Hope in a Highly Regulated River: Native Fish Recovery in the Colorado River

Authors: David L. Rogowski, Jan K. Boyer

Year: 2018

Abstract

In many rivers altered by dams, native fishes have been extirpated or persist at small fractions of their original abundance and distribution, despite extensive recovery programs. The Colorado River is one of the most regulated rivers in the world, and native fish populations in the Colorado River basin have declined due to threats from invasive species and habitat change (e. g., dams, water diversions). However, over the past 19 years within the Grand Canyon, native fishes have increased in abundance and expanded their ranges in the highly regulated Colorado River. We used data from a long term monitoring program (2000-2018) to describe changes in native and nonnative fish abundance and spatial distribution. Fish were sampled using boat electrofishing and baited hoop nets at randomly selected sites (stratified by ~ 8km reaches) between Glen Canyon Dam and Lake Mead (406-503 river kilometers). Catch per unit effort was calculated as an index of relative abundance. Native fishes increased in abundance during the study period; conversely most non-native species decreased in abundance. Non-native species comprise most of the species assemblage in cold tailwater habitat between Glen Canyon Dam and the Little Colorado River Confluence, but downstream of the Little Colorado River, native fish outnumber non-native fish. Western (downstream) reaches of the Grand Canyon appear to provide particularly important habitat for native fish. Native fishes increased in abundance with increasing river mile, and most age-1 and younger fish were captured in western Grand Canyon. We hypothesize that a combination of an increasing amount of riverine habitat as Lake Mead recedes, tributary refugia, impediments to establishment of warmwater predators, and modifications to Glen Canyon Dam operations, has contributed to the native fish recovery observed in the Colorado River in Grand Canyon.