We evaluated the effects of hourly variation in flow caused by power load following at Glen Canyon Dam ("hydropeaking") on the nearshore habitat use and growth of age-0 rainbow trout Oncorhynchus mykiss downstream from the dam in the Colorado River, Arizona. Reduction in the extent of hydropeaking is a common element of restoration efforts in regulated rivers, but empirical support for such a practice is limited. Our assessment was based on a comparison of bundance in shoreline areas determined by electrofishing at different flows as well as analysis of otolith microstructure. The catch rates of age-0 rainbow trout in nearshore areas were at least two- to fourfold higher at the daily minimum flow than at the daily maximum, indicating that most age-0 rainbow trout do not maintain their position within immediate shoreline areas when flows are high. A striping pattern, identified by the presence of atypical daily increments formed every 7 d, was evident in over 50% of the 259 otoliths examined in 2003 but in only 6% of the 334 examined in 2004. The weekly pattern corresponded to a reduction in the extent of hourly flow fluctuations on Sundays during the growing season, which occurred in 2003 but not in 2004. The atypical increments were 25% wider than the adjacent increments and were indicative of significant (F15, 235=19.2, P<0.0001) short-term increases in otolith growth. The somatic growth rate among fish with otoliths where striping was present (11.2 mm/month) was slightly greater than that among fish with otoliths without striping (10.8 mm/month), but the difference was not significant. We provide evidence suggesting that otolith growth improved on Sundays in 2003 because it was the only day of the week when most age-0 fish were found in immediate shoreline areas, where higher water temperatures and lower velocities provided better growing conditions.
This project was supported through a cooperative agreement between the U.S. Geological Survey and Ecometric Research, Inc. We thank Mike Yard, Steve Hall, Jeff Sneep, and Dave Foster for help with electrofishing. Linda Marks, Tania Davignon-Burton, and Jenna Denyes provided the necessary expertise in otolith preparation and interpretation. Thanks to Mike Bradford, Carl Walters, and Eric Parkinson for helpful comments on earlier versions of this manuscript and to Ted Melis, Lew Coggins, Mike Yard, Barbara Ralston, Carol Fritzinger, and Matthew Andersen of Grand Canyon Monitoring and Research Center for their support throughout this project. Two anonymous reviewers and an associate editor from this journal provided suggestions that greatly improved the quality of this manuscript.