Working PaperARAC/87/WP/5
February 1987
COVER
A review of the culture of Sarotherodon melanotheron in West Africa

David Campbell

AFRICAN REGIONAL AQUACULTURE CENTRE, PORT HARCOURT, NIGERIA
CENTRE REGIONAL AFRICAN D'AQUACULTURE, PORT HARCOURT

UNITED NATIONS DEVELOPMENT PROGRAMME
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH

PROJECT RAF/82/009

February, 1987


Hyperlinks to non-FAO Internet sites do not imply any official endorsement of or responsibility for the opinions, ideas, data or products presented at these locations, or guarantee the validity of the information provided. The sole purpose of links to non-FAO sites is to indicate further information available on related topics.

This electronic document has been scanned using optical character recognition (OCR) software. FAO declines all responsibility for any discrepancies that may exist between the present document and its original printed version.


A REVIEW OF THE CULTURE OF SAROTHERODON MELANOTHERON IN WEST AFRICA

DAVID CAMPBELL

ABSTRACT

The paper reviews results of the culture of Sarotherodon melanotheron in West Africa and problems encountered. A parental mouth brooder living in brackish waters from Senegal to Zaire, it tolerates salinity ranges of 0 to 45 ppt but prefers 10 to 15 ppt. Females can mature at 4.0 cm SL and males at 4.5 cm SL. Mean spawning frequency is every 22°C. It will grow and reproduce at pH 3.5 to 5.2. It can feed on phytoplankton, benthic diatoms, filamentous algae, periphyton, detritus, and artificial feeds. 4 to 5 week feeding with Methyltestosterone at 40 mg/kg feed gave over 95% males.

Under culture conditions in ponds, enclosures, and cages, growth is 0.5 to 0.7 g/day until a mean weight of 25 to 35 g in ponds and 50 to 60 g in cages and enclosures when stunting occurs. In ponds, growth is the same if fed wheat bran, chicken mash, pellets at 5% of the biomass, or with an application of manure at 1.8 T/ha. Biomass did not increase beyond 1.3 T/ha, yearly production was 1.9 to 3.5 T/ha/yr; highest values from ponds harvested in the shortest time. In enclosures, males virtually ceased to grow beyond 50g. Growth of tagged fish of both sexes was inconsistent; some males lost weight. In tanks, males were smaller that females at the end of 8 month feeding trials. In cages, growth was 0.1 to 0.2 g/day beyond 50 to 60g mean weight although there was no breeding; a monosex male culture, lower density, and altering quantity and distribution of the feed did not help. Reasons for stunting include sexual maturity, density, feeding, and chemical factors in the water. The fish can be raised efficiently to a weight of 35 to 50g using ponds, enclosures, or cages; a limited market for this size exists.

For larger fish and higher production, the acadja or brush park technique gives production of 7 to 20 T/ha/year with a size range after 1 year of 20 to 560g.

Research recommendations include a low input pond system, density studies in cages and enclosures, and determining optimum quantities of wood in the acadja. The acadja could be adapted to pond conditions.

COMPTE RENDU DE L'ELEVAGE DE SAROTHERODON MELANOTHERON EN AFRIQUE DE L'OUEST

DAVID CAMPBELL

RESUME

Un compte rendu des résultats de l'élevage de Sarotherodon melanotheron en Afrique de l'Ouest et des problèmes rencontrés sont donnés dans le document. S. melanotheron est un incubateur buccal, qui vit dans des eaux saumâtres du Sénégal jusqu'au Zaíre. Ce poisson tolère une salinité de 0 à 45%, mais préfère une salinite de 10 à 15 ppt. La taille de maturité est atteinte dès 4.0 cm de longueur standard chez les femelles et de 4.5 cm LS chez les mâles. La fréquence moyenne de ponte est de 22 jours et la période moyenne d'incubation est de 14 jours. La reproduction s'arrète en dessous de 20 a 23°C. S. melanotheron grandit et se reproduit à un pH de 3.5 à 5.2. et se nourrit de phytoplancton, des diatomés benthiques, des algues filamenteuses, de périphyton, de détritus et d'aliments artificiels. Une alimentation enrichie de 40 mg/kg de méthyltestostérone et distribuée pendant quatre à cinq semaines a sbouti à plus de 95% de mâles.

Dans des conditions d'élevage en étangs, enclos et cages, la croissance est de 0.5 a 0.7 g/jour jusqu'à un poids moyen de 25 à 35 g en étangs et jusqu'à 50 à 60 g en cages et en enclos, après quoi la croissance s'arrète. En étangs, la croissance est la même, si l'on nourrit avec du son du blé, de l'aliment (farine) pour poules, des granulés à 5% de la biomasse ou si l'on traite avec une application de 1.8 Tonne/ha de fumier. La biomasse n'augmente pas au-delà de 1.3 T/ha, la production annuelle était de 1.9 à 3.5 T/ha; les récoltes les plus élevées obtenues en étang l'étaient avec les élevages les plus courts. En enclos, la croissance de mâles s'arrète à 50 g. La croissance des mâles et de femelles marqués était variable; certains mâles avaient perdu du poids. Apprès huit mois d'essais d'alimentation en bacs, les mâles étaient plus petits que les femelles. La croissance en cages était de 0.1 à 0.2 g/jour au-delà des poids moyens de 50 à 60 g bien qu'il n'y avait pas de reproduction; un élevage monosex de mâles, à une petité densite et en changeant la quantite et la distribution des aliments n'ait pas parvenu à augmenter la croissance. Les facteurs responsables pour l'arrêt de croissance sont la maturité sexuelle la densité, l'alimentation et les facteurs chimiques de l'eau. S. melanotheron peut être élevé de façon efficace jusqu'à un poids moyen de 35 à 50g/étangs, enclos ou cages. Il existe un / en marché très limité pour des poissons de cette taille.

Des poissons plus grands et une production plus élevée peuvent etre obtenus avec la technique d'élevage en acadja avec laquelle on obtient une production de 7 à 20 T/ha/an et un évantail de poids après un an variant de 20 à 560 g.

Des sujets de recherche suivants sont suggérés: système d'élevage extensif en étangs, études de densité en cages et en enclos, et détermination des quantités optimales des buissons dans les acadjas. L'acadja pourrait être adapté aux conditions de l'étang.

1. INTRODUCTION

Sarotherodon melanotheron (Ruppel, 1852) is a brackish water tilapiine species found in the estuaries and lagoons of West Africa (Philippart and Ruwet, 1982). A recent review on the taxonomy and biology only briefly mentions aquaculture (Trewavas, 1983).

It is a popular food fish, and the culture potential has been mentioned (Pillay, 1965; FAO, 1969; Sivalingam, 1976; Pauly, 1976). Recent attempts to raise the fish on an experimental and production basis have shown that problems exist which are not found with the culture of other tilapia species (Legendre, 1983; Grino, 1984; Ibrahim, 1985; Cisse, 1985; Hem, inpress; Campbell unpubl.)

This paper reviews available information on the culture of S. melanotheron, including biological data pertinent to fish farming. It is by no means complete, as much of the culture work done has not been published or has received only a limited distribution.

2. IDENTIFICATION

S. melanotheron occurs from Senegal to Zaire in population groups, which differ in small ways (pigmentation, mean number of vertebrea, mean total dorsal rays, and mean lower gill rakers) (Trewavas, 1983). Available culture experience deal largely with the population ranging from Cote d'Ivoire to Cameroon. This population is given a subspecies status by Trewavas (1983) who labels it S. melanotheron. The same fish is labeled a separate species by Thys (1971). For simplicity, the appellation of S. melanotheron is used in this paper.

The species is characterized by its low numbers of vertebrae (26–29, usually 27–28), 12–19 lower gill rakers, 14–16 dorsal spines, deep preorbital bone, paternal brooding habit, and its strong preference for brackish water (Trewavas, 1983).

Coloration varies with location, sexual activity, and environmental background with a protective chameleon effect. Flanks vary from light blue to dark gold. The throat and ventral surfaces are usually white. The black spots (for which the species is named) on the chin and throat vary considerably both within and among populations. These spots may not be present at all or the entire head and ventral surfaces may become black.

Mature males often have a proportionately larger head caused by month brooding and loss of weight during incubation (hence the previous appellation of T. macrocephala).

More complete descriptions of S. melanotheron and the various subspecies are available in the literature (Trewavas, 1983).

3. REPRODUCTION

3.1. Sexual Maturity

The sexes can be determined by genital papilla. In mature males, the operculum is golden; in the female, it is more transparent and the red coloration of the gills can be seen. In Lagos, Nigeria, the smallest mature female measured was 69 mm SL, and the smallest brooding male was 78 mm SL (Trewavas, 1983). In ponds in the Niger Delta, Saeed (1983) found sexually mature females at 10 cm TL. Legendre (1983) reports first maturity at 134 mm. With a stunted population held under adverse conditions, Eyeson (1983) found mature females at 4.0 cm SL and brooding males at 4.5 cm SL. With these later fish, eggs measured 2.0 × 2.5 mm.

3.2. Spawning frequency

In its local habitat, the fish spwns throughout the year, although somewhat less during the rainly season (Trewavas, 1983). Under laboratory conditions, Eyeson (1979) found spawning occurred on an average once in 22 days, although there were periods of inactivity for 2 months or more.

3.3. Spawning

Information on the breeding behavior of this species is relatively complete (see Trewavas, 1983). The male makes a simple shallow depression (10 – 40 cm dia) in the substrate. The female sheds the eggs in a few large batches, the male fertilizes each batch but does not pick up them up until all are shed. If the male is slow in collecting the eggs, the female may collect them and incubate them.

Females produce anywhere from 200 to 900 eggs (Trewavas, 1983). The number of eggs that can be incubated by the male ranges from less than 20 (male of 20 g) to over 700 (male of 310 g) (Campbell, unpubl.).

3.4. Eggs and hatching

The eggs are orange and have a length range from 2.0 to 3.5 mm weighing 10 to 20 mg (Trewavas, 1983). Eyeson (1979) found a range of 1.9 × 2.1 to 2.6 × 3.0 mm with no relation of egg size to female size. Peters (in Trewavas, 1983) found the number of eggs decreased with increasing female size but the egg size increased.

Eggs hatch within 4–6 days at 27°C. The newly hatched fry are 4.1 mm in length and weigh about 20 mg. The fry are released 10 to 11 days after hatching when they measure 8.6 to 11.5 mm and the yolk sac is totally absorbed. The total bucal incubation time is about 14 days (mode), and may continue for as long as 19 days (Eyeson, 1979; Trewavas, 1983). This incubation period is at least 3 days longer than other month brooding tilapias (see Philippart and Ruwet, 1982).

The incubation instinct is very strong. In a flow through system, about 2000 50g fish were held in a 9 m2 fiberglass tank for 3 weeks. Spawning occurred, and as the tank was drained, several of the males released their eggs. Although swimming on their sides in less than 2 cm of water, the rest of the population promptly seized them. The eggs were not swallowed but were found several minutes later in the mouths (Campbell, unpubl.).

The newly released fry form a school and remain briefly under the protection of the male parent, although the length of the schooling and the protection is not known. Fourteen days after re-absorbtion of the yolk sac, fry measure 2 cm TL (Eyeson, 1979)

4. ECOLOGICAL PARAMETERS

4.1. Salinity

The tolerance and optimum salinity range of S. melanotheron has not been studied. The fish produces viable fry in freshwater aquaria if the embryos are held in the mouth, but saline water is needed in artificial incubation, 40% sea water giving the best results (Trewavas, 1983). The fish occurs rarely in freshwater, and only when brackish water is nearby. Pauly (1976) found S. melanotheron the predominant fish in a closed lagoon where the salinity fluctuated from 0 to over 45 ppt. In the Niger Delta, the fish is most common in the middle salinity ranges (10 to 15 ppt) but is present in both higher and lower salinities (Marioghae, pers. comm.).

A direct transfer from freshwater to 30.7 ppt caused 50% mortality, but apparently transfer from high salinities to fresh water is possible (Trewavas, 1983).

4.2. Temperature and pH

S. melanotheron is a relatively stenothermal species. The temperature range in it's natural habitat is about 18 to 33°C (Phillipart and Ruwet, 1982). No breeding occurs below 20 to 23°C (Trewavas, 1983).

In ponds constructed in acid sulfate soils, S. melanotheron grew well and reproduced at pH values ranging from 3.5 to 5.2. The lethal limit has not been studied (Campbell, unpubl.).

5. FEEDING

5.1 Fry and juveniles

Fry are opportunistic feeders, able to use a variety of food sources. Pauly (1976) found that the food of fry measuring 2.5 cm TL was principally harpacticoid copepods with some phytoplankton, benthic diatoms, and zooplankton. At 4 cm TL copepods were only rarely present, being replaced by phytoplankton and detritus.

In brackish water ponds, fry fed largely on phytoplankton with occasional zooplankton and detritus. As the fish grew, the number of gill rakers/cm decreased, and the diet contained progressively less phytoplankton (Hamman 1983). Trewavas (1983) points out that the columnar mucus cells of the pharyngeal apparatus used in preparing the food for digestion only develop when the fish reach 3.5–4.0 cm.

Fry can be carnivorous, and will feed on smaller fish larvae, eggs, and insects. They can be a pest when rearing other species by feeding on newly hatched fry, and they will attack larger, injured fish (FAO, 1969; Brino, 1984). The newly released fry readily accept artificial feeds and have successfully been reared with a variety of formulas. Legendre (1983) used a mixture of powdered milk, egg yolk, and vitamins. A finely ground mix of wheat bran, peanat cake, fish meal, and vitamins gave good results (Campbell, unpubl.).

5.2 Audlts

Adults also obtain food from a wide range of sources. Trewavas (1983) reviewed feeding habits recorded in the literature which include benthic diatoms, filamentous algae, periphyton, and detritus. In a closed lagoon in Ghana, Pauly (1976) found the fish ingesting mud enriched with pennate diatoms and organic detritus. Fagade (1971) reports the fish feeding on bottom mud, algae, detritus, and invertebrates in the Lagos Lagoon. In fish ponds in the Niger Delta, the adults feed on detritus, plankton, insect larvae, and ingest mud and sand (Hamman, 1983). Welcome (1972) found the fish using both periphyton and bottom fauna in Benin.

In Sierra Leone, Payne (1983) found the fish feeding almost exclusively on the algae growing on mangroves, rocks, and other hard surfaces. The stomach pH ranged from 2.0 to 4.0 with the majority of measurements exceeding 2.8. The fish can produce low enough pH for lysis of blue green algae, but these results did not confirm this. The anterior intestine had a pH range of 6.4 to 7.4 and the rest of the intestine was between 8.0 and 8.5 (Payne, 1978).

In concrete tanks in Cote d'Ivoire and Nigeria, adults feed actively on the periphyton and their teeth marks are clearly visible (Campbell, unpubl.). Hem (pers. common.) found S. melanotheron using the abundant periphyton available in submerged piles of brush.

Adults accept artificial feed either ground, in a mash, pelleted, or in the unconditioned state of an agricultural by-product.

6. CULTURE

6.1. Fry production

Reproducing S. melanotheron is not a problem; the fish will easily reproduce in ponds, concrete tanks, and aquaria. Broodfish are usually left for 4 or 5 weeks, and the unit drained and fry separated from the parents.

A system for mass production of S. melanotheron fry was developed in Cote d'Ivoire using large concrete tanks or “raceways” (Campbell unpubl.). The tanks measured 3 × 18 m with a water depth of 30 to 40 cm. Water flow was maintained at 2 to 3 m3/hr, the salinity varied from 3 to 8 ppt, and the water temperature from 26 to 32°C. Broodfish of 100 to 200 g were placed at a density of about 6 fish/m2 (300 to 400 fish/tank) at a sex ratio of 1:1 and fed to satiation once daily in the afternoon with a 28% protein pellet.

Spawning began soon after stocking. Little territorial activity was noticed, but one could easily identify brooding males by the extended lower jaw. About 3 weeks from stocking, the water level in the tank was lowered to leave 2 to 4 cm in the lower end of the tank; the brood, fish swimming on their sides. A small water flow was maintained. The induced stress caused the fish to release the fry, but rarely the eggs. The fry were attracted to the inflowing current, and were collected by using a length of mosquito netting. The brood fish were left in very shallow water until fry were no longer seen. The process was repeated on a weekly basis.

The fry were divided into 2 groups and placed in 3 × 18 m raceways. Automatic clockwork feeders were used to give a 28% protein feed during daylight hours. The diet was a mix of wheat bran, soybean and peanut cake, fish meal, and vitamins. This was passed 4 to 6 times through a hammer mill using a 1.5 mm screen, reducing much of it to a powder.

Water flow was steadily increased to about 15 m3/hr, and after 4 weeks, the two tanks were emptied and the fish sorted using an 8mm rigid plastic mesh which retained fish of 1g and larger. These fish were transferred to cages and the smaller fish returned to a tank. The next week, virtually all were large enough for transfer.

There were 10 breeding and 10 fry tanks. Water supply was 150 m3/hr. The monthly production was 200,000 to 250,000 fry of 1g.

Water was pumped directly from the lagoon where deliberate poisoning using insecticide (Lindane) for fishing was common. When contaminated water entered the system, fry were decimated within minutes, and although the broodfish would usually not succumb, they had to be entirely replaced as both the number and viability of the fry produced after poisoning was greatly reduced.

For one year, hormonal sex reversal using methyltestosterone at 40 mg/kg of feed for 4 to 5 weeks was used. The 100 mg hormone tablets were simply ground with the feed; alchol dissolution wasn't necessary due apparently to the relatively large size (20 mg) of the fry upon parental release. The exact rate of reversal wasn't known as the fish were held in cages until maturity and wild females entered the mesh, but populations were all over 95% males.

6.2 Pond culture

In brackishwater ponds in the Niger Delta, Grino (1984) and Ibrahim (1985) raised S. melanotheron in a monoculture and polyculture with T guineensis. Recovery rates of this later species were so low (less than 5% of total harvest) that the results can be considered as if a monoculture of S. melanotheron. When T. guineensis were recovered, mean weight was at least twice that of S. melanotheron.

Ponds were 0.2 and 0.4 ha, 30 to 40 cm deep. Brackish water (3 to 21 ppt) entered by tidal energy. In a series of trials, fish were stocked at 2 to 12 g mean weight at 1.5 to 5 fish/m2 and harvested 3 to 5 months later. Depending upon availability, a variety of feeds and fertilization regimes were applied. Feeds were always given at 5% of the biomass.

In all trials, growth was excellent (0.3 to 0.5g/day) until mean weight was 25 to 35g or the onset of sexual maturity. From this point, growth virtually stopped and the pond became rapidly over populated. There was no difference in growth when feeding wheat bran, chicken mash, or a pelleted feed, nor when the pond was heavily fertilized with chicken manure (see Table I).

Production varied from 1.9 T/ha/yr to 3.5 T/hr/yr, the higher production from ponds harvested in the shortest time. Biomass did not increase much beyond 1300 kg/ha, inspite of the length of time, and Ibrahim (pers. comm.) recommended that fish be stocked at a high density (5/m2) and cultured only 8 to 10 weeks or until they reach sexual maturity (25 to 35g) to increase yearly production.

Ironically, when conducting monoculture trials of T. guineensis in the same ponds, the few (less than 250/ha) S. malanotheron present grew from about 5g to 250 – 300g in 4 to 5 months while the T. guineensis ceased growing beyond 35g or sexual maturity. The S. melanotheron apparently fed on phytoplankton and some benthic materials (Campbell, unpubl.).

6.3. Enclosure culture

Particularly in Cote d'Ivoire and Benin, enclosure culture in shallow brackish water lagoons is becoming popular. Legendre (1983) used 25 m2 enclosures to evaluate the culture potential of S. melanotheron. The fish were tested in monoculture and polyculture with T. guineensis. they were fed twice daily with pellets, 5% of the biomass, 6 days a week. The feeds used were a mixture of fish meal, peanut and soy cake, wheat bran, maize meal, and vitamins, varying the proportions to obtain 28, 31, or 39% protein diet.

The enclosures were stocked at 10/m2. The actual time that the fish were held and fed in the enclosures is not clear. The mean weight at stocking varied from 11 to 90g. By combining data from several enclosures, theoretical growth curves were created.

The results from enclosures were the same in mono and polyculture. Feed conversions were high (4.7, 4.8) but were attributed to the presence of numerous small T. guineensis fry which had entered through the mesh and consumed an appreciable amount of feed.

Growth curves in the test derived by combining data show a good growth rate to about 50g, and then a sharp decline. This difference is more remarkable when the sexes are considered separately (see Figure 1). There is almost a complete stoppage of growth with the males, which corresponds with the onset of the mouth brooding feature.

Table I

Results of brackish water pond culture of S. melanotheron
(from Grino, 1985)

Trial number1234567
Stocking density (/m2)1.522.52223
Initial mean weight (g)5212518265
Total stocking weight (kg)7540297100375520150
Number of days15314370141104136144
Feed (5 × a week)       
poultry mash (16 % protein)
1912--129012902340-
wheat bran (15 % protein)
--2501140400--
pellets (35 % protein)
--670----
Fertilizers       
Chicken Manure (initial application)500750-7506006001875
Lime (initial application)200250200750750750250
N:P:K 15: 15: 15 (initial application)210-1908550105175
Urea (applied every 2 weeks)-380270375300137350
Triple super phosphate (applied every 2 weeks)---100100100-
Harvest mean weight (g)413143.835424231
Total       
weight (kg)86278711551340112012921280
Production       
Tons/ha/yr1.91.93.53.22.72.12.8

1 all data extrapolated to 1 ha basis from 0.2 and 0.4 ha ponds.

By extrapolation, males only grew to 120g mean weight in about 25 months, where females reached 150g mean weight in about 15 months. During the test period, there was an unexplained stop in growth for two months during the dry season. The reason was not clear, but attributed to climatic influence.

6.4. Tank culture

Cisse (1985) tested growth and food conversion of S. melanotheron using 2 × 2m concrete tanks and a brackishwater (0 to 5 ppt) flow through system in Cote d'Ivoire. Fish were stocked at 10 /m2. About 15 Hemichromis fasciatus were put in each tank to control reproduction. The fish were fed 3 diets of 20, 25, and 37% protein. Feeding rate was 3% of the biomass, 6 times a week. Initial weight was 20 to 39g. Experiments lasted 240 days. Mean survival was 60%; mortality highest among the males. There was no significant difference in growth and feed conversion between the 20 and 37% protein diet. With all diets, the males were significantly smaller at the end of the experiment, the most extreme example was with the 20% diet where males averaged 98g and females 172g. Food conversion varied from 4.1 to 7.8. A reason given for these high conversion values was that incubating males did not eat. Because of the high mortality and the small initial number of fish (40) in each trial, caution is needed when comparing results from other experiments conducted on a larger scale using several thousand fish.

6.5. Magnet and Kouassi (1979) began experiments in Cote d'Ivoire with cage culture of S. melanotheron. Using a feed of 15% protein, Initial growth rates of up to 0.6 g/day were obtained, but generally varied from 0.2 to 0.4 g/day.

In the same country, a commercial brackishwater (5 to 8 ppt) cage farm attempted raising S. melanotheron, T. guineensis and O. niloticus (Campbell, unpubl.). The S. melanotheron were stocked at 1g mean weight in ‘fry’ cages (8mm mesh 36 m3) with a stocking density of 1000/m3. Fish were fed 6 times daily with a 25 to 28% protein pelleted diet at a rate of 6 to 8% of the biomass. Sorting of 14 to 20g fish began after 1 month, the larger fish transferred to 18 mm mesh cages of 50 or 100 m3 in volume at a stocking density of 200/m3.

In the ‘fry’ cages, growth was 0.4 to 0.6 g/day. Food conversion values based on the given artificial feed were quite good; 1.2 to 1.5 and occasionally less than 1. The fish were apparently using plankton available in the cage.

After transfer to the larger mesh rearing cages, comparable growth and food conversion continued until the fish reached 50 to 60g mean weight, at which point the growth came to a virtual standstill (0.1 to 0.2 g/day).

No breeding had been observed, and no eggs or fry were ever seen in the mouths of the caged fish. The fish fed readily on the pelleted feed, and the other tilapia species farmed continued to grow well under the same conditions.

Fig. 1

Fig. 1: Growth rates of S. melanotheron (redrawn from Legendre, 1983)

Attempts to improve the growth of the S. melanotheron in the cages included using a sex-reversed all male population (see above), lowering aensity, altering the feed ration, distribution, and formula; all to no avail. The financial loss incurred led eventually to the abandonment of the farm.

In contrast, a previous experience on the same farm with S. melanotheron indicated that they could do quite well in cages. When the farm was established, about 700 fish of 25 to 45g mean weight were obtained for potential use in hybridization with O. niloticus, (see Trewavas, 1983) as this latter species does not tolerate the conditions of West African lagoons. The S. melanotheron were placed in a 36 m3 cage (4.25 × 4.25 × 2 m submerged) at about 20 fish/m3. The nylon net was 4mm mesh. Unlike most caged tilapias, these fish did not feed actively at the surface and pellets were washed away, so feeding was soon discontinued. After about 4 months, the cate was emptied. The fish ranged in size from 180 to 230g with a biomass of 4 kg/m3; apparently they had fed by filtering phytoplankton and eating the abundant periphyton presented by such a large surface of small mesh net. No fry were present.

At the same farm, 100 m3 18 mm plastic mesh cages that were neither stocked nor fed were found to contain up to 125 kg of S. melanotheron weighting 25 to 75g 8 months after placing the cage in the lagoon. The fish could only enter through the mesh if less than 14g. Apparently they fed on phytoplankton and periphyton.

6.6. Acadja

An ‘acadja’ or brush park is a simple fish culture technique (see Welcome, 1972; Kapetsky, 1982; Hem, inpress). An area of a shallow (1 to 1.5 m) lagoon with little tidal fluxuation is delineated by driving large, sturdy sticks into the substrate, and can be over a hectare in size. Bundles and piles of branches are placed inside at about 1 bundle /m2. The acadja serves as a shelter from various predators but is most important as an abundant source of food, principally the periphyton attached to the branches (Hem, pers. comm.).

The acadja is left for 6 months to 1 year, and no stocking is done. The harvest can be from 7 to 20 T/ha/year, increasing exponentially the longer the acadja is left undisturbed, and with the density of branches. The predominant species harvested is S. melanotheron, making up to 60 to 80% by weight, adlthough in the open waters around the structure this species constitutes less than 1% of the fishery (Welcome, 1972).

To evaluate the acadja, Hem (in press) used 3 nylon 14mm mesh enclosures measuring 25 × 25m. There was one empty control, one with 100 m2 of floating aquatic weeds placed in the center, and one with 100 m2 of acadja. Branches were placed at a rate of 1 bundle per m2. All enclosures were carefully fished with a seine net before the experiment, but it is possible that not all the fish were captured (Hem, pers. comm.). No stocking was done.

After one year, the empty control enclosure yielded 11.7 kg. the pen with aquatic plants yielded 18.2 kg, and the acadja yielded 80.5 kg of fish; S. melanotheron constituting 79% by weight. Mean weight of the females was 210g, males 160 g.

The 14mm mesh restricted movement to only very small fish. In the acadja, a size range of 20 to 560g was harvested with many individuals over 300g (see Figure 2). A few fish may have been inside the enclosure at the beginning of the experiment, but it is obvious that most of the production came from fish entering through the mesh or reproduced within the acadja during the year.

While the arithmetic mean weight describes the entire population, in practical culture terms the weight of the saleable fish is most important. The proportion of the weight of large, saleable fish in the total harvest is quite high (see Figure 2).

The S. melanotheron fed principally on the periphyton attached to the branches (Hem, pers. comm.). Another reason for the success may be the presence of numerous predators (Hemichromis, Heterobranchus spp.) which represented about 15% of the population at harvest. Newly released fry could also leave the enclosure through the mesh.

7. GROWTH

Legendre (1983) followed the growth of individually tagged fish of various sizes (about 30 to 100g) in enclosures. Fish were fed composed, pelleted feeds of 28 to 39% protein. There was a very large variation in individual growth, both between and among sexes. Browth was not continual, but was a succession of slower or faster growing periods. Many males often ceased growing completely or actually lost weight, which was attributed to the incubation period.

In fish farming, the term ‘growth’ usually means the increase in the mean weight of the cultured population. S. melanotheron grows well under certain conditions; in acadjas, in ponds at very low densities, and cages with low density and abundant periphyton (Welcome 1972; Hem, inpress; Campbell, unpubl.). However, there is very poor growth with normal tilapia culture techniques. Growth rates of S. melanotheron under culture conditions are given in Table II.

Fig. 2
Fig. 2

Fig. 2: Weight distribution of S. melanotheron from an Acadja (Redrawn from Hem, in press)

Table II

Sarotherodon melanotheron Growth rates under culture conditions

Weight
initial
(g)
final
time
(days)
growth
(g/day)
conditionssource
18 – 14300.23 – 0.43cages, 25% proteinCampbell, unpubl.
145050 – 700.51 – 0.72cages, 25% proteinCampbell, unpubl.
5065 +90 +0.15 0.20cages, 25% ProteinCampbell, unpubl
2 – 5311430.18 – 0.20ponds, organic and inorganic fertilizersBrino, 1984
5 – 2631 – 4390 – 1500.11 – 0.35ponds, feeding and fertilizationBrino, 1984
20 – 2653 – 691800.18 – 0.23enclosures, 28–39 % proteinLegendre, 1983
11581800.26enclosures, 31–39% proteinLegendre, 1983
37 – 39130 – 1732400.37 – 0.56tanks, 25–37 % protein females onlyCisse, 1985
37 – 3991 – 1092400.22 – 0.30tanks, 25–37 % protein males onlyCisse, 1985
5 – 10150450 (15 mo)0.33enclosures, females, estimateLegendre, 1983
5 – 10120750 (25 mo)0.16enclosures, males, estimateLegendre, 1983
5 – 7210 mean 20–550360 70.57 meanacadja, femalesHem, inpress
5 – 7160 mean 20–440360 70.44 meanacadja, malesHem, inpress

There is excellent growth (0.5 to 0.7 g/day) to 25 to 35 g in ponds and 50 to 60 g in cages and enclosures. Beyond this, growth slows dramatically, with the exception of the acadja, and to some extent tanks (Hem, inpress; Cisse, 1985).

7.2. Stunting under culture conditions

Possible reasons for an abrupt change in the growth rate, or ‘stunting’, include the onset of sexual maturity, density, feeding, and themical factors in the water.

7.2.1 Sexual maturity

Stunting is most often attributed to the onset of sexual maturity and the subsequent change from stomatic growth to gametogenesis, further compounded by the mouth brooding habit of the male (Legendre, 1983; Grino, 1984; Cisse, 1985; Ibrahim, 1985). If females spawn the year around with a mean frequency of once every 22 days, and males spend about 14 days incubating the eggs and fry and do not feed; a substantial amount of growth potential is lost.

First sexual maturity under culture conditions is 25 to 30 g (Legendre, 1983; Saeed, 1984; Campbell, unpubl.). The size of sexual maturity is an individual characteristic; in a population it is over a range of sizes. In ponds and cages, the entire population does not become mature until the mean weight is 50–75 g. This is 2 to 3 months after the first individuals become mature (Saeed, 1984, Campbell, unpubl.). If the stunting were due only to the onset of gametogenesis and breeding, the change in the growth rate should be more gradual.

In the acadja, good growth continued for both males and females even though reproduction occurred (Hem, pers. comm.).

If reproduction does not occur, the males should grow faster, as most tilapia fishes. Hem (pers. comm.) indicates this is true, but the size range and experimental conditions were not available.

However, populations of over 95% males held in cages with no reproduction ceased growing beyond 50 to 60 g (Campbell, unpubl.), indicating something further may be involved.

7.2.2. Density

In cages enclosures, and ponds, the densities used by workers are similar to that practiced with other tilapia species under the same conditions, yet growth is still poor.

In intensive cage culture of tilapia using composed artificial feeds, a stocking density of 200 fish/m3 is considered moderate, and O. aureus, O. niloticus and O. mossambicus grow well (0.5 to 2 g/day) at densities of 400–600/m3 (Coche, 1983; Campbell, unpubl.). S. melanotheron of 1 to 15 g grew very well in small mesh fry cages at densities over 1000 /m3. At 200/m3, good growth continued from 14 g to about 50 to 60 g mean weight, and then virtually ceased.

Cages were 36, 50, and 100 m3. Reproduction did not occur, and fish fed voraciously. Growth did not improve when density was lowered to 100 and 50 fish/m3 (Campbell, unpubl.).

In Cote d'Ivoire, water depth in enclosures is about 1 m. Stocking density was 10/m2, or over 6,000 fish per 25 × 25 m enclosure. Using artificial feeds, this is a very moderate nensity for enclosure culture (Hem, pers. comm.). Again growth slowed considerably after 50 to 60 g. Although reproduction occurred, overpopulation of the enclosure was not mentioned as a problem (Legendre, 1983).

In brackish water ponds, growth rates at stocking densities of 1.5 to 2/m2 were good to the point of sexual maturity, at which point the pond was soon overcrowded (10 to 20 fish/m2) (Grino, 1984; Ibrahim, 1985). This occurs with any pond cultured tilapia if reproduction is not controlled. However, contrary to most tilapia pond production where overpopulation occurs, the total harvested biomass was much lower (1.3 t/ha) than would be expected using similar quality feeds and heavy fertilization rates (see Table I).

Similar poor results were obtained with a monoculture of T. guineensis in the same ponds. The low production may be due to the shallow depth of the ponds (50 cm), the brackish water conditions, or the species (Campbell, unpubl.).

In contrast, in 100 m2 of acadja, Hem (inpress) found 640 fish of 13 species (353 S. melanotheron) weighing 80 kg. Total density was about 6.5 fish/m2. There was good growth and a production of 8 t/ha/yr. The growth and high production is continued for much larger (1 ha) acadja (Welcome, 1972).

7.2.3. Feed

The ability of tilapia to grow well using a variety of artificial feeds is widely known, and the same or similar formulae work well with other tilapia species. Juvenile S. melanotheron grew well on the various feed formulae used in ponds, tanks, cages, and enclosures with food conversion values around 2.0. There is no indication of a radical shift in nutritional needs from juvenile to adult. Once stunting has occurred, all authors report high conversion rates (4.0 to 11.0).

In tanks, cages, and enclosures where the biomass can be closely monitored, fish were fed from 5 to 3 % of the biomass. At similar feeding rates with comparable diets, other tilapia species continue to grow well with food conversion values of between 2 and 3 (Coche, 1983, Campbell, unpubl.). With caged fish of 50 to 60 g, there was no significant difference in growth when the food ration increased (Campbell, unpubl.).

7.2.4. Chemical factors

Other potential reasons for stunting include low dissolved oxygen, ammonia, pH, and salinity (Pullin et. al., 1983). The effects are more likely to manifest themselves in ponds where water exchange is limited, and this may very well be a cause in the poor results obtained in pond culture (see Table I).

In culture systems with a free exchange of water, low DO and a buildup of metabolic wastes is not usually a problem if density and total biomass are kept within reasonable limits. In the trials in cages and enclosures, density and biomass were low, salinity and pH did not vary significantly, yet ‘stunting’ occurred (Legendre, 1983; Campbell, unpubl.).

Organic substances or pheromones excreted by the fish may paly a role. Fryer and Iles (1972) note that juvenile S. melanotheron (as T. macrocephala) produce a “fright substance” (Schreckstoff) when the skin is injured, warning other fry. This is unusual for Perciformes. A similar substance could be creasted when the fish are stressed, inhibiting growth.

8. DISCUSSION

The culture of S. melanotheron then raises some interesting problems. If reproduction occurs, the female grows faster, contrary to most farmed tilapias. There is frequent spawning and an unusually long incubation time, further compounding the slow growth. Beyond 50–60 g, growth of both males and females slows considerably, inspite of high quality feeds and controlled reporduction. The stunting is in all likelihood a combination of factors. The relationship between growth and sexual maturity, reproduction, density, quantity and quality of feed, and chemical necessarily improve growth.

In contrast, results from acadjas are surprisingly good, particularly when one considers that the only input is bundles of branches at the beginning of rearing period. Production of 7 to 20 t/ha/yr is similar or better than well managed freshwater pond culture of tilapia using fertilizers and feeds. The size range of the harvested fish easily meets market demand, and the technology is simple. Results are the same when used on a large scale (Welcome, 1972).

Using normal culture techniques, S. melanotheron can be raised efficiently and in a short time to a weight of 35 to 50 g. It attains this weight faster than other tilapia fish in the same environment (T. guineensis). The embryo and released fry are larger than usual for tilapia, giving it a head start on growth. This kind of culture can be done in ponds, enclosures, or cages, as fish of this size do very well on plankton, benthic materials, and all kinds of artificial feeds.

In West Africa, there is a market for these small fish; over 100 kg a day were consistently sold to lagoon fishermen in Cote d'Ivoire and pond harvests of 400 to 600 kg in the Niger Delta are easily sold in a few days (Campbell, unpubl.). The market is limited, however, and the sale of large quantities would be a problem unless some economical means of canning of preserving is found, which is unlikely in the near future. Fresh fish of at least 150 g are much more popular.

In brackishwater pond culture, the low production may be compensated by using fewer and less expensive inputs such as compost or low rates of fertilization and very simple feeds, and aiming at a lower biomass. This could prove to be very economical if the correct combination of density, fertilizers, and feeds can be determined and if a market exists for 30–50 g fish.

It would be difficult to raise larger fish using the usual tilapia pond culture techniques, as this requires control over density and reproduction. The technique of mono-sex male culture or polyculture with a predator would be less productive due to the male mouth brooding habit, but this needs to be confirmed. There may be some potential in polyculture with other species at very low densities.

In cages and enclosures, the stress factors that cause stunting need to be determine d before large fish can beeconomically reared. Density would appear to be the key. There should be a relationship between stunting and density as expressed in terms of /m2 or /m3, and again between stunting and the total number of fish in a given culture unit.

There is a possibility of using very small mesh (4 to 8 mm) enclosures or cages in eutrophic brackish waters and allowing the fish to feed largely on the periphytom and plankton drift. Unit and mesh size, stocking density, and economics would need to be determined. Polyculture with mullet, catfish or other species should be attempted.

The acadja system is clearly the key to successful culture of this species, and the excellent results raise further questions. It is a viable system, inconveniences include eutrification of the lagoon, siltation, interference with fishing, and the enormous quantities of wood that are needed. However, Kapetsky (1982) and Hem (pers, comm.) show that these effects are limited. The nutrient load and siltation caused by the branches is minuscule compared to the effect of other natural sources. The acadja helps recruitment in the wild population, and fishermen are attracted to the area around the acadja inspite of frequent snags on discarded branches.

The replacement rate of the wood depends upon the salinity. In low salinities, it can be less thatn 100 %/yr (36 %/yr in Benin) or as high as 300 to 400 %/yr in high salinities where there are high concentrations of ship worm (Teredo spp.) (Kapetsky, 1982). If acadja culture were to become wide spread, nearby plantations for wood might be possible (Welcome, 1972; Kapetsky, 1882). In most West African lagoons and estuaries, the large surrounding swamps would yield sufficient wood on a sustainable basis (Hem, pers. comm.).

Fish production increases exponentially with the amount of branches in the acadja (Welcome, 1972), and the optimum quantity should be determined. What species compose the periphyton and what is the nutritional composition? To what extent does the shelter provided by the acadja eliminate stress on the fish, allowing normal growth and high production?

A pond acadja system could be envisaged, particularly in mangrove zones with a large tidal range such as the Niger Delta. Bundles of mangrove branches could be placed in ponds and a water exchange maintained by setting sluice boards somewhat below the level of high tide. This would also deter pouching by cast net. A level of production similar to the more traditional acadja may be achieved.

9. REFERENCES

Cisse, A. 1985. Résultats préliminaires de l'alimentation artificiel de Tilapia guineensis (Bleeker) et Sarotherodon melanotheron (Ruppel) en élevage, Proceedings; IFS Aquaculture meeting, Kisumu, Kenya, 1985.

Coche, A. G. 1983. Cage culture of tilapias, p. 203–246. in: R.S.V. Pullin and R.H. Lowe-McConnell (eds.) The biology and culture of tilapias. ICLARM Conference Proceedings 7, 432p International Center for Living Aquatic Resources Management, Manila, Philippines.

Eyeson, K.N. 1979. Studies on egg production, spawning, and fry development in Tilapia melanotheron. Ghana J. Sci., 17(1): 25 – 34.

Eyeson, K.N. 1983. Stunting and reproduction in pond-reared Sarotherondon melanotheron. Aquaculture, 31: 257–267.

Fagade, S.O. 1971. The food and feeding habits of Tilapia species in the Lagos Lagoon. J. Fish Biol. 3: 152 – 156.

FAO, 1969. Report to the Government of Nigeria on experiments in brackish water fish culture in the Niger Delta, Nigeria, 1963 – 68. Based on the work of K.K. Nair, FAO/UNDF (TA) Inland fishery Biologist (Fish Culture). Rep. FAO/UNDF (FB). (2759) 14 pp.

Fryer and Iles, 1972. The cichlid fishes of the great lakes of Africa. Oliver and Eoyd, Edinburgh.

Grino, E.G. 1984. Tilapia culture in ponds with fertilization/ feeding. Final Report (RAF/82/009). African Regional Aquaculture Centre, Port Harcourt, Nigeria.

Hamman, J. B. 1983. Food and feeding of Sarotherondon melanotheron (Ruppell, 1851) in relation to its morphological structures in a brackish water environment. M. Tech. (Aquaculture) thesis, African Regional Aquaculture Centre, and Rivers State University of Science and Technology, Port Harcourt, Nigeria.

Hem, S. (in press). Resultats préliminaires sur la productivite d'un système d'aquaculture extensite amenagé: Acadja-enclos.

Ibrahim, K.H. 1986. Final Report (RAF/82/009. African Regional Aquaculture Centre, Port Harcourt, Nigeria.

Kapetsky, J.M. 1982. Quelques considerations sur l'aménagement des pêcheries do lagunes côtieres et d'estuaires. FAO Doc. Tech. Peches, (218): 34 p

Legendre, M. 1983. Observations Préliminaires sur la Croissance et le Comportement en élevage de Sarotherodon melanotheron (Ruppel 1852) et de Tilapia guineensis (Bleeker 1862) en Lagune Ebrie (Cote d'Ivoire) lDoc. Sc. Cent. Rech. Oceanogr. Abidjan Vol XIV; No. 2, Decembre 1983, 1–36.

Magnet C. and Kouassi Y. S. 1979. Essai d'élevage de poissons dans les lagoons Ebrie et Aghien: Reproduction en bac cimentés, élevage en cages flottants. Ministere de la Production Animale, Direction des Peches Maritimes et Lagunaires, Republique de Cote d'Ivoire.

Pauly, D. 1976. The biology, fishery and potential for aquaculture of Tilapia melanotheron in a small West African lagoon. Aquaculture 7: 33 – 49.

Payne, A. I. 1978. But pH and digestive strategies in estuarine grey mullet (Mugilidae) and tilapia (Cichlidae). J. Fish Biol. (1978) 13, 627 – 629.

Payne, A.I. 1983. Estuarine and Salt Tolerant Tilapias. p 33–543. in: Proceedings; International Symposium on Tilapia in Aquaculture, Tel Aviv University, Tel Aviv. 624 pp.

Philipart, J-Cl. and J-Cl. Ruwet, 1982. Ecology and distribution of tilapias. p 15–59. in: R.S.V. Pullin and R.H. Lowe-McConnell (eds.) The biology and culture of tilapias. ICLARM Conference Proceedings 7, 432 p International Center for Living Aquatic Resources Management, Manila, Philippines.

Pillay, T.V.R. 1965. Report to the government of Nigeria on investigations of the possibility of brackish water fish culture in the Niger Delta. Rep. FAO/EPTA, No. 1973: 1–32.

Pullin, R.S.V., and Lowe-McConnell, r.H. 1983. (eds.) The biology and culture of tilapias. ICLARM Conference Proceedings 7, 432 P International Center for Living Aquatic Resources Management, Manila, Philippines.

Saeed, L. A. 1983. Studies on maturation and fecundity of Sarotherodon melanotheron and Tilapia guineensis in a brackish water environment. M. Tech. (Aquaculture), African Regional Aquaculture Centre, and Rivers State University of Science and Technology, Port Harcourt, Nigeria.

Sivalingam, S. 1976. The biology of cultivable brackish and marine finfish in Africa. Proc. FAO/CIFA Symp. on Aquaculture in Africa, Accra, Ghana. CIFA Tech. Pap., (4) suppl. 1: 283–291.

Thys van den Audenaerde, D.F.E. 1971. Some new data concerning the Tilapia species of the subgenus Sarotherodon (Pisces, Chichlidae). Rev. Zool. Bot. Afr. 84: 203–216.

Trewavas, E. 1982. Tilapias: Taxonomy and Speciation. P 3–13. in: R.S.V. Pullin and R.H. Lowe-McConnell (eds.) The biology and culture of tilapias. ICLARM Conference Proceedings 7, 432 p International Center for Living Aquatic Resources Management, Manila, Philippines.

Trewavas, E. 1983. Tilapiine Fishes of the genera Sarotherodon, Oreochromis, and Danakilia. The Dorset Press, Dorchester, 583 pp.

Welcome, R. L. 1972. An evaluation of the acadja method of fishing as practised in the caostal lagoons of Dahomey (West Africa). J. Fish Biol., 4: 39–55.

BackCover

Top of Page