9410

• UCREFRP Catalog Number: 9410
• Author: Wydoski, R. S. and E. J. Wick.
• Title: Ecological Value of Floodplain Habitats to Razorback Suckers in the Upper Colorado River Basin.
• USFW Year: 1998.
• USFW - Doc Type: Denver.
• Copyright Material: NO

I

D. Relation of Food to Larval Fish Survival and Year-Class Strenqth. Larval
fish must initiate feeding during the first few weeks after swimup when
they are making the transition from endogenous nutrition (yolk sac) to
exogenous feeding (invertebrates) before the larvae reach "a point of no
return" or the "point of irreversible starvation" that is termed the
"critical period" (Hjort 1914; Houde 1987; Li and Mathias 1982; Miller et
al. 1988). The timing, density, size, and duration of zooplankton
availability must "match" the timing of the swimup stage of fish larvae.
High mortality of larval fish can occur from starvation or reduced growth
if food resources are limited and/or intra- and interspecific competition
is high (Leggett 1986; May 1974; Welker et al. 1994). Horn (1996), Lasker
(1981), Lawler (1965) and others emphasize that the growth rate of larval
fishes is extremely important because smaller fish that are in poor
condition (i.e., starved) with less locomotive ability (Rice et al. 1987)
are more susceptible to predation (Leggett 1986). The highest survival of
larval fish occurs when high densities of zooplankton are present during
the time when larvae begin exogenous feeding (Hjort 1926; Leggett 1986).
Fish larvae can recover quickly from short periods of starvation through
compensatory growth if they encounter high densities of zooplankton before
they reach their "point of irreversible starvation" (Miglavs and Jobling
1989) .

I

I

I

I

I

I

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 larvae. Papoulias and Minckley 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. Razorback sucker larvae had good survival (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 during 50 days after
swimup occurred when the number of food organisms was 527 per fish per day
or higher (lower diagram; Figure 1) .

I

I

I

I

In earthen ponds,' razorback sucker larvae had excellent survival from
swimup to 8 weeks of age (67.4-89.8%) when the mean number of zooplankton
per liter was between 12.5 and 43.3 (Papoulias and Minckley 1992; upper
diagram, Figure 2). There was no significant difference among 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 increases in fertilization and lower
stocking rate of fry (Hamman 1987) .

I

I

I

Recruitment of long-lived fishes with high fecundity is curtailed
primarily from mortality during the larval stage from either starvation,
predation, or both (Houde 1987; Hunter 1981; Lasker 1981). Razorback
sucker larvae that were deprived food in the laboratory showed an initial
increase in length as they utilized remaining yolk reserves. However,
they were significantly less in total length and weight than larvae of the
same age fed ad libitum at temperatures of 14, 18, and 23 C (Horn 1996) .
Many razorback sucker larvae in Lake Mohave survive to swimup but often
have empty guts suggesting starvation is a factor in early life mortality
(Marsh and Langhorst 1988) or reduced growth due to insufficient food
keeps the larvae within a vulnerable size to predation for a longer period

I

I

16

I
I

I