W. L. Minckley

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.

Nitrogen and phosphorus dynamics in hot desert streams of Southwestern U.S.A.

Nitrogen to phosphorus ratios and concentrations of nitrate and soluble reactive phosphate are presented for an array of Southwestern streams as evidence that nitrogen is the limiting nutrient where such limitation occurs. Nitrate uptake in sections of intermittent streams was attributable to autotrophic activity. Uptake of soluble reactive phosphate was unrelated to any indicator of autotrophic activity, thus concentrations of this nutrient in desert and semi-desert stream waters may be controlled by other factors.

Qualitative Characters, Identification of Colorado River Chubs (Cyprinidae: Genus Gila) and the "Art of Seeing Well"

Qualitative scoring is frequently overlooked in preference to counts or measurements of individual characters, particularly for species whose overlap in morphology makes clear separation difficult. Quantitative measurements and counts of single characters were compared to qualitative rankings of selected morphological features of chubs (genus Gila) from the Yampa River, Colorado. Data were collected by technicians with no specialized training in systematics. A high degree of morphological variability confounded identification using the quantitative data set, while principal components analysis of qualitative data clearly separated Gila cypha (humpback chub) and G. robusta (roundtail chub). Totals of 32 G. cypha and 336 G. robusta were identified; no intermediates were indicated. We thus demonstrate that scoring of complex characteristics is useful for endangered fishes that must be minimally handled to insure uninjured release. The clean separation of syntopic G. cypha and G. robusta supports protection of the lower Yampa River, the only location in the upper Colorado River basin in which apparent hybridization between the endangered G. cypha and its congeners remains to be documented. The art of seeing well, or of noticing and distinguishing with accuracy the objects which we perceive, is a high faculty of the mind, unfolded in a few individuals, and despised by those who can neither acquire it, nor appreciate its results. Rafinesque (1820), Ichthyologia Ohioensis.

Relationships between spatio-temporal environmental and genetic variation reveal an important influence of exogenous selection in a pupfish hybrid zone.

The importance of exogenous selection in a natural hybrid zone between the pupfishes Cyprinodon atrorus and Cyprinodon bifasciatus was tested via spatio‐temporal analyses of environmental and genetic change over winter, spring and summer for three consecutive years. A critical influence of exogenous selection on hybrid zone regulation was demonstrated by a significant relationship between environmental (salinity and temperature) and genetic (three diagnostic nuDNA loci) variation over space and time (seasons) in the Rio Churince system, Cuatro Ciénegas, Mexico. At sites environmentally more similar to parental habitats, the genetic composition of hybrids was stable and similar to the resident parental species, whereas complex admixtures of parental and hybrid genotypic classes characterized intermediate environments, as did the greatest change in allelic and genotypic frequencies across seasons. Within hybrids across the entire Rio Churince system, seasonal changes in allelic and genotypic frequencies were consistent with results from previous reciprocal transplant experiments, which showed C. bifasciatus to suffer high mortality (75%) when exposed to the habitat of C. atrorus in winter (extreme temperature lows and variability) and summer (abrupt salinity change and extreme temperature highs and variability). Although unconfirmed, the distributional limits of C. atrorus and C. atrorus‐like hybrids appear to be governed by similar constraints (predation or competition). The argument favouring evolutionary significance of hybridization in animals is bolstered by the results of this study, which links the importance of exogenous selection in a contemporary hybrid zone between C. atrorus and C. bifasciatus to previous demonstration of the long‐term evolutionary significance of environmental variation and introgression on the phenotypic diversification Cuatro Ciénegas Cyprinodon.

Conservation to Stem Imminent Extinction: The Fight To Save Razorback Sucker Xyrauchen texanus in Lake Mohave and Its Implications for Species Recovery

Razorback Sucker Xyrauchen texanus is iconic of the plight of native “big-river” fishes of the Colorado River system of western North America. The species was historically widespread and abundant throughout the basin but has suffered substantial range reductions and population declines now characteristic of Western fishes. By the 1960s the largest remaining population was in Lake Mohave, a lower Colorado River reservoir where casual monitoring began in the mid-1950s and intensified to focus on Razorback Sucker in the late 1970s. The population then was comprised of several hundred thousand large adults, but recruitment past the larval stage was nil presumably because of predation by non-native fishes and potentially worsened by nutritional limitation. Remnant adults began to dwindle in the 1980s and were virtually gone within twenty years. An ad-hoc “Lake Mohave Native Fishes Work Group” initiated investigations to identify and understand the reasons for recruitment failure and launched an informal program to perpetuate Razorback Sucker in the reservoir. The initial goal was to establish a population of 50,000 adults in Lake Mohave, and the group developed an innovative and ultimately successful strategy in which Razorback Sucker larvae produced naturally by wild adults in the lake were harvested, reared in protected off-channel habitats, and repatriated. Demographic monitoring continued and expanded, providing annual census estimates of population abundance and trends of wild and repatriated fish. Critical genetic monitoring was initiated to track spatial and temporal diversity of harvested larvae and captured repatriates. Wild adults now are gone from Lake Mohave, but they have been replaced by a genetically diverse repatriate population of several thousand fish that spawn annually and provide larvae to continue the management cycle. However, the program is stymied by continued post-larval recruitment failure and predation losses of even the largest stocked Razorback Sucker. The program depends on stocking to maintain a repatriate population and for now has preserved the genetic legacy of the species. The species fares no better elsewhere in the basin where historical genetic diversity was lower, and, with the exception of Lake Mead, wild adults have perished and populations are maintained only by stocking of hatchery-produced fish. Naturally self-sustaining populations of Razorback Sucker are unlikely to ever again occupy the lower Colorado River mainstem and the species will remain “conservation-reliant.” A conceptual strategy that integrates use of non-native-free backwaters and the river channel has promise for this and other big-river species, and its implementation should be aggressively pursued.

COMPARISON OF FOUR TECHNIQUES FOR AGING ADULT COLORADO PIKEMINNOW, PTYCHOCHEILUS LUCIUS

Abstract We estimated ages of Colorado pikeminnow, Ptychocheilus lucius, by counting annuli in scales, vertebral centra, whole otoliths, and thin otolith sections. Vertebrae provided the most precise estimator of age and the range of ages obtained were highly correlated with total length. Ages estimated from sectioned otoliths were ranked second in precision and strongly correlated with vertebral ages. Ages read from scales underestimated vertebral ages, and ages estimated from whole otoliths were most variable and least related to total length. Ages estimated using all 4 structures from the same individuals revealed a close relationship between ages estimated with vertebrae and sectioned otoliths.