Lake Roosevelt Pelagic Fish Study, 1998 PDF Format - [334K] ACKNOWLEDGMENTS We would like to thank the Spokane Tribe of Indians Fish and Wildlife Department, Lake Roosevelt Monitoring Program for providing this research opportunity. Also, Keith Underwood, Tom Cichosz, John Shields, Hank Etue, and other tribal staff were particularly helpful in providing guidance, historical data, and field and lab assistance and equipment. Dave Beauchamp (Utah State University) provided valuable planning, editing, and guidance on modeling operations. We would also like to 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); 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). Dennis Rondorf, Tim Darland, and Mark Novick of the U.S. Geological Survey Biological Resources Division, for performing the midwater trawl surveys. Finally, we thank Bonneville Power Administration for funding contract # 94BI32148.

Lake Roosevelt Pelagic Fish Study, 1998 Casey Baldwin Matt Polacek and Scott Bonar November 1999 Abstract Pelagic fishes, such as kokanee and rainbow trout, provide an important fishery in Lake Roosevelt; however, spawner returns and creel results have been below management goals in recent years. Our objective was to identify factors that potentially limit pelagic fish production in Lake Roosevelt including entrainment, food limitation, piscivory, and other abiotic factors. We estimated the ratio of total fish entrained through Grand Coulee Dam to the pelagic fish abundance for September and October, 1998. If the majority of these fish were pelagic species, then entrainment averaged 10-13% of pelagic fish abundance each month. This rate of entrainment could impose considerable losses to pelagic fish populations on an annual basis. Therefore, estimates of species composition of entrained fish will be important in upcoming years to estimate the proportion of stocked pelagic fish lost through the dam. Food was not limiting for kokanee or rainbow trout populations since growth rates were high, and large zooplankton were present in the reservoir. Estimates of survival for kokanee were low (< 0.01 annual) and unknown for rainbow trout. We estimated that the 1997 standing stock biomass of large (>1.1 mm) Daphnia could have supported 0.08 annual survival by kokanee and rainbow trout before fish consumption would have exceeded available biomass during late winter and early spring. Therefore, if recruitment goals are met in the future there may be a bottleneck in food supply for pelagic planktivores. Walleye and northern pikeminnow were the primary piscivores of salmonids in 1996 and 1997. Predation on salmonid prey was rare for rainbow trout and not detected for burbot or smallmouth bass. Northern pikeminnow had the greatest individual potential as a salmonid predator due to their high consumptive demand; however, their overall impact was limited because of their low relative abundance. We modeled the predation impact of 273,524 walleye in 1996, and 39,075 northern pikeminnow in 1997 because diet data revealed predation on salmonids during these years. We could not determine the absolute impact of piscivores on each salmonid species because identification of fish prey was limited to families. Our estimate of salmonid consumption by walleye in 1996 and northern pikeminnow in 1997 shows that losses of stocked kokanee and rainbow trout could be substantial (up to 73% of kokanee) if piscivores were concentrating on one salmonid species, but were most likely lower, assuming predation was spread among kokanee, rainbow trout, and whitefish. Dissolved oxygen was never limiting for kokanee or rainbow trout, but temperatures were up to 6°C above the growth optimum for kokanee from July to September in the upper 33 meters of water. Critical data needed for a more complete analysis in the future include species composition of entrainment estimates, entrainment estimates expanded to include unmonitored turbines, seasonal growth of planktivorous salmonids, species composition of salmonid prey, piscivore diet during hatchery releases of salmonids, and collection of temperature and dissolved oxygen data throughout all depths of the reservoir during warm summer months.