Habitat and Stocking Strategies for Native Fish Recovery in the Verde River Basin, Arizona

Abstract

Native fishes of the Colorado River Basin in the southwest United States rank among the most imperiled faunal groups worldwide. Their decline is linked to widespread habitat alteration and the subsequent proliferation of nonnative fishes that compete with and prey upon native taxa. Despite decades of conservation efforts, many of the native fishes remain at risk of extinction and there is continued interest in effective recovery strategies across the basin. This dissertation presents detailed investigations of native southwestern fishes, with a particular focus on Razorback Sucker Xyrauchen texanus in the Verde River, Arizona, a key tributary within the Gila River Basin. In this dissertation, I 1) review Razorback Sucker abundance, distribution, and conservation efforts within the Gila River Basin from 2004-2021, 2) evaluate and compare the survival and movement patterns of large (> 250 mm TL) and small (< 250 mm TL) Razorback Sucker stocked into mainstem and reservoir environments of the Verde River, 3) detail novel helicopter stocking methods for translocating endangered fishes to remote locations, and 4) assess how native fish abundance and habitat selection changes in response to nonnative piscivores in Fossil Creek, Arizona. More than 11 million Razorback Sucker were stocked into the Gila River Basin between 1980 and 2004; however, a population has failed to establish. From 2004-2021, 17,313 Razorback Sucker were stocked into the Verde River; but < 0.10% have been recapture and success has been limited. In 2022, I initiated a three-year stocking effort in which approximately 2,500 Razorback Sucker of two size classes were released annually in two locations, the mainstem Verde River and Horseshoe Reservoir. The goal was to evaluate whether reservoir stockings could enhance Razorback Sucker survival. In 2022 and 2023, the survival of small and large Razorback Sucker was negligible regardless of stocking location; however, in 2024 the 9-month post-stocking survival of Razorback Sucker increased to 5.7%. Overall, the survival of large fish was significantly greater than that of small fish and survival in the reservoir was higher than in the river. Mortality was attributed to water quality, specifically high-water temperature and stratification in the reservoir, as well as predation by nonnative fishes and piscivorous birds. Nonetheless, the survival of some Razorback Sucker in Horseshoe Reservoir offers hope for the species’ persistence in the Verde River Basin. Stocking Razorback Sucker into the mainstem river environment presented logistical challenges because it is inaccessible to traditional hatchery vehicles. I therefore commissioned a helicopter to transport Razorback Sucker to a remote location on the Verde River and compared the survival of fish stocked by helicopter to those stocked by conventional hatchery truck. The four-week post-stocking survival of fish stocked by helicopter and those stocked by hatchery truck into Horseshoe Reservoir was similar. Many of the observed mortalities were distant from release sites, suggesting fish mortality was not attributable to the stocking practices. Stocking fishes with a helicopter can be useful for fisheries conservation as it did not impact fish survival and allowed for broader distribution of stocked fish into previously inaccessible environments. Finally, I conducted snorkel surveys in Fossil Creek, Arizona, a Verde River tributary with an entirely native fishes upstream and a native and nonnative fishes downstream of a fish barrier to evaluate differences in species abundance and habitat selection in native fish communities with and without nonnative fish. The abundance of all Roundtail Chub Gila robusta, small (< 127 mm TL) Sonora Sucker Catastomus insignis, and all Speckled Dace Rhinichthys oscullus was significantly lower when sympatric with nonnative fishes; however, Desert Sucker Catostomus clarkii abundances were similar between reaches. When black bass were present, native fishes avoided pools and increased their selection for riffles, a likely predator avoidance behavior. These results highlight the distributional and behavioral consequences of native fishes occupying the same stream as nonnative fishes and highlights the importance of maintaining specific habitat-types when nonnative fishes are present in a river. Collectively, this research highlights the persistent challenges of conserving native fishes in the Verde River and offers practical insights to guide and improve future conservation efforts throughout the basin.