Abstract: Experiments were conducted in aquaria tanks (62.5x29.5x30 cm) for a period of 12 weeks to investigate the effect of stocking density on the growth and survival of Clarias gariepinus fingerlings. The fingerlings (average weight of 0.60 g) were stocked in densities of 5, 10 and 15 fish per tank in duplicate and were fed with a diet containing 35% crude protein diet at 5% body weight twice daily. Feed intake increased at increasing densities. The survival, mean body weight, mean total length and specific growth rate were stocking density dependent. Best growth and survival rates were recorded in stocking density 5 than 10 and 15. This study showed that increased stocking density results in increased activity (air breathing, swimming) and decreased aggression.

INTRODUCTION

The production of fast-growing fingerlings is very vital for the development of a viable aquaculture venture. Aquaculture practices in Nigeria have increased drastically as seen in other parts of the world because of the increasing demand of fish protein. Rivers state, Nigeria has a great potential for sustainable aquaculture development, the initiation of fish culture seems to be one means of providing cheap animal protein to the increasing population. However, the increasing demand for fish protein can be met when capture fishery is supplemented by aquaculture. African walking catfish Clarias gariepinus is a successful aquaculture species in Nigeria. The species is widely accepted by fish farmers and consumers because of some biological reasons (high food conversion ratio, fast growth rate, readily accepting artificial feeds, ease of artificial propagation, disease resistance), social reasons (good market price, good table food quality) and physical reasons (tolerant to a wide range of environmental conditions). These catfish is an important contributor to both inland fisheries and aquaculture in Nigeria. Fish growth differs according to culture density. Aggressiveness seems to increase at lower densities and decrease at higher ones in some species while others show greater levels of aggression and even cannibalism when stocking densities increases (Stickney, 1994). There is a clear relationship between stocking density, oxygen requirements and metabolic waste production. The determination of stocking density for catfish culture is essential for the maximization of its production, profitability and sustainability. Stocking density is known to have a profound influence on growth, survival and behavior of fish (Boujard et al., 2002). However, in larvae and early fingerlings of Clarias gariepinus stocking density has an opposite effect (Sugunan and Katiha, 2004). The effects of stocking density on growth and survival have been studied on some African catfishes such as Clarias gariepinus (Haylor, 1992) and Heterobranchus longifilis (Ewa-Oboho and Enyenihi, 1999; Coulibaly et al., 2007).

This study was therefore designed to investigate the stocking density on the growth and survival of Clarias gariepinus fingerlings reared in tanks with a view of complimenting existing information to assist fish farmers on better culture methods. It is hoped that the results will contribute significantly to the knowledge of culture requirements of the species.

MATERIALS AND METHODS

Experimental location and system: The study was conducted at the Rivers State University of Science and Technology, Port Harcourt, Rivers state, Nigeria fisheries laboratory.

Table 1:	Composition of experimental diets

The fingerlings used for the experiment were procured from Jay-Ess Consultants fish farm in Port Harcourt Rivers state. Individuals of fairly uniform size (2.0 mm) and initial body weight of 0.60 g were used. Fish fingerlings were stocked at densities of 5, 10 and 15 per tank. The stocking density trials were conducted in aquarium tanks with a dimension of 62.5x29.5x30 cm supplied with aerated water.

The experimental fish were fed with a diet containing 35% crude protein. Feeding was done at 5% body weight twice daily (Table 1). The ration was split into two and dispensed at 09:00 and 16:00 h. Feed intake increased at increasing densities. During the experimental period weekly behavioral records were made. The fish in each aquaria tank was batch-weighed fortnightly and from the data the quantity of feed to be dispensed was adjusted to reflect the new weight. During this period undigested food particles and waste products were siphoned out with rubber hose daily. The experiment lasted for 12 weeks after which the fish were individually measured for standard length and weight. Dead fish in each tank were recorded and the tanks were uniformly erated. Water temperature, dissolved oxygen (Oxy meter model WTW OXI 330) and pH (pH meter model WTW pH 330) were measured daily at 7 h. After 12 weeks of culture, all surviving fingerlings were collected, weighted and counted from each tank and individual total length and body weight were recorded.

Statistical analysis: Result of the fish growth and specific growth rates were pooled for each treatment computed and analyzed using One-way-Variance (ANOVA) and difference between means was examined using Duncan’s multiple range test.

RESULTS AND DISCUSSION

Water quality characteristics monitored during the study period are shown in Table 2. The mean water temperature was between 26.50±0.12°C and 27.00±0.10°C.

Table 2:	Summary of water quality during the culture period

Temperature values were not affected by treatments (p>0.05). The pH mean values varied between 6.97±0.02 and 7.45±0.01 and decreased with increasing stocking density. Values of dissolved oxygen ranged between 5.19±0.20 and 5.82±0.20 mg L^-1. The water quality is thus considered suitable for fish production.

Stocking density showed a clear influence on final body weight, specific growth rate, food consumption as indicated by the statistical significance (Table 3). Stocking density 5 showed significantly high food intake and growth rate than stocking density 10 and 15 which did not differ significantly from each other. The result of Specific Growth Rate (SGR) showed that higher stocking densities resulted in lower specific growth rate. The best value was obtained in stocking density 5. The mean body weight, mean total length and specific growth rate were stocking density dependent. At the end of the experiment best growth performances were recorded in stocking density 5. Survival rates were high in densities 5 than 10 and 15, ranging between 81.10 and 91.20%.

The water quality parameters observed during the experimentation were within the range not toxic to fish life in pond (Boyd and Litchtkoppler, 1979; Viveen et al., 1986). The growth rates were found to be inversely related to stocking density. The differences in survival rate, growth and food conversion efficiency of Clarias gariepinus at three different densities were not statistically significant (Table 3) (p<0.05). This indicates optimum production performance at stocking density 5, where yield is significantly higher at stocking density 10 and 15. In fact, under crowded conditions at higher stocking densities, fish suffer stress as result of aggressive feeding interaction and eat less resulting in growth retardation (Bjoernsson, 1994). Stocking density has been considered to be chronically stressful to reared animals (Sugunan and Katiha, 2004). Several studies have also demonstrated that increased stocking density has a negative effect on survival and growth (Rowland et al., 2004; Schram et al., 2006) except in some fish species that exhibit schooling behavior. This impaired growth by stocking density may be attributed to reduced food consumption, lower food conversion rates or increased metabolic cost. However, survival rates were highest for stocking density 5 than 10 and 15. This study showed that increased stocking density results in increased activity (air breathing, swimming) and decreased aggression.

Table 3:	Mean growth parameters and feed utilization of Clarias gariepinus in the different stocking densities during the study period

Specific growth rate varies as a result of numerous factors among which the most important are the species, size, type and amount of food ingested, temperature and stocking density.

CONCLUSION

Stocking density had a significant effect on growth, survival and food conversion. Similar study must be realised in different structures such as concrete tanks, cages and ponds in order to determine the appropriate stocking densities for fingerlings.