate population estimates (modified Petersen method) ranged from 1,017 to 2,494 and poststocking survivorship (Program MARK) ranged from 2% to...

Spatiotemporal Variation in Length–Weight Relationships of Endangered Humpback Chub: Implications for Conservation and Management

Abstract The largest population of endangered humpback chub Gila cypha inhabits the lower Little Colorado River (LCR) and the main-stem Colorado River near its confluence with the LCR in Grand Canyon, Arizona. At present, fish in both rivers spawn almost exclusively in the LCR. Flows in the main-stem Colorado River are regulated by Glen Canyon Dam, and water temperature approximates predam winter temperatures year-round. The LCR continues to provide a relatively natural hydrograph and seasonal warming patterns. Length–weight relationships among adult humpback chub from the lower Colorado River basin showed a seasonal pattern of declining condition during spring spawning season followed by recovery of condition during summer through early winter. Fish from the main stem recovered condition more rapidly after reproduction than did fish from the LCR and may have benefited from dam-mediated environmental changes. Grand Canyon Colorado River fish had the greatest weight at length of eight locations sampled in ...