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North American Journal of Fisheries Management 16:795-804, 1996 American Fisheries Society 1996 Establishment of a Fish Community in the Hayden-Rhodes and Salt-Gila Aqueducts, Arizona R GORDON MUELLER U.S. Geological Survey, Denver Field Station Post Office Box 25007, Denver, Colorado 80225, USA Abstract.-Fish populations were studied in the Central Arizona Project's canal system during the first 4 years of aqueduct operation (1986-1989). Ichthyoplankton entering the canal from Lake Havasu averaged 1 larva/m3 during April-June 1987 and 1988. Larval fish densities increased significantly in downstream samples, substantiating diver observations that fish were spawning in the canal system. Of the 16 fish species collected, 14 were assumed to have originated from Lake Havasu and 2 were introduced by anglers from their bait buckets. Initially, the fish community was dominated numerically by threadfin shad Dorosoma petenense (>89%), centrarchids (<10%), cyprinids (<2%), and striped bass Morone saxatilis (<I%). However, as annual water diversions increased from 1.3 X 108 m3 in 1986 to 9.4 X 108 m3 in 1989, community composition shifted from clupeids to centrarchids (70%). Fish densities dropped from an estimated 1,260 fish/ha in 1986 to 17 fish/ha in 1989, and biomass dropped from 116 to 73 kg/ha. Declines were attributed to higher operational velocities, associated scour, deprivation, and predation. Although initial populations adjusted downward to planned operational conditions, the fish community continued to represent a potentially valuable, but as yet unused, resource. • The significance of fish loss from natural streams to irrigation diversions has been recog- nized since the early 1920s (Prince 1923; Clothier 1953). These losses can be reduced by screening diversion structures. Although the passage of larg- er fish can be controlled, hydraulic and economic constraints generally allow smaller fish to be trans- ported and lost. If aquatic conditions are condu- cive, substantial fish communities can establish themselves and flourish in these artificial habitats (Marsh and Minckley 1982; Minckley et al. 1983). Large conveyance systems constitute hundreds of kilometers of waterways in the western United States, but their respective fish communities have received little recognition or use. Fish inventories of large canals in Arizona and southern California suggest that these fish communities can be sub- stantial and in some cases can rival communities found in natural desert streams (Marsh 1981; Minckley et al. 1983; Mueller et al. 1989; Mueller and Liston 1994). However, only limited infor- mation is available regarding entrainment rates, colonization rates, carrying capacities, and spawn- ing success in these artificial riverine habitats. I describe the early evolution, occurrence of spawn- ing, and management opportunities provided by the fish community that developed in one of the largest concrete-lined canal systems in the western hemisphere. Study Site The Central Arizona Project is a single-purpose water project built to convey Colorado River water from Lake Havasu, California-Arizona, to Phoe- nix and Tucson, Arizona (Figure 1). The convey- ance system consists of a 515-km gravity flow ca- nal system, 14 pumping plants, and an off-site stor- age reservoir, Lake Pleasant. The canal is divided into three segments: the Hayden-Rhodes Aque- duct, the Salt-Gila Aqueduct, and the Tucson Aq- ueduct. The investigation focused on the Hayden- Rhodes and Salt-Gila aqueducts, which consist of the first six pumping plants and 400 km of canal. Remaining downstream portions of the aqueduct and its off-site storage facilities were not opera- tional at the time of this study. The Lake Havasu Pumping Plant lifts water 251 m into the Hayden-Rhodes Aqueduct. The first 27.4 km of canal (Reach 1) was designed to be wider than downstream reaches to accommodate 4.9 x 106 m3 of water storage. This storage allows greater flexibility in off-peak power use for pump- ing water from Lake Havasu. Canals are concrete lined and have bank slopes of 1.5:1 and a maxi- mum depth of 5.5 m. Reach 1 has a bottom width of 24.4 m, compared with downstream reaches that are only 7.3 m wide at the bottom. Water velocities range from 0.0-0.6 m/s in Reach 1 (full pool) to 0.0-1.4 m/s in the narrower downstream sections: The maximum diversion capacity of the canal is 85 m3/s. The Bouse Hills Pumping Plant is located at the end of Reach 1 and is the second lift station on the system. Other pumping plants are Little Har- quahala (92 km downstream of Lake Havasu), 795