BULLETIN OF MARINE SCIENCE. 58(2): 385-392. 1996
ENERGY BUDGET OF FEMALE WHITEMOUTH CROAKER
MICROPOGONIAS FURNIERI, DETERMINED FROM
FIELD DATA IN TRINIDAD, WEST INDIES
Sherry C. Manickchand-Heileman
and Nelson M. Ehrhardt
ABSTRACT
The energy budget of female whitemouth
croaker Micropogonias
furnieri
was investigated
in the field in Trinidad, West Indies. Daily ration, determined from the variation in stomach
content weight over a 24-h period and an exponential gastric evacuation rate model was
3.45%. Growth efficiency averaged 7.6% and decreased from 18.8% at age one to 2% at age
seven. Reproductive effort increased steadily from 5.6% at age one to 6.9% at age seven and
averaged 6.4%. Growth efficiency exceeded reproductive effort until age three, after which
the reverse occurred following the attainment of sexual maturity.
The whitemouth croaker Micropogonias furnieri (henceforth croaker) is of significant commercial value in Trinidad, where it is caught by trawling and bottom
longlining. It is also one of the most important components of the soft-bottom
demersal fish community in the coastal waters of Trinidad (Whiteleather and
Brown, 1945; Bianchi, 1990). These communities are typical of tropical continental shelf ecosystems where the species diversity is high and the abundance of
individual species is relatively low (Longhurst, 1969; Lowe-McConnell, 1987).
Manickchand-Heileman and Julien-Flus (1990) found 99 fish species, including
croaker, during a demersal trawl survey off northwest Trinidad. Only the seatrout
Cynoscion jamaicensis comprised more than 20% of the catch on one occasion
during the survey.
Several of these species are commercially exploited, both through targeted fisheries and as bycatch of the penaeid shrimp fishery_ Complex interactions exist
among them, for example, croaker preys on penaeid shrimps, Penaeus sp. (Manickchand, 1990) both of which are simultaneously exploited in the waters off
western Trinidad.
The need for management of such fisheries on a multispecies basis has long
been recognized (Andersen and Ursin, 1977; Laevastu et aI., 1982; Shepherd,
1988). An understanding of species interactions within the ecosystem is essential
for multi species fisheries management. Such interactions may be defined by studies of the bioenergetic requirements of system components and energy flow
throughout the ecosystem, which form the basis of some multispecies fisheries
models (Andersen and Ursin, 1977; Browder, 1981).
The pattern of energy allocation is affected by exploitation of the stock. Daan
(1975) analysed the effects of different levels of fishing mortality (F) and age at
first capture on growth and reproductive efficiencies in the North Sea cod Gadus
morhua. Growth efficiency rose steeply with increasing F while reproductive efficiency decreased, indicating that the population reacted to increased fishing pressure with a higher utilization of food for growth at the expense of reproduction.
A similar effect was observed with decreasing age at first capture.
Bioenergetic studies have not been conducted on croaker in Trinidad. However,
Ciechomski (1981) studied the assimilation of food and Aristizabul (1992) investigated the effects of salinity and weight on routine metabolism of juvenile croaker
in Argentina. The objectives of the present study were to determine food con385
386
BULLETIN
OFMARINE
SCIENCE.
VOL.58. NO.2. 1996
sumption, and hence energy requirement and the pattern of energy allocation to
growth and reproduction in the croaker.
METHODS
Sample Collection.-Croakers were obtained by trawling during research cruises carried out by the
Norwegian research vessel the DR. FRIDTJOF
NANSENin May, August and November 1988. Hauls of
0.5 h each were made at depths of 20-54 m off the north and east coasts of Trinidad. Croakers were
also obtained from a commercial trawler which made hauls of 3-4 h each in depths of 20-40 m
during the months of August, September and November 1988 off the north coast. Because the time
of death could not be ascertained due to the long duration of the hauls by this vessel, only those
croakers which were alive when the gear was retrieved were taken. A total of 498 croakers were
obtained from the trawlers. Croakers were frozen on board the vessels and taken back to the laboratory
for the determination of daily ration. From May 1985 to November 1987, monthly samples of croakers
were obtained from commercial bottom longline catches taken in depths of 20-40 m off the west
coast of Trinidad. Croakers were packed in ice and taken to the laboratory.
Determination of Daily Ration.-Only croakers caught by trawling were used to determine daily ration.
In the laboratory, croakers were examined for signs of regurgitation such as partially digested food in
the mouth, esophagus or gill chamber and flaccid and empty or everted stomachs. In determining daily
ration, all fish with empty stomachs were excluded since the time that stomachs became empty could
not be ascertained. Those fish showing signs of regurgitation were also excluded since regurgitation
can result in the underestimation of food consumption (Bowman, 1986).
Of the 498 croakers examined, only 98 contained food and did not show evidence of regurgitation.
The number of fish used in the analysis for each month of sampling was: May-34; August-36;
September-3; November-25. These fish ranged in size from 29.1-54.3 cm total length. For each
croaker total length to the nearest 0.1 cm, body weight to the nearest 0.1 g, and stomach content
weight to the nearest 0.01 g were recorded. Stomach content weight was expressed as a percentage
of total body weight. Due to the small number of croakers with food in their stomachs and no signs
of regurgitation, all data were pooled to give mean weight of stomach contents at 3-h time-intervals
over a 24-h period.
Sainsbury (1986) described the estimation of daily ration using ingestion and evacuation rates, and
the beginning and end of the feeding and non-feeding periods from a feeding cycle of arbitrary length.
Based on Sainsbury's models, Jarre et al. (1990) developed a model to estimate daily ration of fish
from a 24-h cycle of stomach content data, assuming an ingestion rate constant in time and an
exponential gastric evacuation rate. This model was used to estimate daily ration and ingestion and
gastric evacuation rates of the croaker.
Wet and dry weight of stomach contents of 19 croakers were determined. All dried samples were
combined, ground to a powder in an electric mill and the energy content units of two I-g subsamples
determined using an adiabatic microbomb calorimeter. Assuming that daily ration was similar for
croakers of all sizes, the wet weight of food consumed annually was estimated and converted to dry
weight and energy content.
Somatic Growth.-To determine energy allocated to growth, it was necessary to determine energy
content of the body at each age. This was the sum of the energy contained in the liver, gut, head and
musculature. Manickchand-Heileman and Kenny (1990) found seven age groups in Trinidad croakers.
By regressing the logarithm of total length (TL) on the logarithm of body weight (BW) (N = 904),
the length-weight relationship was found to be BW = 0.012TU98 and was used to convert mean total
length to body weight for each age group. Relationships between total body weight and liver and gut
weights, and head weight as a percentage of body weight were established to estimate weights of
these components at each age. Component weights were subtracted from total body weight to give
the weight of the musculature and remaining skeleton.
In the laboratory, gonads of fish caught by both trawling and handlines were examined and males
discarded. For each female the following were recorded: total length to the nearest 0.1 cm, total body
weight to the nearest 0.1 g, liver, gut (entire intestinal tract), head and ovary weight to the nearest
0.01 g. Samples of liver (N = 20), gut (N = 16), head (N = 10) and vertical body sections (N = 50)
taken just behind the last dorsal fin-ray and assumed to be representative of the musculature were
dried in an oven at 60°C for 4-5 days until constant weight was attained. Subsamples of dried tissues
were ground to a powder and stored in glass vials in a dessicator then dried overnight in a vacuum
oven before calorimetry. The energy content of 1.0 g samples of liver (N = 6), gut (N = 5), head (N
= 4) and vertical body sections (N = 6) was determined using an adiabatic microbomb calorimeter.
Estimated wet weight of each component was converted to percentage dry weight and energy
content for each age group. Energy content of the body at each age was estimated as the sum of
MANICKCHAND-HEILEMAN
AND EHRHARDT: ENERGY BUDGET, WI·IITEMOUTH CROAKER
387
Table I. Mean weight of stomach contents as a percentage of body weight (S) at 3-h intervals over a
24-h period for the whitemouth croaker Micropogoniasfurnieri (N = total number stomachs examined)
Time
1.00
4.00
7.00
10.00
13.00
16,00
19.00
22.00
S
0.58
0,25
0.09
0,12
0.93
0,82
0.97
1.13
j:
:t:
:t:
:t:
:t:
:t:
:t:
:t:
:t:
SD
With food (no)
0.27
0.17
0.11
0.19
0.20
0,30
0.28
0.23
total
N
15
4
18
6
4
23
19
9
50
35
88
80
58
84
74
29
98
498
energy in the head, liver, gut and the rest of the body (excluding ovaries). Energy allocated to somatic
growth was estimated as the increase in total body energy between successive age groups.
Egg Production.-A sample of 150-200 fresh, hydrated eggs were taken from the ovaries of each of
10 randomly selected croakers between May and September 1987. These fish ranged in size from
34.6-55.2 cm total length. Each sample was gently washed under running water, blotted on absorbent
paper towels and weighed on an analytical balance to the nearest 0.001 mg. Eggs were then counted
under a dissecting microscope and the mean wet weight of a single egg determined. Eggs were dried
in an oven at 60°C for 3 days until constant weight was achieved to determine the dry weight of a
single egg. A second sample of hydrated eggs was taken from five of the 10 females. Eggs were dried
as previously described and ground to a powder. The energy content of eggs was determined for one
subsample of 0.5 g from each of the five samples.
Total fecundity was determined for each of the seven age groups using the relationship between
batch fecundity and body weight and a spawning frequency of 12 times a year (Manickchand-Heileman
and Ehrhardt, 1996), Total number of eggs at each age was converted to wet and dry weights and
energy content.
Determination of Energy Budget.-Winberg
orous fish:
(1956) proposed the following energy budget for carniv-
1001 = 60M + 20G + 20E
where I is food ingested and is equivalent to 100%, M is total metabolism, G is surplus energy
(includes both somatic growth (G,), referred to as growth efficiency, and reproduction (G,), referred
to as reproductive efficiency or reproductive effort), and E is egestion and excretion. The quantity
next to each symbol represents the percentage of food consumed that is allocated to each process, In
the present study food consumption, somatic growth and reproductive effort were determined as previously described. Egestion and excretion were assumed to be 20%, following Winberg (1956), Metabolism was determined as the difference between food consumed and the sum of the other components of the energy budget.
RESULTS
Daily Ration,-Pre]iminary examination of the variation of stomach content
weight (as a percentage body weight) with time indicated one feeding and one
non-feeding period within a 24-h time-interval (Tab]e 1). Variations of stomach
content weight estimated by the mode] and of observed values over a 24-h period
are shown in Figure 1. The time at the start of the feeding period was estimated
to be 12.80 h and that of the evacuation period to be 22.90 h, with a feeding
period of about ]0 h. The gastric evacuation rate was 0.27 h-I, the ingestion rate
0.34 g·h-I and daily ration 3.45% body weight per day.
388
BULLETIN OF MARINE SCIENCE. VOL. 58. NO.2.
1996
1.2
U)
Eo<
:z
>Ll
Eo<
:z~
0
.
U.j.J
0.8
:;::
:r:
u:>"
~'8
OPJ
0.6
Eo<
\ ~~~" ---j
U)d{J
Sot
~
0.4
0.2
0
.--,.--13
,
16
----r---
19
22
~-~'---I'--1
4
a'~
~+
1-----'
7
10
TIME (hrs)
Figure 1. Observed and estimated variation in weight of stomach contents as a percentage of body
weight with time in the whitemouth croaker Micropogonias jumieri.
Growth.-The relationships between body weight (BW) and gut (GW), liver
(LW), and ovary (OW) weight were as follows:
= 0.02IBW - 0.677
LW = 0.015BW - 1.590
OW = 0.026BW - 1.847
GW
= 50, R2 = 0.78)
(N = 52, R2 = 0.68)
(N = 451, R2 = 0.80).
(N
Mean head weight as a percentage of body weight was 23.3% ± 1.19. Percentage dry weight and energy g-I dry weight of head, liver, gut and muscle are
given in Table 2. The energy in the gut, liver, head, musculature and total body
energy at each age are given in Table 3.
Energy Cost of Egg Production.-The mean wet weight of a single hydrated egg
of the croaker was 0.043 ± 0.003 mg and the mean dry weight was 0.015 ±
0.001 mg. The mean caloric content of hydrated eggs was 6,242.4 ± 123.1 cal
g-I dry weight. The energy cost of egg production increases steadily with age,
from 61.8 kcal at age one to 1,260.2 kcal at age seven (Table 4).
Energy Consumption.- The mean percentage dry weight of the stomach contents
of the croaker was 21.39 ± 1.96%, while the energy content was 3,579 ± 117.4
cal g-I dry weight. A daily ration of 3.45% body weight was used to determine
Table 2. Mean percentage dry weight and energy content (cal·g-I dry weight) of body components
of the whitemouth croaker Micropogonias furnieri
Component
% Dry wt :!: SD (N)
Head
Liver
Gut
Muscle
32.7
20.6
16.2
27.4
::'::3.5
::'::2.5
::'::1.1
::'::2.4
(10)
(20)
(16)
(50)
Energy :!: SD (N)
4,164.8 ::'::490.1
5,993.8::':: 319.1
4,931.4 ::'::278.2
4,840.7 ::'::425.7
(4)
(6)
(5)
(6)
MANICKCHAND-HEILEMAN
389
AND EHRHARDT: ENERGY BUDGET, WHlTEMOUTH CROAKER
Table 3. Mean energy content (kcal) of liver, gut, head and muscle, summed to give energy of whole
body and increase in energy due to growth in length of the whitemouth croaker Micropogonias furnieri
Age
Liver
Gut
Head
1
2
3
4
5
6
7
0.2
2.6
7.4
12.8
18.6
24.7
33.1
1.4
4.0
7.8
12.7
17.9
23.3
30.8
36.1
86.8
]60.0
252.6
352.3
456.1
600.7
Body
Growth
149.5
355.3
652.0
1,028.0
1,432.4
1,853.6
2,440.3
205.8
296.7
376.0
404.4
421.2
586.7
370.9
Muscle
111.8
261.9
476.8
749.9
],043.6
1,349.5
1,775.7
annual wet weight of food consumed and energy equivalent for each age group
(Table 5),
Energy Budget.- The energy budget by age group of the croaker is given in Table
6, Growth efficiency was 18,8% at age one, decreasing to 2% at age seven,
Reproductive effort increased from 5.6% at age one to 6.9% at age seven, surpassing growth at age four. The average energy budget for the croaker determined
as the mean of all ages was
100!
=
66M
+ 7.6Gs + 6.4Gr + 20E
The average surplus energy or food conversion efficiency was 14%.
DISCUSSION
In Trinidad the whitemouth croaker showed one feeding period a day, from
about mid-day to late evening. This pattern varies from the one observed in
Argentina where the croaker was found to have three periods of intensive feeding
per day from 0400-0600, 1200-1400 and 1800-2000 (Puig, 1986). This may be
due to differences in prevailing oceanographic conditions or in behavioural patterns between populations.
The daily ration of 3.45% body weight was similar to that of juvenile croakers
in Argentina. Ciechomski (1981) experimentally determined that juvenile croakers
consumed an average of 270 mg per gram total weight per week, which is equivalent to a daily ration of 3.86%. These two estimates of daily ration are similar
to the lower end of the range found by Pandian and Vivekanandan (1985) for
tropical fishes. These authors obtained the daily rations of temperate and tropical
fishes from the literature and plotted them against temperature. Daily ration for
tropical species ranged from 4.1-36% and was found to vary with such factors
as temperature, prey type and prey density.
Table 4. Batch fecundity X 1()4 (BF), total fecundity X 1()4 (F), and energy cost of spawning
for ages 1-7 years in the whitemouth croaker Micropogonias furnieri
Age
I
2
3
4
5
6
7
BF
5.5
14.1
27.1
44.3
63.3
83.5
112.2
F
66.0
169.2
325.2
531.6
759.6
1,002.0
],346.4
Energy
61.8
158.4
304.4
497.6
710.9
937.9
1,260.2
(kcal)
390
BULLETIN OF MARINE SCIENCE, VOL. 58. NO.2,
1996
Table 5. Daily and annual weight of food consumed and annual energy consumption for ages 1-7
years in the whitemouth croaker Micropogonias furnieri
Age
Daily g
Annual kg
I
3,92
9.44
17.39
27.47
38.30
49,59
65.31
1.43
3.45
6.35
10.02
13.98
18.10
23.80
2
3
4
5
6
7
Energy
kcal
1,096.7
2,639.1
4,862.5
7,679.1
10,708.8
13,864.0
18,260.6
Several sources of error may have existed in estimating daily ration in this
study. Regurgitation, which is common in fish with closed gas bladders (Amundsen and Klemetsen, 1986), could have been a major factor if undetected. Bowman
(1986) cites regurgitation as a factor which could result in the underestimation of
food consumption
in fish. It is possible
that regurgitation
was not detected
in
some of the croakers used in this study and hence may have resulted in a low
estimate of daily ration.
Another source of error could have been the assumption of a constant value of
daily ration for croakers of all sizes. It is generally accepted that daily ration
decreases with increasing fish size (Popov a, 1978; Windell, 1978). Since most of
the croakers available for this study were relatively large, the estimate of daily
ration could have been biased for smaller fish.
The fact that croakers used for the determination of daily ration were obtained
only during the months of May, August, September and November could have
also biased the estimate of daily ration by masking seasonal differences in feeding
intensity. Haimovici (1977) reported croakers to feed intensively from November
to March in Argentina. In the present study seasonal variation may have been
masked by the pooling of data for the four months sampled and the lack of data
for the other 8 months.
The estimated daily ration is the first such estimate for adult croakers in Trinidad. Since the main prey of the croaker is penaeid shrimps, for which an important fishery also exists in Trinidad, the results can be used as a first step in
assessing the impacts of exploitation on these populations. Studies of species
interactions and daily ration are required for other species within the ecosystem
to develop a model for energy flow throughout the system.
The calculated energy budget shows that the estimated daily ration can sustain
the observed level of somatic growth and reproduction in the croaker. Average
Table 6. Growth efficiency (G,), reproductive effort (G,), and metabolism (M) for the whitemouth
croaker Micropogonias fumieri (as % food consumed). Egestion and excretion was assumed to be
20% food consumed for all age groups.
Age
I
2
3
4
5
6
7
G,
G,
M
18.8
11.2
7.7
5.3
3.9
4.2
2.0
5.6
6.0
6.3
6.5
6,6
6,8
6.9
55.6
62.8
66.0
68.2
69.5
69.0
71.1
MANICKCHAND-HEILEMAN
AND EHRHARDT: ENERGY BUDGET. WHITEMOUTH CROAKER
391
reproductive effort of 6.4% is within the range reported by Brett and Groves (1979),
and by Wootton (1985) who found that estimates have clustered around or below
10%. The average surplus energy of 14% is lower than that of 19.45% reported
by Ciechomski (1981) for Argentinian croakers. This may be because the latter
estimate applies to very young fish (10-27 g) which generally show higher food
conversion efficiencies (Brett and Groves, 1979).
The pattern of energy allocation with age in the croaker is consistent with that
reported for fish in general (Brett and Groves, 1979). In the croaker growth decreases markedly with age. Reproductive effort and metabolism show a gradual
increase with age. Until the age of 3 years, energy allocated to growth exceeds
that allocated to reproduction, after which the reverse occurs. The age at first
maturity in the croaker is 2 years at a total length of about 32 cm (ManickchandHeileman and Kenny, 1990). Therefore it is not unexpected that after reaching
maturity growth rate decreases as more energy is allocated to reproduction.
The pattern of energy allocation can assist in the assessment of the impacts of
exploitation on the population. Intense fishing pressure results in simplification
of the age structure of a fish population, with a predominance of younger age
groups (Weatherley and Gill, 1987). Ware (1985) reported increased growth rates
and a decrease in the age at first reproduction in herring stocks under heavy fishing
pressure. These changes reflected changes in the allocation of surplus energy to
growth and reproduction which accompanied a fall in stock abundance. Daan
(1975) also found that with heavy fishing pressure and a low age at first capture,
more food was utilized for growth at the expense of reproduction in the North
Sea cod. Channelling of surplus energy to growth rather than reproduction can
impair the spawning potential of the stock and lead to stock collapse.
In Trinidad, the croaker has been heavily exploited for the past 10-15 years.
The majority of croakers caught by trawling are under 34 cm total length (Sturm
et aI., 1983) which is equivalent to an age of 2 years. In light of the pattern of
energy allocation with age in the croaker, the preponderance of young individuals
in the population may indicate that most of the surplus energy is being channelled
to growth rather than to reproduction. This could adversely affect the spawning
potential of the population.
ACKNOWLEDGEMENTS
The authors wish to thank the Institute of Marine Affairs, Trinidad, which provided logistic and
other support for this study; The Organization of American States for the fellowship that was awarded
to the first author to pursue a doctorate (on which this study was partially based) at the Rosenstiel
School of Marine and Atmospheric Science, University of Miami; the Chemistry Department (University of the West Indies) for use of a microbomb calorimeter.
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DATEACCEPTED:November 8, 1994.
ADDRESSES:(S.M.H.) Institute of Marine Affairs, P.O. Box 3160 Carenage, Trinidad, West Indies;
PRESENT
ADDRESS: Blvd de los Virreyes No. 155, Lomas Virreyes, CP JIDDDMexico, D. F., Mexico;
(N.M.E.) Rosenstiel School of Marine and Atmospheric Science, Marine Biology and Fisheries Division, 4600 Rickenbacker Causeway, Miami. Florida. 33149-1098.