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to River below ;h were. grown 05 except for )erature (Jo- roducespos- not likely to ;ratures, and temperature An evalua- npback chub account prey the range of )lementation changes at .orado River, y monitored undance and the primary :yon (Valdez if a TCD is the response :ed tempera- ; water tem- the densities experiment, :es increased ming Gorge water tem- 1ge the taxa 001). Com- i the region, n has had a community am (Stevens othermal re- iversity and .ure may be nining local in streams factors such HUMPBACK CHUB BIOENERGETICS as turbidity and periphyton abundance may also be influential (Gore 1977; Vannote and Sweeney 1980). Temperature effects on nutrient cycling and production rates of invertebrates within the lower Colorado River will likely have a strong impact on humpback chub food sources. Temperature is only one of the many hypothe- sized causes of native fish declines in the Colorado River, with flow regimes and nonnative fish intro- ductions also influencing native fish populations. Abundant nonnative fishes such as fathead min- now, channel catfish Ictalurus punctatus, common carp, rainbow trout, and brown trout may be the most important factor in native fish declines (Minckley 1991). Warmer temperatures in the river may cause increased growth rates and possibly in- creased main-stem spawning by nonnative species that currently co-occur with humpback chub (Clarkson and Childs 2000; Robinson and Childs 2001). Warmer main-stem water temperatures may also increase the probability that fish such as striped bass Morone saxatilis will migrate up the Colorado River from downstream reservoirs, pos- sibly increasing predation on humpback chub and other native fishes (Valdez and Leibfried 1999). The most proximate threat to humpback chub from nonnative predation is brown trout whose current mode of distribution is near Bright Angel Creek. Large brown trout have been observed feeding on adult humpback chub (L. Coggins, GCMRC, per- sonal communication). Brown trout are also more tolerant than are rainbow trout to increased tem- peratures and are adapted to forage under low light conditions (Robinson and Tash 1979; Young 1999), commonly observed in the lower Colorado River. The impacts of a TCD would directly influence the growth rates of predators such as rainbow trout and brown trout along with the rate of growth of small humpback chub that are prey to these pred- ators. The size of prey that predators such as rain- bow trout consume is often limited to a specific size range (Ware 1972; Vogel and Beauchamp 1999), so faster growth of prey can limit the du- ration of time that they are vulnerable to the pred- ator. With increasing temperatures or variations in food availability in the lower Colorado River, this "window of vulnerability" would presumably shorten if the growth rate of juvenile humpback chub increased. The window of prey vulnerability depends upon growth rates of both the predators and their prey, which may differ with an altered temperature regime. Valdez and Ryel (1995) made some estimates of the maximum size of humpback 971 chub that could be consumed by brown trout, rain- bow trout, and channel catfish of varying sizes, but they did not estimate the period of vulnera- bility during the year. Bioenergetic or other growth models could be used to evaluate further specific temperature and food scenarios and the potential importance of prey-to-predator size ratios (Cowan et al. 1996). If a TCD is installed and operated, predator and prey sizes should be monitored so specific predation hypotheses can be tested. The model that we fit and applied is largely driven by water temperature and the diet of hump- back chub and thus does not take into account some conditions that are believed to be important for the growth and survival of humpback chub in the lower Colorado River. Turbidity, for example, increases substantially below the Little Colorado River during certain periods and may reduce pri- mary productivity (and therefore food availability, consumption, or both) through increased light at- tenuation (SWCA 1998). Asian tapeworms Both- riocephalus acheilognathi have also been observed in up to 78% of humpback chub and may represent a new threat in Grand Canyon, perhaps reducing the growth rate of native fishes (Clarkson et al. 1997; Brouder 1999). Turbidity and parasitic ef- fects cannot be directly simulated with the bio- energetics model that we employed, although the model could be modified to incorporate these types of variables. Individual-based models, in partic- ular, have been used to examine how turbidity or light influences foraging rate in fishes (Petersen and Gadomski 1994; Vogel and Beauchamp 1999). Beyers et al. (1999) have shown how stress or parasites might be modeled using a bioenergetics framework. As the specific objectives evolve for humpback chub recovery in the lower Colorado River, models and analyses that account for such factors as turbidity and stress may be necessary. The bioenergetic modeling approach was ap- plied to explore a few of the potential interactions and produced some nonintuitive results that can be used to develop testable hypotheses. Bioener- getic or modified models that consider turbidity and predator-prey size ratios could also be used with nonnative fishes such as rainbow trout and brown trout that have well-developed energetic pa- rameter sets. Acknowledgments We appreciate the financial assistance and en- couragement of Denny Fenn, Steve Gloss, Jeff Lovich, Jim Seelye, and Lyman Thorsteinson. Lew Coggins and the staff at the GCMRC offered help-