7799

• UCREFRP Catalog Number: 7799
• Author: Modde, T. and D. Irving.
• Title: Conceptual Management Plan for Habitat Enhancement In Old Charley Wash.
• USFW Year: 1994.
• USFW - Doc Type: \
• Copyright Material: NO

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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 Endanaered Fishes in the UDDer 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, 199Gb, 1991). The first foods of larval razorback suckers in ponds were diatoms, detritus, algae. and

rotifers (Papoulias and Minckley 1992). Soon afterward, the razorback larvae began to select larger

organisms such as zooplankton (primarily c1adocerans). The density of zooplankton needed for larval

razorback sucker survival (20 organisms per liter of water; Papoulias and Minckley 1990) occurred in flooded

bottomland habitats along the Green River but rarely reached that density in backwaters, and never reached

it in the river (Mabey and Shiozawa 1993). The decline in razorback suckers and the near extirpation of the

bonytail in the upper basin may be linked directly to low or lack of larval survival in these species.

Therefore, flooded bottomland habitats are important to some, if not all, of the endangered Colorado River

fishes in the upper basin. Zooplankton produced in productive off-channel habitats such as flooded

bottomlands also provide food of the proper quantity and size at the right time needed for larval survival of

endangered fishes that occupy main channel habitats such as the Colorado squawfish and humpback chub.

F. Loss of Critical Low Water Velocitv Habitats such as Flooded Bottomlands in the UDDer Colorado River

Basin. Flooded bottomland habitats occurred along low gradient river reaches with broad valleys. In the

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