Wood you believe it? Experimental Nonnative Wood Addition Enhances In-Stream Habitat for Native Desert Fishes

Authors: Benjamin Miller, Casey Pennock, Phaedra Budy

Year: 2022

Abstract

Habitat loss and degradation is a major worldwide driver of freshwater biodiversity decline. Habitat simplification is contributing to the decline of native fishes throughout the Colorado River basin; in the San Juan River (SJR, a major tributary of the Colorado River), flow regulation, overallocation of water, and establishment of nonnative riparian vegetation (primarily Russian olive Elaeagnus angustifolias) have all contributed to broad scale habitat simplification. To inform managers of the potential for native fish habitat enhancement using abundant nonnative woody vegetation, we designed an experiment to test the efficacy of using cut Russian olive to enhance in-stream habitat to benefit native fishes. To achieve this goal, we constructed 120 woody structures consisting of existing Russian olive at 12 sites within the main channel of the SJR in fall 2021. To evaluate fish use and potential habitat changes, we subsequently sampled fishes, macroinvertebrates, habitat (depth, velocity, and dominant substrate size), and deployed portable PIT tag antennas at treatment and paired reference reaches that did not receive wood treatments. During the first-year of sampling, we captured a total of 1,216 fishes in treatment reaches (13% native) and 1,149 fishes in reference reaches (9% native species). Native fish and macroinvertebrate densities were higher on average (44% and 160%, respectively) in treatment reaches than in reference reaches. Total unique fishes detected by PIT antennas was higher in reference reaches than in treatment reaches for both native and nonnative species (80% and 71%, respectively). Habitat composition (depth, velocity, and dominant substrate size) did not differ between reference (means, 0.34 m, 0.26 m/s, 38.40 mm, respectively) and treatment (0.34 m, 0.28 m/s, 51.9 mm, respectively) reaches; however, at the scale of woody structures, depth, velocity, and dominant substrate size means were 0.17 m, 0.09 m/s, and 3.61 mm, respectively. High sedimentation rates facilitated the formation of large (~ 0.5 m tall) sand bars behind most structures, which contributed to increases in habitat complexity, but also stranded structures in some instances. Beaver appeared to feed on Russian olive at multiple sites, and in some cases, this led to the dismantling of woody structures. Presently, in year-two of the project, we adapted our experimental design in attempt to mitigate the effects of beaver activity and stranding of structures by constructing fewer, larger woody structures further from shore in treatment reaches. Our year-1 results suggest that addition of nonnative woody structures is an effective management action for increasing habitat complexity and enhancing native fish habitat at the local scale. Ultimately, this experiment aims to better inform managers of the potential for native fish habitat enhancement by exploiting an existing and abundant nonnative vegetation resource.