| Livestock Research for Rural Development 20 (7) 2008 | Guide for preparation of papers | LRRD News | Citation of this paper |
The effect of stocking density on growth, survival of the larvae of the African catfish Chrysichthys nigrodigitatus was studied. The experiment was conducted in circular tanks during 28 days. Seven-days-old larvae were stocked into 1000-L tanks containing 500 L of water (20 cm depth) at five densities 5 larvae/L (D5), 6 larvae/L (D6), 7 larvae/L (D7), 8 larvae/L (D8), and 9 larvae/L (D9). All larvae were fed with a 45% crude protein feed at 10 % of total biomass three times daily.
The results showed that mean body weight, mean total length, specific growth rate, body weight coefficient of variation, and condition factor were stocking density dependent. The best growth performances were recorded in D5 and D6. However survival rates were similar in all stocking densities and ranged from 86.77 ± 1.39 % to 95.91 ± 1.61 %. Hence, the most suitable stocking density for rearing of Chrysichthys nigrodigitatus larvae in tanks is 6 larvae/L (D6).
Keywords: Chrysichthys nigrodigitatus, densities, Ivory Coast, larval rearing, zootechnic performances
The African catfish Chrysichthys nigrodigitatus is a highly valued food fish in Ivory Coast and some countries of West Africa (Hem and Nuñez-Rodriguez 1995). Because of the high demands, the natural stock is reduced. In order to satisfy the populations, this species is cultured since 1980 (Dia et al 1986). A hardy nature and tolerance to adverse ecological conditions facilitate its culture. Then, in Ivory Coast great attention has been paid to Chrysichthys nigrodigitatus culture in lagoon cages and enclosures. Hem and Nuñez-Rodriguez (1995) reported that the production of farmed Chrysichthys nigrodigitatus in Ivory Coast has jumped from 1981 to 1995 to attend 300 tonnes per year for a market value of 684, 932 €. Since 2000, the annual production of this fish is approximately 20 tonnes (FAO 2005). Deficiency of fry production remains one of the main obstacles limiting the expansion of intensive fish production. Moreover, in Chrysichthys nigrodigitatus, the cycle of marketable fish (350-400 g) production is long (18 months) (Hem and Nuñez-Rodriguez 1995). That consequently increases the cost of the marketable fish production. Profitability of Chrysichthys nigrodigitatus culturing depends upon maximising of the production capability.
In intensive larvae and fry culture, several factors influence survival, welfare, growth, and production for example feeding (Kerdchuen 1992; El-Sayed 2002), water quality (Brazil and Wolters 2002), and stocking density (Sahoo et al 2004, Rahman et al 2005, Schram et al 2006). Stocking density is one of the main factors determining the growth (Engle and Valderrama 2001, Rahman et al 2005) and the final biomass harvested (Boujard et al 2002). 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)
Efficiently producing Chrysichthys nigrodigitatus in intensive system requires advanced knowledge of the minimum spatial requirement needed to achieve optimum growth rate, particularly in larvae and fry. The aim of this investigation was to study the effects of stocking density on growth and survival of Chrysichthys nigrodigitatus larvae reared in circular tanks.
Chrysichthys nigrodigitatus larvae used in this study were obtained from culture ponds of Layo Aquaculture Station (5°19’N, 4°19W; Ivory Coast). Before reproduction, males and females were stocked in cages in the Ebrié Lagoon during 90 days. This broodstock was fed at 7% of the biomass twice daily (8 00 and 17 00) with pelleted food containing 35% crude protein manufactured in Layo Aquaculture Station. After the breeding period (July-September 2006), males (730.77±171.59 and 38.90±3.18 cm) and female (618.60±85.74 g and 35.61±1.95 cm) mature were selected for reproduction according to the method described by Otémé (1993). With selected fishes, couples were constituted and confined in the nests during 30 days. Every days, at 7 00 and 17 00, the nests were controlled. After spawning, the couple was removed from the nests; the eggs were collected and transferred in incubator system at 28-30 °C. Hatching was completed between 4 and 5 days of incubation. Thus, the larvae were collected and transferred to a circular tank for rearing.
The experiment was realised in aerated fifteen 1000-L fibreglass circular tanks filled with 500 L of water (20 cm water depth). These tanks were supplied with water of the Ebrié lagoon via a water-tower. Daily water exchange in the tanks was 1.5 L/min. In Chrysichthys nigrodigitatus larvae, the yolk bladder occurs seven days after hatching. Before this period, the larvae use yolk reserves for their metabolism. At the beginning of this experiment, a total of 30 (7-days-old) larvae were taken in duplicate from the common stock in order to record initial lengths and weights. Initial length was recorded to the nearest half millimeter under a compound microscope using an ocular micrometer (10-fold magnification). Similarly, wet weight was recorded via an electronic digital balance (accuracy of ± 0.01 mg). Initial mean total length and individual weight of 30 larvae were 1.54 ± 0.05 cm and 0.026 ± 0.002 g respectively. The larvae were counted and stocked at five densities 2500, 3000, 3500, 4000, and 4500 larvae per tank (5 larvae/L (D5), 6 larvae/L (D6), 7 larvae/L (D7), 8 larvae/L (D8), and 9 larvae/L (D9) respectively). Three replicate tanks were constituted for each stocking density. The larvae were fed at 10% of total biomass three times (8 00, 12 00 and 17 00 h) per day with a floured diet, 45% CP and 18.76 KJ GE/100 g of food.
During the experiment, leftover feed and wastes were removed twice a day in the morning (6 30 h) and in the evening (16 00h). Dead fish in each tank were recorded and the tanks were uniformly aerated. Water temperature, dissolved oxygen (Oxy meter model WTW OXI 330) and pH (pH meter model WTW pH 330) were measured daily at 6 00 h. Water quality as phosphor, nitrate-nitrogen and nitrite-nitrogen were estimated weekly by the spectrometric method. At weekly intervals, 30 larvae were randomly sampled in each tank, weighted, and the total length was measured individually; daily food ration requirement was adjusted. After 28 days of rearing, all surviving larvae were collected, weighted, and counted from each tank and individual total length and body weight were recorded. The survival rate (SR), specific growth rate (SGR), body weight variation coefficient (CV), mean daily weight gain (MDWG), condition factor (K), and apparent food conversion ratio (AFCR) was calculated as follows:
To evaluate the effect of stocking density on production performance with more precision the performance index (PI) was calculated (Zacharia and Kakati 2002, Mohanty 2004). This index was calculated by combining two responses such as growth and survival.
Data were analysed by variance component analysis (Snedecor and Cochran 1967) and difference between means was examined using Duncan’s multiple range test. These analyses were carried out with Statistica 7.1 software.
Water quality characteristics monitored throughout the study period are summarised in Table 1.
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Table 1. Mean±SD of in-tank water temperature (T), pH, dissolved oxygen (DO), nitrite-nitrogen (NO2--N), nitrate-nitrogen (NO3--N) and phosphorus (PO43--P) recorded during larval rearing in tanks. Each mean represents samples collected at daily (temperature) and at weekly (others parameters) intervals during the 28-days rearing period. |
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Stocking density |
D5 |
D6 |
D7 |
D8 |
D9 |
|
T, °C |
29.80±0.20 |
30.10±0.10 |
29.70±0.30 |
29.80±0.20 |
30.00±0.20 |
|
pH |
7.56±0.06 d |
7.44±0.05cd |
7.33±0.06c |
7.01±0.07b |
6.80±0.07a |
|
D O, mg/L |
6.10±0.12c |
5.71±0.18c |
5.11±0.21b |
4.90±0.17ab |
4.47±0.26a |
|
NO2—N, mg/L |
0.021±0.001a |
0.024±0.001a |
0.025±0.002 a |
0.034±0.003b |
0.035±0.002b |
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NO3—N, mg/L |
0.103±0.006a |
0.147±0.009b |
0.175±0.014b |
0.238±0.02c |
0.267±0.018c |
|
PO43—P, mg/L |
0.154±0.011a |
0.185±0.012b |
0.219±0.017b |
0.315±0.023c |
0.343±0.023c |
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ab Mean values with different superscript letters within a row are significantly different (p<0.05) |
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Water temperature was between 29.70 and 30.10°C during the rearing period. This parameter was not affected by treatments (p>0.05). The pH mean values in all tanks were ranged from 6.80 ± 0.07 to 7.56 ± 0.06, and decreased with increasing stocking density. Values of dissolved oxygen were significantly lower (p < 0.05) in D8 and D9 compared to others densities. However, means of phosphor, nitrate-nitrogen and nitrite-nitrogen were significantly higher (p<0.05) in D8 and D9.
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Figure 1.
Growth in total length (a) and in body weight (b) of C.
nigrodigitatus larvae cultured at five densities in tanks for 28 days. Bars represent standard deviations |
There was not much variation in the total length and the wet weight of larvae stocked at different densities during the first half of the rearing period (Figure 1). During the second half of the rearing period, larvae stocked at D5 and D6 attained the significantly (p<0.05) highest mean total length and mean body weight. In return, the lowest values were obtained in the higher stocking densities D8 and D9. The larvae growth was therefore influenced by the stocking density. The performances of larvae obtained at the end of rearing period are presented in Table 2.
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Table 2. Mean±SD of final total length, final body weight, final specific growth rate (SGR), mean daily weight gain (MDWG), final condtion factor, final body coefficient of variation (CV), final apparent food conversion ratio (FCR), survival rate, performance index (PI), and final biomass of Chrisichthys nigrodigitatus larvae reared in tanks at five densities during the 28-days rearing period |
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Stocking density |
D5 |
D6 |
D7 |
D8 |
D9 |
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Total length, cm |
2.70±0.10c |
2.69±0.11c |
2.63±0.12b |
2.51±0.17a |
2.48±0.13a |
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Body weight, g |
0.15±0.03c |
0.15±0.02c |
0.13±0.02b |
0.11±0.02a |
0.11±0.02a |
|
SGR, %/day |
6.15±0.04d |
6.13 ±0.03d |
5.69±0.01c |
5.26 ±0.06b |
5.01 ± 0.06a |
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MDWG, mg/day |
4.30±0.05d |
4.27±0.05d |
3.66±0.01c |
3.15±0.00 b |
2.87±0.07a |
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Condition factor |
0.73±0.05b |
0.74±0.04b |
0.72±0.06b |
0.70±0.06a |
0.69±0.05a |
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CV, % |
13.09±0.72a |
12.99±0.41a |
14.58±0.41b |
17.46±0.17c |
16.84±0.68c |
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AFCR |
1.17±0.00a |
1.16±0.01a |
1.43±0.01b |
1.47±0.01c |
1.65±0.00d |
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Survival rate, % |
95.91±1.61c |
94.29±1.43bc |
91.20±1.06b |
88.94±1.42ab |
86.77±1.39a |
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PI |
0.44±0.01d |
0.44±0.01d |
0.33±0.01c |
0.28±0.01b |
0.26±0.01a |
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Biomass, g/m3 |
747.58±8.72a |
881.36±9.34c |
822.25±10.13b |
813.69±12.81b |
871.96±9.96c |
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ab Mean values with different superscript letters within a row are significantly different (p<0.05) |
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The results of specific growth rate (SGR) and performance index (PI) showed that higher stocking densities result in lower SGR and (PI). The best values were obtained in D5 and D6. On the other hand, total biomass was similar and higher in D6 and D9. At the end of experiment, the coefficient of variation of body weight (CV) was lower and similar in D5 and D6, followed in increasing order by D7, D8, and D9. The condition factor (K) of larvae was lower in D8 and D9 (p<0.05). But K was similar in larvae reared at the stocking densities D5, D6, and D7 (p>0.05).
Stocking density had a significant effect (p<0.05) on apparent food conversion ratio (AFCR). The AFCR was low and similar in D5 and D6, but it was increased from D7 to D9. Consequently, the highest value of AFCR was obtained in D9.
Survival rate was also significantly (P < 0.05) affected by the stocking density. This rate decreased with increasing stocking density. The means survival rates are summarised in Table 2. In all treatments, larvae did not exhibit cannibalism during this experiment. Mortality would be due to the natural death and to the manipulations during the weekly samplings.
During the experimental period, environmental conditions in culture tanks were influenced by the stocking density. High accumulation of excrements and metabolic wastes from the larvae led to significantly higher concentration of nitrogen compounds and simultaneously lowered the dissolved oxygen in D8 and D9, compared to others densities. The decreasing trend of dissolved oxygen in tanks with high stocking densities would be attributed to the gradual increase in biomass, resulting in higher oxygen consumption at varied stocking densities D8 and D9. In this experiment, Chrysichthys nigrodigitatus larvae reared at high stocking densities responded with increasing level of metabolites such as urine and faeces. This stress response in fish changes water quality (Wendelaar Bonga 1997). Water quality, mainly dissolved oxygen and pH levels are considered as the limiting factor in intensive fish culture. High levels of dissolved oxygen increase growth in channel catfish Ictalarus punctatus larvae reared in tanks (Brazil and Wolters 2002). In general, poor growth performance of cultured species takes place at pH < 6.5 (Mount 1973). It’s possibly that decreasing dissolved oxygen and increasing of the others waters quality parameters observed in this study induced stress result by the low growth in high densities.
Growth is the manifestation of the net outcome of energy gains and losses within a framework of abiotic and biotic conditions. In this experiment, the effect of stocking density on growth (SGR and DWG) and performance index (PI) was highly significant at higher stocking density D9, while there was no significant variation among SGR, DWG, and PI at D5 and D6. This indicates optimum production performance at stocking density D6, where yield is significantly higher than at D5 and almost equal to D9. 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). This indicates that in Chrysichthys nigrodigitatus larvae, stocking densities above D6 delay growth. In a study on the influence of population stocking density in Clarias batrachus larvae reared in tanks, Sahoo et al (2004) reported a similar effect of high stocking densities on growth and SGR. The condition factor of Chrysichthys nigrodigitatus larvae also decreased at higher stocking densities (D8 and D9), but it was similar at lower densities (D5, D6, and D7). This result suggests that, at lower stocking densities all larvae received adequate amounts of food, compared to those of higher densities.
On the other hand, the apparent food conversion ratio increased in high stocking densities. Food conversion ration and feed efficiency were negatively correlated with stocking density in Chrysichthys nigrodigitatus larvae. This result indicates that high stocking density reduced feed efficiency. Similar results have been reported in both Cyprinus carpio larvae (Jha and Barat 2005) and Tor putitora larvae (Rahman et al 2005).
A lessening in total mean body weight increment and differential growth as body weight variation was observed at higher stocking densities; this might be due to improper acquisition of food by the larvae at these higher stocking densities, resulting in poor growth. This heterogeneity of growth and social dominance based on size has been also observed in other larvae fish species such as Heterobranchus longifilis (Ewa-Oboho and Enyenihi 1999), Clarias gariepinus (Hengsawat et al 1997), and Clarias batrachus (Sahoo et al 2004). At the beginning of this experiment, larvae body weight variation was not exactly zero. The important body weight variation observed in larvae reared at high stocking densities (D8 and D9) suggest that these stocking densities increase the dependence of growth on the initial weight of individual larvae and thus the advantage gained by the largest in competing for food.
In our study, larvae survival rate was also affected by stocking density. Mortality of the larvae was higher at high stocking densities and dead larvae removed from the tanks were of small size and presented a nearly flattened abdomen, sign of food absence. The death of small larvae would be due to the fact that they were possibly inhibited from adequate feeding because of the presence of larger individuals which take more food. In fact, in the tanks where larvae were stocked at high densities, the larger individuals were seen to exhibit highest swimming activities and waiting-in-feeding-area behaviour both before and during feeding. Moreover, these larvae spent more time eating. In contrast, the smallest were eating from the bottom of the tanks. The effect of stocking density on larval survival depends on the species. In larvae of Rachycentron canadum (Hitzfelder et al 2006), Clarias batrachus (Sahoo et al 2004), and Solea solea (Schram et al 2006) reared in tanks, survival rate decrease as stocking density increase. On the other hand, Haylor (1992) reported that the stocking density did not affect survival of Clarias gariepinus larvae reared in floating cages. Contrary to the catfishes like Heterobranchus longifilis (Ewa-Oboho and Enyenihi 1999), Clarias gariepinus (Hengsawat et al 1997), and Clarias batrachus (Sahoo et al 2004), Chrysichthys nigrodigitatus larvae did not exhibit cannibalism during the early stages of growth. This would justify the high values of survival rate recorded in all treatments. Values of survival rate obtained in this study were higher than that previous reported 78.0 % by Ouattara (1992) for this species in aquaculture system.
Stocking density had a significant effect growth and food conversion. Therefore, the optimum stocking density for Chrysichthys nigrodigitatus larvae is 6 larvae/L (D6). The same study must be realised in different structures such as cages and ponds in order to determine the appropriate one for larvae rearing.
The authors are grateful to the staff of Layo Aquaculture Station, particularly Patrick Danho and Madeleine Adou for their aids and assistance. We would also like to thank Célestin Blé, responsible of Nutrition Laboratory of Aquaculture Department. This study is a part of the project “Aquaculture et Ressources Halieutiques du Centre de Recherches Océanologiques (CRO)” and was financed by Ivorian government.
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Received 28 February 2008; Accepted 2 April 2008; Published 3 July 2008