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,~.. . .-J.,,< ,'; -J~,_:, .",...;. . I i' The population of humpback chub in the mainstem and Little Colorado River in Grand Canyon is thought to be the largest of six populations for the species range-wide (Marsh and Douglas 1992). The importance of the Grand Canyon mainstem to the species is exhibited by the nine aggregations identified by Valdez and Ryel (1995) throughout the mainstem Colorado Rive'r in the project area and are referenced as river miles from Lees Ferry: 30-Mile, Little Colorado River inflows (about RM 61), Lava Canyon to Hance Rapid (about RM 70), Bright Angel Creek mouth (RM 88), Shinimo Creelc mouth (RM 108), Stephens Aisle (about RM 118), Middle Granite Gorge (about RM 128), Havasu Creek mouth (RM 157), and PumplcinSpring (RM 212). Various life stages and age classes of humpbaclc chub use the mainstem and are influenced by seasonal warming, food availability, and flow. Most humpback chub are spawned and successfully recruited into the population due to favorable habitat conditions in the Little Colorado River. Throughout the year, humpback chub of all sizes may leave the Little Colorado River and reside in the mainstem. During some of the winter season, temperatures in the mainstem are wanner than the more shallow, air-influenced tributaries. ~ ~ ~ f' " 'i~ l~ it :':; Limited spawning of humpbaclc chub occur in other tributaries or mainstem locations. In upper Marble Canyon, some success has been reported at wann springs entering the mainstem at RM 30 (Arizona Game and Fish Department 1993, Valdez and Ryel1995). Larval humpbaclc chub have been collected from Cardenas Marsh (RM 71) and several other locations throughout the.. river. Humpbaclc chubs less than 1 calendar year of age have been captured in the mainstem in January and March (Valdez and Rye11995) ;.;: ;1' ~ '. ~.- i(. ~ ~.. ~,.:, " ;:" Humpbaclc chub feed on a variety of aquatic and terrestrial organisms including the amphipod Gammarus lacusrris, immature chironomids and simuliids, and other invertebrates, all which are influenced by nutrients, turbidity, velocity (Valdez and Ryel 1995), and other flow related conditions. (',;' ,:..,. ~.: \:, , ;'-<: L~;: V: Critical habitat in Arizona includes most of the habitat now used by the Grand Canyon population of humpbaclc chub. Designated reaches are the lower 13 km of the Little Colorado River and from Natuloid Canyon (about RM 34) to Granite Park (about RM 208) on the Colorado River (U.S. Fish and Wildlife Service 1994a).. This is a total of 291 km or 48 percent of humpback chub critical habitat. Known constituent elements include water, physical habitat, and biological environment as required for each life stage. The dominant factor influencing critical habitat in the project area is Glen Canyon Dam. Effects of the dam on water quality and quantity attributes such as temperature, nutrients, turbidity, and hydrologic regime; on the physical habitat; and on the biologic environment such as food supply have been detailed in the FEIS, the 1995 biological opinion, and most of the various GCES research reports. Another important factor is the influence of nonnative fishes. Nonnative fishes have been introduced into altered habitats such as reservoirs and cold water rivers and have profoundly affected native fish populations through predation and competition. r: .' ~~~ r.;-:;: , ;t~ ~ k~ ~l:; I, ~ ~! ~~{~ ~':'. > ~~ ! ,;~;2 -' ! Biological and Conference Opinions Glen Canyon BeachlHabitat-Building Flows 2116196 13