FACTORS LIMITING THE DISTRIBUTIONS OF NATIVE FISHES IN THE LITTLE COLORADO RIVER, GRAND-CANYON, ARIZONA

Four native fish species exist in the spring-fed, perennial lower 21 k m of the Little Colorado River (LCR), Gland Canyon, Arizona, but only speckled dace (Rhinichthys osculus) is present in the entire reach. Th e other three species, humpback chub (Gila cypha), bluehead sucker (Pa ntosteus discobolus) and flannelmouth sucker (Catostomus latipinnis), are present only in the lower 14.2 km below Chute Falls. During base f low, unfavorable water chemistry (i.e., high CO2) and the physical obs tacle of Chute Falls may limit the distributions of the three large-bo died native fishes to below Chute Falls. We sampled benthic algae and invertebrates and analyzed water chemistry downstream (Reach 1) and up stream (Reach 2) of Chute Falls to determine whether the disparity in fist distributions could he attributed to these factors. Algal biomass (chlorophyll a) and density and biomass of invertebrates used as food by fishes were greater in Reach 2 than Reach 1. Free carbon dioxide ( CO2) and alkalinity generally decreased from Reach 2 to Reach 1, where as dissolved oxygen, pH and specific conductance generally increased. To determine if water chemistry above Chute Falls was unsuitable to na tive fishes other than speckled dace, we relocated age-0 (post-larval) and age-1 humpback chub and age-1 bluehead suckers from Reach 1 to th ree sites in Reach 2 and held them in cases for 3 days. All age-1 fish survived the experiment, although some individuals of both species ex perienced short-term respiratory stress. Age-0 humpback chub experienc ed significantly higher mortalities, and exhibited more stress behavio rs at the most upstream relocation site than at other sites. We conclu de that neither food resources nor water chemistry limit these native fishes to Reach 1. Translocation of fish to the reach above Chute Fall s, or breaching that barrier, may be feasible management actions to in crease available habitat for the endangered humpback chub and other na tive fishes in the LCR.