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springtime, remaining sedentary in deep water during daytime and moving into shallow, near-shore habitats during darkness (Marsh and Mueller 2000). Association of stream-dwelling chubs with deep pools, undercut banks and shadows in well known, and use physical structure or even turbidity as daytime cover has been reported for several members of the genus Gila in Arizona streams and rivers (Minckley 1973, Bio/West 1994). This behavior may serve to avoid sight-feeding predators in addition to supporting other life functions including feeding and reproduction. Apparent daytime occupation of cover by large bonytail and apparent relegation of smaller fish to open water may have important implications for bonytail management because availability of suitable cover may in part determine habitat carrying capacity. If all available cover is being used and relegation to open water of fish that are denied access to suitable cover results in increased exposure to predators or other mortality factors, then cover becomes a limiting factor at the population level. It might be possible to experimentally investigate the significance of this and other phenomena on bonytail and other native species' population parameters, but suitable field sites to perform such work, replicates of the Cibola HLP, for example, do not currently exist. Regardless, provision of adequate cover should be considered in the selection or design of bonytail management areas such as those used for grow-out, refugium, or long- term population maintenance (e.g. Minckley et al. 2003) Acknowledgements Gordon Mueller, U.S. Geological Survey (USGS) contributed substantially to the design and implementation of this project. Jeanette Carpenter (USGS), Darren Thornbrugh, Robert Colvin and C.O. Minckley (U.S. Fish and Wildlife Service, USFWS), Joe Milosovitch (California Department of Fish and Game} and J.D. Schooley and Michael R. Schwemm (Arizona State University, ASU) provided additional field assistance. Schwemm and B.R. Kesner (ASU) evaluated point sample data, Schooley performed preliminary analysis of directional data, and Thornbrugh implemented the ArcView project to visualize the directional data. Ken Bovee (USGS) provided the map presented as Figure 1. This work was performed under ASU Animal Use and Care Protocol No. 05-767R. Appropriate Arizona, California, and USFWS permits authorized collections. U.S. Geological Survey Biological Resources Division provided funding through Cooperative Agreement No. OOCRAG004 Project Award No. 0004CS003 to Arizona State University. Gordon Mueller served as the USGS Project Officer. Literature Cited BioNVest. 1994. Life history and ecology of the humpback chub (Gila cypha) in the Colorado River, Grand Canyon. Final Report, U.S. Bureau of Reclamation Contract No. O-CS-40-09110.- Bio/West, Inc., Logan, Utah. 168 pages + appendices. 7