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Scientific Investigations Report 2011–5015

Prepared in cooperation with the Arizona Game and Fish Department

Status and Trends of the Rainbow Trout Population in the Lees Ferry Reach of the Colorado River downstream from Glen Canyon Dam, Arizona, 1991–2009

By Andrew S. Makinster, William R. Persons, and Luke A. Avery

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ABSTRACT

The Lees Ferry reach of the Colorado River, a 25-kilometer segment of river located immediately downstream from Glen Canyon Dam, has contained a nonnative rainbow trout (Oncorhynchus mykiss) sport fishery since it was first stocked in 1964. The fishery has evolved over time in response to changes in dam operations and fish management. Long-term monitoring of the rainbow trout population downstream of Glen Canyon Dam is an essential component of the Glen Canyon Dam Adaptive Management Program. A standardized sampling design was implemented in 1991 and has changed several times in response to independent, external scientific-review recommendations and budget constraints. Population metrics (catch per unit effort, proportional stock density, and relative condition) were estimated from 1991 to 2009 by combining data collected at fixed sampling sites during this time period and at random sampling sites from 2002 to 2009. The validity of combining population metrics for data collected at fixed and random sites was confirmed by a one-way analysis of variance by fish-length class size.

Analysis of the rainbow trout population metrics from 1991 to 2009 showed that the abundance of rainbow trout increased from 1991 to 1997, following implementation of a more steady flow regime, but declined from about 2000 to 2007. Abundance in 2008 and 2009 was high compared to previous years, which was likely the result of increased early survival caused by improved habitat conditions following the 2008 high-flow experiment at Glen Canyon Dam. Proportional stock density declined between 1991 and 2006, reflecting increased natural reproduction and large numbers of small fish in samples. Since 2001, the proportional stock density has been relatively stable. Relative condition varied with size class of rainbow trout but has been relatively stable since 1991 for fish smaller than 152 millimeters (mm), except for a substantial decrease in 2009. Relative condition was more variable for larger size classes, and substantial decreases were observed for the 152–304-mm size class in 2009 and 305–405-mm size class in 2008 that persisted into 2009.

First posted June 9, 2011

For additional information contact:

William R. Persons
U.S. Geological Survey
Southwest Biological Science Center
Grand Canyon Monitoring and Research Center
2255 N. Gemini Drive
Flagstaff, AZ 86001


Southwest Biological Science Center

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Suggested citation:

Makinster, A.S., Persons, W.R., and Avery, L.A., 2011, Status and trends of the rainbow trout population in the Lees Ferry reach of the Colorado River downstream from Glen Canyon Dam, Arizona, 1991–2009: U.S. Geological Survey Scientific Investigations Report 2011–5015, 17 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Flow Regimes

Previous Investigations

River Mile Naming Convention in Study Area

Methods of Investigation

Data Compilation

Rainbow Trout Sampling

Electrofishing

Data Collection and Fish Handling

Creel Surveys

Data Analysis

Rainbow Trout Population Metrics

Comparison of Data from Fixed and Random Sampling Sites

Status and Trends of Rainbow Trout Population

Catch per Unit Effort

Proportional Stock Density

Relative Condition

Effects of 2008 High-Flow Experiment

Future Monitoring

Summary

Acknowledgments

References Cited

Appendix 1. Fish Sampling Results for 2009


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