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<br />pockets on rock benches or along steep rock walls, often adjacent to deep and swift water
<br />(Valdez and Clemmer 1982), The young also use backwaters when they are available, Juveniles
<br />use water as deep as 10m over sand-silt and boulder-bedrock substrates with velocities ofless
<br />than 0.3 mlsec (Valdez et al, 1990), In the Colorado River in Grand Canyon, juveniles <175 mm
<br />1'1 used primarily shorelines with vegetation, talus, or debris fans (Valdez and Ryel1997,
<br />Converse 1996), as well as backwaters (Arizona Game and Fish Department 1996a), Selected
<br />. shoreline habitats for fish <75 mm 1'1 had velocities of 0,0-0,3 mis, with an average of about 0,06
<br />(Valdez and Rye11995), Swimming abilities of juvenile humpback chub (73-134 mm 1'1) in the
<br />laboratory were significantly reduced when fish were tested at colder temperatures of 14' C, when
<br />compared to 20' C, Larger fish were not tested, but there is no evidence to suggest that larger
<br />fish are affected by temperature as dramatically as small fish, Average length at the end of the
<br />first year of life for humpback chub in Grand Canyon was 74 mm 1'1 (range, 58-138 mm 1'1)
<br />(Valdez and Rye! 1995); hence, age 0 fish are most affected by the cold thennal regimes of the
<br />Colorado River through Grand Canyon
<br />
<br />Adults are generally found in a variety of depths, occupying shallow shaded areas in the mornings
<br />and evenings and depths of 12 m or more during midday, Humpback chub are frequently found
<br />along eddy lines in low velocity water adjacent to areas of high velocity, Selected habitats of 69
<br />radio tagged adults in Grand Canyon were large recirculating eddies (88% captures, 74% radio
<br />contacts), and subadults used primarily shorelines with vegetation, talus, and debris fans and eddy
<br />return channels (backwaters) (Valdez and Rye11997),
<br />
<br />R~roduction
<br />
<br />Humpback chub are spring spawners; Spawning in Black Rocks occurred from April to June,
<br />depending on water temperature and flow (Valdez and Clemmer 1982, Kaeding et ai, 1990), The
<br />majority of successful spawning by humpback chub in Grand Canyon presently occurs in the LCR
<br />during March-May, at water temperatures of 16-22'C (Kaeding and Zimmerman 1983), Year-
<br />around releases of 8-10' C from Glen Canyon Dam preclude successful mainstem reproduction,
<br />except possibly for isolated reproductive efforts at mouths of seasonally-warmed tributaries or
<br />warm springs; post-larval humpback chub were captured in association with an isolated
<br />aggregation of adults at a warm spring atRM 30 (Valdez and Rye! 1997), indicating localized
<br />reproductive success. Adult humpback chub from the vicinity of the LCR ascend the LCR
<br />annually in February-April to spawn and descend during the summer months. Many young
<br />hatched in the l.CR are disp1acedinto the Colorado River by monsoonal floOds during July-
<br />September, with annua1 estimates of 246,000-3 ,9 million juveniles during 1990-93 (Valdez and
<br />Rye11995), Survival of young to adulthood at age 3 was estimated at 0,001, while estimated
<br />annual survival of adults was 0,93, Predation of juvenile humpback chub was documented for
<br />brown trout (Salmo tnIUI), channel catfish (Jctalurus punctatus), and rainbow
<br />
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