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Effect of stocking density on the growth of estuary grouper, Epinephelus salmoides Maxwell, cultured in floating net-cages
Aquaculture, 15 (1978) 273-287 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
273
EFFECT OF STOCKING DENSITY ON THE GROWTH OF ESTUARY GROUPER, EPINEPHELUS SALMOIDES MAXWELL, CULTURED IN FLOATING NET-CAGES
SENG-KEH TENG and THIA-ENG CHUA School of Biological Sciences,
Universiti Sains Malaysia, Penang (Malaysia)
(Received 21 March 1978; revised 28 August 1978)
ABSTRACT Teng, S.K. and Chua, T.E., 1978. Effect of stocking density on the growth of estuary grouper, Epinephelus salmoides Maxwell, cultured in floating net-cages. Aquaculture, 15: 273-287. Studies on the effect of stocking density on the growth of estuary grouper (Epinephelus salmoides ) were conducted in floating net-cages. Four stocking densities (15, 30, 60 and 120 fish/m3) and two sizes of fish (26 f 0.2 gm and 15.2 f 0.1 gm) were studied. Results of the present study indicate that fish at the stocking density of 60 fish/m3 grew equally fast and showed comparable food conversion ratio and survival rate as those at lower stocking densities of 15 and 30 fish/m3. The net-yield of fish at a stocking density of 60 fish/m’ was 1.9 and 3.5 times that of fish at densities of 30 and 15 fish/m’, respectively. However, at 120 fish/m’, a remarkable decrease in the weight-gain per fish, mean fish weight, efficiency of food conversion as well as survival rate was recorded. The possible effects of stocking density on the growth of estuary grouper under cageculture conditions were discussed.
INTRODUCTION
The culture of marine fish in floating net-cages was started in 1954 by the Japanese for rearing yellowtail (Seriolu quinquerudiata) in the Inland Sea of Japan (Harada, 1970). During the last decade, this method of marine fish culture has been improved and widely used for salmon culture in North America and Norway, for plaice culture in Britain, and for culturing the seawater-acclimatized rainbow trout in Scotland and Australia (Mime, 1972). Fish culture in floating net-cages was introduced to Malaysia in 1973 for rearing the estuary grouper, Epinephelus salmoides and over the last few years, this method has proven to be technically feasible and commercially viable (Chua, 1973; Chua and Teng, 1977). One of the many advantages of rearing fish in floating net-cages is that fish could be stocked at a much higher density compared to other forms of fish culture (Cache, 1976).
274
High stocking density of fish in floating net-cages in coastal waters or rivers is made possible through the flow of water current which brings an adequate supply of dissolved oxygen to the cages and constantly carries away the metabolic waste of the fish, thus reducing the ill-effects of over-crowding (Hickling, 1962; Bardach et al., 1972; MacCrimmon et al., 1974). Optimum stocking density of marine fish in floating net-cages varies with the species, locality of culture, size of fish initially stocked and size and shape of net-cages. In yellowtail culture, the optimum stocking density in cages was found to be 80-200 fish/m3 with size of fish at 5-10 cm in total length (Fujiya, 1976). In marine flatfish, the maximum production density had not been reached when juveniles at 5.0 cm in length were stocked up to 240 fish/m3 (Howard, 1974). The best stocking density for culturing puffer fish in net-cages in Japan was at 0.08-0.5 kg/m3 with fish around 2.5 cm long (Bardach et al., 1972). For Atlantic salmon (Salmo sular), best growth was obtained by stocking the smolts at density of 10.0 kg/m3 in floating cages (Howard, 1974), whilst for striped bass (Morone saxatilis) fingerlings (average weight 131.5-145.2 gm) cultured in cages in brackish water were stocked as high as 300 fish or more per cubic yard (Powell, 1972). The present study was intended to investigate the optimum stocking rate for the culture of young estuary grouper in floating net-cages in the coastal waters of Malaysia. MATERIALS AND METHODS
Experimental fish The estuary groupers used for the present study were collected from a shallow bank in the Straits of Penang, Malaysia. Notes of their ecology and seasons of abundance are given by Ch.ua and Teng (1977). Culture cages The net-cages used for these experiments were suspended from a floating wooden framework of size 10 X 10 m. Each net-cage of size 1.50 X 1.50 X 1.65 m was made of polyethylene netting of mesh size about 12.5 mm. The volume of the net-cage, which was always submerged in water, was measured at 3.33 cubic meters, which was used for the calculation of the initial stocking density of the fish. The cages were suspended in the Western Channel of Penang Straits. Tidal currents facilitate a free exchange of water in and out of the cages. The general layout and maintenance of the cages were described by Chua and Teng (1977) and Teng et al. (1977). Food and methods of feeding Trash fish of anchovy
(Engraulis), sciaenids (Pseudosciaena) and small
275
carrangids (Selaroides) were chopped into small pieces of approximately 13 mm and fed to the fish in the net-cages. Groupers are carnivorous fish and they soon learn to feed on the chopped trash fish in captivity. The experimental fish were fed to satiation in each feeding. Satiation is said to be reached when excess food given is not taken within 10-15 min. Food was given to the fish at a specific corner of the net-cage where a fine-meshed liftnet was placed. Uneaten food was thus collected by this lift-net and weighed and the difference between initial weight of food and the weight of uneaten food was taken as the quantity eaten by the fish. Environmental conditions Water samples were collected at fortnightly intervals inside and outside the cages for determination of some basic environmental conditions during the course of the study. Salinity was determined by a portable Beckman’s conductive salinometer; pH by a battery-operated Metrohm Hersan’s pH meter; air and water temperatures by means of a mercury thermometer, whilst the standard Winkler’s method was used for measuring the dissolved oxygen content (Strickland and Parsons, 1972). Data analysis The data collected were analysed for calculations of the weight-gain per fish, net-yield, mean fish weight, food conversion ratio, condition factor and survival rate of the fish. These terms were defined as follows: (1) Weight-gain per fish: w, - w, (gm) where, wt = mean weight of fish at t days; w, = initial mean weight of fish. (2) Net-yield:
(IV, - W,) / 3.33 (kg/m3)
where, Wt = total weight of fish survived in a net-cage at t days; W0 = initial total weight of fish in a net-cage. (3.33 is the rearing capacity in cubic meters of each net-cage used in the present experiments.) (3) Mean fish weight: the average weight of fish at t days. (4) Food conversion the fish. (5) Condition
factor:
ratio: wet weight of food eaten / wet weight gained by %t/zf
X 1000
where & = mean fish length at t days. The ‘cube law’ of the length in this calculation was based on a formula of length-weight relationship of young estuary groupers caught from the same locality as the fish used in the experiments were collected. The formula is
276
expressed
as follows:
W = 0.01472
L2-98418
N = 334, size range = 35-285
mm in total length (Teng and Chua, 1978).
(6) Survival rate: Nt/iV,-, X 100 (%) where, Nt = total number
of fish at t days; N,, = initial total number
of fish.
Statistical analysis of the data Analysis 1969) was parameters 1972) was parameters
of variance (two-way ANOVA without replication, Sokal and Rohlf, used to test the effect of stocking density on the various growth in both Experiments A and B. Duncan’s Multiple Range Test (Vann, employed to compare the significance of the means of the growth among the stocking densities tested.
Experimental design The fish used for the present study were acclimatized in the net-cages for a few weeks until they were able to take in sliced trash fish voluntarily. Details of the experimental design are shown in Table I. Four stocking densities and two sizes of fish were studied. In Experiment A, the average size of fish ranged from 12.4 to 13.0 cm in total length and 26.3 to 26.8 gm in weight while in TABLE I Experimental
design, initial stocking density, initial biomass and size of fish stocked Initial stocking density Total no. of fish stocked per net-cage*
Initial stocking density (fish/m3)
Experiment A
50 100 200 400
15 30 60 120
Experiment
50 100 200 400
15 30 60 120
B
Initial biomass stocked ( kg/m3 )
Size of fish stocked Mean length (cm)
S.D.**
Mean weight (cm)
SD.*+
0.40 0.80 1.60 3.16
13.0 13.0 12.6 12.4
1.5 1.3 1.7 1.6
26.8 26.6 26.6 26.3
13.6 10.1 10.0 11.4
0.23 0.45 0.92 1.83
10.3 10.3 10.1 10.0
0.9 1.0 1.8 1.8
15.3 15.0 15.3 15.2
4.4 4.5 5.9 4.8
*Volume of the net-cage submerged in water = 3.33 m3. ** Standard deviation.
277
Experiment B, much smaller fish with average length ranging from 10.0 to 10.3 cm and mean weight from 15.0 to 15.3 gm were used. These two sizegroups were chosen because they were the two initial sizes of young groupers commonly used by the fish farmers in Penang for stocking and fattening to marketable size. Both Experiments A and B were carried out and terminated at the same time (21 December 1975 to 29 February 1976). Stocking density was calculated as the number of fish per cubic meter of cage capacity. The total length (to the nearest 1 mm) and weight (to the nearest 0.1 gm) of the fish were measured fortnightly over a period of 70 days. The fish were fed once daily in the evening (around 6-7 p.m.) which had earlier been found to be the most active feeding period of the day. RESULTS
Mean fish weight The mean fish weights at stocking densities of 15, 30, and 60 fish/m3 were significantly higher than those at a stocking density of 120 fish/m’ (P GO.01, Fig. 1 and Table II). The differences amongst the mean fish weights at stocking densities of 15, 30 and 60 fish/m3 for both experiments are relatively small (Table II) and statistically insignificant (P > 0.05). Weigh t-gain per fish When the weight-gains per fish for the four different stocking densities were compared, they could be statistically segregated into two groups (Fig. 2A and B). In Experiment A, the differences amongst the weight-gains at stocking densities of 15, 30 and 60 fish/m3 were statistically insignificant from one another (P 2 0.05), while the weight-gain at stocking density of 120 fish/m3 was remarkably lower than any other stocking densities tested (P < 0.01). In
TABLE II Mean fish weights of the estuary groupers
after 28 and 70 days of culture at four different stocking densities
Stocking density
Mean fish weight (pm)*
(fish/m’
Experiment A
)
Initial 15 30 60 120
26.8 26.6 26.6 26.3
*t Standard deviation.
* ? t ?
13.6 10.1 10.0 11.4
Experiment -. 28 days
70 days
64.8 63.5 58.7 45.6
155.4 146.7 135.2 97.6
f + + *
25.3 21.0 14.3 21.5
t t t t
36.9 31.8 27.0 36.8
B
Initial
28 days
70 days
15.3 15.0 15.3 15.2
40.5 37.8 34.1 26.0
112.9 106.2 101.5 71.5
t 4.4 f 4.5 L 5.9 t 4.8
t t f t
18.8 12.2 14.5 19.2
f 2 i ?
29.5 22.8 20.8 35.0
278
I
0
I4
PO
42
Time
I@
TO
(days1
(A) Experiment
B
I
0
I4
48
18
Time
I*
TO
(days)
(6)
Fig. 1. Mean fish weights of estuary groupers at four different stocking densities plotted against the period of culture for Experiments A and B.
279
Experiment
A
42
limo
(doyd (A)
Experiment
B
141 -E ,o =
I20
.Y L IOC 0 0.
*
IO 20
so
,/;
/ +/ / / ,-
D
I4
20
42
Tim0
50
70
(day*) (8)
Fig. 2. Weight gains per fish for estuary groupers reared at four different stocking densities and plotted against the time of culture in both Experiments A and B.
280
Experiment B, the test also yielded similar results. At the end of both experiments the weight-gain per fish at stocking density of 120 fish/m3 was 50-80s lower than those at other stocking densities tested in Experiment A, but 40-60% in Experiment B. Net-yield or net-production of fish In this study, the term net-yield was expressed as kilograms of fish weight per cubic meter of the rearing capacity of the net-cage (Cache, 1976). It accounts for the actual production of fish weight at a given time. Net-yields of fish for all the stocking densities tested and plotted against the days of culture are shown in Fig. 3 for Experiment A, and Fig. 4 for Experiment B. In both Experiments A and B, net-yields increased steadily with the increase of the stocking density up to the level of 60 fish/m3. With further increase in the stocking density from 60 to 120 fish/m3, the increase in the net-yield is not statistically significant (P > 0.05). At the end of Experiment A, the netyield was 6.16 kg/m3 at a stocking density of 60 fish/m3, 1.84 kg/m3 at 15 fish/m3 and 3.29 kg/m3 at 30 fish/m3. The increase in net-production at stocking density of 60 fish/m3 was 335% over that at 15 fish/m3 and 187% over that at 30 fish/m3. In Experiment B, the respective increases in percentage were 363 and 190%. The net-yield for stocking density of 60 fish/m3 was recorded to be 4.83 kg/m3 and those for the stocking densities of 15 and 30 fish/m’ were 1.33 and 2.54 kg/m3, respectively. Food conversion ratio The food conversion ratios varied from 3.49 to 4.03 in Experiment A and from 3.42 to 4.35 in Experiment B for stocking density between 15 and 60 fish/m3 (Figs 3 and 4). In neither experiment did fish at stocking densities of 15 and 60 fish/m3 show much difference in their efficiency in food conversion (P > 0.05). At stocking density higher than 60 fish/m3, the efficiency of conversion was correspondingly reduced. At a stocking density of 120 fish/m3, the food conversion ratios varied from 5.03 to 5.83 in Experiment A, and from 5.40 to 6.06 in Experiment B. These ratios are significantly higher than those at lower stocking densities (P < 0.01). Survival rate Survival rates of fish during the period of culture were shown in Fig. 3 for Experiment A and Fig. 4 for Experiment B. In both Experiments A and B, fish at stocking density of 120 fish/m3 suffered the highest mortality and a much lower survival rate was recorded. The survival rates of fish stocked at 15, 30 and 60 fish/m3 were not significantly different from one another (P 2 0.05). At the end of Experiment A, survival rates of fish stocked at densities of 15
281
Inlt~ol
3
dsnrlty
II-.
I0
Experiment
A
0-_--J
30
.
I.0 b-b
D
E 0
stocking
Ulsh/m’l
0.0
60
.-.
I20
4.0
E D. L
2.0
.
: O.Or
1
’
0
1
I4
20
Time
42
I
66
TO
(days)
Fig. 3. Experiment A: net yield, food conversion ratio, condition factor and survival rate for estuary groupers maintained at four different stocking densities.
to 60 fish/m3 varied from 93.0 to 96.0% and 88.8% at a stocking density of 120 fish/m3. The respective percentages of survival at the end of Experiment B were 94.0 to 94.5%, for stocking rates between 15 and 60 fish/m3 and 83.0% for stocking density of 120 fish/m3.
282
0.0 a.0 E ‘ii 5 4.0 z 0 0 .P ,fe.o 8 IA. s.0 t z
l-./* l-----.
100 -
=>‘; l O. >‘5 5 a0 v)
- .-.
Fig. 4. Experiment B: net-yield food conversion ratio, condition factor and survival rate for estuary groupers maintained at four different stocking densities.
Condition factor The condition factor of the cultured fish varied slightly among the different stocking densities for both Experiments A and B (Figs 3 and 4). Statistical test indicates no significant differences in the condition factors among the stocking densities tested for either Experiment A or B (P > 0.05)
Analysis of variance Analysis of variance of the effects of stocking density on the various main growth parameters in both Experiments A and B showed that stocking density exerted significant effect (P < 0.01) on the net-yield, mean fish weight, weight-gain per fish, food conversion ratio as well as the survival rate. However there is no significant effect on the condition factor of the fish for either experiment (Table III). Environmental conditions During the period of the study, air temperature of the culture site varied from 28.5 to 31.9”C. Outside the cages, the water temperature ranged from 28.8 to 32.O”C, salinity from 29.63 to 31.27 p.p.t., dissolved oxygen content from 4.31 to 5.61 cc/l, and pH from 7.9 to 8.3. Within the cages, the water temperature ranged from 29.0 to 31.5”C, salinity from 28.56 to 31.42 p.p.t., dissolved oxygen content from 4.10 to 5.34 cc/l, pH from 7.8 to 8.2 (Table IV). As shown in Table IV, the variations of these physical and chemical parameters of the water inside and outside the net-cages were small and insignificant. Thus, the environmental conditions in the culture site during the period of this study were found to be relatively constant. DISCUSSION
Growth and production of fish are, to a certain extent, dependent on the population density (Let Cren, 1965; Backiel and Le Cren, 1967). The harmful effects that higher stocking density had on the culture of fish were the reduction of growth rate, increase of food conversion ratio and lowering of survival rate (Powell, 1972). In the present study, fish stocked at a density of 120 fish/m’ were found to show a significant decrease in the mean fish weight, weight-gain per fish, efficiency of food conversion as well as survival rate when compared to those at the lower stocking densities of 15-60 fish/m3. TABLE
III
Analysis of variance (F-values) of the effect of stocking density on the various growth parameters groupers cultured in floating net-cages
for estuary
Net-yield
Mean fish weight
Weight-gain per fish
Food conversion ratio
Survival rate
Condition factor
Experiment A : Stocking density
8.48**
10.66”1
16.75**
106.11**
27.67’*
2.31 (N.S.)
Experiment B: Stocking density
6.82**
13.52**
107.90**
13.74**
2.72 (N.S.)
**P < 0.01;
N.S. = not significant.
9.438 *
284
Higher stocking density increases crowding effects of fish (Brown, 1946b, 1957). Overcrowding could easily induce such association as ‘size hierarchies’ within a fish population. Once a size hierarchy within a population is established, the smaller fish are inhibited from feeding satisfactorily because of the physical presence of larger individuals (Brown, 1957). Weatherley (1966) stated that, generally, the intensity of competition for food is directly related to the population density. Within a population density, a larger fish eats more food than a smaller one and is therefore a more ‘effective’ competitor by a factor related to its size. Beverton and Holt (1957) also found that the grazing power of fish is proportional to their size. In floating cage culture where fish are stocked in a restricted culture space, the effect of crowding becomes distinct and competition for food and space increases directly with the stocking density. Dominance of feeding and space by a few larger fish in a crowded culture cage may prevent others from feeding adequately while at the same time they have to spend more energy to counter-act the water current that constantly flows through the cage. Miller (1958) suggests that such a mechanism can cause the death of trout by acidosis from lactate build-up or by starvation. These behaviour patterns of fish may account for the reduced growth and higher mortality at the higher stocking density of 120 fish/m3 observed in the present study. Although overcrowding may depress the growth of fish, an optimum degree of grouping exists among fish populations. Such a grouping effect was observed in the present study, in which the fish at the lower stocking densities of 15-30 fish/m3 did not appear to show significantly better growth than fish stocked at a higher density of 60 fish/m3. However, when the stocking density was further increased from 60 to 120 fish/m3, growth of fish was significantly depressed. Fish at stocking density of 60 fish/m3 increased weight by an average of 1.9 and 3.5 times that of fish at densities of 30 and 15 fish/m3, respectively. Further increase of stocking density to 120 fish/m3 did not significantly increase the net-yield or net-production. In fish culture practices, best growth rate and higher production are usually attained at a particular stocking density, beyond which the growth rate and production are considerably reduced and below which the fish do not grow as well as or better than those at the optimal stocking rate (Walter, 1934, cited by Backiel and Le Cren, 1967; Brown, 1946a). This grouping effect in fish has also been noted by many past studies (Andrews et al., 1971; Collins, 1972; Cache, 1976; Kilambi et al., 1977). Weatherley (1976) stated that there is no general certainty that fish will grow more rapidly the lower the population density or intensity of competition for food. Many fish are social creatures and manifest maximum growth rate in the presence of other individuals in numbers which are optimal for growth. From the view point of net-production of estuary-groupers in the present study, the optimum stocking density to produce maximum yield with minimum cost is at 60 fish/m3. The insignificant fluctuation of all the main hydrological parameters in the culture site indicates a constant environmental condition during the period of the study and it does not appear to contribute any limiting effect on the stocking densities tested.
PH
Dissolved oxygen content (cc/I)
Salinity (%,)
Water temperature (“C)
B (8.0::)
(7.9z.l)
4.73 (4.46-4.86)
B
A
4.71 (4.45-5.01)
A
29.60 (28.56-31.20)
B
30.2 -31.0)
30.5 -31.5)
29.85 (28.61-31.31)
(29.0
(29.0
15
30.3 -31.5)
30.4 -31.0)
8.1 (8.0-8.2)
(8.0%)
4.61 (4.46-4.76)
4.78 (4.56-5.34)
29.72 (28.96-31.21)
29.94 (28.97-31.40)
(29.4
(29.2
30
Stocking density (fish/ma)
Inside the net-cages
A
B
A
Experiment
30.9 -31.6)
30.6 -31.3)
(8.0::)
8.05 (7.9-8.2)
4.43 (4.11-4.57)
4.58 (4.38-5.16)
29.84 (29.00-31.42)
29.86 (28.87-31.32)
(30.5
(29.3
60
29.90
30.9 -31.4)
30.6 -31.4)
(7.8-t:)
(7.9%)
4.27 (4.10-4.39)
4.24 (3.89-4.89)
29.86 (29.00-31.37)
(28.76-31.37)
(30.5
(29.4
120
30.6 -32.0)
(7.9~t.13,
4.79 (4.31-5.61)
30.56 (29.63-31.27)
(28.8
Outside the net-cages
Changes in the water temperature, salinity, dissolved oxygen content and pH inside and outside the net-cages used for both Experiments A and B. Each value is the mean of five measurements taken fortnightly over the period of the experiments (21 December 1975-29 February 1976). Values in brackets indicate the ranges
TABLE IV
z ol
286
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to the International Foundation for Sciences (IFS), Sweden and the Universiti Sains Malaysia for providing financial assistance for research on the culture of estuary grouper in floating net-cages. Thanks are also due to Mr Lim Tack Khoon, owner of the present fish farm, for his permission to use some of his facilities in the farm, to Mr Ong Liang Seng, and to Miss Lok Hong Sin for typing the manuscript.
REFERENCES Andrews, J.W., Lee, H.K. and Page, J.W., 1971. Interactions of stocking density and water turnover on growth and food conversion of channel catfish reared in intensively stocked tanks. Prog. Fish Cult., 33 (4): 197-203. Backiel, T. and Le Cren, E.D., 1967. Some density relationships for fish population parameters. In: S.D. Gerking (Editor), The Biological Basis of Freshwater Fish Production. Blackwell Scientific Publications, Oxford and Edinburgh, pp. 261-293. Bardach, J.E., Ryther, J.H. and McLarney, W.O., 1972. Aquaculture -The Farming and Husbandry of Freshwater and Marine Organisms. Wiley-Interscience, New York, N.Y., London, 868 pp. Beverton, R.J.H. and Holt, S.J., 1957. On the Dynamics of Exploited Fish Populations. HMSO, London, 1957, 533 pp. Brown, M.E., 1946a. The growth of brown trout (Salmo trutta Linn). I. Factors influencing the growth of trout fry. J. Exp. Biol., 22: 118-129. Brown, M.E., 194613. The growth of brown trout (Salmo trutta Linn). II. The growth of two-year-old trout at a constant temperature of 11.5”C. J. Exp. Biol., 22: 130-144. Brown, M.E., 1957. Experimental studies on growth. In: M.E. Brown (Editor), The Physiology of Fishes. Academic Press, New York, N.Y., Vol. 1, pp. 361-400. Chua, T.E., 1973. Grouper Culture in Penang, West Malaysia. IDRC Aquaculture Seminar for Southeast Asia, Malacca, West Malaysia; Contribution Paper No. 16, 7 pp. Chua, T.E. and Teng, S.K., 1977. Floating Fishpens for Rearing Fishes in Coastal Waters, Reservoirs and Mining Pools in Malaysia. Ministry of Agriculture, Malaysia, Fish. Bull. No. 20, 36 pp. Cache, G., 1976. A General Review of Cage Culture and its Application in Africa. FAO Technical Conference on Aquaculture, Kyoto, Japan 26 May-2 June 1976, Tech. Pap. No. FIR: AQ/Conf/76/E. 72, 33 pp. Collins, R.A., 1972. Cage culture of trout in warmwater lakes. Am. Fish Farmer, 3 (7): 4-7. Fujiya, M., 1976. Yellowtail (Seriola quinqueradiata) farming in Japan. J. Fish Res. Board Can., 33: 911-915. Harada, T., 1970. The present status of marine cultivation research in Japan. Helgol. Wiss. Meeresunters., 20: 594-601. Hickling, C.F., 1962. Fish Culture. Faber and Faber, London, 295 pp. Howard, K.T., 1974. U.K. trials on rearing systems for marine flatfish cultivation. Fish Farming Int., 3 (3): 22-36. Kilambi, R.V., Adams, J.C., Brown, A.V. and Wickizer, W.A., 1977. Effects of stocking density and cage size on growth, feed conversion and production of rainbow trout and channel catfish. Prog. Fish Cult., 39 (2): 62-66. Le Cren, E.D., 1965. Some factors regulating the size of populations of freshwater fish. Mitt. Int. Ver. Theor. Angew. Limnol., 13: 88-105. MacCrimmon, J.Z., Stewart, J.E. and Brett, J.R., 1974. Aquaculture in Canada: The Practice and the Promise. Bull. 188, Fish. Res. Board Can., Ottawa, 1974, 84 pp.
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Miller, R.B., 1958. The role of competition in the mortality of hatchery trout. J. Fish Res. Board Can., 15: 27-45. Milne, P.H., 1972. Fish and Shellfish Farming in Coastal Waters. Fishing News (Books) Ltd., London, 208 pp. Powell, M.R., 1972. Cage and raceway culture of striped bass in brackish water in Alabama. Proc. 26 Annu. Conf. Southeast. Assoc. Game Fish Comm., 1972, pp. 553-565. Sokal, R.R. and Rohlf, F.J., 1969. Biometry - The Principle and Practice of Statistics in Biological Research. W.H. Freeman and Company, San Francisco, Calif., 776 pp. Strickland, J.D.H. and Parsons, T.R., 1972. A Practical Handbook of Seawater Analysis. Fish. Res. Board Can., Ottawa, Bull. 167 (2nd ed.), 310 pp. Teng Seng-Keh and Chua Thia-Eng, 1978. Length-weight relationship and growth of young estuary grouper, (Epinephelus salmoides Maxwell) caught from the Middle Bank, Penang Straits. ;School of Biological Sciences, Universiti Sains Malaysia, Penang, Technical Report USM/IFS/CTE No. 26,15 pp. Teng Seng-Keh, Chua Thia-Eng and Lai Hoi-Chaw, 1977. Construction and Management of Floating Net-cages for Culturing the Estuary Grouper, Epinephelus salmoides Maxwell in Penang, Malaysia. Joint SCSP/SEAFDEC Workshop on Aquaculture Engineering, Hoilo, Philippines, 27 November-3 December 1977, Contributed Paper SCSPSFDC/77/AEn/CP 33, 16 pp. Vann, E., 1972. Fundamentals of Biostatistics. D.C. Heath and Company, London, 184 pp. Weatherley, A.H., 1966. Ecology of fish growth. Nature, 212 (II): 1321-1324. Weatherley, A.H., 1976. Factors affecting maximization of fish growth. J. Fish Res. Board Can., 33: 1046-1058.
273
EFFECT OF STOCKING DENSITY ON THE GROWTH OF ESTUARY GROUPER, EPINEPHELUS SALMOIDES MAXWELL, CULTURED IN FLOATING NET-CAGES
SENG-KEH TENG and THIA-ENG CHUA School of Biological Sciences,
Universiti Sains Malaysia, Penang (Malaysia)
(Received 21 March 1978; revised 28 August 1978)
ABSTRACT Teng, S.K. and Chua, T.E., 1978. Effect of stocking density on the growth of estuary grouper, Epinephelus salmoides Maxwell, cultured in floating net-cages. Aquaculture, 15: 273-287. Studies on the effect of stocking density on the growth of estuary grouper (Epinephelus salmoides ) were conducted in floating net-cages. Four stocking densities (15, 30, 60 and 120 fish/m3) and two sizes of fish (26 f 0.2 gm and 15.2 f 0.1 gm) were studied. Results of the present study indicate that fish at the stocking density of 60 fish/m3 grew equally fast and showed comparable food conversion ratio and survival rate as those at lower stocking densities of 15 and 30 fish/m3. The net-yield of fish at a stocking density of 60 fish/m’ was 1.9 and 3.5 times that of fish at densities of 30 and 15 fish/m’, respectively. However, at 120 fish/m’, a remarkable decrease in the weight-gain per fish, mean fish weight, efficiency of food conversion as well as survival rate was recorded. The possible effects of stocking density on the growth of estuary grouper under cageculture conditions were discussed.
INTRODUCTION
The culture of marine fish in floating net-cages was started in 1954 by the Japanese for rearing yellowtail (Seriolu quinquerudiata) in the Inland Sea of Japan (Harada, 1970). During the last decade, this method of marine fish culture has been improved and widely used for salmon culture in North America and Norway, for plaice culture in Britain, and for culturing the seawater-acclimatized rainbow trout in Scotland and Australia (Mime, 1972). Fish culture in floating net-cages was introduced to Malaysia in 1973 for rearing the estuary grouper, Epinephelus salmoides and over the last few years, this method has proven to be technically feasible and commercially viable (Chua, 1973; Chua and Teng, 1977). One of the many advantages of rearing fish in floating net-cages is that fish could be stocked at a much higher density compared to other forms of fish culture (Cache, 1976).
274
High stocking density of fish in floating net-cages in coastal waters or rivers is made possible through the flow of water current which brings an adequate supply of dissolved oxygen to the cages and constantly carries away the metabolic waste of the fish, thus reducing the ill-effects of over-crowding (Hickling, 1962; Bardach et al., 1972; MacCrimmon et al., 1974). Optimum stocking density of marine fish in floating net-cages varies with the species, locality of culture, size of fish initially stocked and size and shape of net-cages. In yellowtail culture, the optimum stocking density in cages was found to be 80-200 fish/m3 with size of fish at 5-10 cm in total length (Fujiya, 1976). In marine flatfish, the maximum production density had not been reached when juveniles at 5.0 cm in length were stocked up to 240 fish/m3 (Howard, 1974). The best stocking density for culturing puffer fish in net-cages in Japan was at 0.08-0.5 kg/m3 with fish around 2.5 cm long (Bardach et al., 1972). For Atlantic salmon (Salmo sular), best growth was obtained by stocking the smolts at density of 10.0 kg/m3 in floating cages (Howard, 1974), whilst for striped bass (Morone saxatilis) fingerlings (average weight 131.5-145.2 gm) cultured in cages in brackish water were stocked as high as 300 fish or more per cubic yard (Powell, 1972). The present study was intended to investigate the optimum stocking rate for the culture of young estuary grouper in floating net-cages in the coastal waters of Malaysia. MATERIALS AND METHODS
Experimental fish The estuary groupers used for the present study were collected from a shallow bank in the Straits of Penang, Malaysia. Notes of their ecology and seasons of abundance are given by Ch.ua and Teng (1977). Culture cages The net-cages used for these experiments were suspended from a floating wooden framework of size 10 X 10 m. Each net-cage of size 1.50 X 1.50 X 1.65 m was made of polyethylene netting of mesh size about 12.5 mm. The volume of the net-cage, which was always submerged in water, was measured at 3.33 cubic meters, which was used for the calculation of the initial stocking density of the fish. The cages were suspended in the Western Channel of Penang Straits. Tidal currents facilitate a free exchange of water in and out of the cages. The general layout and maintenance of the cages were described by Chua and Teng (1977) and Teng et al. (1977). Food and methods of feeding Trash fish of anchovy
(Engraulis), sciaenids (Pseudosciaena) and small
275
carrangids (Selaroides) were chopped into small pieces of approximately 13 mm and fed to the fish in the net-cages. Groupers are carnivorous fish and they soon learn to feed on the chopped trash fish in captivity. The experimental fish were fed to satiation in each feeding. Satiation is said to be reached when excess food given is not taken within 10-15 min. Food was given to the fish at a specific corner of the net-cage where a fine-meshed liftnet was placed. Uneaten food was thus collected by this lift-net and weighed and the difference between initial weight of food and the weight of uneaten food was taken as the quantity eaten by the fish. Environmental conditions Water samples were collected at fortnightly intervals inside and outside the cages for determination of some basic environmental conditions during the course of the study. Salinity was determined by a portable Beckman’s conductive salinometer; pH by a battery-operated Metrohm Hersan’s pH meter; air and water temperatures by means of a mercury thermometer, whilst the standard Winkler’s method was used for measuring the dissolved oxygen content (Strickland and Parsons, 1972). Data analysis The data collected were analysed for calculations of the weight-gain per fish, net-yield, mean fish weight, food conversion ratio, condition factor and survival rate of the fish. These terms were defined as follows: (1) Weight-gain per fish: w, - w, (gm) where, wt = mean weight of fish at t days; w, = initial mean weight of fish. (2) Net-yield:
(IV, - W,) / 3.33 (kg/m3)
where, Wt = total weight of fish survived in a net-cage at t days; W0 = initial total weight of fish in a net-cage. (3.33 is the rearing capacity in cubic meters of each net-cage used in the present experiments.) (3) Mean fish weight: the average weight of fish at t days. (4) Food conversion the fish. (5) Condition
factor:
ratio: wet weight of food eaten / wet weight gained by %t/zf
X 1000
where & = mean fish length at t days. The ‘cube law’ of the length in this calculation was based on a formula of length-weight relationship of young estuary groupers caught from the same locality as the fish used in the experiments were collected. The formula is
276
expressed
as follows:
W = 0.01472
L2-98418
N = 334, size range = 35-285
mm in total length (Teng and Chua, 1978).
(6) Survival rate: Nt/iV,-, X 100 (%) where, Nt = total number
of fish at t days; N,, = initial total number
of fish.
Statistical analysis of the data Analysis 1969) was parameters 1972) was parameters
of variance (two-way ANOVA without replication, Sokal and Rohlf, used to test the effect of stocking density on the various growth in both Experiments A and B. Duncan’s Multiple Range Test (Vann, employed to compare the significance of the means of the growth among the stocking densities tested.
Experimental design The fish used for the present study were acclimatized in the net-cages for a few weeks until they were able to take in sliced trash fish voluntarily. Details of the experimental design are shown in Table I. Four stocking densities and two sizes of fish were studied. In Experiment A, the average size of fish ranged from 12.4 to 13.0 cm in total length and 26.3 to 26.8 gm in weight while in TABLE I Experimental
design, initial stocking density, initial biomass and size of fish stocked Initial stocking density Total no. of fish stocked per net-cage*
Initial stocking density (fish/m3)
Experiment A
50 100 200 400
15 30 60 120
Experiment
50 100 200 400
15 30 60 120
B
Initial biomass stocked ( kg/m3 )
Size of fish stocked Mean length (cm)
S.D.**
Mean weight (cm)
SD.*+
0.40 0.80 1.60 3.16
13.0 13.0 12.6 12.4
1.5 1.3 1.7 1.6
26.8 26.6 26.6 26.3
13.6 10.1 10.0 11.4
0.23 0.45 0.92 1.83
10.3 10.3 10.1 10.0
0.9 1.0 1.8 1.8
15.3 15.0 15.3 15.2
4.4 4.5 5.9 4.8
*Volume of the net-cage submerged in water = 3.33 m3. ** Standard deviation.
277
Experiment B, much smaller fish with average length ranging from 10.0 to 10.3 cm and mean weight from 15.0 to 15.3 gm were used. These two sizegroups were chosen because they were the two initial sizes of young groupers commonly used by the fish farmers in Penang for stocking and fattening to marketable size. Both Experiments A and B were carried out and terminated at the same time (21 December 1975 to 29 February 1976). Stocking density was calculated as the number of fish per cubic meter of cage capacity. The total length (to the nearest 1 mm) and weight (to the nearest 0.1 gm) of the fish were measured fortnightly over a period of 70 days. The fish were fed once daily in the evening (around 6-7 p.m.) which had earlier been found to be the most active feeding period of the day. RESULTS
Mean fish weight The mean fish weights at stocking densities of 15, 30, and 60 fish/m3 were significantly higher than those at a stocking density of 120 fish/m’ (P GO.01, Fig. 1 and Table II). The differences amongst the mean fish weights at stocking densities of 15, 30 and 60 fish/m3 for both experiments are relatively small (Table II) and statistically insignificant (P > 0.05). Weigh t-gain per fish When the weight-gains per fish for the four different stocking densities were compared, they could be statistically segregated into two groups (Fig. 2A and B). In Experiment A, the differences amongst the weight-gains at stocking densities of 15, 30 and 60 fish/m3 were statistically insignificant from one another (P 2 0.05), while the weight-gain at stocking density of 120 fish/m3 was remarkably lower than any other stocking densities tested (P < 0.01). In
TABLE II Mean fish weights of the estuary groupers
after 28 and 70 days of culture at four different stocking densities
Stocking density
Mean fish weight (pm)*
(fish/m’
Experiment A
)
Initial 15 30 60 120
26.8 26.6 26.6 26.3
*t Standard deviation.
* ? t ?
13.6 10.1 10.0 11.4
Experiment -. 28 days
70 days
64.8 63.5 58.7 45.6
155.4 146.7 135.2 97.6
f + + *
25.3 21.0 14.3 21.5
t t t t
36.9 31.8 27.0 36.8
B
Initial
28 days
70 days
15.3 15.0 15.3 15.2
40.5 37.8 34.1 26.0
112.9 106.2 101.5 71.5
t 4.4 f 4.5 L 5.9 t 4.8
t t f t
18.8 12.2 14.5 19.2
f 2 i ?
29.5 22.8 20.8 35.0
278
I
0
I4
PO
42
Time
I@
TO
(days1
(A) Experiment
B
I
0
I4
48
18
Time
I*
TO
(days)
(6)
Fig. 1. Mean fish weights of estuary groupers at four different stocking densities plotted against the period of culture for Experiments A and B.
279
Experiment
A
42
limo
(doyd (A)
Experiment
B
141 -E ,o =
I20
.Y L IOC 0 0.
*
IO 20
so
,/;
/ +/ / / ,-
D
I4
20
42
Tim0
50
70
(day*) (8)
Fig. 2. Weight gains per fish for estuary groupers reared at four different stocking densities and plotted against the time of culture in both Experiments A and B.
280
Experiment B, the test also yielded similar results. At the end of both experiments the weight-gain per fish at stocking density of 120 fish/m3 was 50-80s lower than those at other stocking densities tested in Experiment A, but 40-60% in Experiment B. Net-yield or net-production of fish In this study, the term net-yield was expressed as kilograms of fish weight per cubic meter of the rearing capacity of the net-cage (Cache, 1976). It accounts for the actual production of fish weight at a given time. Net-yields of fish for all the stocking densities tested and plotted against the days of culture are shown in Fig. 3 for Experiment A, and Fig. 4 for Experiment B. In both Experiments A and B, net-yields increased steadily with the increase of the stocking density up to the level of 60 fish/m3. With further increase in the stocking density from 60 to 120 fish/m3, the increase in the net-yield is not statistically significant (P > 0.05). At the end of Experiment A, the netyield was 6.16 kg/m3 at a stocking density of 60 fish/m3, 1.84 kg/m3 at 15 fish/m3 and 3.29 kg/m3 at 30 fish/m3. The increase in net-production at stocking density of 60 fish/m3 was 335% over that at 15 fish/m3 and 187% over that at 30 fish/m3. In Experiment B, the respective increases in percentage were 363 and 190%. The net-yield for stocking density of 60 fish/m3 was recorded to be 4.83 kg/m3 and those for the stocking densities of 15 and 30 fish/m’ were 1.33 and 2.54 kg/m3, respectively. Food conversion ratio The food conversion ratios varied from 3.49 to 4.03 in Experiment A and from 3.42 to 4.35 in Experiment B for stocking density between 15 and 60 fish/m3 (Figs 3 and 4). In neither experiment did fish at stocking densities of 15 and 60 fish/m3 show much difference in their efficiency in food conversion (P > 0.05). At stocking density higher than 60 fish/m3, the efficiency of conversion was correspondingly reduced. At a stocking density of 120 fish/m3, the food conversion ratios varied from 5.03 to 5.83 in Experiment A, and from 5.40 to 6.06 in Experiment B. These ratios are significantly higher than those at lower stocking densities (P < 0.01). Survival rate Survival rates of fish during the period of culture were shown in Fig. 3 for Experiment A and Fig. 4 for Experiment B. In both Experiments A and B, fish at stocking density of 120 fish/m3 suffered the highest mortality and a much lower survival rate was recorded. The survival rates of fish stocked at 15, 30 and 60 fish/m3 were not significantly different from one another (P 2 0.05). At the end of Experiment A, survival rates of fish stocked at densities of 15
281
Inlt~ol
3
dsnrlty
II-.
I0
Experiment
A
0-_--J
30
.
I.0 b-b
D
E 0
stocking
Ulsh/m’l
0.0
60
.-.
I20
4.0
E D. L
2.0
.
: O.Or
1
’
0
1
I4
20
Time
42
I
66
TO
(days)
Fig. 3. Experiment A: net yield, food conversion ratio, condition factor and survival rate for estuary groupers maintained at four different stocking densities.
to 60 fish/m3 varied from 93.0 to 96.0% and 88.8% at a stocking density of 120 fish/m3. The respective percentages of survival at the end of Experiment B were 94.0 to 94.5%, for stocking rates between 15 and 60 fish/m3 and 83.0% for stocking density of 120 fish/m3.
282
0.0 a.0 E ‘ii 5 4.0 z 0 0 .P ,fe.o 8 IA. s.0 t z
l-./* l-----.
100 -
=>‘; l O. >‘5 5 a0 v)
- .-.
Fig. 4. Experiment B: net-yield food conversion ratio, condition factor and survival rate for estuary groupers maintained at four different stocking densities.
Condition factor The condition factor of the cultured fish varied slightly among the different stocking densities for both Experiments A and B (Figs 3 and 4). Statistical test indicates no significant differences in the condition factors among the stocking densities tested for either Experiment A or B (P > 0.05)
Analysis of variance Analysis of variance of the effects of stocking density on the various main growth parameters in both Experiments A and B showed that stocking density exerted significant effect (P < 0.01) on the net-yield, mean fish weight, weight-gain per fish, food conversion ratio as well as the survival rate. However there is no significant effect on the condition factor of the fish for either experiment (Table III). Environmental conditions During the period of the study, air temperature of the culture site varied from 28.5 to 31.9”C. Outside the cages, the water temperature ranged from 28.8 to 32.O”C, salinity from 29.63 to 31.27 p.p.t., dissolved oxygen content from 4.31 to 5.61 cc/l, and pH from 7.9 to 8.3. Within the cages, the water temperature ranged from 29.0 to 31.5”C, salinity from 28.56 to 31.42 p.p.t., dissolved oxygen content from 4.10 to 5.34 cc/l, pH from 7.8 to 8.2 (Table IV). As shown in Table IV, the variations of these physical and chemical parameters of the water inside and outside the net-cages were small and insignificant. Thus, the environmental conditions in the culture site during the period of this study were found to be relatively constant. DISCUSSION
Growth and production of fish are, to a certain extent, dependent on the population density (Let Cren, 1965; Backiel and Le Cren, 1967). The harmful effects that higher stocking density had on the culture of fish were the reduction of growth rate, increase of food conversion ratio and lowering of survival rate (Powell, 1972). In the present study, fish stocked at a density of 120 fish/m’ were found to show a significant decrease in the mean fish weight, weight-gain per fish, efficiency of food conversion as well as survival rate when compared to those at the lower stocking densities of 15-60 fish/m3. TABLE
III
Analysis of variance (F-values) of the effect of stocking density on the various growth parameters groupers cultured in floating net-cages
for estuary
Net-yield
Mean fish weight
Weight-gain per fish
Food conversion ratio
Survival rate
Condition factor
Experiment A : Stocking density
8.48**
10.66”1
16.75**
106.11**
27.67’*
2.31 (N.S.)
Experiment B: Stocking density
6.82**
13.52**
107.90**
13.74**
2.72 (N.S.)
**P < 0.01;
N.S. = not significant.
9.438 *
284
Higher stocking density increases crowding effects of fish (Brown, 1946b, 1957). Overcrowding could easily induce such association as ‘size hierarchies’ within a fish population. Once a size hierarchy within a population is established, the smaller fish are inhibited from feeding satisfactorily because of the physical presence of larger individuals (Brown, 1957). Weatherley (1966) stated that, generally, the intensity of competition for food is directly related to the population density. Within a population density, a larger fish eats more food than a smaller one and is therefore a more ‘effective’ competitor by a factor related to its size. Beverton and Holt (1957) also found that the grazing power of fish is proportional to their size. In floating cage culture where fish are stocked in a restricted culture space, the effect of crowding becomes distinct and competition for food and space increases directly with the stocking density. Dominance of feeding and space by a few larger fish in a crowded culture cage may prevent others from feeding adequately while at the same time they have to spend more energy to counter-act the water current that constantly flows through the cage. Miller (1958) suggests that such a mechanism can cause the death of trout by acidosis from lactate build-up or by starvation. These behaviour patterns of fish may account for the reduced growth and higher mortality at the higher stocking density of 120 fish/m3 observed in the present study. Although overcrowding may depress the growth of fish, an optimum degree of grouping exists among fish populations. Such a grouping effect was observed in the present study, in which the fish at the lower stocking densities of 15-30 fish/m3 did not appear to show significantly better growth than fish stocked at a higher density of 60 fish/m3. However, when the stocking density was further increased from 60 to 120 fish/m3, growth of fish was significantly depressed. Fish at stocking density of 60 fish/m3 increased weight by an average of 1.9 and 3.5 times that of fish at densities of 30 and 15 fish/m3, respectively. Further increase of stocking density to 120 fish/m3 did not significantly increase the net-yield or net-production. In fish culture practices, best growth rate and higher production are usually attained at a particular stocking density, beyond which the growth rate and production are considerably reduced and below which the fish do not grow as well as or better than those at the optimal stocking rate (Walter, 1934, cited by Backiel and Le Cren, 1967; Brown, 1946a). This grouping effect in fish has also been noted by many past studies (Andrews et al., 1971; Collins, 1972; Cache, 1976; Kilambi et al., 1977). Weatherley (1976) stated that there is no general certainty that fish will grow more rapidly the lower the population density or intensity of competition for food. Many fish are social creatures and manifest maximum growth rate in the presence of other individuals in numbers which are optimal for growth. From the view point of net-production of estuary-groupers in the present study, the optimum stocking density to produce maximum yield with minimum cost is at 60 fish/m3. The insignificant fluctuation of all the main hydrological parameters in the culture site indicates a constant environmental condition during the period of the study and it does not appear to contribute any limiting effect on the stocking densities tested.
PH
Dissolved oxygen content (cc/I)
Salinity (%,)
Water temperature (“C)
B (8.0::)
(7.9z.l)
4.73 (4.46-4.86)
B
A
4.71 (4.45-5.01)
A
29.60 (28.56-31.20)
B
30.2 -31.0)
30.5 -31.5)
29.85 (28.61-31.31)
(29.0
(29.0
15
30.3 -31.5)
30.4 -31.0)
8.1 (8.0-8.2)
(8.0%)
4.61 (4.46-4.76)
4.78 (4.56-5.34)
29.72 (28.96-31.21)
29.94 (28.97-31.40)
(29.4
(29.2
30
Stocking density (fish/ma)
Inside the net-cages
A
B
A
Experiment
30.9 -31.6)
30.6 -31.3)
(8.0::)
8.05 (7.9-8.2)
4.43 (4.11-4.57)
4.58 (4.38-5.16)
29.84 (29.00-31.42)
29.86 (28.87-31.32)
(30.5
(29.3
60
29.90
30.9 -31.4)
30.6 -31.4)
(7.8-t:)
(7.9%)
4.27 (4.10-4.39)
4.24 (3.89-4.89)
29.86 (29.00-31.37)
(28.76-31.37)
(30.5
(29.4
120
30.6 -32.0)
(7.9~t.13,
4.79 (4.31-5.61)
30.56 (29.63-31.27)
(28.8
Outside the net-cages
Changes in the water temperature, salinity, dissolved oxygen content and pH inside and outside the net-cages used for both Experiments A and B. Each value is the mean of five measurements taken fortnightly over the period of the experiments (21 December 1975-29 February 1976). Values in brackets indicate the ranges
TABLE IV
z ol
286
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to the International Foundation for Sciences (IFS), Sweden and the Universiti Sains Malaysia for providing financial assistance for research on the culture of estuary grouper in floating net-cages. Thanks are also due to Mr Lim Tack Khoon, owner of the present fish farm, for his permission to use some of his facilities in the farm, to Mr Ong Liang Seng, and to Miss Lok Hong Sin for typing the manuscript.
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