Controls on native fish diversity in the Big Bend and Forgotten Reach of the Rio Grande and implications for management

Authors: Brian Laub, Demitra Blythe, Jack Schmidt, Phaedra Budy

Year: 2019

Abstract

Flow alteration impacts native river biota through multiple stressors, including altered disturbance regimes, water quality, and physical habitat. To understand the relative impacts of these different stressors on native fish communities in the Rio Grande, we examined how native fish richness responded to variation in flow, water quality, physical habitat, and non-native fish relative abundance in two reaches of the Rio Grande that differed in their degree of flow alteration. Flows in the Forgotten Reach (FR) have been completely altered from historic conditions, the historic channel has narrowed by 90%, and water quality is severely degraded. In the Big Bend, flows from the Rio Conchos provide higher baseflows than in the Forgotten Reach and periodic channel reset floods that temporarily widen the channel. In each reach, we identified all available fish sampling records at several common sampling points, and for each fish sample compiled information on flow levels, specific conductivity, water temperature, channel width, and nonnative fish relative abundance. We then developed and parameterized a structural equation model for each reach relating fish richness to hydrology, water quality, habitat, and non-native abundance variables. Comparison of the models between the two reaches revealed that width variation is a major factor driving native fish richness in Big Bend, where channel-widening floods occur, and where water quality is less impaired. In the Forgotten Reach, where channel-widening floods do not occur, small flow spikes and water quality are equally as important as habitat changes for native richness. Though additional research is needed, results suggest flow management to maintain channel width in the Big Bend is warranted if water quality is protected, whereas in the Forgotten Reach, habitat management may be ineffective without improvements in water quality.