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<br />119 ~ COrl vu s&) et AI
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<br />REGULATED RIVERS: RESEARCH & MANAGEMENT
<br />Regul. Rivers: Res. Mgmt. 14: 267~284 (1998)
<br />
<br />HABITAT RELATIONSHIPS OF SUBADULT HUMPBACK CHUB IN
<br />THE COLORADO RIVER THROUGH GRAND CANYON: SPATIAL
<br />VARIABILITY AND IMPLICATIONS OF FLOW REGULATION
<br />
<br />YVETTE K. CONVERSP'*, CHARLES P. HAWKINSb AND RICHARD A. VALDEZc
<br />
<br />a Department of Fisheries and Wildlife/Ecology Cenler, Utah State University, Logan, UT 84322-5210, USA
<br />b Department of Fisheries and Wildlife/Ecology Center/Watershed Science Unit, Utah State University, Logan, UT 84322-5210, USA
<br />c Bio/Westlncorporated, 1063 West 1400 North, Logan, UT 84321, USA
<br />
<br />ABSTRACT
<br />
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<br />We examined subadult humpback chub densities along 24 kms of the Colorado River in the Grand Canyon to: (I)
<br />identify geomorphic conditions in the study area; (2) determine associations between subadult humpback chub ( < 200
<br />mm TL) habitat use and geomorphic differences; and (3) determine how discharge, during base flow conditions, was
<br />related to subadult humpback chub habitat conditions.
<br />Habitat was categorized at two nested spatial scales: geomorphic reach and shoreline type. Within reaches,
<br />shoreline types were categorized according to geomorphology. We measured water depth, velocity and cover
<br />attributes along all shoreline types over a range of discharges to determine if habitat quality of reaches and shoreline
<br />types varied with discharge.
<br />Reaches I and 3 had narrow, deep corridors, whereas Reach 2 was a wide, shallow reach. Among shoreline types,
<br />depth, velocity and cover varied; however, differences were not consistent between reaches. Fish densities also varied
<br />among shoreline types and reaches. Vegetation, talus and debris fan shorelines had the highest densities of subadult
<br />humpback chub in a pattern similar to that of cover. In addition, subadult humpback chub presence was associated
<br />with a high frequency of cover regardless of shoreline designation. However, these relationships explained little of the
<br />overall variation in subadult densities.
<br />Lack of a strong association between fish density and geomorphology may be partially due to effects of discharge
<br />on habitat quality. The overall trend among shorelines (without regard to type) showed that cover decreased with
<br />increasing discharge, whereas depth and velocity increased. However, no consistent pattern between discharge and
<br />depth, velocity and cover among individual shoreline types was evident.
<br />Vegetated shorelines, consisting mainly of non-native tamarisk (Tamarix chinensis), had nearly twice the fish
<br />densities of talus and debris fan. Reasons are discussed as to why subadult humpback chub occupy naturalized
<br />habitat like vegetated shorelines in greater densities than natural habitats. The relationships observed in this study
<br />have important implications for humpback chub recovery and management of the Colorado River through Grand
<br />Canyon. @ 1998 John Wiley & Sons, Ltd.
<br />
<br />KEY WORDS: humpback chub; habitat; geomorphology; shoreline; cover; flow; regulation; Colorado River; Grand Canyon
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<br />INTRODUCTION
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<br />In 1967, the US Fish and Wildlife Service (USFWS) listed the humpback chub (Gila cypha) as endangered
<br />following a decline in numbers in the Colorado River (Minckley, 1991). Federal listing prompted a
<br />number of studies to describe the ecology and determine the status of humpback chub (Vanicek et aI.,
<br />1970; Holden and Stalnaker, 1975; Carothers and Minckley, 1981; Valdez and Clemmer, 1982; Kaeding
<br />and Zimmerman, 1983; Berry and Pimentel, 1985; Miller and Hubert, 1990; Valdez et aI., 1992). However,
<br />humpback chub are difficult to study because of their rarity and their residence in swift, turbid and
<br />inaccessible riverine environments; as a result, 25 years of investigation has yielded little information on
<br />this fish, particularly at early life-history stages (Gorman et al., 1994; Valdez and Ryel, 1995).
<br />
<br />* Correspondence to: Aquatics Section, Utah Division of Wildlife Resources, 1594 West North Temple, Suite 2110, Box 146301, Salt
<br />Lake City, UT 84114-6301. e-mail: nrdwLyconvers@state.uLus
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<br />Contract grant sponsor: United States Bureau of Reclamation
<br />
<br />CCC 0886-9375/98/030267-18$17.50
<br />I() 1998 John Wiley & Sons, Ltd.
<br />
<br />Received 17 March 1997
<br />Accepted 22 December 1997
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<br />'b 10-:)
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