7996

• UCREFRP Catalog Number: 7996
• Author: Wydoski, R. S. and E. D. Wick.
• Title: Ecological Value of Flooded Bottomland Habitats to Endangered Fishes in the Upper Colorado River Basin.
• USFW Year: 1996.
• USFW - Doc Type: \
• Copyright Material: NO

the quality and quantity of food organisms during the early life stages (Hjort
1914, 1926) . High mortality of larval freshwater fishes has been attributed to
this critical period (Li and Mathias 1982). The density, size, timing, and
duration of zooplankton availability must "match" the timing of the swimup stage
of fish larvae. The highest survival of larval fish occurs when high densities
of zooplankton are present during the time when larvae begin exogenous feeding
(Hjort 1926). During years with optimum environmental conditions, high survival
of 1 arval and juvenil e fi sh produces strong year cl asses. The endangered
Colorado River fishes are long-lived and are known to produce strong year classes
(Henrickson and Brothers 1993; McCarthy and Minckley 1987, Miller et al. 1982;
Minckley et al. 1991; Vanicek 1967). The ultimate year-class strength of many
ri veri ne fi shes depends on the magnitude and durat i on of overbank fl oodi ng (i. e. ,
the flood pulse) of floodplain habitats (Bayley 1991).

Starvation as factor in mortality 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 between 8 and 19 days to survive. The "point of no
return" or "point of irreversible starvation" when the fish died even though
sufficient food of the right size became available occurred between 19 and 23
days for razorback sucker 1 arvae. Papoul i as and Mi nckl ey reported that the
minimum quantity of food required for survival of the razorback sucker larvae
during the critical period was 30-60 brine shrimp nauplii per fish per day to
survive when the larvae must be exposed to the right size and abundance of food
organisms.

Razorback sucker 1 arvae had good survi va 1 (80-90%) if the number of food
organisms available to each fish was 58 per day or higher (upper diagram; Figure
1) . However, the best growth duri ng 50 days after swimup occurred when the
number of food organisms was 527 per fish per day or higher (lower diagram;
Figure 1). In earthen ponds, razorback sucker larvae had excellent survival from
swimup to 8 weeks of age (77-98.8%) when the mean number of zooplankton per liter
was between 12.5 and 43.3 (upper diagram; Figure 2). There was no significant
difference between the treatments. However, growth of razorback sucker larvae
during the eight week period increased significantly with the density of
zooplankton (lower diagram; Figure 2). In another study, the survival of
razorback sucker fry in hatchery ponds at the Dexter National Fish Hatchery, New
Mexico, increased from 10.8-35.7% to 87.8-98.6% with changes in the fertilization
regime and stocking rate of fry (Hamman 1987).

IX. FOOD AVAILABILITY FOR LARVAL ENDANGERED FISHES
IN THE UPPER COLORADO RIVER BASIN

The larvae and juveniles of all endangered Colorado River fishes feed on
zooplankton (U.S. Fish and Wildlife Service 1987, 1990a, 1990b, 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 cladocerans).
The density of zooplankton needed for larval razorback sucker survival (30-60
organisms per fish per day; Papoulias and Minckley 1990) occurred in flooded

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