HABITAT RELATIONSHIPS OF SUBADULT HUMPBACK CHUB IN THE COLORADO RIVERTHROUGH GRAND-CANYON - SPATIAL VARIABILITY AND IMPLICATIONS OF FLOW REGULATION

We examined subadult humpback chub densities along 24 kms of the Color ado River in the Grand Canyon to: (1) identify geomorphic conditions i n the study area; (2) determine associations between subadult humpback chub (< 200 mm TL) habitat use and geomorphic differences; and (3) de termine how discharge, during base flow conditions, was related to sub adult humpback chub habitat conditions. Habitat was categorized at two nested spatial scales: geomorphic reach and shoreline type. Within re aches, shoreline types were categorized according to geomorphology. We measured water depth, velocity and cover attributes along all shoreli ne types over a range of discharges to determine if habitat quality of reaches and shoreline types varied with discharge. Reaches 1 and 3 ha d narrow, deep corridors, whereas Reach 2 was a wide, shallow reach. A mong shoreline types, depth, velocity and cover varied; however, diffe rences were not consistent between reaches. Fish densities also varied among shoreline types and reaches. Vegetation, talus and debris fan s horelines had the highest densities of subadult humpback chub in a pat tern similar to that of cover. In addition, subadult humpback chub pre sence was associated with a high frequency of cover regardless of shor eline designation. However, these relationships explained little of th e overall variation in subadult densities. Lack of a strong associatio n between fish density and geomorphology may be partially due to effec ts of discharge on habitat quality. The overall trend among shorelines (without regard to type) showed that cover decreased with increasing discharge, whereas depth and velocity increased. However, no consisten t pattern between discharge and depth, velocity and cover among indivi dual shoreline types was evident. Vegetated shorelines, consisting mai nly of non-native tamarisk (Tomarix chinensis), had nearly twice the f ish densities of talus and debris fan. Reasons are discussed as to why subadult humpback chub occupy naturalized habitat like vegetated shor elines in greater densities than natural habitats. The relationships o bserved in this study have important implications for humpback chub re covery and management of the Colorado River through Grand Canyon. (C) 1998 John Wiley & Sons, Ltd.