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<br />trout (Oncorhynchus mykis&), and may be a major reason for low survival of chubs in Grarid
<br />Canyon,
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<br />Female humpback chub produce a variable number of eggs depending on size and age of fish,
<br />Hamman (1982) found an average of2,523 eggs per tisn for eigltt females neld in a I1atcnery (2,8
<br />nun average egg diameter), Females typically broadcast their eggs over cobble/gravel riftles and
<br />the semi-adhesive eggs attach in well-aerated interstitial spaces, protected from swift currents and
<br />predation, The eggs water harden in about 48 hours and total incubation time is typically 4-5
<br />days at water temperatures of21-2Z.C (Hamman 1982), The eggs become eyed in about 78
<br />hours and recently-hatcned larvae are 6-7 mmlong (Snyder 1981),
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<br />Ave and Growth
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<br />The humpback chub is a slow-growing, long-lived fish, Hendrickson (1993) identified up to 22
<br />annular rings from otoliths of humpback chub from the LCR. Other aging studies have not been
<br />done with this species, because only structUres (Le" otoliths) that can be obtained through lethal
<br />means have shown to be reliable for older fish, Nevertheless, the species probably lives well over
<br />ZO years of age, Valdez and Ryel (1995), using scale back-ca1culation techniques for juveniles,
<br />detennined average lengths of96, 144, and 186 mm TL for fish at 1,2, and 3 annular rings; annuli
<br />complete development in March, about the same time as hatching, and hence, coincide with age.
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<br />Larval humpback chub are about 7 mm long at hatching (Muth 1990), Average 30-day growth
<br />rate for first year fish in the LCR at 16-22.C was 10,30 nun (from scale back-calculations; Valdez
<br />and Ryell995), compared to 10,63 mm for laboratory fish at 20.C (Lupher and Clarkson 1994),
<br />Growth rate in the LCR was greatly reduced following maturity; average monthly growth rates
<br />were 1.42 mm, 1.33 mm, 1.08 mm, and <1 mm for fish 150-200 mm TL, 200-250 nun TL, 250-
<br />300 mm TL, and >300 mm TL, respectively (Minckley 1992), Average 30-day growth rate of
<br />subadults in the mainstem Colorado River in Grand Canyon at 10-12. C was 4,00 nun (Valdez
<br />and RyeI1995), comparable to 2,30 mm for laboratory fish at 10.C (Lupher and Clarkson 1994),
<br />However, growth rates of adults were considerably higher in the colder mainstem than in the
<br />LCR; i,e" 2,25 mm, 2,79 mm, 2,50 mm, and 1.20 mm for fish 150-200 mm n, 200-250 nun n,
<br />250-300 nun n, and >300 nun TL, respectively (Valdez and Rye11995),
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<br />Humpback chub typically eat a variety of insects, crustaceans, and organic matter (Le" plants,
<br />seeds berries). Diet of fish from the LCR was dominated by immature Chironomidae (midges)
<br />and Simullidae (blacldlies) (Kaeding and Zimmerman 1983). Aduhs from the mainstem Colorado
<br />River in Grand Canyon consumed freshwater amphipods (f'JlII1UY\anJs lacustris. 45%), black flies
<br />(simuliidae, 40%), terrestrial invertebrates (9%), midges (chironomidae, 5%), and other aquatic
<br />invertebrates (1 %), Humpback chub in the upper Green River subbasin ate large numbers of
<br />Mormon crickets (,A.nabrns simplel(), which is a 7-year locust (Tyus and Minckley 1988),
<br />Humpback chub in the LCR were reported as being cannibalistic at high population densities
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<br />18
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