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Chapter III Affected Environment and Environmental Consequences 33 If a change were to occur, there are some indications that the system is fairly resilient to short disturbances. Reclamation found that even though the 1996 Beach Habitat Building Test Flows strongly scoured the benthic foodbase in the mainstream, recovery was extremely rapid. Native and Endangered Fishes -The native fishes of the Colorado River make up one of the most unusual assemblages of fish specially adapted to their environment found anywhere in the world. However, recent history has introduced new challenges by modifying the fish's evolutionary environment. Major dams have modified streamflow extremes, cleared and cooled the waters, converted rivers to lakes, blocked natural movement corridors, and permitted the introduction of non-native fish that compete with and/or prey upon the natives. Of the eight species of native fish, three (Colorado squawfish, bonytail, and roundtail chub) have been extirpated from Glen and Grand Canyons, two (humpback chub, and razorback sucker) are listed as endangered, one (flannelmouth sucker) is a candidate species for listing under the Endangered Species Act. One (bluehead sucker) is common only in tributary inflows and one (speckled dace) is relatively common throughout the canyon. Cold water temperature is thought to be an overriding constraint for larval and young of the year native fish in the Colorado River mainstem. Cold temperatures prevent spawning or, if spawning occurs, limit egg and larvae survival in both native and warmwater non-native fishes. Clarkson et al. (1994) suggests that temperature modification is the only way to alleviate the known restriction by cold-water temperatures to successful mainstem reproduction and recruitment of native fishes. Releases of l50C (590F) in the late summer should warm as the water flow downstream and provide optimum temperatures of l60C to 220C for spawning, incubation, and growth in the mainstem. Valdez reported that humpback chub, flannelmouth sucker, bluehead sucker, and speckled dace require l60C to 220C for spawning, egg incubation, and survival of larvae, while razorback sucker spawn successfully at 100C to 220C. Temperatures released from the dam are about 80C (460F) and warm longitudinally (240 miles downstream) to about l60C (61 OF) in summer. These temperatures are not quite sufficient for spawning of native fish. All documented spawning of native fishes has occurred in warm tributaries and springs. Humpback Chub - Humpback chub is a native fish which evolved in the Colorado River before water development and regulation by dams. Studies report that the Little Colorado River is the main spawning area for the humpback chub in Grand Canyon. The Little Colorado River is a small, unregulated (natural) tributary to the Colorado River located about 77 miles below Glen Canyon Dam. Spawning is suspected in warm springs and warm tributary inflows. Adult chub in the mainstem spawn in the lower 9 miles of the Little Colorado River from March through May. Adults stage in large eddies in February and March and make spawning runs up the Little Colorado River from March through Mayas flows decrease, warm, and clear (Valdez and RyeI1995). Young humpback chub either remain in the Little Colorado River or move into the mainstem, where mortality is believed to be high because of cold- water temperatures, thermal shock, and predation. Small numbers of chubs spawned the previous year may be present in the mainstem the following spring.