Rare habitats, rare species, and invasive predators highlight management complexities in the Colorado River system

Abstract

Long-term drought caused Lake Powell, a reservoir on the Colorado River (USA), to decline to its lowest elevation in >50 years during 2022–2023, allowing warm water to pass through intakes of Glen Canyon Dam and facilitating invasion by non-native Smallmouth Bass ( Micropterus dolomieu ). Establishment of bass downstream of the dam could threaten persistence of several native fishes, including two federally listed species. Subsequent detection of larval Smallmouth Bass in a spring-fed slough (river mile -12 slough) connected to the river in Glen Canyon National Recreation Area (NRA) increased urgency to stem further invasion. The National Park Service is evaluating proposed actions to limit effects from non-native predators on native species in the Colorado River, including potentially channelizing the slough. This locally rare, spring-fed waterbody provides habitat for other species, including Western Tiger Salamanders ( Ambystoma mavortium subsp.) of uncertain origin. We found salamanders from the slough had two distinct mitochondrial DNA haplotypes identical to sequences from nearby Arizona Tiger Salamander ( A. m. nebulosum ) populations, confirming they are the native genotype. We detected Red-spotted Toads ( Anaxyrus punctatus ) and Woodhouse’s Toads ( A. woodhousii ) from three other sites in Glen Canyon NRA and 34 sites in adjacent, downstream Grand Canyon National Park (spanning ∼464 km of river) with environmental DNA and traditional surveys. However, we did not detect salamanders elsewhere, matching prior information that salamanders are rare in the Colorado River corridor below Glen Canyon Dam. Based on this information, we discuss management options for the local population of Arizona Tiger Salamanders.