SECTION III: SYSTEMS AND TECHNIQUES OF AQUACULTURE (contd.)

REARING OF THE NILE CATFISH, Clarias lazera, TO MARKETABLE SIZE IN EGYPTIAN EXPERIMENTAL PONDS

by

A.R. El Bolock
Institute of Oceanography and Fisheries
Academy of Scientific Research and
Technology, Cairo, Egypt

Abstract

Clarias lazera is considered an important cultivable fish in Egypt. Propagation of this species as well as production of great quantities of fingerlings for pond stocking have been successfully carried out. Experiments on rearing Clarias lazera in its first year of life under different conditions of stocking and feeding are described. Results indicate that, with the absence of supplementary feeding, loss due to cannibalism is high and production per hectare increases with the stocking rate. Best production results were obtained when supplementary food was given; food conversion values ranged from 3.2 to 6.7 according to the quality of food given. The lowest conversion was obtained with a mixture of rice bran, animal blood and chicken offals. The cost of food given compared with fish production was found to be economical and profitable. The average sizes obtained after rearing for about 266 days are considered marketable.

Résumé

En Egypte, l'on considère Clarias lazera une espèce importante de poisson cultivable. On a entrepris la propagation de cette espèce, ainsi que la production de grandes quantités d'alevins pour en stocker les étangs, avec succès. On décrit l'élevage expérimental de Clarias lazera dans sa première année de vie, dans différentes conditions de stockage et d'alimentation.

Les résultats indiquent que, sans alimentation supplémentaire, les pertes dues au cannibalisme sont élevées et la production par hectare augmente avec le taux de stockage. Les meilleurs résultats de production ont été obtenus avec la distribution d'aliments supplémentaires; les taux de conversion de la nourriture s'échelonnent de 3,2 à 6,7 selon la qualité des aliments distribués. Le taux le plus faible de conversion a été obtenu avec un mélange de son de riz, sang animal et déchets de poulet. Le côut de la nourriture supplémentaire comparé à la production de poisson a été retenu économique et profitable. Les tailles moyennes obtenues à la fin d'un élevage de 266 jours sont considérées des tailles marchandes.

1. INTRODUCTION

Catfishes constitute some of the most important species used in warm-water fish culture. The intensive culture of catfishes, especially Ictalurus punctatus and other related species, on a commercial basis, in the U.S.A. is good evidence of the contribution of such fishes to fish production. Swingle (1957, 1959) reported that the production of the channel catfish, Ictalurus punctatus, in ponds ranges between 1 070 and 2 624 kg/hectare depending on the rate of stocking and methods of feeding. The culture of Clarias spp. especially Clarias batrachus and C. macrocephalus in Thailand has yielded production of up to 107 tons/hectare (Pawapootanon, 1965; Sidthimunka et al., 1966).

It has been established (Aboul-Ela et al., 1973) that successful spawning and propagation of Clarias lazera is now possible and several methods have been developed for this purpose. Besides, Clarias lazera has a rapid rate of growth and accepts a variety of food items. The weight of the unconsumable body parts of this species is also very small as compared with the total fish weight. However, literature on the culture of Clarias lazera in ponds is very scarce.

The present work was carried out to obtain some preliminary data on the rearing of this species in ponds during its first year of life with an aim to determine the best production methods.

2. MATERIAL AND METHODS

Clarias fry and fingerlings were obtained by controlled breeding of adults according to methods described by Aboul-Ela et al. (1973). Fry of 40 mm total length and 1 g average weight were used for stocking the ponds since they are hardy and can easily adapt to artificial feeding. Stocking was carried out in June, July and August and the ponds were harvested during February and March of the same year.

Two sets of experiments were conducted in five separate ponds, two at El Serow¹ and three at the Barrage farm², and are referred to in the text as the first, second, third, fourth and fifth pond, respectively (Table I). Pond areas ranged between 400 and 1 822 m² and were approximately 1 m deep.

Before stocking, the ponds were completely drained and exposed to the sun for about one month to ensure the elimination of any predaceous and competitive animal life, and to condition pond soil. Stocking rates are shown in Table I; no artificial food was offered in the first and third ponds, whereas the remaining ponds were supplied with supplemental food six days per week. The food was offered on special wooden tables adjusted at 30 cm below the water level. Supplemental food was composed of fresh minced chicken offals (mainly intestines and lungs) or rice bran mixed with fresh animal blood.

At the end of the experiments all ponds were drained and the fish were collected by hand, counted, measured and weighed. For comparison and convenience, results obtained from both the El Serow and Barrage farms are recorded together in tables and histograms.

3. RESULTS

In the experiments conducted at the El Serow farm, the average gain in length and weight per individual fish was higher in the second (200 mm and 109 g) than in the first pond (180 mm and 82 g) although the fish were stocked at the same rate and reared for nearly the same period (Table I, Fig. 1). Consequently the total gain in weight and final production per hectare in the second pond was 1.8 times that in the first. This relatively high production is most probably due to the supplementary food offered to the fish in the second pond. The natural food in the first pond was probably exhausted gradually, resulting in a lower production as well as higher percentage loss (39.2 percent). The latter is presumed to be due to cannibalism.

In the experiments conducted at the Barrage fish farm, the lowest average gain in length and weight was obtained in the third pond where no supplementary food was offered, while the highest average values were observed in the fifth pond (Table I); moderately high gain was recorded from the fourth pond. The total gain in production per hectare was highest in the fifth pond which received larger amounts of supplementary food. It should be noted that the three ponds were stocked at nearly the same rate and the rearing period was longer in the fourth than in the third and fifth ponds. However, the difference in total gain is very large and cannot be attributed to the difference in the rearing period in the three ponds. It can only be a result of the extra supplementary food offered. It should also be noted, by comparing production from the first and third ponds which were deprived of supplementary food, that the percentage loss, as well as production (total gain), increased with the stocking rate.

The relative wellbeing of the fish is estimated by the condition factor or coefficient of condition “K”, calculated as follows:

(where W = weight in g and L = total length in mm).

¹ Near Manzallah Delta-Lake.
² Near Cairo.

TABLE I

Results of preliminary experiments on pond-rearing of Clarias lazera

¹ 1 Egyptian pound = 1 000 millims = U.S.$ 2.3

TABLE II

Condition factor (“K”) of produced Clarias lazera

The values of “K” for the fish produced in each experiment (using average total length and weight) are shown in Table II. It is clear that the fish which were offered supplementary food had higher “K” values; the highest value (0.84) was obtained where the greatest amount of supplementary food was given. At the same time, external examination of the fish showed that those offered supplementary food were robust and healthy. Dissection also revealed that fed fish were more fatty than unfed ones.

3.1 Food conversion (“S”) rate

In fish ponds variable amounts of natural food organisms are available and when supplementary food is added, the weight of fish produced is due to the combined effect of natural and artificial feed offered. In dealing with pond culture and when supplementary feeds are being compared, it is suggested by Swingle (1958) that the symbol “S” represents the food conversion where no correction can be, or is, made for natural production. Consequently the food conversion or

The weight of fish produced equals the total fish weight obtained at the end of the experiment less the weight originally stocked. In the present work “S” values for Clarias lazera were determined according to the above formula.

In determining food conversion values (Fig. 2), the total weight of both kinds of diets was used. It is clear from Fig. 2 that the highest fish production was produced in the fifth pond where the greatest amount of additional food was introduced. In this experiment the food conversion value was lower (3.21) than in the fourth pond (3.46). In the second pond, where only rice bran mixed with animal blood was offered, 6.72 kg of this diet produced 1 kg of fish flesh.

4. DISCUSSION

Certain members of the family Ictaluridae, Siluridae and Clariidae have shown excellent results in pond culture with reference to production per hectare of water surface.

In the present work the results obtained from rearing experiments with Clarias lazera in the first year of life, in both the El Serow and Barrage farm ponds, proved that healthy fingerlings, good environment and suitable feeding programmes are obvious criteria for profitable production.

Clarias lazera fingerlings 40 mm in length were found convenient for stocking rearing ponds since at this stage they are sturdy enough to endure handling and transfer from the nursery ponds and can be easily trained on supplementary food. According to Sidthimunka et al. (1966), in Thailand, fry of Clarias batrachus and Clarias macrocephalus can be transferred to stocking ponds at the size of 15–43 mm. Fry of 25 mm length collected from natural waters are also sold for stocking. Farmers, however, prefer to stock the bigger fry (40–50 mm) because the mortality rate is lower and the time required for culture to market size is shorter.

Sidthimunka et al. (1966) stated that the rate of stocking of Clarias spp. in ponds 2–3 m deep should be 50 fish/m² water surface when 7–10 cm fish are used, or more if smaller fish are stocked. In the present primary experiments, the ponds are stocked at a rate of 125–274 fish/100 m² (12 500–27 400/ha). Although it is difficult to compare the results obtained at the El Serow and Barrage ponds due to the difference in conditions in both areas, it has been noted that in the absence of supplementary food, production per hectare increases with the rate of stocking. This agrees with observations made by Brown (1951), Swingle (1959) and Snow (1962) on the production of channel catfish. In Europe sheatfish, Silurus glanis (40–60 mm in length) are stocked in rearing ponds at a rate of 15–20 fish per 100 m² (1 500–2 000/ha) to reach 50–100 g by the end of the first summer (Woynarovich, 1966). Fielding (1966) found that the catfish Ictalurus nebulosus marmuratus can be stocked at a rate of 74 fingerlings/100 m² (7 400/ha) to produce 1 012 kg of marketable fish at the end of 13 months. Sneed et al. (1970) reported stocking rates for channel catfish fingerlings of 300–500/100 m² (30 000–50 000/ha) in rearing ponds.

In the present experiments, fish loss did not exceed 46 percent and lower mortality rates were obtained in ponds where supplementary food was given. The highest mortality rate occurred in ponds where no supplementary food was offered. This is most probably due to predation. High losses - due to cannibalism and predators - which may have exceeded 50 percent were also noted in the case of the flathead catfish, Pylodictis olivaris, reared from fry to fingerlings in troughs (Sneed et al., 1961). Woynarovich (1966) reported a loss percentage between 20 and 50 when rearing the sheatfish, Silurus glanis, in its first year of life. The loss percentage with Clarias lazera in the present experiments is within the range reported as compared with those for other reared catfishes in other countries. However, such loss is influenced by the number of fish stocked, the amount of feed, frequency of feeding, size grading and other protective management measures.

The food utilization shown in Table I and Fig. 2 illustrates the efficiency of the Nile catfish, Clarias lazera, in converting supplementary food into flesh. Values of food coefficient, or conversion factor “S”, in the present experiments ranged from 6.72 to 3.21 according to the quality of diet used. The diet composed of a mixture of rice bran and animal blood gave the highest conversion values (6.72) while that composed of the above diet together with chicken offals gave lower values (3.46 and 3.21) according to the quantity of chicken offals given. These lower food coefficient values may be due to the fact that the diet used contains the constituents needed for such a carnivorous fish. According to Imam (1970), the animal protein percentage found in chicken offals (79.42) and in animal blood (83.18) nearly equals that found in the flesh of Clarias lazera itself (80.49). This high protein content is probably responsible for the lower food coefficient. Further, the presence of carbohydrates in rice bran in a convenient percentage (about 45.38) plays an important role in providing the energy required for fish growth (Tal and Hepher, 1966). Similar results of high growth in Clarias lazera experimentally fed on a fish meat diet were obtained by Imam et al. (1970).

The average size per individual fish (290–310 mm) obtained at the end of the present experiments in the fourth and fifth ponds can be considered as a harvestable size in culture ponds in Egypt. In Thailand, Clarias spp. are harvested when about 250 mm long and 200 g in weight, a size usually attained after rearing for about four months (Sidthimunka et al., 1966).

An important question which has to be answered, when feeding is considered, is whether feeding is economical. According to the present experiments, the production of 1 kg of Clarias lazera costs about 67 millims when rice bran and animal blood were offered. When chicken offals were added, the cost of food per kg of fish produced ranged from 18–20 millims. One kg of marketable size of Clarias lazera sells for about 150 to 200 millims. This underlines the importance of chicken offals as an economical and growth-accelerating diet in Clarias lazera culture.

6. REFERENCES

Aboul-Ela, I., F. Amer and A.R. El Bolock, 1973 Studies on spawning and spawning behaviour of Clarias lazera Cuv. and Val. in fish farms of A.R. Egypt. Bull.Zoolog.Soc. of Egypt, No.25:25–32

Brown, W.H., 1951 Results of stocking largemouth black bass and channel catfish in experimental Texas ponds. Trans.Amer.Fish.Soc., Vol.80:210–7

Fielding, J.R., 1966 New systems and new fishes for culture in the United States. Proceedings FAO World Symposium on Warm-water Pond Fish Culture. FAO Fish.Rep., No. 44, Vol.5:143–61

Imam, A., 1970 Rearing of catfish in artificial ponds and biological changes induced by low dose total body Gama irradiation. MSc. Thesis, Faculty of Science, Cairo Univ.

Imam, A. et al., 1970 Feeding of catfish, Clarias lazera in experimental ponds. Bull.Inst.Ocean. and Fish., No. 1:207–22

Pawapootanon, O., 1965 The cultivation of the Pladuk (Clarias batrachus Linn.) in the vicinity of Bangkok. Thesis, Fish. Faculty, Kastsart University, Bangkok, Thailand

Sidthimunka, A. et al., 1966 The culture of catfish (Clarias spp.) in Thailand. Proceedings FAO World Symposium on Warm-water Pond Fish Culture. FAO Fish.Rep., No.5:196–204

Sneed, K.E., H. Dupree and O.L. Green, 1961 Observations on the culture of flathead catfish (Pylodictis olivaris) fry and fingerlings in troughs. Proceedings of Fifteenth Annual Conference, Southeastern Association of Game and Fish Comm., Atlanta, Georgia

Sneed, K.E. et al., 1970 Report to the fish farmers. The status of warm-water fish farming and progress in fish farming research. Bureau of Sport Fish. and Wildlife Public., No.83, 124 p.

Snow, J.R., 1962 A comparison of rearing methods for channel catfish fingerlings. Progressive Fish Cult., 25(3):112–8

Swingle, H.S., 1957 Preliminary results on the commercial production of channel catfish in ponds. Proc.Southeast.Assoc. Game Fish Comm., 10:160–2

Swingle, H.S., 1959 Experiments on growing fingerling channel catfish to marketable size in ponds. Proc.Southeast.Assoc. Game Fish Comm., 12:63–72

Tal, S. and B. Hepher, 1966 Economic aspects of fish feeding in the Near East. Proceedings FAO World Symposium on Warm-water Pond Fish Culture. FAO Fish.Rep., No. 44, Vol.3: 285–325

Woynarovich, E., 1966 New systems and new fishes for culture in Europe. Proceedings of FAO World Symposium on Warm-water Pond Fish Culture. FAO Fish.Rep., No.44, Vol.5:162–81

Fig. 1. Histogram showing gain in length (mm) and weight (g) of Clarias fry as well as the stocking rates and percent loss in five pond-culture tests.

Fig. 2. Histogram showing the total gain in weight of reared Clarias lazera, amount of food offered and food coefficient (in parenthesis).

PRELIMINARY RESULTS CONCERNING THE CULTURE OF Clarias lazera IN CAMEROON

by

H. Hagendoorn and R. Wieme
FAO/UNDP Fish Culture Development Project
Foumban, Cameroon

Abstract

The paper presents the results of preliminary tests on the reproduction and pond culture of Clarias lazera undertaken at the National Fishculture Centre, Foumban, following introduction of this species into the country from the Central African Republic in 1972. The fish has been spawned successfully in ponds with and without hormone (Doca) injection but the number of fingerlings produced per female remains low and variable. Production trials with pond fertilization and/or supplementary feeding have yielded growth rates up to 5 g/day; best results were achieved with both fertilizer application and supplementary feeding. Future work is aimed at the intensification of the breeding tests and continued pond-rearing trials.

Résumé

Le document présente les résultats d'essais préliminaires sur la reproduction en pisciculture en étang de Clarias lazera entrepris au Centre National de Pisciculture de Foumban, à la suite de l'introduction de l'espèce dans le pays, en provenance de la République Centrafricaine en 1972. Le poisson a été reproduit avec ou sans injection d'hormone (Doca) mais à ce jour la production du nombre d'alevins pour chaque femelle est restée faible et variable. Des essais de production par fertilisation d'étang et/ou alimentation supplémentaire ont donné des taux de croissance atteignant jusqu'à 5 g/jour; les meilleurs résultats ont été obtenus au moyen de fertilisant et d'alimentation supplémentaire combinés. Les travaux ultérieurs tendent à l'intensification des essais de reproduction et à la continuation des essais d'élevage en étang.

1. INTRODUCTION

Clarias lazera (Clariidae) was introduced into Cameroon in 1972. The fish was obtained from Bangui, Central African Republic, where it had given promising results in aquaculture.

Under the limnological conditions existing at the National Fishculture Centre in Foumban, i.e., negligible hardness, relatively low water temperature (15–23°C) and little seasonal variation, the results thus far obtained in the culture of this species have been quite satisfactory. The fish has been successfully spawned and approximately 1 000 fry reared from eggs; these will serve as future broodstock. Good growth rates have been achieved with artificial diets compounded with local by-products; better results were achieved with supplementary feed when pond productivity was high. Heavy fish losses occurred occasionally, presumably due to the escape of this "walking catfish"; bamboo fences are presently being tried to control this phenomenon.

2. REPRODUCTION

In 1973, at the age of two years, 12 Clarias (7 females and 5 males) reproduced naturally with a rather low return of fingerlings. The fish were held together in a 350 m² pond fertilized with pig manure from an adjacent piggery. It was thus shown that Clarias lazera would reproduce at high altitude (1 400 m) under Cameroon conditions.

In 1974, efforts were undertaken to obtain more accurate data on the reproduction of this species. In the course of these experiments, a total of 1 716 fingerlings were obtained under conditions indicated in Table I.

Due to this narrow base of experience, no conclusions can as yet be drawn, but trials are being continued in line with the following observations:

1. Hormonal (Doca) induction of spawning, though not strictly necessary, appears to increase the spawning success (Table I).

2. Shading (Trials 1 and 3, Table I) and shelter (Trial 6) may be beneficial factors. Plankton availability is not to be ruled out as a contributing factor to the comparatively favourable results obtained in these trials.

3. Pond area does not seem to affect spawning success as much as the final fingerling density affects the growth rate.

Future plans are aimed at intensification of the breeding programme as permitted by the availability of Clarias broodstock. In 1975, Clarias will be spawned in two interconnected small ponds with different depths. The upper pond will be vegetated and the hypothesis is that the fish will come up into it to spawn and descend into the deeper part afterwards. This would allow early and easy separation of fry and broodfish. Spawning mats will be tried to possibly collect eggs and arrive at a form of fry culture in order to attempt to increase the survival rate.

3. PRODUCTION

Until mid 1974, Clarias were reared either in ponds that were heavily fertilized with pig manure and/or fed a dried wheat bran-blood mixture. The latter gave growth rates of up to 5 g/day.

In order to determine the feasibility of more elaborate, though incomplete, diets, an experiment was started in October 1974. Beer wastes, wheat bran, cotton seed cake, palm cake, blood, bone, meal, ground fish and wheat flour were mixed in different proportions resulting in 7 different diets with a total digestable protein content ranging from 25 to 35 percent. The trials were carried out with Clarias averaging 8.65 g and the growth ranged from 38 to 103 g in 4 months (0.32–0.85 g/day). Better results were obtained in a pond that was heavily fertilized with the daily wash water of the blood receptable. Here the Clarias grew from 2 to 123.7 g in 2.5 months (1.6 g/day) under very rich plankton conditions.

Guided by the above results, the significance of abundant plankton is presently being tested by feeding cotton seed cake only and/or utilizing cow manure to fertilize rearing ponds. Clarias averaging 78 g and mature Tilapia nilotica of 44 g average weight were mixed in some ponds to determine the predation efficiency of the former and the effect of tilapia fingerlings on the growth rate of Clarias. The results of the experiment are given in Table II. A two week acclimatization period was allowed and the figures pertain to week 2 through 5 (to date). This coincided with the appearance of tilapia fingerlings.

Considering the short duration of the tests, no conclusions can be made at this time. However, the following observations were made:

1. Eutrophic pond conditions alone do not allow for a satisfactory growth of Clarias of this size (78 g).

2. A simple local waste product as cotton seed cake can be successfully utilized as food given a sufficient quantity of plankton.

3. The availability of tilapia fingerlings as prey appears to enhance the growth of Clarias.

4. The polyculture of Tilapia nilotica and Clarias lazera for production purposes appears to be feasible. The predation capacity of Clarias needs to be quantified to determine suitable stocking rates of predator and prey. A strict stocking schedule will be necessary to gain maximal benefit from predator/prey culture.

5. An intermediate mortality due to an infestation with Dactylogyrus in many young Clarias lazera was successfully counteracted with formalin, using a standing bath treatment of 200 ppm for one hour.

TABLE I

Results of Clarias lazera breeding trials

(a) The same females were used as in trial number 4

TABLE II

Results of Clarias lazera rearing trials

OBSERVATIONS ON THE GROWTH OF Tilapia nilotica L. IN TROPICAL FRESHWATER FISH PONDS TREATED WITH DIFFERENT FERTILIZERS

by

T.T. George
Fisheries and Hydrobiological Research Section
Khartoum, Sudan

Abstract

Experimental trials were carried out during 1969–70 in twelve fish ponds of the Experimental Fish Farm at Shagarra to test the effect and economic value of four fertilizers (cow-dung, poultry manure, lime and triple-superphosphate) in increasing fish production and to determine the dose and frequency of their application.

Highest production of fish was obtained when a combination of superphosphate and cowdung or poultry manure were used. When applied singly, superphosphate and poultry manure gave better results than lime or cow-dung. Although the use of superphosphate is made difficult by high costs (U.S.$ 0.1/kg) and problems of availability, it is suggested that returns in terms of increased fish production justify the costs involved. The biological and economic implications of the results are discussed.

Résumé

Des essais ont été enterpris, à titre expérimental, en 1969–70 dans douze étangs à poisson de l'Etablissement de pisciculture de Shagarra en vue d'analyser les effets et la valeur économique de quatre fertilisants (bouse, fumier de volaille, chaux et superphosphate triple) et d'en déterminer les doses et fréquences d'application pour augmenter la production de poisson.

La production la plus elevée a été obtenue par l'utilisation combinée de superphosphate et bouse ou fumier de volaille. En application simple, le superphosphate et le fumier de volaille donnent de meilleurs résultats que la chaux ou la bouse. Bien que l'emploi de superphosphate soit limité par le coût élevé (U.S.$ 0.1/kg) et la disponibilité, il est estimé que le profit retiré en termes d'augmentation de la production de poisson justifie cette dépense. On discute les implications biologiques et économiques des résultats.

1. INTRODUCTION

In Sudan, Tilapia nilotica is the only indigenous species cultivated in ponds because:

1. it is capable of reproducing naturally in ponds;

2. its offspring are hardy and tolerate long-distance transportation;

3. the species is low in the food chain, i.e., non-predacious and can convert organic matter directly into edible fish flesh;

4. it can be readily adapted to crowded conditions; and

5. it is a highly relished fish.

At the ponds of the Shagarra Experimental Fish Farm, production tests with T. nilotica, carried out during 1954–68, relied mostly on natural pond productivity; supplementary feeding and pond fertilization were not practised. Under these conditions, it was difficult to produce marketable fish measuring at least 23–25 cm in length, even in a two-year period; a 5 cm long T. nilotica will reach 18–20 cm in 12 months and 25 cm in 18–24 months. Yearly yields of 250–500 kg/ha have been seldom reached and the best yield attained during 1956–57 was 165 kg/ha/year.

The beneficial effects of fertilizers on fish production under various regimes of fertilization have been summarized and reviewed by Swingle (1947) and Mortimer and Hickling (1954). Swingle and Smith (1939a, 1939b) obtained a significant increase in plankton and fish production by combining inorganic nitrogen, phosphorus, potassium and lime. Smith and Swingle (1940) also reported increased plankton as well as fish production using both organic and inorganic fertilizers with and without superphosphate (Smith and Swingle, 1943). Although such experiments carried out under temperate and other climatic conditions in various parts of the world helped establish some standards for fish pond fertilization, relatively little information is available on this subject with reference to tropical conditions in Africa. Much basic and background knowledge is required before fertilizers can be used to the best advantage in Africa (Lingen, 1966).

The present paper reports on the use of organic and inorganic fertilizers in ponds of the Expertimental Fish Farm at Shagarra, lying about six miles south of Khartoum on the White Nile. The experimental trials involved the use of four types of fertilizers, applied separately and in combination, in twelve fish ponds for a duration of six months during 1969–70.

The objectives of the study were:

1. to test the effect of organic and inorganic fertilizers on increasing production of fish food organisms and, indirectly, production of marketable-size fish;

2. to determine whether lime, cow-dung or poultry manure has any beneficial effect when applied with superphosphates;

3. to justify the use of superphosphate from an economic point of view as compared with other cheaper and readily available organic fertilizers such as cow-dung and poultry manure.

2. MATERIAL AND METHODS

T. nilotica was obtained from the White Nile at Gabel Aulia Dam. Mixed sizes of fish, ranging from 7 to 12 cm total length, were stocked in two series of ponds (Table I). Series A ponds consisted of six small earthen ponds, each measuring 21.4 × 9.4 m, while Series B ponds consisted of an equal number of bigger ponds, each measuring 55 × 27.5 m. Water level was kept constant at 55 cm and 75 cm in Series A and B ponds respectively by making up evaporative water loss with water taken from the White Nile. Fish losses observed in the control pond of Series A two days after stocking, were replaced by fish of the same size.

The experimental design (Fig. 1) consisted of a control pond for each series, four ponds for treatment with separate fertilizers and six for mixed fertilizers. The fertilizers used were cow-dung, poultry manure, lime and triple-superphosphate. Fertilizer application rates are shown in Table II.

Monthly applications of fertilizers were carried out; half the dose was applied at the beginning and the middle of each month. The superphosphate was first completely dissolved in a bucket of water and then spread evenly over the water surface to reduce the loss of phosphorus due to the formation of insoluble phosphate; lime was also dissolved before spreading over the water surface. Cow-dung was placed in a sack at one corner of each pond, allowed to be soaked and then squeezed; poultry manure was spread over the water surface.

Water analysis for oxygen content, pH and temperature variations, was carried out every two weeks at 09.00 h in all ponds. All samples were taken at the pond sluice outlet in Series A and the sluice inlet in Series B. The oxygen content was determined by the Winkler method, the pH value by a Beckman pocket pH meter, and the water surface temperature was recorded with a simple thermometer. The quantitative evaluation of plankton samples for the last month (May) in Series A and B ponds was determined by the water displacement method. The volume of plankton in each sample was expressed as the volume in ml/l of water. The qualitative composition of those samples was also analysed. The number, total length and weight of fish was recorded at the end of a six-month period by draining the ponds totally and cropping the fish (Fig. 2).

3. RESULTS

3.1 Fish production

Table III shows total production from each pond against the initial weight stocked. Production (weight fished out - weight stocked), expressed as percentage increment, is also given in Fig. 3. The best results were obtained with mixed applications of phosphate plus cow-dung (B6) or phosphate plus poultry manure (B5).

The length/weight relationship in Series A and B ponds at the end of the six-month period is shown in Table IV, Figs. 4 and 5. These show that when poultry manure was used, most of the fish were in the 26–27 cm size group and to a lesser extent in the 23 cm group, the maximum length reached being 27 cm. But with superphosphate, most of the fish were in the 19–21 cm size group, with a maximum length of 28 cm. Similarly, most of the fish ranged between 29–31 cm and 29–30 cm when superphosphate plus poultry manure and superphosphate plus cow-dung were used, the maximum length recorded being 32 cm (Fig. 6) and 31 cm respectively.

3.2 Water characteristics

Average water temperatures, during the experimental period, are shown in Fig. 7; fluctuations in water chemistry (pH, O₂ and colour of water) are given in Table V. Quantitative and qualitative data on plankton are given in Tables VI and VII.

4. DISCUSSION

The effect of fertilizers on the productivity of fish ponds is of considerable biological and economic interest.

4.1 Biological aspects

4.1.1 Fish production

The stocking rate in the present study was not taken into consideration since “fertilizer studies are better kept separate from stocking rate studies” (Prowse, 1966a). This is why the yield is expressed in weight/area and not in weight/ha/year.

From Table III, it is clear that there is a difference in yield between the two control ponds. This is attributed to the difference in the initial number of fish stocked and to the “living space” theory. Brockway (1950) and Kawomoto and Nakanishi (1957) have shown that small ponds may inhibit the growth rate of fish; Prowse (1963) and Chen and Prowse (1964) also reported that growth rate increases with pond area.

Considering the biological aspects of superphosphate, cow-dung and poultry manure, the present results agree with Smith and Swingle (1943) who reported that the effectiveness of organic manures can be increased by the addition of an inorganic fertilizer like superphosphate. However, when lime was applied separately or in combination with cow-dung or poultry manure, production was less than that of other fertilizers. Muller (1957, 1961a, 1961b) found no effect of liming on pond fish growth, and Swingle (1947) recorded even a decline in fish growth when lime was administered to pond water showing an alkaline reaction. According to Vinberg and Liakhnovich (1965), liming is unnecessary and even harmful in alkaline water; it speeds up decomposition of organic matter and thereby releasing CO₂ from the bottom sediments (quoted by Wolny, 1966). These findings account for the results obtained in the present study when lime was used.

From the present investigations, it is noted that when poultry manure is used separately, it has the same, and even better effect as superphosphate. This agrees with the conclusion of Sklower (1951). However, according to Huet (1952), “the disadvantage of cow-dung and poultry manure treatments is the risk of de-oxygenation of the water and the liability of gill-rot disease among fish when using poultry manure”; Sklower (1951) stated that “cow-dung is objectional because it contains so much ineffective cellulose which resist decomposition and shuts off the pond soil.” But, in spite of these facts, no fish mortality occurred; fish were healthy, had good taste, did not contain nematode worms in their hearts or showed gill-rot disease and the yield was significant, particularly when using poultry manure alone or in combination with superphosphate. These conclusions agree with Buschkiel (1937) who mentioned that “it is characteristic of a good tropical fish pond that very heavy fertilization with poultry manure can be done without harm to fish because the high temperature of the water, 22–23°C and even higher, accelerates the development of an enormous number of bacteria which starts the breakdown of the organic material.”

4.1.2 Water characteristics

Hutchinson (1957) remarked that a series of oxygen determinations along with a knowledge of turbidity and colour of water could provide more information about the nature of water than any other chemical data. The water colour (Table V) became dense green during February and April in ponds A2, A5, B5 and B6 due to the presence of a thick yellowish-green scum on the water surface.

The dissolved oxygen was influenced by the photosynthetic activities of phytoplankton, submerged aquatic weeds and the seasonal variation in temperature. The cause of low values of oxygen, particularly during April and May, may be mainly due either “to the decay of organic matter on the bottom of the pond or to excessive fertilization, leading to heavy growth of phytoplankton that form green batches on the water surface or bottom macrophytes” (Prowse 1963, 1964, 1966b; T.F.C.R.I. Report, 1961–62). In fact, from the present observations, the formation of green batches on the water surface, the growth of Charra and the low oxygen content are indications of over-fertilization effect. Consequently, these observations emphasize the need for much basic and background knowledge to test phosphate and poultry manure at several levels by “trial and error” so as to demonstrate the over-fertilization effect and thus standardize the technique to the best advantage.

During May, the oxygen level in pond A5 fell to 1.2 ppm but no fish mortality occurred. Ellis (1937) and Hora and Pillay (1962) stated that the lethal level for most fishes is below 3 ppm. However, the present condition agrees with Bishai (1965) who found that T. nilotica can resist high temperature and very low oxygen content.

It is observed from Fig. 7 that water temperature was lowest (16°C) during January and reached a maximum value during May (25°C). This temperature range was beneficial to fish production and agrees with the findings of Buschkiel (1937) and also Banerjea (1967) who has stated that “the significant effect of higher temperature is the increased rate of biochemical activity of the microbiota so that the release of nutrients by decomposition of organic matter at bottom is more at higher temperature with consequent increase in the nutrient status of water. Growth rate of fish is also much higher at higher temperature in the tropics.”

The pH range during the experimental period was 8.8–11.0 and 8.0–10.4 in Series A and B ponds respectively. The limit below or above which pH has a harmful effect is given by Ohle (1938) as 4.8 and 10.8. Swingle (1961) stated that pH 11.0 may be taken as the alkaline death point practically for all pond fishes. During February, the pH value reached 11.0 in ponds A2 and A5 and 10.8 in the control pond A1, but no fish mortality occurred. According to Swingle (1947), water containing heavy growth of phytoplankton and/or submerged higher plants may increase in pH to 9.5 or even 10.0 (Smith, 1952) as the available free CO₂ and HCO₃ are utilized in photosynthesis. On the other hand, Neess (1946) maintained fish production at pH 12.0, the high pH being due to photosynthesis. Since the control pond A1 contained the submerged macrophytic weed Charra and A2 also contained Charra and phytoplankton while the rest contained phytoplankton, the high pH values may be attributed to photosynthesis.

Previous work (Abu Gideiri, 1969) carred out in the fish ponds of the Experimental Fish Farm at Shagarra has shown that plankton populations decrease during April but start to increase in May. Considering this fact, variations in plankton quantity and composition shown in Tables VI and VII are mainly attributed to the effect of fertilizer application. Thus, the maximum fish lengths and size ranges attained in these trials, and the quantity and composition of plankton in the respective experimental ponds, show the biological effects of fertilizer treatments.

4.2 Economic value

The economic value of fertilizer application is assessed by the increased production of market-size fish. Demol (1925) stated that “the best index of the efficacy of any manural treatment in a fish pond is not the effect on the plankton, bottom fauna, or on the rooted vegetation, but on the increase in fish crop over and above natural productivity.” As the experimental ponds in each Series A and B were initially stocked with the same number of fish, then the effectiveness of the fertilizer is judged by the greater weight and length of fish taken out of the fertilized pond as compared with the untreated controls.

Considering quantity and quality, it becomes clear that the yield due to poultry manure is greater than that due to superphosphate because most of the fish attained greater length. However, better results were obtained with superphosphate plus poultry manure than superphosphate plus cow-dung, as indicated by the greater fish length achieved. It is worth mentioning that a very high rate of breeding took place when superphosphate plus cow-dung were used. Most of the offspring were 7–17 cm in length at cropping; this accounts for the greater yield from pond B6.

Huet (1959) mentioned that “a 5-cm T. nilotica in the Sudan will reach 18–20 cm in twelve months and 25 cm in eighteen to twenty four months.” In the present investigations, most of the fish attained or exceeded marketable size, 25 cm, in six months only. This underlines the economic value of fertilizer application.

The results obtained with phosphate confirm Hickling's statement (1962) that “it is unlikely that a case exists where phosphate would not be beneficial.” But, the only disadvantage of superphosphate under the present experimental conditions and in most parts of Africa is its high cost and the fact that it is not readily available. Calculations of cost and profit showed that superphosphate is more expensive than poultry manure and cow-dung as it is imported and costs U.S.$ 0.1/kg (P.T. 4). In fact, Bishai (1963) and Hora and Pillay (1962) have shown that under tropical conditions, preference is given to organic manures on account of the ease with which they can be obtained and their low cost as compared to import restrictions which limit the supplies of inorganic fertilizers.

5. ACKNOWLEDGEMENTS

I am most grateful to Dr. Y.B. Abu Gideiri, Head of the Zoology Department, University of Khartoum and Dr. H.M. Bishai, Senior Lecturer, University of Cairo, for their comments and useful suggestions. Thanks are also due to Dr. A.I. El Magrabi, Acting Head of the Hydrobiological Research Unit, University of Khartoum and Mr. F. Sinada, Lecturer, Botany Department, University of Khartoum, for identification of plankton and to Prof. M. Huet for his encouragement to publish the results of this work.

6. REFERENCES

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TABLE I

Stocking of Series A and B ponds with T. nilotica

TABLE II

Dose of fertilizers and frequency of application in Series A and B ponds

TABLE III

Percentage increment in weight of T. nilotica at the end of six months in Series A and B ponds

TABLE IV

Length/weight relationship of T. nilotica in Series A and B ponds at cropping