8136

• UCREFRP Catalog Number: 8136
• Author: Bestgen, K. R.
• Title: Growth, survival, and starvation resistance of Colorado squawfish larvae.
• USFW Year: 1995.
• USFW - Doc Type: \
• Copyright Material: YES

198

et al. 1970, Kaeding & Osmundson 1988, Tyus &
Haines 1991).
Several hypotheses have been forwarded regard-
ing mechanisms which control recruitment of fishes
early in life (Houde 1987). Starvation, presence and
duration hf a critical period during ontogeny, and
biotic interactions such as competition and preda-
tion may all affect survival of larvae. Outcomes of
these processes often depend on size and growth
rates of larvae, which may in turn be affected by en-
vironmental factors such as water temperature,
temperature fluctuations, and food abundance
(Houde 1987, Weatherly & Gill 1987, Miller et al.
1988). Understanding effects of these environmen-
tal factors on growth, survival, and starvation resist-
ance may be essential in evaluating the complexity
of mechanisms that control recruitment of Colora-
do squawfish in the wild.
Research on fishes in the upper Colorado River
Basin has emphasized effects of altered flow re-
gimes on distribution and abundance of endan-
gered fishes such as Colorado squawfish (Tyus &
Karp 1989, Tyus & Haines 1991). However, few in-
ferences can be made about direct effects of tem-
perature, habitat availability, and food abundance
on growth, survival, and starvation resistance of
Colorado squawfish larvae because scant empirical
laboratory or field data are available that describe
their ecology. Black & Bulkley (1985) studied ef-
fects of constant 15, 20, 25 and 300 C temperatures
on growth of yearling Colorado squawfish and
found fastest growth at 250 C when provided with
unlimited food. However, food availability may
vary and not always be unlimited in fluctuating riv-
erine backwaters of the Colorado River Basin and
optimal growth conditions for larvae may be differ-
ent than for yearlings. I conducted laboratory stud-
ies to determine growth and survival of larvae in
constant and fluctuating temperature regimes at
several different temperature and food abundance
levels. Because food abundance may be seasonally
low in the turbid and fluctuating habitat of the Col-
orado River Basin, starvation resistance of Colora-
do squawfish larvae was also examined.

Methods

Growth and survival experiments

Colorado squawfish embryos were obtained from
Dexter National Fish Hatchery and Technology
Center, Dexter, New Mexico (Hamman 1981). Em-
bryos were from paired matings of 20 broodfish of
each sex (R. Hamman personal communication).
Test animals were held in 2.01 aquaria (20 cm x
10 cm x 12.5 cm deep). Flow-through conditions in
each aquarium maintained dissolved oxygen at
5-6 mg 1-\ and prevented accumulation of waste
products. Photoperiod was 14 h lightlO h dark.
Healthy, two-day old embryos after hatching
were randomly selected and counted into groups of
20, randomly assigned to one of seven experimental
treatments, placed in aquaria, and acclimated to
test temperatures at a rate of about 20 C h-1. These
free embryos and later larvae were subjected to
fluctuating or constant temperatures (hereafter
called regimes), with nominal constant temper-
atures of 18,22,26, or 300 C and nominal fluctuating
temperatures of 18, 22, or 260 C (diel fluctuation of
:t 2.50 C, 50 C total). Embryos hatched in 1991 were
used for the 18, 22, and 260 C fluctuating and 220 C
constant-temperature-treatment experiments, and
1992 embryos were used for the 18, 26, and 300 C
constant and a second 220 C constant-temperature-
treatment experiments.
Fluctuating temperatures were tested because
diel temperature fluctuations of 50 C or more are
common in the Yampa and Green rivers in Colora-
do and Utah during late June through August (Nes-
ler et al. 1988, United States Geological Survey re-
cords, published annually). The fluctuating temper-
ature cycle mimicked the natural cycle as follows:
lowest temperatures occurred from 1:00 to 7:00 h
(e.g., 19.50 C for the 220 C fluctuating treatment);
warming occurred at a constant rate from 7:00 to
13:00 h; temperatures were highest from 13:00 to
19:00 h (e.g., 24.50 C for the 220 C fluctuating treat-
ment); and cooling occurred at a constant rate from
19:00 to 1:00 h. Observed mean water temperatures
(SE) in the constant and diel fluctuating regimes
were as follows: constant nominal 180 C = 18.2
(0.07), 220 C = 21.95 (0.05), 260 C = 26.0 (0.02), and

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