Assessing Pre- and Post- Flood Fish Abundance, Population Structure, and Habitat Use in an Arizona River

Abstract

The highly endemic and unique fishes’ native to the American Southwest are suffering substantial population declines due to changes in natural flow regimes, alteration to instream habitat, and a legacy of nonnative fish introductions. Here I quantify habitat for native and nonnative Arizona fishes and how fish use of habitat features changed over time, following an extreme flood. In 2019, the Verde River of central Arizona, experienced its single-largest flooding event of the past decade. This provided me the unique opportunity to quantify fish abundance, population structure, and habitat utilization pre- and post- a high magnitude flood in a major river of this region. Using previously collected habitat use-nonuse data from 2017 and comparing that with data from 2019, I assessed change to the fish community and interannual variability in fish microhabitat selection. Additionally, using data collected through PAED electrofishing and a literature review, I developed type-I and type-III habitat suitability criteria (HSC) for fishes of the Verde River, Arizona. Following the flood of February, native fish captures increased from 0.6% of total catch to 55.6% of total catch (P < 0.00). The increase in native fish catch per unit effort (CPUE) was exclusive to age-0 Roundtail Chub and age-0 Sonora Sucker. Native fish captures were concentrated at upstream river locations. Catch per unit effort of nonnative fishes remained stable. My evaluation of habitat utilization by fish of the Verde River indicates that microhabitat utilization is subject to interannual variability; however, this variability was minimal, with the utilization of microhabitats remaining within a relatively narrow range across sampling years and locations. I identified ontogenetic shifts in habitat use within species and differential habitat use among species. Native adult Roundtail Chub, Sonora Sucker, and Desert Sucker were found to occupy the deepest of pool habitats with flow velocities of 0.00 m3/s to 0.47 m3/s; however, Desert Sucker utilize larger substrates and flows up to 0.53 m3/s. Adult nonnative fishes, Common Carp and Black Bass were found in shallower and slower water than native fishes, but evidence from gill net surveys suggests the use of deeper pool environments. Age-0 fishes of species; Roundtail Chub, Sonora Sucker, Desert Sucker, and Black Bass selected for shallower and slower waters than their adult counterparts, displaying remarkable similarity in habitat use. The crowding of multiple species < 100 mm TL in shallow, slow velocity environments suggests that habitat selection of small-bodied fishes may be a response to predation risk and is likely to increase the occurrence of negative interspecies interactions. My research provides an additional case-history that the protection of flood is vital to the conservation of the imperiled native fishes of the American Southwest, while my evaluation of the interannual variability in habitat use suggests HSC remains stable from year-to-year even after an extreme disturbance event. Defining HSCs for both native and nonnative fishes of the Verde River provides a template for habitat restoration and conservation activities in a future defined by climatic and hydrologic uncertainty.