The Colorado River below Glen Canyon Dam has been altered by dam-induced modifications to the river’s flow, temperature, and sediment supply. Nonnative species have also changed the natural system. Nonnative fish are thought to prey on and compete with native fish, including the endangered humpback chub (Gila cypha). These impacts have likely changed both the amount and sources of energy that fuel the aquatic food web and the flows of energy within the food web. Installation of the dam created a relatively clear, cool aquatic environment below the dam that now allows aquatic plants to capture the sun’s energy, and they in turn are now consumed by a few species, including scuds (Gammarus lacustris), midges (Family: Chironomidae), blackflies (Simulium arcticum), and New Zealand mudsnails (Potamopyrgus antipodarum). The first three species can provide food for both native and nonnative fishes, but fish cannot digest the New Zealand mudsnail.
GCMRC scientists and their university cooperators (University of Wyoming, Idaho State University, and Loyola University, Chicago) are studying the kinds of organic matter (for example, algae and leaf litter) and invertebrate communities (for example, black flies and bloodworms (Families: Lumbricidae and Tubificidae)) in the Colorado River below Glen Canyon Dam and how the sun’s energy is captured and passed from one species to another. Collectively, organic matter and the aquatic invertebrates that consume it largely constitute the food base for fish in the Colorado River ecosystem. Current research efforts focus on the temporal patterns, multi-year trends, and factors that affect the amount and sources of food for humpback chub and rainbow trout (Oncorhynchus mykiss). This information is used to understand the role that food plays in determining the distribution, population density, and growth of these animals. A better understanding of the aquatic food web of the Colorado River will allow natural resource managers to describe the conditions that would be expected to support the Grand Canyon population of humpback chub, the economically important Lees Ferry trout fishery, and other fish species.
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