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Body composition of Mugil cephalus, Liza dumerili, Liza richardsoni and Liza tricuspidens (Teleostei: Mugilidae) caught in the Swartkops Estuary
Aquaculture, 10 (1977) 75-86 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
75
BODY COMPOSITION OF MUGIL CEPHA LUS, LIZA DUMERILI, AND LIZA TRICUSPIDENS (TELEOSTEI: MUGILIDAE) CAUGHT IN THE SWARTKOPS ESTUARY
LIZA RICHARDSONI
J.F.K. MARAIS and T. ERASMUS Department of Zoology, (South Africa)
University ofPort Elizabeth, P.0, Box 1600, Port Elizabeth
(Received 24 August 1976)
ABSTRACT Marais, J.F.K. and Erasmus, T., 1977. Body composition of Mugil cephalus, Lisa dumerili, Liza richardsoni and Liza tricuspidens (Teleostei: Mugilidae) caught in the Swartkops Estuary. Aquaculture, 10: 75-86. Body composition (protein, fat, ash and moisture) and energy content of four mullet species Mugil cephalus, Liza dumerili, Liza richardsoni and Lisa tricuspidens were determined monthly for 13 months in specimens taken from the Swartkops Estuary. Six size classes (<2.5, 2.6-6.0, 6.1-10.0, 10.1-15.0, 15.1-23.0 and >23 cm SL) were analysed separately. Statistical analyses of results could only be performed on three size classes of M. cephalus, L. dumerili and L. richardsoni due to inavailability of fish during certain months. Regression equations were calculated for those body components that were significantly correlated, viz. : energy and moisture, fat and moisture, protein and moisture and energy and fat. The influence of species, size as well as seasonal effects and reproductive cycle on body composition were investigated. It was found that in general Liza dumerili had the lowest mean body energy and fat values and Liza tricuspidens the highest. Smaller size fish were lower in energy and fat and larger size fish lower in moisture. Overall mean values showed that body energy reserves were the highest during the winter. Sexually mature specimens of Liza dumerili showed a build-up of energy before the commencement of the spawning season.
INTRODUCTION
Atwater (1888, cited by El Saby, 1934), determined body composition by chemical analysis during the last century as an indication of the nutritive value of food fishes. Atwater, as well as other research workers, stressed the necessity of performing body composition determinations over an extended period in order to incorporate the many factors which would influence the final result. In the present study, analyses were performed on the full size range of the four most abundant mullet species netted in the Swartkops Estuary near Port Elizabeth over a 13-month period. This was done in an effort to relate
76
differences in body composition to size, species, season, feeding level and spawning time. Information on the energy content of fish in their natural environment also provides base line data for energy flow studies as well as useful information on their nutritional value. Whole fish composition (including the alimentary canal stripped of its contents) was used since it can be regarded as a more reliable reflection of total body nutrients than analysis of muscle alone. Suppes et al. (1967) indicated that per gram weight, viscera were the major storage area for fat. In spite of this, muscle, which represents the edible part of the fish, is more often analysed. In the determination of the body components it was assumed that the mass of an empty vertebrate body (alimentary canal contents removed) could be factorized into four components: water, fat, protein and mineral matter or ash. Carbohydrate content can be ignored since there is general agreement in the literature that it is present in very small amounts in the vertebrate body, mainly as glycogen (Kleiber, 1961; Maynard and Loosli, 1962). Various workers have commented on the fact that moisture content of the fish body increases as fat content decreases (El Saby, 1934; Brandes and Dietrich, 1953; Gerking, 1955; Brown, 1957; Suppes et al., 1967; Habashy, 1972; Niimi, 1972; Blaber, 1973; Denton and Yousef, 1976) without establishing the exact relationship between these two body components. The relationship between body moisture and body fat and between moisture and protein, has led to the determination of regression equations by which fat and protein content of farm animals can be calculated from a knowledge of body water content alone (Blaxter, 1962). From a large number of body composition determinations on Mugil cephalus Linnaeus, 1758; Liza dumerili (Steindachner, 1869) and Liza richardsoni (Smith, 1846), regression equations were calculated by which energy, fat and protein content of these species can be estimated with a high degree of accuracy using the easily determinable parameter of body moisture. METHODS
Mullets were obtained monthly (December 1972 to December 1973) from four stations (see Fig. 1) along the banks of the Swartkops Estuary by means of drag, cast and scoop netting. The drag net (used at Stations 1,2 and 3) was a 35 X 1.7-m nylon net covered on the inside with a shade cloth having mesh size 1 cm. The rocky nature of the substratum at Station 4 necessitated the use of two cast nets, one with diameter 300 cm and mesh size 3 cm and a smaller one with diameter 180 cm and mesh size 1.5 cm. A scoop net with diameter 53 cm and mesh size 2 mm was used at Stations 1 and 4 for fry. (All mesh measurements are stretched mesh.) After identification according to Smith (1975), mullet of the same species were separated into the size ranges <2.5 cm, 2.6-6.0 cm, 6.1-10.0 cm,
10.1-15.0 cm, 15.1-23.0 cm and >23 cm standard length (SL), irrespective of locality caught. Because of the time involved to identify fry <2.5 (Wallace and Van der Elst, 1973), fish in this size range were identified as Mugilidae only and lumped together.
SWARTKOPS
Causeway
ESTUARY
Amsterdamhoek
Redhouse
9
!
;
ALGOA BAY
$km
Fig. 1. Map of Swartkops Estuary showing location of sampling sites.
Fish in the different size ranges were homogenised with an electrical meat mincer. Moisture determinations were performed on 2-g duplicate samples of this material (24 h at 100°C). About 50 g of the remaining material was freeze dried (4 days). For fry and small numbers of fish in the smaller size ranges, moisture content of whole fish in the fresh state was determined by weight loss during freeze drying. Comparative determinations revealed that differences in moisture content between the two methods were less than 0.5% and could be ignored. The freeze dried material, including scales and bones, was finely milled with a Wiley Mill (mesh size 2 mm). The following determinations were carried out on this material: crude fat (goldfish apparatus), crude protein (nitrogen by micro-Kjeldal method X 6.25), energy (adiabatic bomb calorimeter) and ash (the completely combusted inorganic remains of samples used for energy determinations). Variance analyses, correlation coefficients and regression coefficients were computed by the abbreviated Doolittle method (Bennett and Franklin, 1954) for Mugil cephalus, Liza dumerili and Liza richardsoni belonging to the size ranges 6.1-10.0 cm, 10.1-15.0 cm and 15.1-23.0 cm SL. To test for significant differences in energy content between the different months (all species and size ranges combined), the multiple comparison procedure, developed by Tukey (Snedecor, 1956), was used. The other size ranges and Liza tricuspidens (Smith, 1935) could not be included in the computer programme because material was obtained too infrequently. The results obtained, however, are presented.
78 RESULTS
AND DISCUSSION
The different body components and energy content (kcal/lOO g) for M. cephalus, L. dumerili, L. richardsoni and L. tricuspidens of different size ranges are presented in Table I. Pooled fry composition was determined and used for the <2.5-cm size range because of the time involved in individual identification. Data presented in Table I represent mean combined values of all the different months. Monthly data for Liza dumerili in the size range 15.1-23.0 cm SL is presented in Table II. This was the only species and size range in which monthly energy content showed a significant pattern. Table II also contains the monthly energy content of all species and size ranges combined to indicate seasonality of data. Interrelationships of the various body components General tendencies observed in mean body composition of all the mullet species analysed are best illustrated in Liza richardsoni. A greater number of determinations over the full size range were carried out for this species than for any of the other individual species. Fig. 2, indicating the proportions of different body components of whole specimens of Liza richardsoni at different sizes, was constructed from Table I. NATURAL
BASIS
la)
DRY
BASIS
FAT
lb)
L
6
6
IO
12
Y
BASIS
(c 1
MOISTURE
MOISTURE
2
FREE
16
18 20 22 2L
L
6
8
x)
STANDARD
12 lL
16
LENGTH
18 20 22 2L
L
6
6
10 12 14
16
16 20
22 24 26
I cm)
Fig. 2. Changes in body composition with age expressed on (a) natural basis, (b) dry basis, (c) fat free basis.
Kleiber (1961) and Maynard and Loosli (1962) indicated that protein and ash rapidly increase in very young farm animals and that moisture decreases and fat increases later on as the animal ages. The same tendencies are demonstrated for fish in Fig. 2a. The general increase of fat with age after an initial decrease is best illustrated in Fig. 2b where body components are expressed on a moisture free basis. The slight initial decrease in fat content can probably be ascribed to change over from one type of diet to another. Odum (1970) stated that juvenile mullet are primarily carnivorous until they reach a standard length of about 30 mm, which supports earlier work of
of mullet
6.0
2.6-
2.6-
6.1-10.0
Mea”
M.
caught
mean
Overall
moisture
free
8
4
All sizes
All sizes
mean
Overall
All sizes
cephalus L. dumerili L. richardsoni L. ticuspidens
1 11 9
1 3
13 2
8
6 11 8 8
2
3
1 2
I
5 11 1 2
8
6 5
10 1
9
8
6 11 9
12
8 10 14 12
105
6 11
11 13
number
Estuary
fish per month
Mean
in the Swartkops of months
11 12
13 9
11 13
Number sampled
All sizes AU sizes
>23.0
Mea”
hf.
>23.0 >23.0
L. richardsoni L. tricuspidens
15.1-23.0
> 23.0
Melt”
cepholus
15.1-23.0 15.1-23.0 15.1-23.0 15.1-23.0
M.
10.1-15.0
Mea”
M. cephalw L. dumerili L. richardsoni L. tricuspidens
cephalus L. dumerili L. richardsoni L. tricuspidens
10.1-15.0 10.1-15.0 10.1-15.0 10.1-15.0
6.1-10.0
MIX”
M.
6.1-10.0 6.1-10.0 6.1-10.0
cepholw L. dumerili L. richardsoni L. tricuspidens
6.0
6.0 6.0 6.0
<2.5
(4
Size range
composition
2.62.62.6-
fty
body
1
cephalus L. dumerili L. richardsoni L. tricuspidens
M.
Pooled
Species
Mean
I‘AHLE
of
49.22
65.43 38.0 64.36 31.63
298.81
302.04 297.74 258.40
111.47
16.7 18.2
82.5 119.3 118.2
11.19
9.1
12.4
13.2 11.1
25.0
24.5 25.1 24.1
17.76
18.6
17.8
12.31
121.90
40.56
12.72 12.03 12.39 10.40
41.06 37.88 21.27
7.94
44.22
10.00
6.99
8.61 8.12 7.87
5.1
4.8 5.7
2.29 3.17 2.64
5.5 4.8
2.0
(cm)
466.0
120.6
126.1
124.7
119.2 117.9
177.5
160.1
187.3 174.2
123.4
116.1 124.9 130.3 90.6
119.8
119.2 122.2
110.4 119.4
118.5
123.9 114.9 113.1 124.9
121.1
121.6 136.4
131.5 108.2
108.1
Energy kcal/lOO
basis)
Mean SL
on a natural mass
3.99 2.43
0.15
14.27 9.86 9.35 5.94
(g)
Mean
(expressed
g
74.23
74.06
74.51 74.19 73.38
67.22
67.95 66.98 70.53
73.55
74.38 73.34 72.39 79.49
74.35
73.65 73.50
75.70 73.78
68.32
17.61
17.39
17.53 17.71 18.17
18.82
16.12 19.72 15.61
18.04
17.88 18.47 18.49 14.55
17.83
18.39 18.22
17.26 17.88
17.82
18.02
73.90 74.28
17.67 17.72 17.94
16.80
17.03 17.61
16.82 16.01
15.11
protein
Percent
74.05 74.30 74.74
74.97
75.06 73.02
74.74 76.31
78.12
Percent moisture
11.46
2.96
2.72 2.56 3.10 3.95
9.12
11.55 8.31 8.36
3.01
1.65
2.50 2.97 3.30
2.41
2.43 4.77
2.05 2.70
2.71
2.16 2.29 3.49
3.25
3.27
4.66
5.09 2.32 2.98
3.27
fat
Percent
19.24
4.96
4.66
4.94 5.29 5.04
4.14
4.65 4.77 4.54
5.10
5.13 4.40
5.23 5.14
5.19
5.38 3.83
4.93 5.41
5.14
5.19 4.85
4.80 5.60
4.52
4.62
3.95 4.79 4.28
3.70
Percent ash
99.04
99.76
99.70 99.75 99.69 100.06
99.90
100.27 99.78 99.04
99.72
99.99 99.92 99.31 100.09
99.86
99.86 100.32
99.94 99.77
100.00
100.26
99.77 99.78 100.24
99.58
99.43 99.35 99.91
100.60
100.18
Total
N
13 12 7 8 11 12 15 15 14 15 12 5 9
11
Date
12/1972 111973 211973 311973 411973 5/1973 6/1973 7/1973 811973 g/1973 10/1973 11/1973 1211973
Mean
+ f f f + + t f f * * * f
22.62 23.62 27.12 24.79 17.48 24.14 22.47 15.22 14.20 19.92 14.16 13.12 35.78
*Mugil cephalus, Liza dumerili
82.5
92.2 81.5 88.1 79.9 76.0 89.2 74.5 79.2 76.7 76.8 80.1 89.0 89.7
content
+ 1.25 +_ 1.30 f 1.79 f 1.62 f 1.25 * 1.50 f 1.83 + 1.01 f 0.90 f 1.15 * 0.95 * 1.00 f 1.75
and Liza richardsoni
16.7
17.1 16.5 17.0 16.7 16.3 17.1 16.4 16.5 16.2 16.5 16.6 17.3 17.0
SL + SD (cm)
as well as the energy
Mass + SD (g)
composition
II
Mean body
TABLE Liza dumerili
119.0
121.0 114.0 114.9 114.8 120.9 131.6 130.6 117.4 117.9 110.0 121.5 116.2 116.5
10.1-15.0
Energy - spp. g and size ranges combined*
mature
of size ranges 6.1-10.0,
124.9
116.4 119.8 119.9 116.6 130.9 130.9 137.0 126.6 137.3 125.1 127.2 126.0 110.4
Energy keal/lOO
of sexually
18.47
20.55 18.34 18.36 17.78 .19.16 18.70 16.95 18.74 17.52 18.59 17.90 19.57 19.02
Percent protein
2.97
1.20 1.23 2.39 3.55 3.02 3.96 4.81 3.78 4.56 2.97 3.21 2.71 1.25
Percent fat
cm SL), expressed
and 15.1-23.0.
73.34
73.21 75.08 73.66 73.89 72.15 72.87 72.08 73.77 72.47 73.07 72.88 72.82 75.42
Percent moisture
(15.1-23.0
5.14
5.55 5.55 5.68 5.68 5.49 4.83 4.78 4.54 4.68 5.29 4.97 5.02 4.73
Percent ash
on a natural
99.92
100.51 100.21 100.09 100.90 99.82 100.36 98.62 100.83 99.23 99.92 98.96 99.12 100.42
Total
basis
81
Kuthalingham (1966). Allowing for inter-specific differences, the diet of larger size mullet seems to consist mainly of micro-algae including epiphytic and benthic forms, decaying plant detritus and inorganic sediment particles (Thomson, 1954; Masson and Marais, 1975). The relatively small changes that occur in actual protein and ash content of the body after the very young stage is illustrated in Fig. 2c when body composition is expressed on a fat free basis. This was also demonstrated for Egyptian food fishes by El Saby (1934). Denton and Yousef (1976) found that body ash content of rainbow trout remained relatively constant during the first 14 months of life. They also found that although body protein content increases with age, it remains constant when expressed as a percent of total solids. Body composition of mullets from the Swartkops is rather similar to data for Mugil cephalus, Liza ramada and Liza saliens (literature quoted by Thomson, 1966), although only the edible portion of these fish was analysed. In most of the literature cited by Thomson, moisture varied between 73 and 78%, protein 17 and 21%, fat 2 and 4% and ash 1 and 2%. As could be expected from the inclusion of scales and bones in the present determinations, TABLE III Correlation coefficients (r), number of determinations (N), intercepts (a), regression coefficients (b), standard errors of b (SE) and computed t values of different body constituents determined separately for three mullet species (Y = bX + a) Species Percent
N moisture
a
r
33 38 36
Percent
(X) and percent
-0.867* -0.869* -0.932*
M. cephalus L. dumerili L. richardsoni
33 38 36
Percent
(X) and percent
moisture
M. cephalus L. dumerili L. richardsoni Percent
33 38 36
-0.588* -0.562* -0.426*
-0.684* -0.525* -0.728s
fat (X) and energy
M. cephalus L. dumerili L. richardsoni *p = 0.01
t
(X) and energy kcal/lOO g (Y)
M. cephalus L. dumerili L. richardsoni moisture
SE
b
33 ‘38 36
613.3654 573.8048 643.4895 protein
-6.6466 -6.1527 -7.0998
0.6853 0.5828 0.4720
9.6991* 10.5571* 15.0406*
-0.3007 -0.4706 -0.2628
0.0742 0.1156 0.0956
4.0521* 4.0725* 2.7489*
(Y)
40.0714 52.7593 37.6061 fat (Y)
45.8690 28.1463 38.9214
-0.5791 -0.3461 -0.7280
-0.1111 -0.0935 -0.4923
5.2142* 3.6996* 6.1912*
kcal/lOO g (Y) 0.922* 0.759* 0.832*
95.1182 98.4703 95.7567
8.3362 8.1485 9.3720
0.6306 1.1644 1.0706
13.1297* 6.9980* 8.7544*
82
ash values were much higher (from a mean of 4.66% for the species with the lowest to 5.29% for the species with the highest) and protein slightly lower (17.39% in species with lowest and 18.17% in species with highest) (see Table I). A notable feature of all the body determinations over the 13-month period was the correlation that existed between the various body components (see Fig. 2). This enabled the calculation of correlation coefficients. All those that proved to be significantly correlated, viz.: moisture and energy, moisture and fat, moisture and protein, and energy and fat are given in Table III. Table III also presents regression equations by which energy, fat and protein content of fish bodies can be calculated from a knowledge of moisture content only. Regression equations to calculate energy from fat content are also included. The high precision of these estimates is indicated by the small standard errors of the regression coefficients. Factors influencing body composition Species Mean values for the body components of M. cephalus, L. dumerili and L. richardsoni were rather similar (see Table I). Variance analysis performed on body components of these three species (all sizes combined) revealed that the only body component that differed significantly (p = 0.05) was ash of Mugil cephalus (4.94%) and Liza dumerili (5.29%). Liza tricuspidens exhibited higher energy and fat values and was thus lower in ash than the other three species. Statistical analyses were not performed on these results because of the relatively small numbers of Liza tricuspidens obtained. Size comparison within species The effect of size on body composition within the three species used for statistical treatment is first considered (for size ranges 6.1-10.0, 10.1-15.0 and 15.1-23.0 cm SL). Liza richardsoni. Liza richardsoni in the size range 15.1-23.0 cm had a significantly (p = 0.05) higher energy content (130.3 kcal/lOO g) than fish falling in the size ranges 10.1-15.0 cm (119.2 kcal/lOO g) and 6.1-10.0 cm (113.1 kcal/lOO g; p = 0.01). As expected, moisture content in the larger size range (72.39%) was significantly (p = 0.05) lower than in the middle (73.65%) and smaller size ranges (74.74%; p = 0.01). Fat in the larger size range was also significantly (p = 0.05) higher than in the two smaller size ranges. Although the >23-cm size range was not included in the statistical analysis, it must be noted that this size range had much higher energy (174.2 kcal/lOO g) and fat (8.31%) values than the smaller size ranges had. Fat content was more than twice as much as in the 15.1--23.0~cm size range and more than three times as much as in the 6.1-lO.O-cm and lO.l-15.0-cm SL size ranges.
83
Liza dumerili. Other statistically Liza dumerili where ash content
significant differences within species was in of fish falling in the 15.1-23.0-cm size range (5.14%) was significantly lower than in the smaller size range (5.60%). This can probably be attributed to the higher fat percentage in the larger size range (2.97% compared to 2.16%; not statistically significant). As was the case with Liza richardsoni, energy and fat content of larger size ranges were higher than in the smaller size ranges.
Mugil cephalus and Liza tricuspidens. In these two species differences
in energy and fat between the different size ranges smaller than 23 cm SL showed no fixed pattern. This can probably be ascribed to inavailability of material during many months. As was indicated for Liza richardsoni >23 cm, energy and fat content of Mugil cephalus (187.3 kcal/lOO g and 11,55%) and Liza tricuspidens (160.1 kcal/lOO g and 8.36%) were considerably higher in fish falling in the largest size range than in fish falling in any of the other size ranges (see Table I).
Size comparison between species 2.6-6.0 cm SL. In this size range Liza dumerili had by far the lowest energy fat and protein content and the highest moisture and ash. However, differences in this size range were not tested for significance due to inavailability of Mugil cephalus and Liza tricuspidens in most months.
6. l-l 0.0 cm SL. Only one statistically significant difference occurred in this size range. Mugil cephalus (4.80%) had significantly less ash than Liza dumerili (5.60%; p = 0.05). 10.1-15.0 cm SL. Mugil cephalus falling in the lO.l--15.0~cm size range had a significantly higher moisture content (75.70%) than both Liza dumerili (73.78%) and Liza richardsoni (73.65%; p = 0.01). It also had a significantly lower protein content (17.26%) than Liza richardsoni (18.39%; p = 0.05). 15.1-23.0 cm SL. The only significant difference in range was that Mugil cephalus (74.38%) had a higher Liza richardsoni (72.39%; p = 0.05). The energy and tricuspidens obtained during only 2 of the 13-month much lower than that of the other species (see Table
the 15.1-23.0-cm size moisture content than fat content of Liza sampling period, was I).
>23 cm SL. Too few determinations of body composition were performed in the >23-cm size range to make valid inter-specific comparisons. Nevertheless Mugil cephalus (187.3 kcal/lOO g) appeared to have the highest mean energy content followed by Liza richardsoni (174.2 kcal/lOO g) and Liza tricuspidens (160.1 kcaljlO0 g).
a4
Effect of season Love (1957) indicated that differences in season, depending on the availability of food at different times of the year, have a considerable effect on the tissue components, particularly fat. Changes in the reproductive cycle also have a marked effect on body composition. He further mentioned that fish, like other animals, store fat to supply energy needed during food scarcity and reproductive phases so that one might expect the greatest concentration at the end of the prolific feeding in the summer and the least at the end of the winter. With the multi le comparison of Tukey (Snedecor, 1956) it was shown, in accordance witR Love, that fish contained more (p = 0.05) energy during the winter months (see Table II), May (131.6 kcal/lOO g) and June (130.6 kcal/lOO g) than during the early summer month, September (110.0 kcal/ 100 g) and January (114.0 kcal/lOOg), February (114.9 kcal/lOOg) and March (114.8 kcal/lOO g) (species and size ranges combined for this comparison). Apart from the significant differences in energy content indicated for the different months (pooled data), no variation in body composition attributable to season was found for any individual species or size range with the exception of Liza dumerili 15.1-23.0 cm SL. Most specimens of Liza dumerili falling in this size range can be regarded as sexually mature. Both Marais (1976) and Van der Horst (1976) have shown that sexual maturity in male Liza dumerili commences at between 14 and 15 cm SL and at between 16 and 17 cm SL in females. Comparison with Mugil cephalus and Liza richardsoni of the same size range which are mostly immature (Marais, 1976), revealed that these species did not experience the same general increase in fat and energy during the winter months. Sexually mature specimens of Liza dumerili thus had considerably higher energy reserves during the off-season than during the sexually active season commencing in September and sustained until February-March (Marais, 1976; Van der Horst, 1976). This is in agreement with published data. Valenzuela (1928) indicated that Phillipine food fishes (including mullets) deposited the maximum amount of fat just before the spawning season and that they had the minimum fat content a few weeks after spawning. This is also supported by data of El Saby (1934) for Egyptian food fishes and is in agreement with general comments of Love (1957). Reduction in fat content during the spawning season has been recorded for cod (Dambergs, 1964), channel catfish (Suppes et al., 1967), brown trout (Lusk, 1968), mirror carp (Habashy, 1972) and the three-spined stickleback (Wootton, 1974). Mature Liza dumerili thus followed the same general trend as other species, viz. building up of body reserves in the sexually inactive period in preparation for the spawning season. CONCLUSIONS
From the body composition determinations it was shown that the different body components of mullet experienced the same general changes with age as
85
have been established for farm animals (Blaxter, 1962) and rainbow trout (Denton and Yousef, 1976). During the development of the very young animal, percentage moisture decreases and fat and protein increase. As the animal ages, the most pronounced change is a decrease in moisture as body fat increases. Expressed on a dry basis, it was conclusively demonstrated that both protein and ash percentage decrease as fat increases. However, when expressed on a fat free basis, it was shown that ash and protein content remain fairly constant in ageing fish after the initial increase in very young fish. Correlation coefficients were thus calculated for the different body components. For those that were significantly correlated, regression equations were calculated, viz. for moisture and energy, moisture and protein, moisture and fat (all negatively correlated) as well as for fat and energy (positively correlated) in M. cephalus, L. dumerili and L. richardsoni. Estimation of biologically important parameters such as protein, fat and energy from a relatively simple moisture determination can thus be effected by highly significant (p = 0.01) regression equations (Table III). Interspecific body composition comparisons revealed that mean body composition (all size ranges combined) of the four species was fairly similar, although considerable differences in energy and fat content occurred in the 2.6-6.0 and 15.1-23.0~cm size ranges. The combined data revealed that Liza tricuspidens had the highest overall energy (126.1 kcal/lOO g) and fat (3.95%) and lowest ash (4.66%) and protein content (17.39%), and that Liza dumerili had the lowest energy (117.9 kcal/lOO g) and fat (2.56%) and highest ash content (6.29%). It is surprising that the earlier maturing Liza dumerili had lower energy values than the larger size range (>23 cm SL) of Mugil cephalus (immature), Liza richardsoni (just mature) or Liza tricuspidens (immature) at all times during the 13-month collection period (Table II). This can probably be accounted for by the fact that this species ingested food material of a considerably lower nutritive value (Marais, 1976). ACKNOWLEbGEMENTS
Grateful acknowledgement for financial support for this investigation is paid to: the South African Council for Scientific and Industrial Research and the University of Port Elizabeth. I also want to thank Mrs L. Botha for preparing the figures; Dr S.G. Reinach and Mr G.P. Melville for their assistance in the statistical analyses of the results and Mr H.W. Lombard who helped me with the body composition determinations.
REFERENCES Bennett, C.A. and Franklin, U.L., 1954. Statistical Analysis in Chemistry and the Chemical Industry. John Wiley and Sons, London, 124 pp. Blaber, S.J.M., 1973. The-Ecology of Juvenile Rhobdosurgus holubi (Steindachner) (Teleostei : Bparidae). Thesis, Rhodes University, Graham&own, 123 pp.,mimeographed.
86
Blaxter, K.L., 1962. The Energy Metabolism of Ruminants. Hutchinson Scientific and Technical, London, 329 pp. Brandes, C.H. and Dietrich, R., 1953. A review of the problem of fat and water content in the edible part of herring. Fette Seifen, 55: 533-541. Brown, ME. (Editor), 1957. The Physiology of Fishes I. Academic Press, New York, N.Y., 447 pp. Dambergs, N., 1964. Extractives of fish muscle, 4. Seasonal variations of fat, water solubles, protein and water in cod (Gadus morhia L.) fillets. J. Fish. Res. Board Can., 21: 703709. Denton, J.E. and Yousef, M.K., 1976. Body composition and organ weights of rainbow trout, Salmo gairdneri. J. Fish Bioi., 8: 489-499. El Saby, M.K., 1934. Dietetic value of certain Egyptian food fishes. Rapp. P. ‘J. Reun. Cons. Int. Explor. Mer, 8: 127-143. Gerking, S.D., 1955. Influence of rate of feeding on body composition and protein metabolism of bluegill sunfish. Physiol. Zool., 28: 267-282. Habashy, A.P., 1972. Seasonal variations of moisture, protein, fat and ash in the mirror carp, Cyprinus carpio, Linnaeus, 1758, reared in the A.R.E. Zool. Listy, 22: 85-89. Kleiber, M., 1961. The Fire of Life. An Introduction to Animal Energetics. John Wiley and Sons, London, 454 pp. Kuthalingam, M.D.K., 1966. A contribution to the life history and feeding habits of Mugil cephalus L. Treubia, 27 : 11-32. Love, R.M., 1957. The biochemical composition of fish. In: M.E. Brown (Editor), The Physiology of Fishes I. Academic Press, New York, N.Y., pp. 401-418. Lusk, S., 1968. Changes in the gonad weight and amounts of fats in the brown trout, Salmo truth, m. fario L. in the course of a year. Zool. Listy, 18: 67-80. Marais, J.F.K., 1976. Comparative Studies on the Nutritional Ecology of Mullet in the Swartkops Estuary. Theses, University of Port Elizabeth, Port Elizabeth, 136 pp., mimeographed. Masson, H. and Marais, J.F.K., 1975. Stomach content analyses of mullet from the Swartkops estuary. Zool. Afr., 10: 193-207. Maynard, L.A. and Loosli, J.K., 1962. Animal Nutrition. McGraw-Hill, Toronto, 533 pp. Niimi, A.J., 1972. Total nitrogen, non-protein nitrogen and protein content in large-mouth bass (Micropterus salmoides) with reference to quantitative protein estimates. Can. J. Zool., 50: 1607-1610. Odum, W.E., 1970. Utilisation of the direct grazing and plant detritus food change by the striped mullet, Mugil cephalus. In: J.H. Steele (Editor), Marine Food Chains. University of California Press, Berkeley, Calif., pp. 222-240. Smith, M.M., 1975. Common and Scientific Names of Fishes of Southern Africa. Part I: Marine Fishes. Rhodes University, Grahamstown, 178 pp. Snedecor, G.W., 1956. Statistical Methods. Iowa State University Press, Ames, Iowa, 534 pp. Suppes, C., Tiemeier, O.W. and Deyoe, C.W., 1967. Seasonal variation of fat, protein and moisture in channel catfish. Trans. Kans. Acad. Sci., 70: 349-358. Thomson, J.M., 1954. The organs of feeding and the food of some Australian mullet. Aust. J. Mar. Freshwater Res., 5: 469-485. Thomson, J.M., 1966. The grey mullets. Oceanogr. Mar. Biol. Ann. Rev., 4: 301-335. Valenzuela, A., 1928. Composition and nutritive value of Philippine food fishes. Philipp. J. Sci., 36: 235-242. Van der Horst, G., 1976. Aspects of the reproductive biology of Liza dumerili (Steindachner, 1869) (Teleostei : Mugilidae) with special reference to sperm. Thesis, University of Port Elizabeth, Port Elizabeth, 00 pp., mimeographed. Wallace, J.H. and Van der Elst, R., ! 973. Identification of juvenile mullet. S. Afr. Assoc. Mar. Biol. Res. Bull., No. 10: 26-2’i. Wootton, R.J., 1974. The inter-spawning interval of the female three-spined stickleback, Gasterosteus acufeatus. J. Zool. (London), 172: 331-342.
75
BODY COMPOSITION OF MUGIL CEPHA LUS, LIZA DUMERILI, AND LIZA TRICUSPIDENS (TELEOSTEI: MUGILIDAE) CAUGHT IN THE SWARTKOPS ESTUARY
LIZA RICHARDSONI
J.F.K. MARAIS and T. ERASMUS Department of Zoology, (South Africa)
University ofPort Elizabeth, P.0, Box 1600, Port Elizabeth
(Received 24 August 1976)
ABSTRACT Marais, J.F.K. and Erasmus, T., 1977. Body composition of Mugil cephalus, Lisa dumerili, Liza richardsoni and Liza tricuspidens (Teleostei: Mugilidae) caught in the Swartkops Estuary. Aquaculture, 10: 75-86. Body composition (protein, fat, ash and moisture) and energy content of four mullet species Mugil cephalus, Liza dumerili, Liza richardsoni and Lisa tricuspidens were determined monthly for 13 months in specimens taken from the Swartkops Estuary. Six size classes (<2.5, 2.6-6.0, 6.1-10.0, 10.1-15.0, 15.1-23.0 and >23 cm SL) were analysed separately. Statistical analyses of results could only be performed on three size classes of M. cephalus, L. dumerili and L. richardsoni due to inavailability of fish during certain months. Regression equations were calculated for those body components that were significantly correlated, viz. : energy and moisture, fat and moisture, protein and moisture and energy and fat. The influence of species, size as well as seasonal effects and reproductive cycle on body composition were investigated. It was found that in general Liza dumerili had the lowest mean body energy and fat values and Liza tricuspidens the highest. Smaller size fish were lower in energy and fat and larger size fish lower in moisture. Overall mean values showed that body energy reserves were the highest during the winter. Sexually mature specimens of Liza dumerili showed a build-up of energy before the commencement of the spawning season.
INTRODUCTION
Atwater (1888, cited by El Saby, 1934), determined body composition by chemical analysis during the last century as an indication of the nutritive value of food fishes. Atwater, as well as other research workers, stressed the necessity of performing body composition determinations over an extended period in order to incorporate the many factors which would influence the final result. In the present study, analyses were performed on the full size range of the four most abundant mullet species netted in the Swartkops Estuary near Port Elizabeth over a 13-month period. This was done in an effort to relate
76
differences in body composition to size, species, season, feeding level and spawning time. Information on the energy content of fish in their natural environment also provides base line data for energy flow studies as well as useful information on their nutritional value. Whole fish composition (including the alimentary canal stripped of its contents) was used since it can be regarded as a more reliable reflection of total body nutrients than analysis of muscle alone. Suppes et al. (1967) indicated that per gram weight, viscera were the major storage area for fat. In spite of this, muscle, which represents the edible part of the fish, is more often analysed. In the determination of the body components it was assumed that the mass of an empty vertebrate body (alimentary canal contents removed) could be factorized into four components: water, fat, protein and mineral matter or ash. Carbohydrate content can be ignored since there is general agreement in the literature that it is present in very small amounts in the vertebrate body, mainly as glycogen (Kleiber, 1961; Maynard and Loosli, 1962). Various workers have commented on the fact that moisture content of the fish body increases as fat content decreases (El Saby, 1934; Brandes and Dietrich, 1953; Gerking, 1955; Brown, 1957; Suppes et al., 1967; Habashy, 1972; Niimi, 1972; Blaber, 1973; Denton and Yousef, 1976) without establishing the exact relationship between these two body components. The relationship between body moisture and body fat and between moisture and protein, has led to the determination of regression equations by which fat and protein content of farm animals can be calculated from a knowledge of body water content alone (Blaxter, 1962). From a large number of body composition determinations on Mugil cephalus Linnaeus, 1758; Liza dumerili (Steindachner, 1869) and Liza richardsoni (Smith, 1846), regression equations were calculated by which energy, fat and protein content of these species can be estimated with a high degree of accuracy using the easily determinable parameter of body moisture. METHODS
Mullets were obtained monthly (December 1972 to December 1973) from four stations (see Fig. 1) along the banks of the Swartkops Estuary by means of drag, cast and scoop netting. The drag net (used at Stations 1,2 and 3) was a 35 X 1.7-m nylon net covered on the inside with a shade cloth having mesh size 1 cm. The rocky nature of the substratum at Station 4 necessitated the use of two cast nets, one with diameter 300 cm and mesh size 3 cm and a smaller one with diameter 180 cm and mesh size 1.5 cm. A scoop net with diameter 53 cm and mesh size 2 mm was used at Stations 1 and 4 for fry. (All mesh measurements are stretched mesh.) After identification according to Smith (1975), mullet of the same species were separated into the size ranges <2.5 cm, 2.6-6.0 cm, 6.1-10.0 cm,
10.1-15.0 cm, 15.1-23.0 cm and >23 cm standard length (SL), irrespective of locality caught. Because of the time involved to identify fry <2.5 (Wallace and Van der Elst, 1973), fish in this size range were identified as Mugilidae only and lumped together.
SWARTKOPS
Causeway
ESTUARY
Amsterdamhoek
Redhouse
9
!
;
ALGOA BAY
$km
Fig. 1. Map of Swartkops Estuary showing location of sampling sites.
Fish in the different size ranges were homogenised with an electrical meat mincer. Moisture determinations were performed on 2-g duplicate samples of this material (24 h at 100°C). About 50 g of the remaining material was freeze dried (4 days). For fry and small numbers of fish in the smaller size ranges, moisture content of whole fish in the fresh state was determined by weight loss during freeze drying. Comparative determinations revealed that differences in moisture content between the two methods were less than 0.5% and could be ignored. The freeze dried material, including scales and bones, was finely milled with a Wiley Mill (mesh size 2 mm). The following determinations were carried out on this material: crude fat (goldfish apparatus), crude protein (nitrogen by micro-Kjeldal method X 6.25), energy (adiabatic bomb calorimeter) and ash (the completely combusted inorganic remains of samples used for energy determinations). Variance analyses, correlation coefficients and regression coefficients were computed by the abbreviated Doolittle method (Bennett and Franklin, 1954) for Mugil cephalus, Liza dumerili and Liza richardsoni belonging to the size ranges 6.1-10.0 cm, 10.1-15.0 cm and 15.1-23.0 cm SL. To test for significant differences in energy content between the different months (all species and size ranges combined), the multiple comparison procedure, developed by Tukey (Snedecor, 1956), was used. The other size ranges and Liza tricuspidens (Smith, 1935) could not be included in the computer programme because material was obtained too infrequently. The results obtained, however, are presented.
78 RESULTS
AND DISCUSSION
The different body components and energy content (kcal/lOO g) for M. cephalus, L. dumerili, L. richardsoni and L. tricuspidens of different size ranges are presented in Table I. Pooled fry composition was determined and used for the <2.5-cm size range because of the time involved in individual identification. Data presented in Table I represent mean combined values of all the different months. Monthly data for Liza dumerili in the size range 15.1-23.0 cm SL is presented in Table II. This was the only species and size range in which monthly energy content showed a significant pattern. Table II also contains the monthly energy content of all species and size ranges combined to indicate seasonality of data. Interrelationships of the various body components General tendencies observed in mean body composition of all the mullet species analysed are best illustrated in Liza richardsoni. A greater number of determinations over the full size range were carried out for this species than for any of the other individual species. Fig. 2, indicating the proportions of different body components of whole specimens of Liza richardsoni at different sizes, was constructed from Table I. NATURAL
BASIS
la)
DRY
BASIS
FAT
lb)
L
6
6
IO
12
Y
BASIS
(c 1
MOISTURE
MOISTURE
2
FREE
16
18 20 22 2L
L
6
8
x)
STANDARD
12 lL
16
LENGTH
18 20 22 2L
L
6
6
10 12 14
16
16 20
22 24 26
I cm)
Fig. 2. Changes in body composition with age expressed on (a) natural basis, (b) dry basis, (c) fat free basis.
Kleiber (1961) and Maynard and Loosli (1962) indicated that protein and ash rapidly increase in very young farm animals and that moisture decreases and fat increases later on as the animal ages. The same tendencies are demonstrated for fish in Fig. 2a. The general increase of fat with age after an initial decrease is best illustrated in Fig. 2b where body components are expressed on a moisture free basis. The slight initial decrease in fat content can probably be ascribed to change over from one type of diet to another. Odum (1970) stated that juvenile mullet are primarily carnivorous until they reach a standard length of about 30 mm, which supports earlier work of
of mullet
6.0
2.6-
2.6-
6.1-10.0
Mea”
M.
caught
mean
Overall
moisture
free
8
4
All sizes
All sizes
mean
Overall
All sizes
cephalus L. dumerili L. richardsoni L. ticuspidens
1 11 9
1 3
13 2
8
6 11 8 8
2
3
1 2
I
5 11 1 2
8
6 5
10 1
9
8
6 11 9
12
8 10 14 12
105
6 11
11 13
number
Estuary
fish per month
Mean
in the Swartkops of months
11 12
13 9
11 13
Number sampled
All sizes AU sizes
>23.0
Mea”
hf.
>23.0 >23.0
L. richardsoni L. tricuspidens
15.1-23.0
> 23.0
Melt”
cepholus
15.1-23.0 15.1-23.0 15.1-23.0 15.1-23.0
M.
10.1-15.0
Mea”
M. cephalw L. dumerili L. richardsoni L. tricuspidens
cephalus L. dumerili L. richardsoni L. tricuspidens
10.1-15.0 10.1-15.0 10.1-15.0 10.1-15.0
6.1-10.0
MIX”
M.
6.1-10.0 6.1-10.0 6.1-10.0
cepholw L. dumerili L. richardsoni L. tricuspidens
6.0
6.0 6.0 6.0
<2.5
(4
Size range
composition
2.62.62.6-
fty
body
1
cephalus L. dumerili L. richardsoni L. tricuspidens
M.
Pooled
Species
Mean
I‘AHLE
of
49.22
65.43 38.0 64.36 31.63
298.81
302.04 297.74 258.40
111.47
16.7 18.2
82.5 119.3 118.2
11.19
9.1
12.4
13.2 11.1
25.0
24.5 25.1 24.1
17.76
18.6
17.8
12.31
121.90
40.56
12.72 12.03 12.39 10.40
41.06 37.88 21.27
7.94
44.22
10.00
6.99
8.61 8.12 7.87
5.1
4.8 5.7
2.29 3.17 2.64
5.5 4.8
2.0
(cm)
466.0
120.6
126.1
124.7
119.2 117.9
177.5
160.1
187.3 174.2
123.4
116.1 124.9 130.3 90.6
119.8
119.2 122.2
110.4 119.4
118.5
123.9 114.9 113.1 124.9
121.1
121.6 136.4
131.5 108.2
108.1
Energy kcal/lOO
basis)
Mean SL
on a natural mass
3.99 2.43
0.15
14.27 9.86 9.35 5.94
(g)
Mean
(expressed
g
74.23
74.06
74.51 74.19 73.38
67.22
67.95 66.98 70.53
73.55
74.38 73.34 72.39 79.49
74.35
73.65 73.50
75.70 73.78
68.32
17.61
17.39
17.53 17.71 18.17
18.82
16.12 19.72 15.61
18.04
17.88 18.47 18.49 14.55
17.83
18.39 18.22
17.26 17.88
17.82
18.02
73.90 74.28
17.67 17.72 17.94
16.80
17.03 17.61
16.82 16.01
15.11
protein
Percent
74.05 74.30 74.74
74.97
75.06 73.02
74.74 76.31
78.12
Percent moisture
11.46
2.96
2.72 2.56 3.10 3.95
9.12
11.55 8.31 8.36
3.01
1.65
2.50 2.97 3.30
2.41
2.43 4.77
2.05 2.70
2.71
2.16 2.29 3.49
3.25
3.27
4.66
5.09 2.32 2.98
3.27
fat
Percent
19.24
4.96
4.66
4.94 5.29 5.04
4.14
4.65 4.77 4.54
5.10
5.13 4.40
5.23 5.14
5.19
5.38 3.83
4.93 5.41
5.14
5.19 4.85
4.80 5.60
4.52
4.62
3.95 4.79 4.28
3.70
Percent ash
99.04
99.76
99.70 99.75 99.69 100.06
99.90
100.27 99.78 99.04
99.72
99.99 99.92 99.31 100.09
99.86
99.86 100.32
99.94 99.77
100.00
100.26
99.77 99.78 100.24
99.58
99.43 99.35 99.91
100.60
100.18
Total
N
13 12 7 8 11 12 15 15 14 15 12 5 9
11
Date
12/1972 111973 211973 311973 411973 5/1973 6/1973 7/1973 811973 g/1973 10/1973 11/1973 1211973
Mean
+ f f f + + t f f * * * f
22.62 23.62 27.12 24.79 17.48 24.14 22.47 15.22 14.20 19.92 14.16 13.12 35.78
*Mugil cephalus, Liza dumerili
82.5
92.2 81.5 88.1 79.9 76.0 89.2 74.5 79.2 76.7 76.8 80.1 89.0 89.7
content
+ 1.25 +_ 1.30 f 1.79 f 1.62 f 1.25 * 1.50 f 1.83 + 1.01 f 0.90 f 1.15 * 0.95 * 1.00 f 1.75
and Liza richardsoni
16.7
17.1 16.5 17.0 16.7 16.3 17.1 16.4 16.5 16.2 16.5 16.6 17.3 17.0
SL + SD (cm)
as well as the energy
Mass + SD (g)
composition
II
Mean body
TABLE Liza dumerili
119.0
121.0 114.0 114.9 114.8 120.9 131.6 130.6 117.4 117.9 110.0 121.5 116.2 116.5
10.1-15.0
Energy - spp. g and size ranges combined*
mature
of size ranges 6.1-10.0,
124.9
116.4 119.8 119.9 116.6 130.9 130.9 137.0 126.6 137.3 125.1 127.2 126.0 110.4
Energy keal/lOO
of sexually
18.47
20.55 18.34 18.36 17.78 .19.16 18.70 16.95 18.74 17.52 18.59 17.90 19.57 19.02
Percent protein
2.97
1.20 1.23 2.39 3.55 3.02 3.96 4.81 3.78 4.56 2.97 3.21 2.71 1.25
Percent fat
cm SL), expressed
and 15.1-23.0.
73.34
73.21 75.08 73.66 73.89 72.15 72.87 72.08 73.77 72.47 73.07 72.88 72.82 75.42
Percent moisture
(15.1-23.0
5.14
5.55 5.55 5.68 5.68 5.49 4.83 4.78 4.54 4.68 5.29 4.97 5.02 4.73
Percent ash
on a natural
99.92
100.51 100.21 100.09 100.90 99.82 100.36 98.62 100.83 99.23 99.92 98.96 99.12 100.42
Total
basis
81
Kuthalingham (1966). Allowing for inter-specific differences, the diet of larger size mullet seems to consist mainly of micro-algae including epiphytic and benthic forms, decaying plant detritus and inorganic sediment particles (Thomson, 1954; Masson and Marais, 1975). The relatively small changes that occur in actual protein and ash content of the body after the very young stage is illustrated in Fig. 2c when body composition is expressed on a fat free basis. This was also demonstrated for Egyptian food fishes by El Saby (1934). Denton and Yousef (1976) found that body ash content of rainbow trout remained relatively constant during the first 14 months of life. They also found that although body protein content increases with age, it remains constant when expressed as a percent of total solids. Body composition of mullets from the Swartkops is rather similar to data for Mugil cephalus, Liza ramada and Liza saliens (literature quoted by Thomson, 1966), although only the edible portion of these fish was analysed. In most of the literature cited by Thomson, moisture varied between 73 and 78%, protein 17 and 21%, fat 2 and 4% and ash 1 and 2%. As could be expected from the inclusion of scales and bones in the present determinations, TABLE III Correlation coefficients (r), number of determinations (N), intercepts (a), regression coefficients (b), standard errors of b (SE) and computed t values of different body constituents determined separately for three mullet species (Y = bX + a) Species Percent
N moisture
a
r
33 38 36
Percent
(X) and percent
-0.867* -0.869* -0.932*
M. cephalus L. dumerili L. richardsoni
33 38 36
Percent
(X) and percent
moisture
M. cephalus L. dumerili L. richardsoni Percent
33 38 36
-0.588* -0.562* -0.426*
-0.684* -0.525* -0.728s
fat (X) and energy
M. cephalus L. dumerili L. richardsoni *p = 0.01
t
(X) and energy kcal/lOO g (Y)
M. cephalus L. dumerili L. richardsoni moisture
SE
b
33 ‘38 36
613.3654 573.8048 643.4895 protein
-6.6466 -6.1527 -7.0998
0.6853 0.5828 0.4720
9.6991* 10.5571* 15.0406*
-0.3007 -0.4706 -0.2628
0.0742 0.1156 0.0956
4.0521* 4.0725* 2.7489*
(Y)
40.0714 52.7593 37.6061 fat (Y)
45.8690 28.1463 38.9214
-0.5791 -0.3461 -0.7280
-0.1111 -0.0935 -0.4923
5.2142* 3.6996* 6.1912*
kcal/lOO g (Y) 0.922* 0.759* 0.832*
95.1182 98.4703 95.7567
8.3362 8.1485 9.3720
0.6306 1.1644 1.0706
13.1297* 6.9980* 8.7544*
82
ash values were much higher (from a mean of 4.66% for the species with the lowest to 5.29% for the species with the highest) and protein slightly lower (17.39% in species with lowest and 18.17% in species with highest) (see Table I). A notable feature of all the body determinations over the 13-month period was the correlation that existed between the various body components (see Fig. 2). This enabled the calculation of correlation coefficients. All those that proved to be significantly correlated, viz.: moisture and energy, moisture and fat, moisture and protein, and energy and fat are given in Table III. Table III also presents regression equations by which energy, fat and protein content of fish bodies can be calculated from a knowledge of moisture content only. Regression equations to calculate energy from fat content are also included. The high precision of these estimates is indicated by the small standard errors of the regression coefficients. Factors influencing body composition Species Mean values for the body components of M. cephalus, L. dumerili and L. richardsoni were rather similar (see Table I). Variance analysis performed on body components of these three species (all sizes combined) revealed that the only body component that differed significantly (p = 0.05) was ash of Mugil cephalus (4.94%) and Liza dumerili (5.29%). Liza tricuspidens exhibited higher energy and fat values and was thus lower in ash than the other three species. Statistical analyses were not performed on these results because of the relatively small numbers of Liza tricuspidens obtained. Size comparison within species The effect of size on body composition within the three species used for statistical treatment is first considered (for size ranges 6.1-10.0, 10.1-15.0 and 15.1-23.0 cm SL). Liza richardsoni. Liza richardsoni in the size range 15.1-23.0 cm had a significantly (p = 0.05) higher energy content (130.3 kcal/lOO g) than fish falling in the size ranges 10.1-15.0 cm (119.2 kcal/lOO g) and 6.1-10.0 cm (113.1 kcal/lOO g; p = 0.01). As expected, moisture content in the larger size range (72.39%) was significantly (p = 0.05) lower than in the middle (73.65%) and smaller size ranges (74.74%; p = 0.01). Fat in the larger size range was also significantly (p = 0.05) higher than in the two smaller size ranges. Although the >23-cm size range was not included in the statistical analysis, it must be noted that this size range had much higher energy (174.2 kcal/lOO g) and fat (8.31%) values than the smaller size ranges had. Fat content was more than twice as much as in the 15.1--23.0~cm size range and more than three times as much as in the 6.1-lO.O-cm and lO.l-15.0-cm SL size ranges.
83
Liza dumerili. Other statistically Liza dumerili where ash content
significant differences within species was in of fish falling in the 15.1-23.0-cm size range (5.14%) was significantly lower than in the smaller size range (5.60%). This can probably be attributed to the higher fat percentage in the larger size range (2.97% compared to 2.16%; not statistically significant). As was the case with Liza richardsoni, energy and fat content of larger size ranges were higher than in the smaller size ranges.
Mugil cephalus and Liza tricuspidens. In these two species differences
in energy and fat between the different size ranges smaller than 23 cm SL showed no fixed pattern. This can probably be ascribed to inavailability of material during many months. As was indicated for Liza richardsoni >23 cm, energy and fat content of Mugil cephalus (187.3 kcal/lOO g and 11,55%) and Liza tricuspidens (160.1 kcal/lOO g and 8.36%) were considerably higher in fish falling in the largest size range than in fish falling in any of the other size ranges (see Table I).
Size comparison between species 2.6-6.0 cm SL. In this size range Liza dumerili had by far the lowest energy fat and protein content and the highest moisture and ash. However, differences in this size range were not tested for significance due to inavailability of Mugil cephalus and Liza tricuspidens in most months.
6. l-l 0.0 cm SL. Only one statistically significant difference occurred in this size range. Mugil cephalus (4.80%) had significantly less ash than Liza dumerili (5.60%; p = 0.05). 10.1-15.0 cm SL. Mugil cephalus falling in the lO.l--15.0~cm size range had a significantly higher moisture content (75.70%) than both Liza dumerili (73.78%) and Liza richardsoni (73.65%; p = 0.01). It also had a significantly lower protein content (17.26%) than Liza richardsoni (18.39%; p = 0.05). 15.1-23.0 cm SL. The only significant difference in range was that Mugil cephalus (74.38%) had a higher Liza richardsoni (72.39%; p = 0.05). The energy and tricuspidens obtained during only 2 of the 13-month much lower than that of the other species (see Table
the 15.1-23.0-cm size moisture content than fat content of Liza sampling period, was I).
>23 cm SL. Too few determinations of body composition were performed in the >23-cm size range to make valid inter-specific comparisons. Nevertheless Mugil cephalus (187.3 kcal/lOO g) appeared to have the highest mean energy content followed by Liza richardsoni (174.2 kcal/lOO g) and Liza tricuspidens (160.1 kcaljlO0 g).
a4
Effect of season Love (1957) indicated that differences in season, depending on the availability of food at different times of the year, have a considerable effect on the tissue components, particularly fat. Changes in the reproductive cycle also have a marked effect on body composition. He further mentioned that fish, like other animals, store fat to supply energy needed during food scarcity and reproductive phases so that one might expect the greatest concentration at the end of the prolific feeding in the summer and the least at the end of the winter. With the multi le comparison of Tukey (Snedecor, 1956) it was shown, in accordance witR Love, that fish contained more (p = 0.05) energy during the winter months (see Table II), May (131.6 kcal/lOO g) and June (130.6 kcal/lOO g) than during the early summer month, September (110.0 kcal/ 100 g) and January (114.0 kcal/lOOg), February (114.9 kcal/lOOg) and March (114.8 kcal/lOO g) (species and size ranges combined for this comparison). Apart from the significant differences in energy content indicated for the different months (pooled data), no variation in body composition attributable to season was found for any individual species or size range with the exception of Liza dumerili 15.1-23.0 cm SL. Most specimens of Liza dumerili falling in this size range can be regarded as sexually mature. Both Marais (1976) and Van der Horst (1976) have shown that sexual maturity in male Liza dumerili commences at between 14 and 15 cm SL and at between 16 and 17 cm SL in females. Comparison with Mugil cephalus and Liza richardsoni of the same size range which are mostly immature (Marais, 1976), revealed that these species did not experience the same general increase in fat and energy during the winter months. Sexually mature specimens of Liza dumerili thus had considerably higher energy reserves during the off-season than during the sexually active season commencing in September and sustained until February-March (Marais, 1976; Van der Horst, 1976). This is in agreement with published data. Valenzuela (1928) indicated that Phillipine food fishes (including mullets) deposited the maximum amount of fat just before the spawning season and that they had the minimum fat content a few weeks after spawning. This is also supported by data of El Saby (1934) for Egyptian food fishes and is in agreement with general comments of Love (1957). Reduction in fat content during the spawning season has been recorded for cod (Dambergs, 1964), channel catfish (Suppes et al., 1967), brown trout (Lusk, 1968), mirror carp (Habashy, 1972) and the three-spined stickleback (Wootton, 1974). Mature Liza dumerili thus followed the same general trend as other species, viz. building up of body reserves in the sexually inactive period in preparation for the spawning season. CONCLUSIONS
From the body composition determinations it was shown that the different body components of mullet experienced the same general changes with age as
85
have been established for farm animals (Blaxter, 1962) and rainbow trout (Denton and Yousef, 1976). During the development of the very young animal, percentage moisture decreases and fat and protein increase. As the animal ages, the most pronounced change is a decrease in moisture as body fat increases. Expressed on a dry basis, it was conclusively demonstrated that both protein and ash percentage decrease as fat increases. However, when expressed on a fat free basis, it was shown that ash and protein content remain fairly constant in ageing fish after the initial increase in very young fish. Correlation coefficients were thus calculated for the different body components. For those that were significantly correlated, regression equations were calculated, viz. for moisture and energy, moisture and protein, moisture and fat (all negatively correlated) as well as for fat and energy (positively correlated) in M. cephalus, L. dumerili and L. richardsoni. Estimation of biologically important parameters such as protein, fat and energy from a relatively simple moisture determination can thus be effected by highly significant (p = 0.01) regression equations (Table III). Interspecific body composition comparisons revealed that mean body composition (all size ranges combined) of the four species was fairly similar, although considerable differences in energy and fat content occurred in the 2.6-6.0 and 15.1-23.0~cm size ranges. The combined data revealed that Liza tricuspidens had the highest overall energy (126.1 kcal/lOO g) and fat (3.95%) and lowest ash (4.66%) and protein content (17.39%), and that Liza dumerili had the lowest energy (117.9 kcal/lOO g) and fat (2.56%) and highest ash content (6.29%). It is surprising that the earlier maturing Liza dumerili had lower energy values than the larger size range (>23 cm SL) of Mugil cephalus (immature), Liza richardsoni (just mature) or Liza tricuspidens (immature) at all times during the 13-month collection period (Table II). This can probably be accounted for by the fact that this species ingested food material of a considerably lower nutritive value (Marais, 1976). ACKNOWLEbGEMENTS
Grateful acknowledgement for financial support for this investigation is paid to: the South African Council for Scientific and Industrial Research and the University of Port Elizabeth. I also want to thank Mrs L. Botha for preparing the figures; Dr S.G. Reinach and Mr G.P. Melville for their assistance in the statistical analyses of the results and Mr H.W. Lombard who helped me with the body composition determinations.
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