Adaptive Management of an Imperiled Catostomid in Lake Mohave, Lower Colorado River, USA

Lake Mohave, a man-made reservoir in the lower Colorado River, USA, was once home to the largest wild population of the endemic and endangered razorback sucker Xyrauchen texanus, estimated at 60,000 individuals in the late 1980s. Individuals of this population were 25 years or older because recruitment was precluded by the removal of larval production by introduced centrarchid species. A repatriation program was initiated in the 1990s to replace the aging population with young fish by capturing larvae from the reservoir and raising them in hatcheries and protected lakeside backwaters until they were released back into the reservoir. Although more than 200,000 fish have been repatriated to Lake Mohave, the repatriate population has remained at a few thousand fish. The wild population is now functionally extinct. The program has adapted to new threats to the population, political realities, and technological advances. Management shifted in 2006 to the Lower Colorado River Multi-Species Conservation Program, which has politicized the process. The aim of this chapter is to describe the initial, informal adaptive management strategy for razorback sucker in Lake Mohave, the transition to a formal program, and the inherent pitfalls that formalization entails

Assessment of Potential Augmentation and Management Strategies for Razorback Sucker \u3cem\u3eXyrauchen texanus\u3c/em\u3e in Lake Mead and Grand Canyon: A 2021 Science Panel Summary

Razorback Sucker Xyrauchen texanus is a large-bodied, long-lived species endemic to the Colorado River Basin. This species historically ranged throughout the basin from the Colorado River delta in Mexico to Wyoming and Colorado. Currently, the species persists ,in a small portion of its historical range with the help of intensive management efforts including augmentation. Recruitment to adult life stages is extremely limited in the wild, but is documented consistently in Lake Mead. Research and monitoring efforts in Lake Mead are ongoing since 1996 and have recently expanded to include the Colorado River inflow area and portions of lower Grand Canyon. Despite evidence of recruitment, the current population size in Lake Mead and Grand Canyon is believed to be small (data) and susceptible to stochastic effects. This raised interest in the potential to augment the population to prevent loss of genetic diversity and increase abundance and distribution in general, as well as explore recruitment bottlenecks. To address critical uncertainties surrounding this management option and to brainstorm other potential options, a Planning Committee and Steering Committee made up of representatives of state (Arizona, Nevada), tribal (Hualapai Tribe, Navajo Nation), and federal (Bureau of Reclamation, National Park Service, and U.S. Fish and Wildlife Service) management agencies convened an Expert Science Panel (ESP; 2021), to consider augmentation and management strategies for Razorback Sucker in Lake Mead and Grand Canyon. The purpose of this report is to summarize those findings

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

<p>Larval 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 <i>Xyrauchen texanus</i> 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 <i>Lepomis macrochirus</i> and in 14% and 15% of Green Sunfish <i>L. cyanellus</i> 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 mitigation of their impacts is to provide nonnative-free refugia for native large-river fishes.</p> <p>Received June 17, 2016; accepted September 19, 2016 Published online January 19, 2017</p

Data from: Retention of ancestral genetic variation across life-stages of an endangered, long-lived iteroparous fish

As with many endangered, long-lived iteroparous fishes, survival of razorback sucker depends on a management strategy that circumvents recruitment failure that results from predation by non-native fishes. In Lake Mohave, AZ-NV, management of razorback sucker centers on capture of larvae spawned in the lake, rearing them in off-channel habitats, and subsequent release ('repatriation') to the lake when adults are sufficiently large to resist predation. The effects of this strategy on genetic diversity, however, remained uncertain. After correction for differences in sample size among groups, metrics of mtDNA (number of haplotypes, NH, and haplotype diversity, HD) and microsatellite (number of alleles, NA, and expected heterozygosity, HE) diversity did not differ significantly between annual samples of repatriated adults and larval year-classes or among pooled samples of repatriated adults, larvae, and wild fish. These findings indicate that the current management program thus far maintained historical genetic variation of razorback sucker in the lake. Because effective population size, Ne, is closely tied to the small census population size (Nc = ~1,500-3,000) of razorback sucker in Lake Mohave, this population will remain at risk from genetic as well as demographic risk of extinction unless Nc is increased substantially

RBS_mtDNA

This is an Arlequin file that contains counts of each mtDNA haplotype (311 bp cytb) in each collection, including annual samples from 1997-2010 for repatriated adults, larval cohorts, and wild fish (pooled)

RBS_Haps

This file contains DNA sequences for 34 mitochondrial DNA haplotypes (311 bp fragment of cytochome b)

Carson et al 2016 RBS msat data

This file contains genotypic data 13 microsatellite loci for all individuals, including annual collections of repatriated adults (1997-2010), larval year classes (1997-2010), and the pooled sample of wild fish (1997-2010)