Idea Transcript
Bangladesh] Fish. Res., 10(1), 2006:13-24
Effects of socking density on ammonia excretion and the growth of Nile tilapia ( Oreochromis Niloticus L.) M.S. Ali 1' *, S.M. Stead and D.F. Houlihan Department of Zoology, University of Aberdeen, Aberdeen AB24 2TZ, Scotland, UK 1 Present address & corresponding author: Bangladesh Fisheries Research Institute, Freshwater Station, Mymensingh 2201, Bangladesh
Abstract The effects of stocking density (10, 15, 50 & 75 fish in 65L tank) and ammonia excretion on the growth of Nile tilapia, Oreochromis mloticus (12.19 ± 1.21 g) were investigated. Increasing stocking density of Nile tilapia from 15 fish/tank (2.81 g fish/L) to 75 fish/tank (14.07 g fish/L) resulted in associated increase in ammonia level (1.48 ± 0.87 mg/L to 26.44 ± 11.4 mg/L) and significantly lower growth rates. Significantly better feed conversion ratios were found for fish reared at lower (15 fish/tank) stocking densities compared to higher (75 fish/tank) stocking densities. Individual growth rates were significantly better for fish reared at a lower stocking density 15 fish/tank compared to higher stocking density 75 fish/tank and size variation (coefficient of variation in weight) were positively correlated with stocking density. Although water exchange did not have a significant effect on the growth of Nile tilapia for fish stocked at 10 fish/tank (1.88 g fish/L) and 50 fish/tank (9.38 g fish/L), however, the fish in the higher stocking density (9.38 g fish/L) groups and without water exchange, significantly changed the coloration of their bodies (silver to black) which may be due to the lower oxygen levels combined with higher ammonia levels. Ammonia level increased with increasing stocking density and without water exchange. In this study, it may be suggested that when fish reared at higher stocking densities then water exchange must be taken in to consideration so as to help avoid environmental and physiological stress to the fish. Key words: Nile tilapia, Stocking density, Ammonia excretion, Growth
Introduction In areas where suitable land and water are limiting for fish culture purposes and the market demand for fish is high, semi-intensive or intensive tank culture systems have been recommended (Sin and Chiu 1983) and found to be economically viable (Liao and Chen 1983). Intensive culture systems generally employ high stocking densities in order to maximise production with minimal water usage. High densities however, have implications for fish welfare. For example, in pond culture of tilapia where water can be stagnant, stocking densities up to 10 fishm 2 might be employed (Balarin and Haller 1982). Densities beyond 8 fish/m 2 have been found to be detrimental to fish due to the build-up of waste metabolites in the pond water (Zohar et al. 1984).
M.S. Ali et al.
High density culture of tilapia has been shown to be successful (Balarin and Haller 1982), but comparing results with studies conducted on tilapia maintained at lower stocking densities is difficult because individual studies do not address difficulties that arise when there are so many interactive factors involved. Horner et al. (1987) worked with juvenile and adult tilapia and reported that high densities (12 fish/L) fostered faster growth but with slightly greater variance than lower densities (1 fish/L). Stocking density and, therefore, the volume of water per fish is a significant factor in determining optimum production in tank culture systems. Increasing stocking density results in increased ammonia excretion and may lead to stress (Harris et al. 1998) which can result in enhanced energy requirements causing reduced growth rates and utilisation of food (Rusmussen and Korsgaard 1996). While a number of studies have examined growth, survival, and production of various tilapia species under different stocking densities (Suresh and Lin 1992; Siddiqui and Al-Barbi 1995), little information is available on the relationships between water quality such as dissolved oxygen and ammonia excretion with growth performance, stocking density, and size variation (i.e. social hierarchy). The objectives of this study were twofold. First, to assess the effects of stocking density on growth and size variation of Nile tilapia 0. niloticus. Second, to establish the relationships between stocking density and ammonia excretion by rearing tilapia at high densities with optimal water quality to test the hypothesis that high density would have a detrimental effect on growth but reduce the variance of size. Materials and methods
Experimental set up The experiment was conducted between August and November 2001 at the Department of Zoology, University of Aberdeen, Scotland, UK. Nile tilapia ( Oreochromis n1Joticus L.) fingerlings of the same age and size (12.19 ±1.21 g) were obtained from the Stirling University Aquaculture Unit, Stirling, Scotland, UK. Prior to the start of the experiments, all fish were reared in the freshwater unit (water temperature 27-30°C) in a large circular stock tank (water volume 150L) at the Department of Zoology, University of Aberdeen, Scotland, UK. Before conducting any experiments, the tanks (65L water volume) were prepared as the tank water passed through a box-type gravel clinoptilite external water filter. The water temperature was kept between 27 and 30°C, and the photoperiod was regulated by a time clock adjusted so that there was 12 hour light and 12 hour darkness.
Food and feeding The fish were fed a commercial pelleted diet (45% protein, oil 18%, ash 8.5%, fiber 2%; "Ewos" fish food company, UK). The dry weight of the feed was measured following the procedure described in AOAC (1983) and 1 g of wet feed corresponded to 0.9549 g of dry feed. The fish were fed at a ration level of 2.5% body weight once a day and everyday (7 days a week) between 9:00-9:30 h. The ration was adjusted after every sampling 14
Effects of socking density on ammonia excretion of Nile tilapia
occasion. The tanks were cleaned daily (where necessary) after one hour of feeding. Three different experiments were conducted to determine the optimum stocking density of juvenile Nile tilapia 0. mloticus under tank culture condition. The three experiments were as follows:
Experiment I: Effect of stocking density and ammonia excretion on growth of Nile tilapia when oxygen levels are different Experiment 2: Effects of stocking density and ammonia excretion on growth of Nile tilapia when oxygen levels are similar Experiment 3: Changes in individual growth performance and size variation of Nile tilapia under different stocking densities
Experiment I: The effect of stocking density was assessed under 2 varying conditions as uniform water exchange and without water exchange. Here note that, the oxygen levels of different tanks were not similar although oxygen supply was the same. The experiment was carried out between 6 August and 25 August 2001, for a duration of 20 days. Two stocking densities replicated twice with or without water exchange for this experiment. The experimental tanks were designated as: T 1: Lower density (10 fish/65L tank i.e. 1.88 g fish/L) with water exchange daily, T 2 : Lower density (10 fish/65L tank i.e. 1.88 g fish/L) with no water exchange, T 3 : Higher density (50 fish/65L tank i.e. 9.38 g fish/L) with water exchange daily and T 4 : Higher density (50 fish/65L tank i.e. 9.38 g fish/L) with no water exchange. Experiment 2: The effect of stocking density was assessed under 2 varying conditions as uniform water exchange and without water exchange. Here note that, oxygen level was similar in all tanks. This experiment was carried out between 7 September and 25 September 2001, for a duration of 19 days. The experimental protocol for this experiment was same as experiment 1 with the only difference being that the oxygen was maintained at a similar level. This was achieved by more aeration (air stone) being provided in tanks where necessary. The procedures used were the same as for experiment 1 except for the higher density (50 fish/65L tank) tank where additional aeration was provided with more air-stones to keep oxygen level similar with lower stocking density · (10 fish/65L tank). The experimental tanks were designated as: T 1: Lower density (10 fish/65L tank i.e. 1.88 g fish/L) with water exchange daily, T 2 : Lower density (10 fish/65L tank i.e. 1.88 g fish/L) with no water exchange, T 3 : Higher density (50 fish/65L tank i.e. 9.38 g fish/L) with water exchange daily and T 4 : Higher density (50 fish/65L tank i.e. 9.38 g fish/L) with no water exchange.
Experiment 3: The main objectives of this experiment were to determine the growth variation between individual fish and between fish reared at lower and higher stocking densities. The lower stocking density for this experiment was increased from SD-10 used in experiments 1 and 2 to SD-15 because the fish reared at lower stocking density (SD10) in experiments 1 and 2 showed aggressive behaviors. Higher stocking density was 15
M.S. Ali et al.
used for this experiment i.e. increased from SD-50 (used in experiments 1 and 2) to SD75 because in experiments 1 and 2 there were no significant differences in growth of Nile tilapia observed between higher and lower stocking densities (SD-10 and SD-50 fish/65L tank, respectively). The increased range between minimum (SD-15) and maximum (SD75) stocking densities was used to investigate if significant differences in growth could be observed. This experiment was conducted between 4 and 25 November 2001, for a duration of 22 days. Two stocking densities were replicated twice for this experiment. Approximately 30% of the tank water was exchanged daily and oxygen levels were kept similar in all tanks in order to observe the effects of ammonia excretion on growth of Nile tilapia under lower and higher stocking densities. The experimental tanks were designated as: T 1: Lower stocking density (15 fish/65L tank i.e. 2.81 g fish/L) and T 2 : Higher stocking density (75 fish/65L tank i.e. 14.07 g fish/ L).
Water quality measurements Water quality parameters were monitored based on daily measurements between 8:00 and 9:00 h in the morning. In tanks where water exchange was necessary, water quality parameters were measured before any exchange of water. Dissolved oxygen (DO, mg/L) was measured using a portable Microprocessor auto cal. DO meter (Model HI 9143, Sensitivity±0.01 mg/L, HANNA Instruments, Portugal). pH was measured with a portable pH meter. For measuring ammonia by using pH meter (Unicam, 9450 pH Meter, Sensitivity± 0.001/0.01, Cambridge, UK) and ammonia electrode (Model IS 570NH3, Sensitivity 57±-2m V, Philips), triplicate water samples from each tank were collected and immediately neutralized by acid and stored at -20°C for later analysis.
Treatments ofdata and statistical analyses The equation used to calculate the wet weight specific growth rate (SGR, %/day Ricker 1979). The coefficient of variation of weight (CV weight) was calculated to determine the homogeneity of the size range of fish in each tank. cvweight was obtained according to the following equation: CVweight (%) = SD/X l 00; Where, CVweight is the coefficient of variation of mean weight, SD is the standard deviation of mean wet weight and X is the mean wet weight (g) of fish (Gomes et al. 2000). The feed conversion ratio (FCR) was calculated (Suresh and Lin 1992) using the following formula: FCR = total mg dry weight feed fed/g wet weight gain (fish final wet weight in g - initial fish wet weight in g). Differences in growth under different stocking densities, dissolved oxygen and ammonia levels in response to with and without water exchange were analyzed by ANOVA using SPSS 9.0 statistical computer package for Windows 95/98. Where ANOV A showed significant differences between or within the treatments, Scheffe's multiple comparison tests were performed. Data are presented as means with their standard errors (± S.E). A probability level of 5% was considered for significant differences in all tests.
16
Effects of socking density on ammonia excretion of Nile tilapia
Results
Experiment 1: Effect of stocking density and growth (with and without water exchange) with varying levels of oxygen There were no significant differences in the mean initial body weights of the four experimental groups of fish used in this experiment (Table 1). By the end of the experiment the groups of fish reared at lower stocking density (SD-10) both with and without water exchange had higher growth rates (SGR, %/day) than fish reared at higher stocking densities (SD-50). Growth rate however, was not significantly different between stocking density or between tanks with and without water exchange (Fig. 1). Feed conversion ratios for the group of fish reared at a lower stocking density were significantly better than those fish at a higher stocking density (Table 1). The variation of size in weight (CVweighP %) of fish significantly increased with increasing stocking density (Table 1). During the period of experiment 1, dissolved oxygen (DO, mg/L) level significantly decreased with increasing stocking density (Fig. 2b ). Levels of ammonia increased significantly with increasing stocking density (Fig. 2a). Here note that, in this experiment the group of fish reared at a higher stocking density (SD-50) and without water exchange exhibited changes in body coloration (from silver to black). In this experiment, it was observed that fish stocked at the lower density (both with and without water exchange) showed better growth rates (not significant) than those reared at a higher stocking density (where oxygen level was lower and ammonia level was higher). The question arises then why is rate of growth of fish at the higher stocking density lower than fish at lower stocking density? Could it be the higher levels of ammonia or due to the lower oxygen level? To answer this question the following experiment was conducted where oxygen levels were kept similar in all tanks. (b) (a) o Initial Wt. (g) o Final Wt. (g) Water Exchange
No Water Exchange
Water Exchange
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Fig. 1. Comparison of mean initial and final weights (g) in (a) and mean specific growth rates (SGRm, %/day) in (b) of fish reared under different stocking densities (SD-10 and SD-50) with and without the water being exchanged during the experimental period (Expt. 1).
17
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Table 1. Mean initial and final weights (g), mean specific growth rates (SGRm, %/day), size variations (CV wcighv %) and mean feed !; conversion ratios (FCRm, mg dry food/g wet weight of fish) of fish reared under different stocking densities (Exp .1, 2 & 3). ~ Mean values sharing a common superscript (down columns) are not statistically different at the 5% (p < 0.05) level of :-significance (Scheffe's multiple comparison test). Experiment
Stocking density (fish/65L tank)
Expt. I.Varying levels of oxygen Water exchange 10 No water exchange 10 Water exchange 50 No water exchange 50 Expt. 2. Oxygen level similar Water exchange 10 No water exchange 10 Water exchange 50 50 No water exchange Expt. 3. Individual growth study Water exchange 15 Water exchange 15 Water exchange 75 Water exchange 75
Initial weight (g)
Final weight (g)
SGRm (%/day)
CV weight (%)
FCRm (mg/g)
12.72 ± 0.37" 12.19 ± 1.21" 12.22 ± 1.51" · " 12.09 ± 1.60 3
19.88 ± 3.02 a 18.53 ± 2.52 3 18.05 ± 3.10" 17.16 ± 3.34"
2.34 ± 0.29 3 2.20± 0.73" 2.05 ± 0.50 3 l.S4 ± 0.68"
12.38 ± 1.39 a 12.29 ± 1.44" 19.85 ± 3.5lb 21.46 ± 4.llb
943±174 2 1019±326" 1151±36lb 1176±26b
24.23 ± 4.26" 22.51 ± 6.36 a 21.54 ± 5.98 3 23.74 ± 7.36"
36.53 ± 6.33 a 32.29 ± 4.73 a 29.55 ± 8.54" 28.81 ± 6.82 a
2.57 ± 0.42 a 2.26±0.18 3 1.98 ± 0.93 a 1.86 ± 1.14"
17.74 ± 2.68 a 18.64 ± 3.09 a 31.13 ± 7.87b 30.16 ± 7.98 b
859 ± 283 3 974±243" 1134 ± 156 b 1265 ± 366 b
13.06 ± 1.51 a 12.89 ± 1.57 3 12.73 ± 1.39 3 12.86 ± 1.66 2
29.19 ± 8.16 3 28.05 ± 5.07 3 21.48 ± 3.92 b 21.98 ± 5.41 b
2.74 ± 0.83 a 2.74 ± 0.61 a 1.86 ± 0.54 b 1.85 ± 0.63 b
22.53 ± 7.23 a 20. 77 ± 6.92 a 43.58 ± 14.35 b 42.70 ± 16.23 b
732 ± 69 3 764 ± 92 3 1161±122 b 1139±lllb
Effects of socking density on ammonia excretion of Nile tilapia
(b) 7 mys 0xygm rnta (a) 7 days Ammonia data -+-ffi.10-L'-ffi.10 ~
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Fig. 2. Levels of ammonia (mg/L) in (a) and dissolved oxygen (mg/L) in (b) measured 7 days for fish reared under different stocking densities (SD-10 and SD-50) with and without the water being exchanged during the experimental period (Expt. 1).
Experiment 2: Relationships between stocking density and growth where oxygen levels in the water remain similar Although no significant differences in growth rates of fish were observed between fish reared at lower (SD-10) and higher (SD-50) stocking densities, there was a general trend of increased growth with decreasing stocking density (Fig. 3). In this experiment dissolved oxygen levels was similar in all tanks (Fig. 4b). Ammonia levels increased significantly with increased stocking density (Fig. 4a). Significantly better FCRs were observed for fish at a lower stocking density and the variation of size in weight (CVweight,%) of fish significantly increased with increasing stocking density (Table 1). Here it was also found that water exchange did not show significant differences in growth rates of the fish reared at lower or higher stocking densities, (a) o lritia w.
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Fig. 3. Comparison of mean initial and final weights (g) in (a) and mean specific growth rates (SGRm, %/day) in (b) of fish reared under different stocking densities (SD-10 and SD-50) with and without water being exchanged during the experimental period (Expt. 2).
19
M.S. Ali et al.
(a) 10 daysllffimniadata -+-- SD-10 _,, _ SD-10
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Fig. 4. Levels of ammonia (mg/L) in (a) and dissolved oxygen (mg/L) in (b) measured 10 days for fish reared under different stocking densities (SD-10 and SD-50) with and without the water being exchanged during the experimental period (Expt. 2).
Experiment 3: Changes in individual growth performance and size variation of Nile tilapia in relation to different stocking densities From the results of experiments 1 and 2, it was observed that increasing stocking density from 10 fish/6SL tank (1.88 g fish/L) to SO fish/60L tank (9.38 g fish/L), had no significant effect on growth rate. Also exchanging part of the water did not have a significant effect on growth rate. In experiment 3, significantly higher growth rates of fish were observed for fish reared in lower stocking density (SD-lS) compared to higher stocking density (SD-7S) groups (Fig. Sa & Sb). Significantly better FCRs were observed in fish stocked at a lower density and the variation of size in weight (CVweight' %) of fish significantly increased with increasing stocking density (Table 1). Oxygen levels in the water were similar for all tanks (Fig. 6b) and ammonia levels (Fig. 6a) increased significantly with increased stocking density. Fig. 7 shows the relationships between specific growth rates and ammonia and specific growth rates and stocking density, respectively (data are combined from the above three experiments). There was a significant negative correlation found between specific growth rates and ammonia (Fig. 7). (a)
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Fig. 5. Comparison of mean initial and final weights (g) in (a) and mean specific growth rate (SGRm, %/day) in (b) of fish reared under different stocking densities (SD-15 and SD-75) with water being exchanged and oxygen levels similar in all tanks during the experimental period (Expt. 3).
20
Effects of socking density on ammonia excretion of Nile tilapia
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Fig. 6. Levels of ammonia (mg/L) in (a) and dissolved oxygen (mg/L) in (b) measured over 22 days for fish reared under different stocking densities (SD-15 and SD-75) with water being exchanged during the experimental period (Expt. 3). Comparison of Tilapia Growth under different Ammonia levels --+- SGR _,,_Ammonia
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Fig. 7. Companson or specwc growth rates (:iLr.K, %/day) with 1eve1s ot ammoma (mg/ LJ produced by fish reared under different stocking densities (fish /65L tank) and experimental conditions (with and without the water being exchanged).
Discussion The results show that increasing stocking density from 1.88 g fish/L to 9.38 fish g/L in both with and without water exchanged groups of fish had no significant effect on growth although in general growth decreased with increasing stocking density. A significantly better growth rate was observed in groups of fish stocked at a density of 2.81 g fish/L when compared to 14.07 g fish/L. The coefficient of variation in size (weight) was significantly influenced by increasing stocking density up to 14.07 g fish/L. Feed conversion ratios were negatively correlated with stocking density. Several studies have found a close relationship between stocking density and growth rates of fish, e.g., negative relationship were observed for the African catfish, Cfarias ga1iepinus (Burchell) fry (Baylor 1991), Nile tilapia, Oreochromis niloticus fry (Huang and Chiu 1997), and 21
M.S. Ali eta!.
also in the present study. Papst et al. (1992) suggested that in intensive aquaculture the stocking density is an important factor that determines the economic viability of the production system. Furthermore, the size variation of fish may also be affected by stocking density. A linear relationship is usually found between stocking density and growth rate of fish. When the relationship model is valid, it is useful for predicting the optimal stocking density in aquaculture. The effect of stocking density on size variation has been shown to be positive in red tilapia (Watanabe et al. 1990) but negative in Arctic charr (Wallace et al. 1988). In the present study, coefficient of variation in size (weight) significantly increased with increasing stocking density. The significant and positive size variations in weight between higher and lower stocking densities in this study support the results obtained by Rubenstein (1981) and Watanabe et al. (1990). The effects of stocking density on size variation also depend on water quality and the biological characteristics of fish (Miao 1992). Also, size variation is related to the social interaction of fish (Macintosh and De Silva 1984). In crowding conditions, fish that are bigger and maintain a dominant position grow faster. The hypothesis of the present study is that if fish are reared under high density, the threshold for fight is elevated by frequent interactions which lowers discomfort and stress which in turn results in a less persistent dominance hierarchy, more uniform growth and potentially more rapid growth than at low density. However, high stocking density rates may result in high size variation in the present study which is in agreement with the results reported by Huang and Chiu (1997). In the present study FCR was better for lower stoking density indicate less competition and quite life increases aquaculture production whereas at higher density perhaps discomfort, competition, aggression and stress combindly decrease the production efficiency. In addition, tilapia are territorial and aggressive fish (Ali 2001), so density influences growth and size variation which might be by their competition for territories, similar cases were found for African catfish (Baylor 1991). In the present study, it was found that oxygen concentration of the water did not affect the growth of tilapia although oxygen concentration decreased with increasing stocking density (Fig. 1). In the present study, oxygen concentration of the water was always above 5.0 mg/L and kept similar for all fish at different stocking densities (Figure 4b). Although, in the present study, water exchange did not effect the growth and size variation of 0 mlotkus between higher (9.38 g fish/L) and lower (1.88 g fish/L) stocking density groups, fish reared at the higher stocking density without water exchange (19 days rearing) changed the coloration of their body (from silver to black). Color change was only found for fish in experiment 1, where stocking density was 9.38 g fish/L (without water exchange), dissolved oxygen was low and ammonia level was high. \Xfhy higher stocking density (without water exchange) groups of fish changed coloration of their body is unknown, but it may be that water with higher ammonia concentrations caused by crowdedness and lower dissolved oxygen level could cause this physiological response as a result of environmental perturbation (Wootton 1990). High density culture of tilapia has been employed to reduce the number of young produced (Allison et al. 1979), increase cannibalism of small fish (Pantastico et al. 1988), and delay age at sexual maturation (Allison et al. 1979). High density also may be useful 22
Effects of socking density on ammonia excretion of Nile tilapia
in changing behavior of fish from antagonistic to schooling behavior and this was found to be in the present study where in case of lower stocking density (1.88 g fish/L) fish showed aggressive behavior but in case of higher stocking density (9.38 g fish/L & 14.07 g fish/L) fish there were no such aggressive behavior observed. In the present study, the levels of ammonia in water increased with increasing stocking density, it was also found that levels of ammonia were significantly higher in fish reared at a higher stocking density without water exchange groups (Figure 6a). Ammonia is the principal nitrogenous compound excreted by aquatic animals and being toxic to fish can limit fish growth (Allan et al. 1990). Excess ammonia can induce detrimental changes in tissue structure, cell function, osmoregulation and reproductive capacity of fish (Harris et al. 1998). In the present study, it was found that increase in stocking density from 15 fish/tank to 75 fish/tank resulted in associated increase in ammonia level (1.48±0.87 mg/L to 26.44±11.4 mg/L) and significantly lower growth rates of 0 mloticus. From a management point of view, the price of fish is determined partly by the market demand for supply (size and production) which in turn depends on fish growth and survival rates. The production is the summation of individual weights of all reared fish, or a gross-product of the number of surviving fish and their mean weight (Miao 1992). If the present results would later be used to estimate the production (total biomass in that tank i.e. summation of individual weight of all reared fish) then production should be higher in higher stocking density. Although water exchange did not effect the growth of 0 niloticus, it has been shown that a higher stocking density without~water exchange had a lower growth tendency associated with higher ammonia lev~l (F±g. 7). Comparing the results from the present study under different stocking densities as already mentioned that no significant differences in growth rates were observed fish reared at 10 fish tank 1 compared to 50 fish/tank. However, considering higher production (total biomass will be higher for fish reared in higher density i.e. 50 fish/tank compared to lower density, 10 fish/tank), it may be suggested that when higher stocking density (50 fish/tank) will be maintained then water exchange must be taken in to consideration to reduce the ammonia level and to increase the level of oxygen, so as to avoid any physiological and environmental stress. References AOAC (Association of Official Agricultural Chemists), 1983. Official Methods of Analysis, 13'h Edition, AOAC, Washington, DC. Ali, M.S., 2001. The effects of salinity and food consumption on growth and protein turnover in Nile tilapia ( Oreochromis 111Joticus L.). PhD Thesis. Department of Zoology, University of Aberdeen, UK. Allan, G.L., G.B. Maguire and S.J. Hopkins, 1990. Acute and chronic toxicity of ammonia to juvenile J!1etapenaeus macleayi and Penaeus monodon and the influences of low dissolved oxygen levels. Aquaculwre 91: 265-280. Allison, R., R.0. Smitherman and J. Cabrero, 1979. Effects of high density culture and form of feed on reproduction and yield of T aureus. In: Advances in Aquaculture (eds. T.V.R. Pillay and W.A. Dill), pp. 168-170. Fishing News Books, Farnham, Surrey, England. Balarin, J.D. and R.D. Haller, 1982. The intensive culture of tilapia in tanks, raceways and cages. In: Recent Advances in Aquaculture, (eds. J.F. Muir and R.J. Roberts), pp. 473-483. Croom Helm, London.
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Gomes, L.C., B. Baldisserotto and J.A. Senhorini, 2000. Effects of stocking density on water quality, and growth oflarvae of the matrinxa, Bqcon ceplwlus (Characidae) in ponds. Aquaculwre 183: 73-81. Harris, J.O., G.B. Maguire, S. Edward and S.M. Hindrum, 1998. Effects of ammonia on the growth rate and oxygen consumption of juvenile greenlip abalone, Haliotis laevigata Danovan. Aquaculwre 160: 259-272. Baylor, G .S., 1991. Controlled hatchery production of Clarias gaiipinus (Burchell 1922): growth and survival of fry at high stocking density. Aquaculture and Fisheries .Management 22: 405-422. Honer, G., H. Rosenthal and G. Kruner, 1987. Social interaction of juvenile S. galilaeus in laboratory aquaria . .fournal ofAquaculture and the Tropics 2: 45-57. Huang, W-B. and T-S. Chiu, 1997. Effects of stocking density on survival, growth, size variation, and production ofTilapia fry. Aquaculwre Research 28: 165-173. Liao, I-C. and T-P. Chen, 1983. Status and prospects of tilapia culture in Taiwan. In: Proceedings of the Imerm1tional Symposium on Tilapia in Aquaculture, (eds. I. Fishelson and Z. Yaron), pp. 588-598. Tel Aviv University, Tel Aviv, Israel. Macintosh, D.J. and S.S. De Silva, 1984. The influence of stocking density and food ration on fry survival and growth in 0. mossambicus and 0. niloticus x 0. aureus male hybrids reared in a closed circulated system. Aquaculture 41: 345-358. Miao, S., 1992. Growth and survival model of red tail shrimp Penaeus penicillatus (Alock) according to manipulating stocking density. Bulletin of the Institute of Zoology, Academia Sinica 31: 1-8 Pantastico, J.B., M.M.A. Dangilan and R.V. Eguia, 1988. Canabalisms among different sizes of tilapia fry/ fingerlings and the effect of natural food. In: The 2nd Imernational Symposium on Tilapia in Aq1wculture (eds. R.V.S Pullin, T. Bhukaswan, K. Tonguthal and J.L. Maclean), pp. 465-468. Bangkok, Thailand. Papst, M.H., T.A. Dick, A.N. Amason and C.E. Engel, 1992. Effect of rearing density on the early growth of juvenile Arctic ch arr, Salvelinus Blpinus (L.). AquBculture Bnd Fisheries ManBgement 23, 41-47. Ricker, W.E., 1979. Growth rates and models. In: Plsh Physiology Vol. VIII, (eds. W.S. Hoar, D.J. Randall and J.R. Brett), pp. 677-743. Academic press Inc., New York. Rubenstein, D.I., 1981. Individual variation and competition in the Everglades pygmy sunfish . .founwl of Aninwl Ecology50: 337-350. Rusmussen, R.S. and B. Korsgaard, 1996. The effect of external ammonia on growth and food utilisation of juvenile turbot, Scophthalmus maxim us. Journal ofE<:pe1imental 111mine Biology and Ecology 205: 3548. Siddiqui, A.Q. and A.H. Al-Barbi, 1995. Evaluation of three species of tilapia, red tilapia and hybrid tilapia as culture species in Saudi Arabia. Aquaculwre 138: 145-157. Sin, A.W. and M.T. Chiu, 1983. The intensive monoculture of the tilapia hybrid, Sarotherodon 111Jotica (male) x S. mossambica (female) in Hong Kong. In: Proceedings of the International L~vmposium on Tilapia in Aquaculwre, (eds. I. fishelson and Z. Yaron), pp. 506-516. Tel Aviv University, Tel Aviv, Israel. Suresh, A.V. and C.K. Lin, 1992. Effect of stocking density on water quality and producLion of red tilapia in a recirculated system. Aquaculwre Engineering 11: 1-22. Wallace, J.C., A.G. Kolbeinshav and T.G. Reinsnes, 1988. The effects of stocking density on early growth in Arctic charr. Aquaculture 73: 101-110. Watanabe, W.0., J.H. Clark, J.B. Dunkham, R.I Wicklund and B.L. Olla, 1990. Culture of Florida red tilapia in marine cages: The effects of stocking density and dietary protein on growth. Aquaculture 90: 123-134. Wootton, R.]., 1990. Reproduction. In: Ecology ofTeleost Fishes (ed. R.J. Wootton), pp. 157-195. Chapman Hall, London & New York. Zohar, G., U. Rappaport, Y. Avnimelech and S. Sarig, 1984. Results of the experiments carried out in the Genosar experimental Station in 1983. Cultivation of Tilapia in high densities and periodic flushing of the pond water. Bamidgeh 36: 63-69.
(Manuscript received 28 February 2006)
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