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

摘要

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 mid1950s 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 nonnative 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 Wild adults now are gone from Lake Mohave, but they have been replaced by a genetically diverse repatriate population of several thousand fish that spawn annually and provide larvae to continue the management cycle.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.