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<br />14 . Executive Summary
<br />
<br />approximately double the growth rates reported for
<br />adults from the LCR (Minckley 1992) of 1.4, 1.3,
<br />1.1, 0.4, 0.5, and 0.1 rom for fish less than 200 Il1l11,
<br />200-250 nun TL, 250-300 rom TL, 300-350 nun
<br />TL, 350-400 nun TL, and over 400 rom TL,
<br />respectively. Average monthly growth of PIT-
<br />tagged humpback chub in Westwater Canyon was
<br />1.08 and 1.35 nun for 200-250 nun TL and 250-
<br />300 nun TL, respectively (T. Chart, pers. corom.,
<br />UDWR), which was comparable to the growth rates
<br />reported from the LCR, but only about half the
<br />growth rate reported by this investigation for
<br />mainstem fish.
<br />
<br />Higher adult growth rates in the mainstem are
<br />explained by greater availability of living space,
<br />greater consistency and abundance of food supplies,
<br />and the more stable year-round habitat than in the
<br />LCR Despite suboptimal temperatures, the growth
<br />rate of adults in the mainstem may be higher than
<br />growth rates of any other population in the Colorado
<br />River Basin, Average total length of mainstem
<br />Grand Canyon humpback chub is significantly
<br />greater (P<.05) than the average length offish from
<br />the LCR, and probably greater than lengths of any
<br />other population. Relatively high condition factor
<br />and growth rates suggest that the region occupied by
<br />the LCRI aggregation was below canying capacity
<br />for adults.
<br />
<br />Age of adults was not determined during this
<br />investigation, but Hendrickson (1993) reported a
<br />maximum age of 21 years for a humpback chub
<br />from the LCR that was 461 rom TL. Maximum
<br />sizes of fish from the mainstem were 460 rom TL
<br />(1,122 g) for males and 480 rom TL (1,165 g) for
<br />females, indicating that the fish aged by
<br />Hendrickson was close to maximum longevity.
<br />
<br />REPRODUCTIVE CAPACITY AND
<br />SUCCESS
<br />
<br />The only self-sustaining population of humpback
<br />chub in Grand Canyon was found in the lower 14.9
<br />km of the LCR and the adjacent 13.5 km of the
<br />mainstem Colorado River (6.9 km upstream and 6.6
<br />km doWnstream of the inflow). The majority of
<br />mainstem adults (~200 rom TL) of the LCRI
<br />aggregation ascended the LCR for presumed
<br />spawning from March through May. Adults in other
<br />disjunct aggregations reached a peak in spawning
<br />readiness in May through June, nearly 2 months
<br />
<br />Final Report
<br />
<br />later than the LCR population, but consistent with
<br />historic mainstem temperatures and timing of
<br />spawning by other mainstem populations, such as
<br />Black Rocks (Valdez and Clemmer 1982, Kaeding
<br />et a!. 1990), Cataract Canyon (Valdez 1990), and
<br />Yampa Canyon (Karp and Tyus 1990). Hence,
<br />mainstem temperatures were sufficient for normal
<br />gonadal maturation as reported by Kaeding and
<br />Zimmerman (1982), but apparently too cold for
<br />survival of eggs and larvae.
<br />
<br />The only definitive evidence of mainstem
<br />reproduction during this investigation was the
<br />discovery of about 100 post-larval humpback chub
<br />(14 captured, range, 18-31 nun TL) in a warm
<br />spring plume at RM 30.8 on July 12, 1994 (Valdez
<br />and Masslich In Review). Water temperature at the
<br />source of the spring was relatively constant at
<br />21.5 DC, compared to lODC in the adjacent main
<br />channel; the :fish were in a plume with a temperature
<br />of 15-19 DC. These young :fish belonged to the 1994
<br />year class, and probably hatched from eggs
<br />deposited in the warm spring plume, since mainstem
<br />water temperature was too cold for survival of eggs
<br />or larvae (Hamman 1982, Marsh 1985), These fish
<br />were about 36 days old (hatched about June 8,
<br />1994), based on age to length relationships of larvae
<br />and post-larvae (Muth 1990).
<br />
<br />Young humpback chub were reported 21.7 km (13.5
<br />mi) downstream of this spring by Arizona Game
<br />and Fish Department in 1993 (AGF 1994), i.e., 20
<br />yay (range, 20-50 nun TL) were captured at RM
<br />44.3. These findings also suggest past spawning
<br />attempts by humpback chub, either in springs in the
<br />vicinity of Fence Fault (30-Mile area), in the Paria
<br />River, or in unknown springs near RM 44, It is
<br />unlikely that these young fish originated from the
<br />Paria River, since adult humpback chub have not
<br />been reported in that tributary, and a large number
<br />of young would be necessary to supply a distant
<br />backwater with 20 individuals, where fish are
<br />expected to become more dispersed with distance
<br />downstream. Possibly, other warm springs exist
<br />downstream of Fence Fault that may be underwater
<br />and largely undetected. The likelihood of survival
<br />by eggs and larval from such springs is low because
<br />of the lack of cover along the river bottom and
<br />sizeable numbers of potential predators.
<br />
<br />Although it is unlikely that larval humpback chub
<br />could survive the thermal shock of a transition from
<br />
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