Growth of Endangered Humpback Chub in Relation to Temperature and Discharge in the Lower Colorado River

Abstract

Assessments of growth can provide information needed to understand how fish populations respond to changing environmental conditions and management actions, including ecosystem experimentation. We estimated growth rates and parameter uncertainty from otoliths of endangered Humpback Chub Gila cypha from the Colorado River in Grand Canyon, Arizona. We then compared growth of Humpback Chub , age 2 that were 1) occupying the mainstem Colorado River during a period of variable discharge and cooler water temperatures (1980-1998; epoch 1), 2) occupying the Colorado River during a period of moderate discharge variability and warmer water (2001-2011; epoch 2), and 3) occupying the unregulated Little Colorado River. Because growth rates of juvenile Humpback Chub (, age 2) may be more sensitive to changes in environmental conditions than adult fish, we used analysis of covariance and linear models to compare growth of juvenile fish (slopes) between epochs and capture sites (mainstem Colorado River vs. Little Colorado River). Our analysis of covariance results were ambiguous (age 3 epoch 3 site interaction; P ¼ 0.06). on growth and growth variability useful for parameterizing models to assess population viability and 2) provide empirical information on how growth of juvenile and adult Humpback Chub growth may respond to changing environmental conditions.