Effects of nursery environmental cycles on larval red drum (Sciaenops ocellatus) growth and survival

TWRI Special Report 02-02, January 2002
Effects of nursery environmental cycles on larval red drum
(Sciaenops ocellatus) growth and survival
RAFAEL PÉREZ-DOMÍNGUEZ, AND JOAN G. HOLT
Fisheries and Mariculture Laboratory, Marine Science Institute, The University of Texas at Austin, 750 Channel
View Dr., Port Aransas, TX 78373, USA (rperez@utmsi.utexas.edu)
SUMMARY: Red drum early larval stages migrate through coastal inlets and settle into shallow
seagrass meadows within estuaries. This study describes environmental rhythms (ER) in red drum
nursery habitats and evaluates their role in larval growth. Well-defined diel ER were observed in
temperature (amplitude: 2 to 4.5ºC) and dissolved oxygen (DO) (range: 2.9-7.5 mg O2 L-1), and sporadic
cooling caused by cold fronts. We exposed groups of settlement sized larvae (4.9 mm standard length)
to two oscillating temperature treatments (amplitudes: 3 and 6ºC; daily mean 27ºC), an oscillating DO
treatment (range: 2.4-6.1 mg O2 L-1; daily mean 4.2 mg O2 L-1) and a control (no cycles; daily mean
27ºC, 6.4 mg O2 L-1). Relative to controls, growth was significantly reduced in the DO treatment but not
in the temperature treatments. Survival was similar in all treatments. Fish previously exposed to
temperature cycles maintained faster growth rates and higher food intake than control fish when
exposed to a simulated cold front. These results suggest that (1) ER may impart a physiological
advantage to fish, (2) acclimation to oscillating DO environments is unlikely, and (3) field estimates of
environmental characteristics based upon averaged daily point samples are inadequate for predicting
fish growth.
KEY WORDS: red drum, settlement, recruitment, temperature, dissolved oxygen, diel
rhythms
INTRODUCTION
Seagrass beds are structurally complex and
highly productive habitats which provide shelter for
Fish recruitment is a central issue for
larval red drum as well as abundant food to fuel
understanding fish population dynamics. Fish
their rapid growth. However, these shallow
nursery areas are thought to play a critical role in
estuarine habitats experience substantial
determining adult population size by influencing
fluctuations in environmental characteristics to
year class recruitment.1,2) Red drum is a valuable
which settled larvae will be exposed.7)
resource along the Gulf and East coast of North
Environmental parameters may fluctuate widely in
America. From late August through October, red
estuaries as a result of diel and tidal cycles, and
drum produce numerous planktonic larvae in
stochastic events (storms and cold fronts). Diel
offshore waters near estuarine inlets. After two to
temperature cycles of 3 to 5ºC have been reported
three weeks in the plankton, the larvae settle in
in shallow estuarine environments.8) Similarly,
seagrass beds within estuaries.3,4) Seagrass beds
water cooling associated with frontal systems is
serve as settlement and primary nursery habitat for
common during the settlement season. Dissolved
young red drum. Rapid larval growth during this
oxygen (DO) oscillations may be large in shallow
extremely vulnerable period greatly increases the
subtropical seagrasses due to photosynthesis-
probability of survival of larvae,5) and therefore
respiration rhythms of benthic communities.8)
recruitment.6)
Temperature and DO fluctuations in seagrass beds

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are perhaps the most important abiotic factors
connected to a 150 L reservoir for water
controlling growth during the larval period and
conditioning. Water was recirculated through each
hence recruitment to adult stocks.
system 1.2 times hour-1 to ensure a homogeneous
Although the effects of temperature and DO on
environment between the three replicate tanks
growth and survival have been studied intensively
within treatments. Temperature, DO, pH and
in numerous species, very few studies have
salinity were recorded in at least one tank from each
addressed the effect of fluctuating environmental
treatment at 15 min intervals by YSI
conditions (diel and tidal rhythms) and short-term
multiparameter water quality data sondes.
atmospheric events (storms, cold fronts) on fish
Preliminary tests have shown that due to the high
growth. The aim of the present study was two fold:
recirculation rate used, there are no differences in
1) describe naturally occurring environmental
water quality among replicate tanks. To estimate
rhythms (range and patterns of variation) within
treatment effects on fish growth a total of 20-25 fish
prospective red drum nursery habitats during the
from each tank were sampled at regular intervals
settlement period, and 2) determine the effects of
throughout the experiment and the standard length
temperature and DO cycles on larval growth and
(SL) measured to the closest 0.1 mm.
survival in laboratory studies.
Diel temperature cycles. Settlement sized red
drum larvae (4.9 mm standard length; SL) were
MATERIALS AND METHODS
exposed to two oscillating temperature treatments
(OSC TLo and OSC THi) (amplitudes: 3 and 6ºC;
Identification of cyclical environmental patterns
daily mean 27ºC) and their growth and survival
in red drum nursery habitats (seagrass beds):
compared to larvae held at constant temperature
Environmental data surveys were compiled from
(CONTROL) (no cycles; daily mean 27ºC).
three locations in the Aransas-Corpus Christi
Temperature cycles were simulated using timer-
estuary system during the fall of 2000. Two stations
controlled heaters (total 1500 watts) and a water
were located in shallow seagrass beds (SG1 and
chiller unit connected to the reservoir. The
SG2) within the estuary where red drum larvae
experiment was repeated twice; once to test the
have been found previously.3) A third station
high fluctuation and the other the low fluctuation
(INLET) was located in the Aransas Pass Ship
regimen. The experiments lasted for 20-22 days.
Channel linking the estuary to the Gulf of Mexico.
Diel DO cycles . Settlement sized red drum
Temperature, dissolved oxygen (DO), pH,
larvae (5.2 mm standard length; SL) were exposed
conductivity, turbidity and water height in the
to an oscillating dissolved oxygen treatment (OSC
seagrass stations were recorded at 30 min intervals
DO) (range 2.4 to 6.1 mg O2 L-1) and growth and
for six weeks by YSI multiparameter water quality
survival compared to larvae held in constantly well-
data sondes placed within the seagrass canopy. The
oxygenated water (CONTROL) (6.4 mg O2 L-1).
sondes were checked and data downloaded weekly
The experiments lasted for 22 days. An oxygen
to ensure proper working conditions and to prevent
depletion column was placed between the reservoir
data loss. Environmental data from the INLET was
and the experimental tanks to generate the desired
obtained from the automated monitoring program
fluctuating DO conditions. DO levels were
established at the University of Texas Marine
controlled by the flow of nitrogen injected into the
Science Institute Pier Laboratory. The study was
column. All tanks were kept at constant 27ºC.
coincident with the peak period of larval red drum
settlement to the seagrass beds.
Effects of storm related cooling on the growth
and survival of red drum grown in stable and
Effects of diel temperature and DO fluctuation
oscillating temperature:
on the growth and survival of settlement sized
red drum:
Fish previously exposed to temperature cycles
(OSC T) and fish grown at constant temperature
Red drum larvae were initially raised in 300 L
(CONTROL) were subjected to a simulated cold
circular tanks. Larvae of approximately 5-6 mm SL
front. Water temperature was dropped in all tanks
(19-22 d) were randomly assigned to experimental
from 27ºC to 17ºC over a 36 hour period. The
tanks at a density of 5-10 larvae L-1. Two groups of
temperature was kept at 17ºC for 48 hours (day 2
three replicate tanks (150 L) were used in all
and 3) and then raised back to 27ºC on day 4. The
experiments. Tanks from each treatment were
experiment lasted for six days. Water parameters

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were monitored at 15 min intervals as described
Effects of diel temperature and DO fluctuation
previously. Samples were obtained five days before
on the growth and survival of settlement sized
the beginning of the experiment (from the
red drum:
conditioning tanks) and on days 0, 3 and 6. Groups
of 25-30 fish from each tank were measured on
Since the fish were exposed to different thermal
each sampling date. Food level was adjusted daily
treatments a cumulative degree index (15 min
ensuring that fish ate to satiation. Every morning
interval) was used. Relative to controls, growth was
the tanks were carefully siphoned and the
significantly reduced (p<0.01) in the DO treatment
unconsumed food collected and dried at 60ºC for 36
(Fig. 2) but no differences in growth relative to
h. Food consumption was then estimated by
control fish were observed in either of the two
subtracting the food recovered (corrected for
temperature cycles used (Fig. 3). Survival was
leaching) from the total food provided during the
similar in all treatments.
day.
35
30
Dissolved Oxygen
RESULTS
25
20
15
Identification of cyclical environmental patterns
SL (mm)
10
in red drum nursery habitats (seagrass beds):
5
0
degr -1
ee 0
i 0
n 00
dex
10000
30000
50000
Well-defined diel environmental rhythms were
Fig.2 Growth of red drum under cyclic DO conditions (OSC DO;
observed in temperature (amplitude: 2 to 4.5ºC) and
close circles) and stable conditions (CONTROL; open circles).
dissolved oxygen (DO) (range: 2.9-7.5 mg O
Means ± SE (n=3).
2 L-1)
in seagrass beds (Fig. 1). DO levels were high
35
during daytime and decreased during the night to
30
Temperature
25
low levels (2-4 mg/L). Sporadic cooling episodes
20
with temperature drops greater than 10ºC in two to
15
SL (mm)
10
three days were recorded
5
0
30
28
Seagrass
Inlet
degr -10
ee i 0
n 00
dex
10000
30000
50000
26
Fig.3 Growth of red drum under cyclic temperature conditions
24
(OSC T
22
Hi; close circles) and stable constant (CONTROL; open
circles). Means ± SE (n=3).
Temperature (oC)
20
Effects of storm related cooling on the growth
12
) 10
and survival of red drum grown in stable and
-1
8
6
oscillating temperature:
4
DO (mg L
2
0
Fish previously exposed to temperature cycles
12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 time
maintained faster growth rates during the cooling
Fig. 1 Temperature and DO levels in the INLET and SG1. The
arrows indicate the passage of cold fronts. Clear rectangles indicate
phase (days 0-3) of the cold front (p<0.05). The
daytime.
difference in growth was no longer apparent by day
six (Fig. 4). Food consumption was on average
during the passage of cold fronts. Much less
higher for fish previously grown under cycling
variability was observed in the others parameters in
conditions than control fish during the simulated
the survey (data not shown). Relative to seagrass
cold front (p<0.01) (Fig. 5). No fish died during the
beds, diel patterns and overall variability were
6-day experiment.
greatly reduced in the INLET. Diel temperature
fluctuation was only 1 to 1.5ºC. Diel DO cycles
were absent in the inlet with DO levels remaining
near to saturation levels at all times (Fig. 1). All
other parameters remained nearly constant during
the study (data not shown).

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8
sized red drum changed significantly faster in
larvae stocked in the field than siblings held in
6
laboratory and concluded that metabolic turnover
4
*
(changes in isotopic composition not explained by
/degree index)
growth) was accelerated in the caged fish. It can be
Growth rate 3
2
inferred that the cost of growth (i.e. energy used per
(mm 10
unit weight gain) was higher in the natural
0
Day 0
0-3
3-6
environment. Since no predation and unrestricted
Experimental interval
food access was assumed in both caged and
Fig.4 Growth rate of juvenile red drum during the simulated cold
laboratory fish, it seems possible to hypothesize
front experiment. Black columns represent fish previously exposed
that environmental fluctuations may drive these
to temperature cycles. White columns represent CONTROL fish.
physiological differences in growth.
Means ± SE (n=3). The asterisk indicates significant difference
between groups (p<0.05).
Fish in the OSC DO grew significantly less than
CONTROL fish, while survival was not affected.
150
Taylor and Miller11) reported growth reduction in
100
southern flounder (Paralichthys lethostigma)
cyclically exposed to nocturnal hypoxia (2.8 mg O2
(mg/fish)
50
Food intake
L-1). Although the lowest DO concentration used in
0
the experiment (2.4 mg O2 L-1) was similar to the
0
1
2
3
4
5
lowest observed values in the seagrass, these levels
Experimental day
were reached only occasionally in the seagrass
Fig.5 Food consumption during the simulated cold front
beds. The levels administered in the laboratory
experiment. Black columns represent fish previously exposed to
were well into the range where oxygen acts as a
temperature cycles. White columns represent CONTROL fish.
limiting factor resulting in retarded growth. A
Means ± SE (n=3).
reduced growth potential is predicted in estuarine
DISCUSSION
areas where strong diel DO cycles are present even
when daytime DO levels are high. These
Naturally occurring environmental cycles are
experiments also indicate that the use of field
linked to different patterns of energetic input within
estimates of environmental characteristics based
the relatively small nursery habitat. Circadian
solely upon averaged daily samples is inadequate
rhythms arise from day-night and tidal cycles while
for predicting fish growth in the pristine seagrass
stochastic fluctuation are linked to meteorological
beds studied.
disturbances. Diel temperature and DO cycles
The benefit of evolving an estuarine dependent
weaken or completely disappear on cloudy days.
lifestyle strongly argues for an overall increased
Both cycles were clearly related to irradiation. In
growth potential in these fluctuating environments
shallow, productive seagrass beds there is an
and the possibility for specific adaptations to meet
extensive primary productivity (photosynthesis) 9)
environmental challenges of the nursery habitat.
and respiration confined to a small volume of water,
Larval growth and survival in fluctuating
resulting in large diel DO changes.
environments of estuaries is a crucial aspect for
Red drum larvae are very tolerant of temperature
understanding recruitment variability/mechanism
fluctuations. Diel temperature fluctuation in excess
and ultimately fish population dynamics in
of that found in our environmental surveys did not
estuarine dependent species like red drum.
impair growth in the laboratory. Nevertheless the
simulated cold front experiment seems to suggest
ACKNOWLEDGMENTS
that exposure to diel temperature rhythms may
impart a physiological memory to the fish that
This work has being supported in part by
allows for faster growth compared to CONTROLS
contributions from Perry R. and Nancy Lee Bass,
during rapid cooling events. Food consumption
and by the Texas Water Resource Institute
during that experiment was on average higher in
(TAMU). The authors thank M.C. Alvarez, S.
fish previously exposed to diel temperature
Applebaum, S. Holt, I. McCarthy, C. Pratt, and M.
fluctuation. However, no significant differences
Tanaka for their comments and continuous support
were detected at any particular day. Herzka 10)
during this study.
found that the isotopic composition of settlement

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