A review of the biology and culture of Tilapia guineensis

David Campbell

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

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

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A REVIEW OF THE BIOLOGY AND CULTURE OF TILAPIA GUINEENSIS

DAVID CAMPBELL

ABSTRACT

The paper reviews the identification, range, reproduction, fecundity, feeding habits, physiological parameters, fry rearing techniques, and the culture in ponds, enclosures, tanks, and cages of Tilapia guineensis. The fish is euryhaline, found in estuaries and lagoons of West Africa. It is a nest building substrate spawner. 2.7 mm eggs hatch in 2 days (26 C) forming 5–6 mm larvae. Number produced is 200–3000. First external food is not known. Juveniles and adults feed on benthic materials and leaves and will readily accept artificial feed. Temperature range is 14–33C. Oxygen consumption decreases from 625 to 100 mg/kg/hr with increasing weight, lower lethal oxygen level is 0.2 mg/l. Lower pH limit is 3.4. Fry are produced in ponds, tanks, and cages. Pond culture yields 1.5 to 3.3 t/ha/yr using fertilizers and feeding. In enclosure farming, growth was 0.38 g/day; growth stopped for 2 months lowering values. In tanks and cages, best growth was 0.6 g/day. Recommendations to improve growth rates include genetic selection and nutritional studies.

LA REVUE DE LA BIOLOGIE ET L'ELEVAGE DE TILAPIA GUINEENSIS

DAVID CAMPBELL

RESUME

Cette publication passe en revue l'identification, la zone de répartition, la reproduction, la fecondité, les habitudes alimentaires, les paramètres physiologiques, les techniques d'alévinage et l'élevage en étang, enclos, bacs et cages de Tilapia guineensis. Le poisson est euryhalin et se ren∅contre dans les estuaires et les lagunes de l'Afrique de l'Oquest. C'est un pondeur sur substrat, qui construit un nid. Les oeufs ont 2,7 mm de diamètre et éclosent après deux jours (26°C), à l'eclosion, les larves ont 5 – 6 mm de longueur. Le nombre de larves produits par ponte est de 200–3000. La premère nourriture consommée n'est pas connue. Les juvéniles et les adultes se nourrissent de matériaux benthiques et de feuilles et acceptent aisément la nouŕriture artificielle. Les variations de température acceptées sont de 14–33°C. Les consommations d'oxygène diminuent de 625 – 100 mg/kilo/heure avec l'augmentation de poids et le taux léthal inférieur d'oxygene est de 0,2 mg/L. La limite inferieure de pH est de 3,4. Le frai est produit en étang, bac, ou cage. L'élevage en étang produit de 1,5 a 3,3 T/ha/an, en utilisant des engrais et de la nourriture. En enclos, la croissance du poisson est de 0,38 g/jour, quoique la croissance s'était arrêtee pour deux mois. En bac et en cage, la meilleure croissance obtenue était de 0,6 g/jour. Des recommandations pour améliorer la croissance doivent porter sur une sélection génétique et des études de nutrition.

1. INTRODUCTION

Tilapia guineensis (Bleeker, 1862) is a euryhaline species found along the West Coast of Africa (Philippart and Ruwet, 1982). There is an increasing interest in this fish for aquaculture purposes, particularly in areas of high or wariable salinities, characteristic of the estuaries and extensive lagoon systems which constitute its natural range. In this habitat, other species more traditionally used in ‘tilapia’ culture are either not locally available (Oreochromis mosambicus) or do not tolerate the prevailing saline conditions (O. niloticus). T. guineensis shares much the same range and habitat as Sarotheromis melanotheron, but neither species is well known for aquaculture purposes.

Legendre (1983) and Cisse (1985) of the Centre de Recherches Oceanographiques in Cote of Ivoire have recently published information on the culture of this species. However, an important amount of work has been done by students and researchers in West Africa, that for a variety of reasons has not been published or has received a limited distribution. An example of this are postgraduate theses of the African Regional Aquaculture Centre (ARAC), Nigeria, and research under taken by the staff. Another source is private fish farms. This paper is a synthesis of available information on the biology and culture of this species. Much of this comes from unpublished data and personal communi cations. This paper is by no means complete and an emphasis is placed on material that may be of use to fish culturists or researchers using this species.

For the purposes of this paper, larvae are defined as the newly hatched fish until the point of re-absorbtion of the yolk sac. Fry are defined as fish of 2 mg and 5 to 7 mm total length, or as soon as the yolk sac is absorbed, to fish of 500 mg (total length 25 mm). Juveniles are fish from 0.5 to 25 g, or the size of first sexual maturity (standard length 8 to 9 cm). Adults are any larger fish.

2. IDENTIFICATION

The usual coloration of T. guineensis is shiny, dark greenish yellow on the back and flanks becoming lighter in shade near the abdomen. The lower lip is white. The ventral part is usually white although in some specimens black and red coloration appears. All scales on the flanks have a black spot at the base. The anal fin is grey and the ventral fins are grey or black and marked by a white line in the anterior edge. The dorsal fin is gray or transparent with the black “tilapia” mark very prominent. The tail is bluish grey and banded with lighter colored spots and a distinctly shaded upper and lower portion.

T. guineensis is closely related to T. zilli. It is most easily differentiated from the latter by the mean number of spines in the dorsal fin (T. zilli = 15; T. guineensis = 16) and coloration (T. zilli has two horizontal dark bands, T. guineensis does not). More complete descriptions are available in the literature (Thys, 1966; Daget and Iltis, 1965).

Philippart and Ruwet (1982) noted that T. guineensis is geographically separated from other similar species (T. zilli, T. rendalli, T. tholloni, T. congica) and that they exclude each other. T. guineensis is found in the coastal waters of Africa from Senegal to Angola.

2.1 Classification:

3. LIFE HISTORY

3.1 Reproduction:

T. guineensis is a typical substratum spawner. There is a firm pair bonding with prolonged association, at least during one breeding cycle. The pair establishes a territory and both defend it. Inside the territory is a spawning nest. The nest consists of a series of holes, the form and dimensions varying considerably depending on the nature of the substrate and the size of the individual fish. In sandy clay soils, a typical nest will be in the from of a large basin, diameter 50 cm, with a series of holes leading from the bottom, each hole 10 to 15 cm in diameter with a maximum recorded depth of 1 meter (Legendre, 1983). It is not known if one or both of the sexes take part in building the nest.

When spawning, an area near one hole is cleaned of debris, and the female passes laying a single line of eggs. The male follows immediately and fertilizes them. The female then deposits another line, and the process continues until all eggs are spawned. The entire process takes any where from 5 to 15 minutes. When guarding the eggs, the black ‘tilapia’ mark disappears in the male but is retained by the female (Fryer and Iles, 1972).

T. guineensis will easily spawn in aquaria, the eggs adhering to the glass. Fertilized eggs have also been observed attached to dock pillars and mangrove roots, particularly when the natural substrate is very soft mud. On the fish farm, in 100 m3 cages, feed bags were partially filled with sand and placed in the lower corners of the cage to retain the cubic form. Using the bag as a substrate, T. guineensis couples spawned even at densities as high as 200 fish/m3 or 20,000 fish per cage.

3.2 Eggs and hatching:

Hanon (1975) reports that the length of the egg is 2.7 mm and the larvae are 5 to 5.5 mm at hatching. In the Niger Delta, eggs measure 1 × 2 mm and the larvae are 5 to 6 mm at hatching.

The eggs are yellowish-green in color and adhesive. The eggs can be moved from hole to hole in the nest by the female. The female, and to some extent the male, guard and ventilate the eggs. As the fry hatch, the female removes them with her mouth and places them at the side of one or more of the holes in the nest where the young larvae remain attached through the use of the head gland.

At temperatures greater than 26 C the eggs hatch in two days and the yolk sac is absorbed 4 to 5 days later. The newly hatched fry weigh 1 to 2 mg.

Both parents continue to guard the fry after the yolk sac is absorbed and the fry begin actively feeding, although the exact length of time the association continues is unknown. The parents and brood will leave the nest and travel extensively in shallow water (less than 10 cm) where the fry begin feeding. Estimations based on the size of the fry captured with continuing parental care indicate that the care continues at least 10 days after re-absorbtion of the yolk sac.

3.3 Fecundity

Fecundity is defined here as the number of fry produced per spawn. The number of fry increases with increasing size of the female, although there is considerable variation per spawn. Using the number of eggs in the ovaries as an indication. Saeed (1983) found an exponential value of 2.5 for increasing fecundity with increasing total length. Campbell et. al. (1986) used actual fry counts from females of 8 to 5 cm standard length which were spawned in controlled concrete tanks. A linear relationship gave the best fit, with Y = 219 × - 1019.5 (r = 0.635) where Y is the number of fry produced and × the standard length. There was a range of 200 to 3116 fry produced per spawn with an average value of 1355. Dadzie (1981) used hapas and fish of 30 to 66 g and found a range of 200 to 1532 fry with an average of 1202.

Issac-Harry (1986) investigated the various relationship between the number of eggs in the gonad, the gonad weight, the standard length and weight of the female, and the gonado-stomatic index. There was a significant correlation (r = 0.5873) between the number of eggs and the weight of the ovary as would be expected, and there was a negative correlation (r = -0.384) between the gonado-stomatic indeed and increasing standard length. There was a surprisingly random relationship between the number of eggs in the ovary and the standard length and weight of the female as well as the weight of the gonad and female weight.

3.4 Spawning frequency

With a relatively stable water temperature greater than 26 C and if the fry are removed from the female as soon as the yolk sac is absorbed or 6 to 7 days after spawning, the female will spawn again in as little as 3 weeks (Issac-Harry, 1986). The number of fry can be either more or less in subsequent spawns. If two females are stocked in a hapa with one male, the male will breed at 10 to 14 day intervals with both females (Dadzie, 1981)

It is probable that in the extreme northern range of the species that lower temperatures during certain seasons will increase the spawning interval, and there may be definite spawning seasons under these conditions. In most of its range, it spawns the year around.

3.5 Size at first maturity:

Legendre (1983) under culture conditions in enclosures found first sexual maturity in females of 154 mm fork length. In brackish water ponds in the Niger Delta, first maturity begins at 25 to 35 g (standard length 8 to 9 cm). However, specimens from the wild in the Nigeria Delta will spawn at smaller sizes (10 to 15 g). One individual female actually spawned viable eggs in an aquarium at 8.5 g.

4. FEEDING HABITS

4.1 Larvae and first feeding

First feeding is defined as the transitional phase where the larvae shift from the yolk sac to external sources of nourishment. Feeding habits and preferences of larvae T. guineensis at this stage are poorly understood. Where as other tilapia species at first feeding will accept and digest either zooplankton or plant material (Bodganova, 1970) and there is usually no problem at this stage using artificial compound feeds, this is not the case with T. guineensis. Poor success using compound feeds has discouraged several workers. Legendre (1983) noted a heavy mortality at first feeding using a mixture of egg yolk, powdered molk, and vitamins. In the Niger Delta, there was a poor survival with a variety of mixtures using crab meal, powdered molk, and powdered cereal baby rood. Although survival rates are not available, growth was very poor using other artificial diets (see Table I). These same or very similar formulae are often used successfully with other species of tilapia, indeed other genera of fishes (Legendre, 1983, Campbell, unpubl.).

The only available data on first feeding using natural feeds comes from fish that have been held in some kind of captivity, either aquaria or concrete tanks out of doors.

TABLE I
GROWTH RATES OF TILAPIA GUINEENSIS FRY

In these cases, survival is poor (5 to 50 %) and gut analysis was inconclusive as the stomachs were either empty or contained the algae most prevalent in the container at the time of stocking. There was however 100 % survival when large numbers of mosquito larvae were present, both in the container and stomachs. A survival rate of 60 to 80 % with good growth (Table I) were found when the larvae were stocked in a previously fertilized pond (Aleem, unpubl.).

It would appear then that the larvae of T. guineensis require a more specific food at first feeding, probably zooplankton of a particular size range. In practical aquaculture terms, the difference in the results obtained from pond rearing the larvae and all other attempts using artificial feeds is so striking that for now, it appears that ponds are necessary for successful and efficient rearing of this species at this life stage.

4.2 Fry and juveniles

In the estuaries of Sierra Leone, Payne (1978) reported that the fish of less than 6 cm in length fed on algae, and principally filamentous blue greens which they are able to digest. In brackish water ponds in the Niger Delta receiving only inorganic fertilizers, fish of 1.1 to 5.5 cm standard length are benthic feeders showing a strong preference for rotifers (Limnia, Pholidina, Branchionus sp.). They also fed on copepod nauplii, small (2 to 5 micron) benthic diatomes, and detritus (Mahatane, 1986).

4.3 Adults

Apparently the adults of the species consume and are able to digest a variety of natural and artificial feeds. Cisse (1985) considers the fish a benthic grazer, and these results are confirmed from stomach analysis in the Niger Delta (Mahatane, 1986). However, Payne (1978) found adults feeding on decaying leaves in the estuaries in Sierra Leone, and this author considered this species the only true estuarine leaf chewer (Payne, 1983). Fagade (1971) found the fish feeding on algae, detritus, sand, and invertebrates in the Lagos Lagoon. Philippart and Ruwet (1982) considered the species to be macrophagous. One can only conclude that T. guineensis is an opportunistic feeder, apparently able to consume and digest a variety of food items. The stomach pH values are extremely low; 1.0 to 3.7 with 75% of the observations being less than 2.0. A pH value of less than 2 will considerably help in the digestion of algae and bacteria (Payne, 1978). The fish has been observed actively eating holes in the leaves of the aquatic weed Nynphea lotus.

4.4 Artificial feeds

Juveniles and adults accept any variety of artificial feeds under a variety of forms and presentations. The fish will readily accept hard or soft pellets, feed in ground and powdered form, as a wet mash, or simply in the unconditioned state of an agricultural by-product. They feed actively on the surface or bottom of ponds and generally accept the feed as soon as they are aware of its presence.

No work has been done on digestibility of certain feed stuffs nor the specific nutritional needs of this species.

Fig. 1

Oxygen consumption of T. guineensis
(Data from Wokoma, 1986)

5. PHYSIOLOGICAL PLARAMETERS

5.1 Temperature

T. guineensis is considered a relatively stenothermal species with a temperature ranged of 14 to 33 C (Philippart and Ruwet, 1982) At high temperatures mortality is sporadic, and the species poorly supports sharp changes in temperature. A 25 % mortality occurred when the temperature fell abruptly from 18 to 8 C and the fish were left at this low temperature for 3 hrs (Malard and Philippart, 1981).

5.2 Oxygen

Using a sealed respirometer, Wokoma (1986) estimated the lower lethal Dissoloved Oxygen concentration for all sizes of T. guineensis to be between 0.1 to 0.2 mg/l. The fish goes into an anaerobic stage before death occurs. Oxygen consumption varies with the size of the fish, with smaller individuals (0.5 to 0.7 g) consuming 625 to 446 mg/ kg/hr depending on the ambient dissolved oxygen concentration. Larger individuals (7.5 g and above) consume about 90 to 100 mg/kg/hr (Fig. 2) (Wokoma, 1986).

5.3 pH

T. guineensis shows a remarkable resistance to low pH values The lower lethal range is between 3.0 and 3.3 (50 % mortality in 7 days) with fish surviving 7 days at pH 3.4 (Wokoma, 1986). In ponds constructed in acid sulfate soils, T. guineensis grows and reproduced at pH values ranging from 3.5 to 5.2. Some acclimatization is necessary. A direct transfer of 250 mg fry from pH values of 7 to 8 to a pond with a pH value of 3.7 caused 100% mortality in 24 hours.

5.4 Salinity tolerance

T. guineensis is a true euryhaline species; growing and reproducing in salinities of 0 to 35 ppt. The optimum salinity range has not been studied. In certain situations T. guineensis is the predominant tilapiine species in fresh water (eg. Lake Anyama; Aghien Lagoon, Cote d' Ivoire). In the Niger Delta, there are three main tilapiine species (S. melanotheron, T. guineensis, T. mariae. In low salinities (5 to 10 ppt) T. guineensis is by far the predominant species, but in the middle salinity ranges (10 to 15 ppt), the ratio of T. guineensis to S. melanotheron is roughly 1 : 1 and no T. mariae are present. In higher salinities, T. guineensis again predominates with a ratio of T. guineensis to S. melanotheron of 2 : 1 (Marioghae, pers. comm.).

6. AQUACULTURE

6.1 Fry production

6.1.1 Fry production in ponds

The fish will readily reproduce in ponds, however, the newly hatched fry are relatively small and easily fall prey to other species, particularly S. melanotheron fry and larger T. guineensis juveniles (FAO, 1969; NIOMR, 1981; Grino, 1984). In the intertidal mangrove zone where the burrowing grapsid crab Sesarma huzardi are found in concentrations of between 5 and 8 /m2, the dikes of the breeding ponds are easily perforated and it is extremely difficult to control the invasion of wild fish. Attempts to spawn T. guineensis in large numbers under such conditions are largely unsuccessful. Cannibalism on the newly hatched fry is also a problem, particularly in ponds with a low level of primary productivity and clear water where the fry are easily seen. Fish that are over one week old will soon begin to prey on the smaller newly hatched fry. In view of the above problems, attempts have been made to spawn the fish using other means.

6.1.2 Fry production in small concrete tanks

In the Niger Delta, T. guineensis was bred in small concrete tanks with a surprising degree of synchronized spawning (Campbell et. al., 1986). Concrete tanks of 0.4 m3 were filled with water from tidal ponds, the salinity varying from 11 to 20 ppt, temperature from 26 to 34 C. Broodfish were captured from unused tidal ponds and one male and one female of roughly the same size were introduced into each tank. As soon as spawning occurred, the male was removed to avoid cannibalism. The fry were collected by siphon as soon as the yolk sac was absorbed.

The wild fish spawned in an average of 3.3 days, and 62% of the couples stocked spawned within the first 7 days from stocking. In each 2 week cycle using 25 tanks, 10,000 to 35,000 fry were produced. Those produced within one week could be stocked into a single pond with no fear of cannibalism.

6.1.3 Fry production in hapas

Successful spawning in small, fine mesh cages of “hapas” has also been carried out in the Niger Delta (Dadzie, 1981). In terms of mortality of the broodfish, percentage of spawning, and ease of manipulation, the best cage size appeared to be 1 m3. The cages were made with a wooden frame and an outer cover of 2 cm plastic mesh (Netlon). The fine 1 mm mesh material of the hapa itself was separated from the larger mesh by the wooden frame, protecting it from swimming crabs. A small shallow wooden box was filled with mud and placed inside the fine mesh hapa to serve as a spawning substrate.

The best sex ratio was 2 females to 1 male. Spawning usually occurred 2 to 3 weeks after stocking. The male would breed with both females.

6.1.4 Fry production in large concrete tanks

T. guineensis has been bred successfully in Cote d'Ivoire using large concrete tanks or “raceways”. The tanks measured 3 × 18 m with a water depth of 30 to 40 cm. Water flow was maintained at 2 to 3 m3/hour, the salinity varied from 6 to 8 ppt, and the water temperature from 26 to 32 C. Broodfish were placed at a density of 4 to 6 fish/m2 at a sex ratio of 1:1. Along each side of the tank concrete bricks were placed at 2 to 3 meter intervals to allow for territoriality.

Spawning began 7 to 10 days. A couple would create a territory around a brick and defend it. As soon as the yolk sac was absorbed, the fry were collected by using a 1.5 meter length of mosquito netting and trapping the entire brood.

Spawning continued sporadically until the tank was drained. This became necessary as the few fry than would be missed when collecting the broods would soon grow to a size where they become extremely cannibalistic and preyed on the newly hatched fry of other broods. Production varied considerably, from 20,000 to over 100,000 fry/tank and per month. The variation could not be explained (Campbell, unpubl.).

6.2 Pond culture

In the Niger Delta, experiments on rearing T. guineensis in brackishwater ponds using only inorganic fertilizers have been conducted (Campbell, unpubl.). The 0.4 ha ponds were build in acid sulfate soils of the mangrove zone thus the pond water pH values were quite low; pH 4.5 ± 0.5. Salinity varied from 6 to 12 ppt during the experiments. The ponds were initially fertilized at a rate of 125 kg/ha with N:P:K 15:15:15 and boosted at 45 day intervals with urea (250 kg/ha) and triple superphosphate (50 kg/ha). Fish were stocked at a rate of 2 fish/m2 (8000 fish/pond).

The phytoplankton bloom that developed consisted almost entirely of blue green algae (Anacystes sp.) however stomach analysis showed that the fish were feeding almost exclusively on benthic materials and were not directly exploiting the plankton bloom (Mahatane, 1986). In spite of low pH values, an initial growth rate of 0.4 g/day was obtained. Growth then slowed almost to nothing as the fish reached 25 g mean weight and reproduction occurred. At the end of the 120 day rearing period, the total harvest was poor, yielding only 200 and 230 kg/pond (.5 to 1.725 t/ha/yr.). Low pH values undoubtedly influenced the results.

In another series of experiments, a suplemental feed made from brewers waste, palm kernet cake, and brewers yeast in a retio of 2:2:1 was given in addition to the fertilization regime described above. The feed was approximately 25% protein, the ingredients were simply mixed and were not milled or pelleted. A growth rate varying from 0.4 to 0.6 g/day was obtained. The means total harvest after 120 days was 450 kg/pond (3.3 T/ha/yr.). Food conversion varied from 2.4 to 3.2. About half of the harvest in each pond were unsaleable fry of less than 15 g. Again growth stopped as soon as the fish reached sexual maturity, or at 35 to 45 g mean weight.

An attempt to raise manually sexed males in a polyculture with Clarias gariepinus was successful until the predator disappeared when the salinity increased beyond 15 ppt. The few accidental females in the pond promptly overpopulated the pond and growth stopped.

6.3 Enclosure culture

Legendre (1983) tested growth of T. guineensis in small 25 m2 enclosures. 10 g fish were stocked at 10/m2. T. guineensis was tested by itself and with S. melanotheron. The fish were fed a 39% protein diet twice daily, 6 ×/week at 5% of the biomass. The growth of T. guineensis was slightly better in enclosure than S. melanotheron. The results were the same in monospecific and mixed species culture. There was a significant difference in growth rates of the males and females, 0.38 g/day and 0.30 g/day respectively.

The data is somewhat misleading as there was a stop in growth for two months during the dry season. Food conversions were high (4.7 : 4.8) but were attributed to the presence of numerous small T. guineensis fry which has entered the enclosures through the mesh, were trapped, and consumed an appreciable amount of feed. The fish also caused problems with its nest building habits. Fish would hide in nest holes when seining the enclosures.

6.4 Tanks

Cisse (1985) tested growth and food conversion of T. guineensis using 2 × 2 m 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 30% protein. Feeding rate was 3% of the biomass, 6 ×/week. Initial weight was 36.5 g. Experiments lasted 240 days. Mean survival was 80%. The best growth and feed conversion were with the 25% protein diet, where the males reached 185.2 g (0.6 g/day) and the females 111.5 g (0.3 g/day). Food conversion was 3.32. The author felt one of the reasons for the high food conversion value was that feed distributed during courtship and reproduction was not eaten.

6.5 Cages

Melard and Philippart (1981) raised T. guineensis in cages placed in heated industrial waste water over a 30 day period. With an all male culture, a 46% protein diet gave a growth rate of 0.6 d/day with a food conversion of 2.9.

In Cote d'Ivoire at a commercial brackishwater (5 to 8 ppt) cage farm, T. guineensis were raised with S. melanotheron and O. niloticus. From 1 to about 50 g, growth was similar to the two other species (0.4 to 0.6 g/day). Beyond 50 g, growth of T. guineensis varied from 0.6 to 0.8 g/day with males growing faster than females. The O. niloticus grew still faster, increasing to 2.0 g/day. However this later species succumbed to high mortality (Vibrio sp.) which did not affect the T. guineensis. Beyond 50g. The growth rate of S. melanotheron dropped considerably (0.2 g/day) (Campbell, unpubl.)

In both the above cage culture attempts, O. niloticus was preferred because of faster growth.

7. MISCELLANEOUS

7.1 Jumping

A 200 g T. guineensis can easily leap over 1.5 meters from the water. This can cause unpleasant surprises by being hit in the face when pulling a seine net, or watching market size fish leap over a normally adequate barrier when harvesting a cage or enclosure.

In tidal filled ponds with water control gates set to allow some tidal fluctuation, T. guineensis can be seen jumping over screens and boards, particularly at night or early morning. At high spring tide, fish are found stranded on the tops of dikes. The fish are attracted in the same direction as the tidal 'flow; when the water is flowing into the ponds, wild fish will try to enter.

When the tide reverses, those inside the pond will attempt to leave. To some degree, this may explain the poor recovery rates of this species noted by some workers who advocate constant free tidal flushing of ponds (Grino, 1984, Ibrahim, pers comm.).

7.2 As a pest fish

T. guineensis can be a pest when trying to raise other species, particularly in freshwater or low salinity areas. Legendre (1983) noted that small fish entered enclosures through the mesh and disrupting feeding trials, and that they ‘spontaneously’ colonized small ponds. In cages, the fish entered in such numbers that they seriously competed for the feed supplied to the farmed fish. A partial solution was to stock a few predatory fish (Clarias, large Chrysichthys) which apparently freghtened most of them away (Campbell, unpubl.).

If the substrate is firm, the fish can cover the entire pond enclosure bottom with nests causing problems when draining. This does not occur if the bottom is very soft mud.

8. CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER RESEARCH

T. guineensis shows good aquaculture potential in the various brackishwater environments of West Africa. It has been successfully raised in ponds, enclosures, cages, and tanks. It accepts the high density necessary for economical culture, unlike other available species such as S. melanotheron (Campbell, unpubl.).

However, when compared to other tilapiine species, all workers report relatively slow growth rates and improvement is needed. The information now available raises some interesting questions.

8.1 Reproduction and genetics

At this time, several fry production systems have been described which can be easily adapted to local conditions. As with all Tilapia species, there is a great potential for enhancing growth and production through genetic selection. Significant results with other tilapia species have been already been obtained (Bondari et al. 1983).

T. guineensis produces relatively large numbers of fry/spawn. Several couples can be made to spawn at the same time (Campbell et. al., 1986). This would allow the comparison of the same age fish from varying genetic strains under identical culture conditions.

8.2 Physiology

Work is needed to determine what physiological mechanisms are involved in low oxygen and low pH tolerance. The optimum salinity for growth needs to be determined.

8.3 Nutrition

The feed needed at the transitional stage from yolk sac to external sources needs to be studied, as well as producing such a feed in brackish waters.

An acceptable artificial feed for the post-larval stage would allow culture in controlled conditions and hormonal sex reversal could then be tried.

Beyond the size of 0.5 g, feed acceptance is not a problem. Of interest is the variety of feed stuffs reported as consumed by the juveniles and adults. Nutritional studies could be done on such things as fresh or dried cassava leaves and other inexpensive agricultural by-products. To what extent does it use bacteria present in the benthos?

8.4 Enclosures and cage farming

With cage or enclosure farming, T. guineensis currently depends upon artificial feeding (Legendre, 1983, Campbell, unpubl.). The nutrition of T. guineensis under cage and enclosure conditions.

Certain areas of the West African coastal lagoons are highly subtrophic with a mud substrate. Under these conditions, enclosure farming using such a benthic feeder could be envisioned.

8.5 Pond rearing

Pond fertilizers should be applied to raise benthic organisms as is done in Southeast Asian Chanos chanos culture. In a monospecific culture, T. guineensis does not utilize all the available pond resources, and culture trials were not particularly encouraging in terms of total production of salable fish. As a benthic feeder, the fish could be cultured in an association with phytoplankton feeders (mullet). In a polyculture system, the entire field of correct species ratios and proper feeding and/or fertilization is yet unexplored.

As a Tilapia species, prolific breeding can quickly cause overpopulation in ponds. The more common African predators used in fish culture (Hemichromis, Clarias) do not survive about about 15 ppt salinity (Nwigwe, 1985, Campbell, unpubl.) and work is needed to determine appropriate species and stocking rates under these conditions.

Monosex male populations are preferred due to their faster growth. Hormonal sex reversal has not been attempted. Even slight errors in manual sexing can lead to overpopulation due to the large numbers of fry produced per spawn.

The cannibalism noted in small (2 to 4 are) fry production ponds could perhaps be used to advantage in larger grow out ponds. Fish size, density, and pond conditions need to be studied before this becomes feasible.

9. REFERENCES

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