Evaluation of Limiting Factors for Stocked Kokanee and Rainbow Trout in Lake Roosevelt, Washington Roosevelt, 1999 PDF Format - [1.7MB] ACKNOWLEDGMENTS We would like to thank the Spokane Tribe of Indians (STI) and the Bonneville Power Administration for funding this research. Keith Underwood, Tom Cichosz, John Shields, Hank Etue, and other Spokane tribal staff were particularly helpful in providing guidance, historical data, and field and lab assistance and equipment. We thank other agencies and people who provided historical data and/or facilities including Richard LeCaire (Colville Confederated Tribes); Mitch Combs [Washington Department of Fish and Wildlife (WDFW), Sherman Creek Hatchery], Mike Lewis (WDFW Spokane Hatchery); Aulin Smith, Ray Duff, Brian Trickle, Scott Young, and Lois Blanchette (WDFW Region 1); Al Scholz, Holly McLellan, and Jason McLellan (EWU); Scott Bonar, Craig Burley, Bob Gibbons, and Ross Fuller (WDFW Olympia); the National Park Service for parking, camping, and use of boat ramps; Craig Sprankle (Bureau of Reclamation). We thank Dr. Al Scholz, Holly McLellan, Jason McLellan, Deanne Pavlik (STI), and Heather Woller (WDFW) for reviewing drafts of this report.

Evaluation of Limiting Factors for Stocked Kokanee and Rainbow Trout in Lake Roosevelt, Washington, 1999 Casey Baldwin and Matt Polacek March 2002 Abstract Hatchery supplementation of kokanee Oncorhynchus nerka and rainbow trout O. mykiss has been the primary mitigation provided by Bonneville Power Administration for loss of anadromous fish to the waters above Grand Coulee Dam (GCD). The hatchery program for rainbow trout has consistently met management goals and provided a substantial contribution to the fishery; however, spawner returns and creel survey results for kokanee have been below management goals. Our objective was to identify factors that limit limnetic fish production in Lake Roosevelt by evaluating abiotic conditions, food limitations, piscivory, and entrainment. Dissolved oxygen concentration was adequate throughout most of the year; however, levels dropped to near 6 mg/L in late July. For kokanee, warm water temperatures during mid-late summer limited their nocturnal distribution to 80-100 m in the lower section of the reservoir. Kokanee spawner length was consistently several centimeters longer than in other Pacific Northwest systems, and the relative weights of rainbow trout and large kokanee were comparable to national averages. Large bodied daphnia (> 1.7 mm) were present in the zooplankton community during all seasons indicating that top down effects were not limiting secondary productivity. Walleye Stizostedion vitreum were the primary piscivore of salmonids in 1998 and 1999. Burbot Lota lota smallmouth bass Micropterus dolomieui, and northern pikeminnow Ptychocheilus oregonensis preyed on salmonids to a lesser degree. Age 3 and 4 walleye were responsible for the majority (65%) of the total walleye consumption of salmonids. Bioenergetics modeling indicated that reservoir wide consumption by walleye could account for a 31-39% loss of stocked kokanee but only 6-12% of rainbow trout. Size at release was the primary reason for differential mortality rates due to predation. Entrainment ranged from 2% to 16% of the monthly abundance estimates of limnetic fish, and could account for 30% of total mortality of limnetic fishes, depending on the contribution of littoral zone fishes. Inflow to GCD forebay showed the strongest negative relationship with entrainment whereas reservoir elevation and fish vertical distribution had no direct relationship with entrainment. Our results indicate that kokanee and rainbow trout in Lake Roosevelt were limited by top down impacts including predation and entrainment, whereas bottom up effects and abiotic conditions were not limiting.