Steady as she flows: assessing the response of native fish to a steady flow experiment from Glen Canyon Dam.

Authors: Mike Dodrill, Bill Pine, Brandon Gerig, Colton Finch

Year: 2013 (xlv)

Abstract

Management actions in aquatic ecosystems are frequently directed at restoring or improving specific habitats in order to benefit fish populations. In the Grand Canyon reach of the Colorado River, experimental flow operations as part of the Glen Canyon Dam Adaptive Management Program create habitat types (i.e., sandbars and associated backwaters), and physical conditions which are believed to be critical for juvenile native fish conservation. One of the objectives of a recent experiment evaluating steady flow operations from Glen Canyon Dam was to understand how river discharge, through its interaction with physical habitat structure, influences survival rates and habitat use of native fish in the Colorado River in Grand Canyon. The key fish species of interest was humpback chub, Gila cypha, currently listed as “Endangered” under the US Endangered Species Act. The “Nearshore Ecology” (NSE) project was designed to assess the response of native fish species to experimental flow operations in 2009 - 2011 and improve our knowledge of humpback chub population ecology. The NSE project found that annual apparent survival of juvenile humpback chub (size at tagging < 100-mm total length, TL) did not differ significantly between the extant fluctuating flows and the experimental steady flow treatments in September and October. The study also documented that juvenile humpback chub were able to survive and rear in the mainstem Colorado River even at small sizes of 40 - 100 mm TL. Additionally, ultrasonic telemetry of larger humpback chub (about 180 - 190 mm TL) found that habitat selection and daily movements did not change between fluctuating conditions and the steady flow experiment. The NSE study also quantified abundance and density of juvenile humpback chub, bluehead sucker, Catostomus discobolus, flannelmouth sucker, Catostomus latipinnis, and speckled dace, Rhinichthys osculus, in different shoreline habitats (cliff, talus, debris fan, sand, and backwater). Backwaters contained the highest densities of native fish compared to other habitats, however they supported a small proportion of total abundance in the study area because the spatial extent of this habitat was very limited. Thus management actions directed at manipulating backwaters will likely only affect a small proportion of the overall abundance of native fish in this reach of the Colorado River. The results of the NSE project suggest that juvenile humpback chub survival and habitat use did not change in response to the fall steady flows observed during 2009 - 2011. However, our ability to generalize inferences from the fall steady flow experiment either temporally (i.e., summer steady flows) or spatially (i.e., other aggregations of humpback chub) may be limited due to confounding aspects of the experimental design. Nonetheless, the development and application of methods to assess the response of juvenile native fish to flow operations in the mainstem Colorado River are key new additions to the knowledge available for managing the Colorado River.