From Bad to Worse: High Nonnative Densities & Monsoonal-Induced Habitat Deterioration in Backwater Nurseries may Explain the Recruitment Bottleneck of Imperiled Fishes in the San Juan River of NM, CO, & UT

Authors: Blake Hansen, James Whitney, Amber Bell, Sasha Ewing, Michaela Fishback, Linden Jones, Josh Holloway

Year: 2022

Abstract

The Razorback Sucker, Xyrauchen texanus and Colorado Pikeminnow, Ptychocheilus lucius are federally-endangered species occurring in the San Juan River of NM, CO, and UT. These species have shown little natural recruitment in this system, with a lack of high-quality nursery habitats being one potential explanation for this phenomenon. The young-of-year of both species prefer backwaters, including those that form in secondary channels or in association with islands. However, it is unknown how factors that could limit imperiled fish recruitment, such as nonnative densities, resource availability, and physicochemical features, differ between secondary channel and island backwaters. Furthermore, how these environmental features vary during the critical post-spawning window of the July-September monsoon season within and across years is also poorly understood. As such, we compared nonnative and native densities, chlorophyll-a concentrations, turbidity, percent silt substrate, and mean depth between the two backwater types across 20 sites (i.e., 10 of each type) sampled on five occasions each in 2021 and 2022. We found that nonnatives attained higher densities in secondary channel than island backwaters, and in both backwater types they had much higher densities than natives. Furthermore, both backwater types had low chlorophyll-a concentrations that likely resulted from high turbidity and coverage of unstable silt substrate. Furthermore, silt coverage increased throughout the monsoon season, which resulted in habitat aggradation and declining depth. Lastly, flow events that were likely detrimental to recruitment occurred in both study years, including a fish kill following a monsoonal flow pulse in 2021, and a flow reversal in 2022 characterized by extreme low flows that caused backwater drying during July 09-25 (mean discharge = 387 cubic feet/second) followed by high flows that caused backwater flushing during July 26 – August 08 (mean discharge = 1,985 cubic feet/second). Our results suggested that both backwater types had low nursery quality that became even poorer throughout the monsoon season, which may explain the recruitment bottleneck. Identifying strategies for improving backwater nursery quality (e.g., environmental flows management) will be paramount in helping to alleviate the recruitment bottleneck of imperiled Razorback Sucker and Colorado Pikeminnow in the San Juan River, thus aiding in their recovery.