The effect of stocking density on yield, growth and mortality of African catfish (Clarias gariepinus Burchell 1822) cultured in cages

ELSEVIER

Aquaculture 152 (1997) 67-76

The effect of stocking density on yield, growth and mortality of African catfish ( Clarias gariepinus Burchell 1822) cultured in cages Khwuanjai Hengsawat

a, F.J. Ward b,*, Pornchai Jaruratjamorn

a

aDepartment of Fisheries, Khon Kaen Uniuersity, Khon Kaen, Thailand b Depanment of Zoology, Uniuersity of Manitoba, Winnipeg, Man. R3T 2N2, Canada Accepted 3 1 December

1996

Abstract African catfish (CZurius gariepinus Burchell 1822) were cultured at four different densities based on fish biomass per cubic metre in cages suspended in a dugout pond during the summer of 1991. Catfish fingerlings (mean weight 32 g) were stocked at densities of 1.66, 3.44, 4.65 and 6.40 kg per cage or 50, 100, 150 and 200 fish per cage, respectively. At the end of 8 weeks harvest weights were, respectively, 16.6, 32.7, 51.2 and 63.5 kg per cage. Mean fish weights per cage were highest at the lowest density. The daily instantaneous growth rates were not significantly different, but mean weights decreased with increasing density. Instantaneous mortality rates were low. Harvests and production estimates increased with increasing stocking density. Growth and mortality of African catfish cultured in cages at these stocking densities were not affected by initial density, but total harvest and production were directly related to stocking density. 0 1997 Elsevier Science B.V. Keywords: African catfish; Stocking

density; Cage culture; Harvest;

F’roduction

1. Introduction The African catfish or sharptooth catfish, Chrius guriepinus Burchell 1822, is tolerant of a wide range of temperatures, as well as low oxygen and high salinity levels (Bovendeur et al., 1987). Because African catfish grow quickly, are omnivores and are desirable as food, they are a valuable species, worldwide. They are extensively cultured

* Corresponding

author.

0044-8486/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO44-8486(97)00008-Z

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in Thailand by commercial fish farms, by local government fisheries stations and also for research purposes at Khon Kaen University in northeastern Thailand. Cage culture is one of the major priorities of the Department of Fisheries, Royal Thai Government, especially in the northeastern region where there are many reservoirs suitable for culturing fish in cages. In addition, fish can be reared in cages placed in lakes and dugout ponds. Stocking density and, therefore, the volume of water per fish is a significant factor in determining production in cages. Increasing stocking density results in stress (Leatherland and Cho, 1985) which leads to enhanced energy requirements causing reduced growth and food utilization. Consequently, identifying the optimum stocking density for a species may be a critical factor in designing an efficient cage culturing system. The major objective of this study was to determine the relationship between stocking density and the production of African catfish in cages. Other related objectives were to determine the effect of stocking density on the growth of the fish, on mortality, and finally the feasibility of African catfish as a species suitable for cage culture in northeastern Thailand. This information is presently unavailable. The primary design of the study was to vary the initial stocking density of African catfish per cage based on the biomass (kg) and/or the number of fish per cubic metre while maintaining food levels constant relative to the biomass of fish in each cage.

2. Materials and methods 2.1. Location and experimental

design

The study was carried out in 1991 in a dugout pond (6400 m2, mean depth 1.5 m> in the Fisheries Division, Department of Fisheries, Faculty of Agriculture, Khon Kaen University, Thailand (16.26”N, 102.50”E), 391 km northeast from Bangkok (Fig. 1). The rectangular cages measured 1 X 1 X 1.5 m and were made of black polyethylene netting of 5-mm mesh size, square measure. The submerged volume of each cage was 1 m3. Cage frames were made of split bamboo. The cages were suspended from a bamboo structure fixed by cotton-nylon cords to a walkway from shore. Plastic bottles, attached along the four sides of each cage, were used as floats. The experiment was a completely randomized design (CRD). There were four treatments using four stocking densities and there were three replicates of each treatment. Twelve cages were used in the experiment. Five female catfish were spawned at the Fisheries Division, Department of Fisheries, Khon Kaen University on April 28, 199 1. Catfish larvae were held in a circular concrete pond for 33 days. During the first 2 weeks the fish were fed zooplankton (Moina spp., collected from an earthen pond on the university campus and Artemia salina, a product of China). They were then fed pellets. Fingerlings were kept in cages for an adaptation period of 3 weeks before being allotted to the experiment. African catfish were treated with a solution of formalin (200 ppm) for 3-5 min before being placed in cages at the experimental site. A total of 1500 fish were stocked on June 23, 1991 at 32.9 g average weight per individual fish at four different densities (1.66, 3.44, 4.65 and 6.40 kg mm3

K. Hengsawat et al./Aquaculture

I, L.-

i

NORTH

152 (1997) 67-76

69

LAOS

i

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l

Udon Thani

*-.

. Khon Kaen

BURMA

1

NORTH-EAST

--?

Ubon ” Ratchathani ,. I . ( :

. Nakhon Ratchasima

CAMBODIA

N t

Gulf of Thailand t’

t

Fig. 1. Map of Thailand showing the North-East.

or 50, 100, 150 and 200 fish mW3> (Table 1) and harvested 56 days later on August 18, 1991. The densities selected were based on a preliminary experiment conducted in 1990. Results indicated that densities needed to be much higher if an optimum stocking level was to be determined. The highest density used in 1990 was the lowest used in 1991, 1.66 kg m-3.

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Table 1 Stocking and harvesting data for African catfish reared in cages for 56 days at four stocking 95% confidence intervals are shown with means (n = 3)

densities a. The

Densities (kg per cage) 1.66 kO.38 No. stocked Initial mean weight (g) Harvest (no. fish) Final mean weight per fish (g) Harvest (kg per cage)

50 33.13+7.67 43.00 + 6.55 385.75 +25.39a 16.58 + 1.92a

a Values with the same letter are not significantly

3.44 + 0.63 100 34.42 89.67 364.93 32.73

+ f + f

6.27 13.65 11.47b 5.52b

4.65 +0.80 150 31.02k5.35 143.33 + 13.65 357.13 * 10.81bc 51.19*5.35c

6.40* 200 32.90+ 183.00 k 346.81 + 63.47 +

1.04

6.38 6.56 1.33~

2.09d

different.

2.2. Food and feeding

Two catfish food pellets, manufactured by Chareon Pokpand Ltd., of known nutrient content were used. The main differences between the two pellets was in crude protein content and in pellet size. Other components (lipid, fibre, ash and carbohydrate) were at similar levels. Fish were hand fed initially with 9910 (small particle size and high crude protein, 33.2% by weight) at 10% of body weight (BDW) 4 times day-’ (7:00, 12:00 a.m., 5:00 and 11:OO p.m.) at the amounts of 25, 20, 25 and 30% of the daily ration. The highest amount was given at night and the lowest at midday. Satiation feeding was employed in the first day of each sampling period for adjusting the amount of food offered to percent of body weight per day. Following sampling and the replacement of fish, a low dose of the antibiotic oxytetracycline was mixed with the food of the first meal at the rate of 50 mg kg- ‘. This antibiotic was used as a preventative agent for stress-induced cytophaga-like bacterial infections caused by sampling and handling. All treatments received the same low dosage; therefore the procedure was not believed to be an added variable. The total biomass of fish in each cage was used to readjust the food quantity downwards from 10 to 7 and 5% BDW for the 2nd and the 4th week, respectively, according to the calculated fish biomass. The reduction was based on changes in satiation feeding. Older fish were observed to eat less than younger ones. After the fourth week, the food was changed to the 9912 formula. Larger pellet size and a lower crude protein level (27.7% by weight) was appropriate for the larger fish. Subsequently, the amount was kept constant at 5% BDW until the 7th week of the experiment. During the last week (8th) fish were fed at 7% of the total biomass. 2.3. Sampling Twenty percent (by number) of the fish in each cage was randomly sampled bi-weekly by partially lifting the cage netting and removing a sample of fish with a dip net. The purpose was to determine fish growth in length and weight. On each sampling day individual fish from each cage were weighed in grams using a l-kg spring balance manufactured by Tanica Co. Ltd. The scale was calibrated in 5-g gradations. The total length in cm of each weighed fish was also measured. All fish in each cage were weighed to find the actual total biomass at 2-week intervals, using a 7-kg spring balance.

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Mean fish weight at each period was calculated by dividing the total biomass by the number of fish in each cage. The number of fish in each cage was also recorded to provide an estimate of mortality. Dissolved oxygen and temperature (using the Orion 820 dissolved oxygen meter) were measured every 2 weeks at 2:00 p.m. The pH of the water was determined at the same time by using a Coming pocket pH meter. After 56 days the total weight (kg) of survivors in each cage was recorded. 2.4. Analyses The final average weight (g) of individual fish in each cage was estimated by dividing the total final biomass in the cage by the number of survivors. Total weight increments, AB,, (kg) in each cage was estimated by comparing total final biomasses with initial biomass stocked in the cage; AB,=B,-B. where B, = total final biomass (kg); B, = total initial biomass (kg). The average weight increment, Aw,, (per fish in each cage, per day) was estimated from the difference between initial and final weight after 56 days; Aw, = W, - We/t where

W, = final

mean

fish weight

(g); W, = initial

mean

fish weight

(g); t = time

(days). Average instantaneous growth rates, G,, per day for all cages, based on individual weight increments (Aw,), were calculated according to Ricker (1975). Total instantaneous mortality rates (Z), determined from the initial numbers stocked and the number of fish surviving to harvesting, were calculated (Ricker, 1975). Production, P,, values were based on the Chapman (1968) method using estimates of initial, and final biomasses and the instantaneous growth rates. Net income was determined by the difference between the sale price of the fish after harvest and the costs of fingerlings and food. Analyses of variance (ANOVA) using SAS programs (SAS Institute Inc., 1988) were employed to test the effect of stocking density on various growth parameters. Regression procedure was used to estimate relationships between final mean weight, stocking density and harvest and stocking density (SAS Institute Inc., 1988). Tukey’s Studentized range (HSD) test was employed to compare the significance of differences between the means of the various growth parameters at the four stocking densities (SAS Institute Inc., 1988). The 95% confidence interval of means at each density was calculated for the basic parameters.

3. Results 3.1. Harvests

and final fish weights

Harvests (final biomasses) stocking density, F = 110.54,

from cages increased directly with increasing mean P > F = 0.009, R2 = 0.973 (Fig. 2(A)). Harvests from

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2

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3

4

5

3

4

5

6

400 390

S 380 E .cp Q) 370

3

350 340 1

2

6

7

Stocking Density (kg cage-‘) Fig. 2. (A) The relation between final harvest and stocking density. The 95% confidence interval for each mean is shown. (B) The relation between mean weight per fish at harvest and stocking density. The 95% confidence interval for each mean is shown.

cages with mean stocking densities of 6.40 kg per cage, the highest density, was 3.8 times the lowest, 1.9 times the second lowest, 1.2 times that of the second highest. There was an overall significant difference, at harvest, between cages (F = 466.11, P > F = 0.0001, R2 = 0.994). Furthermore, there were significant differences between cages stocked at different densities (Table 1). Final mean fish weights decreased with increasing mean stocking density, F = 64.04, P > F = 0.02, R2 = 0.955 (Fig. 2(B)). There were differences in mean fish weight between densities of 1.66 kg per cage (lowest) and the other three higher densities (Table 1). There were also significant differences between the densities of 3.44 and 6.40 kg per cage, but not between densities of 3.44 and 4.65 kg per cage. There was no difference in final mean fish weight between the second highest (4.65 kg per cage) and the highest (6.40 kg per cage) (Table 1). 3.2. Growth, mortality and production Daily increments per fish (Aw,) were significantly different overall (F = 25.58, P > F = 0.0002, R2 = 0.906). In general, daily increments decreased with increasing

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Table 2 Production (P,) data, daily weight increments (Aw,), daily instantaneous growth rates in weight CC,) and daily mortality rates (Z) for African catfish reared in cages for 56 days at four stocking densities a. The 95% confidence intervals are shown with means (n = 3) Densities (kg per cage) 1.66kO.38

Aw (g) Gv Z P, (kg per cage)

3.44kO.63

6.30+0.36a 0.04 + O.OOa 0.003 f O.OOa 16.30+2.09a

4.65 + 0.80

5.90 + 0.29b 0.042 i- O.OOa 0.002 + O.OOa 31.19*3.59b

a Values with the same letter are not significantly

5.82+O.lObc 0.044 + O.OOa 0.001+ O.OOa 48.81* 3.74~

6.40+

1.04

5.61 f0.13~ 0.042 * O.OOa 0.002 & O.OOa 60.31 f 2.76d

different.

stocking density but not all differences were significant (Table 2). The average daily increment of 6.3 g for fish held at the lowest density was significantly different from the other three densities (Table 2). There were also significant differences between the second lowest density and the highest density but not between Density 2 and Density 3 nor between Density 3 and the highest density. In contrast, there were no significant differences between average daily instantaneous growth rates (G,) at the four mean stocking densities (F = 1.42, P > F = 0.305, R* = 0.348) (Table 2). Daily instantaneous mortality rates were very low and the differences in rates between mean stocking densities were not significant, but production values were directly affected by stocking density (Table 2). Total production increased significantly with increasing densities (F = 712.65, P > F = 0.0001, R2 = 0.996). 3.3. Economics

Net profits were directly related to stocking density. The highest density provided the highest profit per cage; although the cost of fingerlings was also high (Table 3).

Table 3 Economic information for African catfish reared in cages for 56 days at four stocking confidence intervals are shown with means (n = 3)

densities.

The 95%

Densities (kg per cage) 1.66 No. fish stocked No. fish harvested Harvest (kg per cage) Food used (kg) Fingerling cost (2.5 Bht per fish) Food cost (12.5 Bht kg-‘) Total cost (Bht) Value of harvest (30 Bht kg- ’ ) Net profit (Bht)

50 43.00 + 6.55 16.58 + 1.92 20.75 125.00 259.36 384.36 497.40 113.04

3.44 100 89.67+ 13.65 32.73 + 5.52 37.25 250.00 465.59 715.59 981.90 203.31

4.65 150 143.33 + 13.65 51.19k5.35 56.49 375.00 706.08 1081.08 1575.33 454.65

6.40 200 183.00+6.56 63.47 k 2.09 70.62 500.00 882.76 1382.76 1903.98 521.22

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4. Diiussion Hogendoom and Koops (1983) also found that the highest biomass (harvest) was achieved at the highest stocking density for African catfish cultured in ponds. Culture of 0. niloticus in cages showed that the highest stocking density (100 fish me3) achieved the highest biomass after 53 months (Daungsawasdi et al., 1986). Similar results were obtained with channel catfish, Zctdurus punctutus (Storck and Newman, 1988). Final mean weights were inversely proportional to stocking density, which was particularly evident when average weight of fish held at the lowest and highest densities were compared; however, only the average weight of fish reared at the lowest stocking density was significantly different from weights of fish reared at the higher densities. Stocking density also affected the growth of C. macrocephalus X C. gariepinus hybrids cultured in concrete ponds at three different densities (Jarimopas et al., 1992). Fish reared at the highest density had the lowest final mean weight. Various studies on African catfish report differences according to the type of culture. Viveen et al. (1984) reported that growing catfish in tanks required 24-28 weeks to reach a size of 300-500 g. In ponds in which the fish were fed for the same time, catfish grew to a weight of 200 g; however, Hogendoom and Koops (1983) found that the fish, under field conditions, reached 300 g in only 22 weeks. During the same period, but in fertilized ponds and without supplemental food, catfish reached a maximum weight of 135 g (Bok and Jongbloed, 1984). Results from the present study, showed that catfish reached weights ranging from 346 to 385 g in only 8 weeks when stocked at an average weight of 32 g. Instantaneous growth rates were unrelated to stocking density. Growth rates were high even at the highest density. These results agree with those reported for African catfish raised in aquaria (Machiels and Van Dam, 1987) for channel catfish cultured in raceways (Woiwode and Adelman, 1989) and for Nile tilapia (0. niloticus) in cages (Daungsawasdi et al., 1986). Our results from fingerling stocking are different from those obtained by Haylor (1991) from experiments with African catfish fry. He found that growth rates were inversely related to stocking density. Similarly, Steffens (1989) found that growth rates in rainbow trout were inversely related to stocking density. Differences between our results and Steffens’ were probably caused by differences in the biology and environmental requirements of the two species. Mortality rates were not related to stocking density as might be expected. Hogendoom and Koops (1983) also reported that the survival rate of African catfish in ponds was not clearly influenced by stocking density. Similarly, mortality of Nile tilapia raised in cages was not dependent upon by stocking density (Daungsawasdi et al., 1986); however, Haylor (1991) found that mortality rates in African catfish fry were directly related to stocking density. Apparently, fingerlings are less sensitive than larvae to stress induced by crowding. There was a strong trend for both production and final harvests to increase with increasing stocking density. These results agree with those of Cruz and Ridha (1989) from studies on tilapia (Oreochromis spilurus) reared in cages. Our results also agree with those of Teng and Chua (1979). Production estimates which are based on biomass estimates adjusted for mortality and

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corrected for growth rate (Chapman, 1968) are the basis for estimating the economic yield from both fish culture operations and from natural fish populations. Because both growth and mortality rates were low, production and harvest values were similar and were independent of stocking density. While final harvest and production values were directly related to stocking density, there must be some density at which mortality is severe for a variety of causes and growth rate is reduced. When this occurs production will be reduced. This critical level was not reached in our experiment although the stocking density of 6.40 kg per cage was high. One reason for the ability of African catfish to maintain high production levels when cultured at high densities, but provided with sufficient food, may be their adaptation for aerial respiration. Aerial breathing was observed to increase at the higher stocking densities. In contrast, increased mortality in larval African catfish was associated with the onset of aerial respiration (Haylor, 1992); however, aerial breathing in larvae may not be a cause of mortality, but a relatively unsuccessful method of reducing stress induced mortality during a highly sensitive stage. The apparent independence of production from stocking density has to be treated with caution. At some high density, mortality may become extremely severe leading to a major, if not total loss of production. This sudden, extreme mortality may be caused by a number of density related agents; for example, behavioural changes or a rapid spread of a virulent pathogen (Cruz and Ridha, 1991). Catfish stocked at a weight of 32 g per fish as was the case in this study should have been harvested soon after the sixth week at an average weight between 200 and 265 g, the preferred market size, instead of approximately 364 g after 8 weeks. If this had been done, the value per kilogram would have been higher. Consequently, the highest economic yield may be determined not by maximum production but by preferred market size and price (Zonneveld and Fadholi, 1991). Furthermore, additional crops per year could have been reared. The larger fish can be sold to large restaurants, but these outlets constitute a small market. In conclusion, results were positive but did not indicate the upper limit of stocking densities for African catfish cultured in the system used. Additional experiments could be conducted to determine optimal stocking density of catfish in small cages and also densities which would produce the maximum number of fish of the desired size. Other experiments could be carried out to determine optimum food levels at various densities.

Acknowledgements We wish to thank the following members of the Faculty of Agriculture, Khon Kaen University: Drs Phanna Waikakul, Manochai Keerati-Kasikom, Jaowamam Khajaroen and Mr Prapast Chalorkpuntut. This research could not have been completed without the assistance of students of the Fisheries Club, Faculty of Agriculture, Khon Kaen University. We are grateful to the Canadian International Development Agency for providing funds for this research through the linkage programme between Khon Kaen University and the University of Manitoba (CIDA/ILP KKU-UM Fishery Project).

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