Brian R. Kesner

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...

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...

Decline of the Razorback Sucker in Lake Mohave, Colorado River, Arizona and Nevada

Abstract The 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 on their larvae by nonnative fishes has precluded measurable recruitment for nearly half a century. At the current rate of decline, extirpation is anticipated within this decade because, at this time, there is no practicable method to remove the continuing threat of nonnative predators...

Effective size, census size, and genetic monitoring of the endangered razorback sucker, Xyrauchen texanus

For the past several decades the Lake Mohave population of the federally endangered razorback sucker, Xyrauchen texanus, has had no natural recruitment because of intensive predation on larvae by non-native fishes. In response to impending extirpation, a repatriation program was implemented where larval razorbacks are collected from the wild following natural spawning, reared in protective custody, and then repatriated at a much larger size. In this study, we estimated annual (Nbf) and generational female effective size (Nef) of the spawning stock by characterizing temporal genetic changes in mtDNA among larval cohorts, and then compared these estimates to the estimated number of reproductively capable (adult) females in Lake Mohave (Nf). Razorback suckers have life history and mortality schedules that could yield values of Nef/Nf as low as 10−5 due to match–mismatch recruitment (or the “Hedgecock effect”) that increases variance in reproductive success. Average Nbf was estimated to be 160, Nef was 706, and the ratio Nef/Nf was 0.29 and 0.38 for arithmetic and harmonic mean Nf, respectively. Our findings indicate that (i) larval sampling in Lake Mohave sufficiently encompasses temporal and spatial variation within annual larval cohorts as to be representative of the spawning stock, (ii) roughly 3–16% (about 8% on average) of adult females contribute genetically to larval cohorts each year, and (iii) repatriated fishes appear to be contributing genetically to larval cohorts. Simultaneous genetic and demographic monitoring offers insights that neither approach can provide alone into effects of population decline and management practices in this species.

Time-series analysis reveals genetic responses to intensive management of razorback sucker (Xyrauchen texanus).

Time‐series analysis is used widely in ecology to study complex phenomena and may have considerable potential to clarify relationships of genetic and demographic processes in natural and exploited populations. We explored the utility of this approach to evaluate population responses to management in razorback sucker, a long‐lived and fecund, but declining freshwater fish species. A core population in Lake Mohave (Arizona‐Nevada, USA) has experienced no natural recruitment for decades and is maintained by harvesting naturally produced larvae from the lake, rearing them in protective custody, and repatriating them at sizes less vulnerable to predation. Analyses of mtDNA and 15 microsatellites characterized for sequential larval cohorts collected over a 15‐year time series revealed no changes in geographic structuring but indicated significant increase in mtDNA diversity for the entire population over time. Likewise, ratios of annual effective breeders to annual census size (Nb/Na) increased significantly despite sevenfold reduction of Na. These results indicated that conservation actions diminished near‐term extinction risk due to genetic factors and should now focus on increasing numbers of fish in Lake Mohave to ameliorate longer‐term risks. More generally, time‐series analysis permitted robust testing of trends in genetic diversity, despite low precision of some metrics.

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.

Use of a Molecular Assay to Detect Predation on an Endangered Fish Species

AbstractMastication, digestion, and rapid evacuation rates make visual identification of larval fish remains in the digestive tracts of predatory fishes problematic. Recent advances in molecular technology, however, have increased the likelihood of identifying remnants of partially digested larval prey, thereby enabling assessment of predator impacts on local populations. Conducting controlled laboratory experiments, we evaluated the utility of quantitative polymerase chain reaction (qPCR) for identification of Razorback Sucker Xyrauchen texanus larvae in the digestive tracts of Green Sunfish Lepomis cyanellus and Western Mosquitofish Gambusia affinis. Primers and a probe were developed to amplify a fragment from Razorback Sucker mtDNA. Tests using a suite of potential predators and prey indicated that these primers and probes amplified only mtDNA from Razorback Sucker and Flannelmouth Sucker Catostomus latipinnis, an allopatric and allochronic species in the lower Colorado River. Amplification with prime...

Use of Molecular Techniques to Confirm Nonnative Fish Predation on Razorback Sucker Larvae in Lake Mohave, Arizona and Nevada

AbstractLarval predation by nonnative fishes has long been implicated in the decline of western native large-river fishes, but visual assessment of predation is difficult due to the postconsumption degradation of fragile larvae. Molecular techniques were used to demonstrate predation on the larvae of endangered Razorback Suckers Xyrauchen texanus by nonnative juvenile centrarchids in field samples from Lake Mohave, Arizona–Nevada, where larvae are seasonally abundant. Razorback Sucker DNA was detected in the gut contents of 7% and 9% of Bluegills Lepomis macrochirus and in 14% and 15% of Green Sunfish L. cyanellus that were captured during spring 2014 and 2015, respectively. There was no significant (α = 0.05) effect of predator TL, predator species, or year on the presence of larval DNA. Juvenile centrarchids have the potential to consume substantial numbers of Razorback Sucker larvae and thus to impact recruitment. The control of nonnative fishes may be impractical, but a viable alternative for mitigati...

Positive Phototaxis in Larval Bonytail

Abstract We conducted an experiment to determine phototaxis in bonytail Gila elegans, a Colorado River basin endemic fish. We tested groups of 50 protolarvae (6.1–8.9 mm total length) in darkness for 2 h or 4 h with illuminated (lighted) and nonilluminated (unlighted) traps suspended in aquaria in pairs (one lighted and one unlighted, two lighted, or two unlighted); we repeated each trial three times. Among all trials, lighted traps captured 60% (359 of 600) of available larvae, which was significantly more than the 0% (1 of 600) from unlighted traps. Results demonstrate positive phototaxis and suggest larval light-trapping could be used to detect the presence of larval bonytail or to monitor or collect individuals for management of this critically imperiled species.