7798

• UCREFRP Catalog Number: 7798
• Author: Richardson, S.
• Title: Conceptual Management Plan for Razorback Sucker Habitat Enhancement in Flooded Bottomlands, Escalante Ranch, Jensen, Utah.
• USFW Year: 1994.
• USFW - Doc Type: \
• Copyright Material: NO

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and Severn 1994b}. The mean number of cladocerans and copepods
from a backwater of the Gunnison River at the Escalante State
Wildlife Area, about 5 miles downstream from Delta, colorado, was
11 organisms per liter of water and the mean number from an open
water wetland was 25 organisms per liter (Cooper and Severn
1994c). Zooplankton (cladocerans and copepods) samples from a
bottomland (Old Charlie Wash) on the Ouray National Wildlife
Refuge contained a mean of 31 organisms per liter of water
(Cooper and Severn 1994d). Samples taken from the middle Green
River and a backwater on the refuge did not contain any
cladocerans or copepods.

C. Maior Limitinq Factor Affectinq Endanqered Colorado River
Fish Populations. The major limiting factor affecting the
populations of endangered Colorado River fishes is the lack of
successful recruitment (U.S. Fish and wildlife Service 1987,
1990a, 1990b, 1991). In the Lower Colorado River Basin, studies
have demonstrated that predation by non-native fishes and the
lack of food may be limiting the survival of larval and juvenile
endangered fishes (Marsh and Langhorst 1988; Papoulias and
Minckley 1990, Papoulias and Minckley 1992). Similar conclusions
have been reached for the endangered Colorado River fishes in the
upper basin (Miller et ale 1982; Maddux et ale 1993).

D. Relation of Food to Larval Fish Survival. Year-class
strength of marine fisheries is often related to the Match or
Mismatch Principle where environmental conditions such as water
temperature as well as the quality and quantity of food organisms
must match the hatching of larval fishes to realize high survival
(Hjort 1914, 1926). In other words, the density, size, time and
duration of availability of zooplankton must "match" the timing
of the swimup stage of fish larvae. When the yolk sac absorbed,
food organisms of the right size must be available to fish larvae
or they will starve. This principle was suggested by Marsh and
Langhorst (1988) for razorback sucker larvae in Lake Mohave and
documented for the razorback sucker larvae in the laboratory
(Papoulias and Minckley 1990). Razorback sucker larvae of about
10 mm total length were maintained in the laboratory at 18 C.
Unfed razorback larvae died in 10 to 30 days. Razorback larvae
must find food of the right size and density in 8 to 19 days to
survive. The "point of no return" when the fish died even though
sufficient food of the right size was available was between 19
and 23 days for razorback sucker larvae. Papoulias and Minckley
reported that the quantity of food required for survival of the
razorback larvae was 20 brine shrimp nauplii per fish or high
mortality occurred.

E. Food Availabilitv for Larval Endanqered Fishes in the Upper
Colorado River Basin. The larvae of all endangered Colorado
River fishes feed on zooplankton early in life (U.S. Fish and
Wildlife Service 1987, 1990a, 1990b, 1991). The first ~oods of
larval razorback suckers in ponds were diatoms, detritus, algae,

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