8033

• UCREFRP Catalog Number: 8033
• Author: Lentsch, L. D., et al.
• Title: Endangered Fish Interim Management Objectives for the Upper Colorado River Basin Recovery And Implementation Program -Final Report.
• USFW Year: 1998.
• USFW - Doc Type: 55,
• Copyright Material: NO

1
11
t
LI
1

1

t

Ir ?
u
1

1
the Green River (LTDWR, unpublished; Modde et at. 1996), however significant recruitment to the
adult population is doubtful, because few young fish have been found and adults were considered
rare (Modde et aL 1996; Chart et al. 1997). Razorback sucker was declared endangered by the
USFWS in 1991 due to declining numbers and lack of documented recruitment of natural
populations (USFWS 1997).
Demographic information in the UCRB is sketchy. Sex ratios have been reported from relict
populations in Lake Mojave at 3 males to 2 females (Bozek et al. 1984) and in lotic environments
as 2 males to 1 female (Tyus and Karp 1990). Minckley (1983) and McAda and Wydoski (1980)
reported from 1812 and 1166 eggs/cm of total length. When translated to an average sized female
(500 mm TL), fecundity is approximately 82,500 eggs/female. No specific estimates on
survivorship are available.
Lanigan and Tyus (1989) estimated a population size of 948 individuals in the Green river above
Desolation Canyon but felt the number of captures in the lower Green River was too low to
estimate population size. Current population estimates suggest approximately 500 mature
individuals exist in the Green River (including middle and lower) Modde et al (1996). It is not
clear whether this discrepance represents a population decrease or differences in estimation
techniques. Crowl and Bouwes (1998) assume an initial population of 50 for the Yampa River
based on reports of very low numbers, because no estimate exists for the Yampa River population.
Humpback Chub The USFWS has recognized five populations of humpback chub in the Colorado
River Basin (Valdez and Clemmer 1982; USFWS 1990). Presently, populations are found in
canyon reaches of the Colorado River system. The largest and most stable population is also the
only population remaining in the Lower Colorado River Basin and resides in Grand Canyon in and
near the confluence of the Little Colorado River (LCR). The other populations are in
WestwaterBlackrocks Canyons and Cataract Canyons of the Colorado River, Desolation/Gray
Canyon of the Green River and in Yampa Canyon of the Yampa River. In addition, aggregations
of humpback chub or roundtail/humpback hybrids occur sporadically throughout the basin within
confined canyon reaches.
Humpback chub have been difficult to study because of their rarity and residence in swift, turbid
and inaccessible riverine environments. The only sex ratios reported suggest they are
approximately equal and that fecundity averaged 3, 677 eggs/ female in the Grand Canyon of the
lower Colorado River basin (Valdez and Ryel 1995). Egg survival is optimal between 16 °C and
22 °C and significantly reduced below temperatures of 12 °C which could affect reproductive
success of mainstem spawning in the Grand Canyon (Marsh 1985). In Grand Canyon studies, age-
s fish were noted to disperse one to three months after emergence (Valdez and Ryel 1995).
Survivorship in years 0, 1 and 2, collectively was 10% but most likely later life stages survived
better. Adult survivorship has been reported as 60% in the upper Colorado River basin and 75%
in the Grand Canyon (Kesler 1992; Valdez and Ryel 1995). Humpback chub mature in two to
three years (approximately 200 mm in total length), and they may five 20 to 30 years (Valdez et al.
1992).
12
IJ