by
M.A. Afinowi
Federal Department of Fisheries
P.M.B. 12529, Victoria Island
Lagos, Nigeria
Abstract
The ecology, reproduction and growth of Anadara senilis were investigated in three areas along the West African coast. There was little difference in temperature between the various stations all the year round. A. senilis occurs naturally in water with salinity over 10‰ and in which diurnal and seasonal fluctuations are minimal. The deposits in which A. senilis were found ranged from coarse sand to black mud. It was absent from channels with strong currents. At all the stations, the period of lowest oxygen content coincided with the high salinity season. Trygon (= Dasyatis) margarita is the only known predator. A study of the effect of salinity fluctuation on the life cycle of G. gasar is discussed. Gaps in knowledge of cultivable shellfish and future guide lines are discussed.
Résumé
L'écologie, la reproduction et la croissance d'Anadara senilis out été etudiées dans trois zones cotières de l'Ouest Africain. On a noté peu de différence de température entre les stations au cours de l'année. A. senelis se rencontre naturellement dans des eaux à salinité supérieure à 10‰ et où les fluctuations diurnes et saisonnières sont minimales. On a trouvé A. senelis dans des zones de sable grossier mais aussi dans des vases noires. Elle est absente des zones à fort courant. A toutes les stations la période du plus bas niveau d'oxygène dissoue correspond à la saison des hautes salinités. Trygon (= Dasyatis) margarita est le seul prédateur connu. Une étude de l'action des fluctuations de salinité sur le cycle de G. gasar est discutée. Les difficultés dues au manque de connaissances sur les mollusques cultivables et les futures lignes de recherches sont revues.
Several mollusc species have been listed from the west coast of Africa (Buchanan, 1954; Nickles, 1950), but not all are necessarily cultivable. Going by the recent concept of aquaculture, for a species to qualify as cultivable it probably would have to satisfy the following conditions: occurs freely and easy to obtain; reproduces easily (or be easily induced to reproduce); grows fast; sufficiently large to be accepted as a worthwhile protein source; with controllable ecological parameters, and be commercially important.
Two bivalve molluscs occurring along the West African coast may be considered. They are the cockle clam, Anadara (Senilia) senilis, and the mangrove oyster, Gryphaea gasar (Adanson) Dautzenberg. The latter is synonymous with Crassostrea gasar.
Aquaculture aims at establishing conditions most suited to the settlement, growth and survival of the species that is being cultivated. Achieving this objective entails altering the balance of environmental factors in such a way as to favour the cultivated species. Stated in a different way, aquaculture aims at establishing a community dominated by the species under cultivation, in various stages from juvenile to adult when it is ready for the market. However, to be able to establish an environment that favours the cultivable species especially, the aquaculturist requires a thorough knowledge of the biology of the particular species. Aspects of biology that are of importance in this regard are: ecology, i.e., the life of the animal in relation to its biotic and physical environment; reproduction, and growth of the animal. With particular reference to bivalve molluscs, the basic anatomy is very similar and much has been written on the subject. In fact, most of the literature on A. senilis is concerned with its shell characteristics, classification and geographical distribution (Adanson 1757; Linne, 1758; Gray, 1847; Reeve, 1844; Thiele, 1935; Reinhart, 1935; Nickles, 1950, Yonge, 1955) gave a short account of its habits and the pattern of its ciliary currents.
Very little, however, is known about the ecology, growth and reproduction of this species which forms an important source of protein for many coastal tribes in West Africa. The two papers by Yoloye (1974, a and b) attempt to fill the gap in our knowledge of this economically important bivalve.
Sandison and Hill (1966) and Sandison (1966) have reviewed the ecological parameters and growth of Gryphaea gasar.
Anadara senilis is common in many West African estuaries and lagoons. It is endemic to West Africa and occurs from Rio-de-Ore in the north to Angola in the south (Nickles, 1950). Its collection in the wild is not regulated, thus the species had been eliminated from places where it once flourished. As yet A. senilis is not cultivated but is in fact cultivable. The ecological studies reported in this paper were undertaken in an attempt to save the fishery from total collapse.
The investigation was done in three main areas along the coast of West Africa (Fig. 1).
The Ghana lagoon
The Lagos lagoon system
The Niger Delta
The investigation in Lagos was done between October 1962 and December 1965 with a view to determining the conditions under which A. senilis flourishes.
Data were collected from the Lagos lagoon system fortnightly between November 1962 and January 1965. At Okrika and Andoni Flats (Rivers State, Nigeria) data were collected from January to December 1963. The three Ghana lagoons - Sakuma, Nyayanu and Princesstown lagoon - were visited in March 1963 at the peak of the dry season, and in August when the rains were heaviest.
Data were collected on the following ecological parameters: salinity, temperature, oxygen content, turbidity, nature of substratum, and associated macrofauna of each locality.
Below are the results obtained from the different stations:
2.1 Temperature
There was little difference in temperature between the various stations all the year round. The temperature of 31°C was obtained in March at the peak of the dry season, while during the rains (May to November), the temperature gradually fell to 26°C. The annual range was, therefore, 5°C (Figs. 6 and 7).
2.2 Salinity
The results confirm earlier observations by Olaniyan (1957), Hill and Webb (1959) and Sandison and Hill (1966) that there are two salinity seasons yearly in Lagos lagoon system (Fig. 2). The salinity was high from December to May, but low between June and December because of the influx of fresh water from the rivers. The waters of the creeks (except those without an opening to the sea) undergo a diurnal fluctuation in salinity.
Kurano Water and Onijegi lagoon (Figs. 2 and 8) have a salinity of over 10‰ all the year round. The lowest salinity of 11‰ was recorded in October 1963 while 25.5‰ was recorded as the highest salinity in March 1964. The maximum diurnal fluctuation recorded in Kurano Water was 2‰. Onijegi lagoon showed no diurnal fluctuation since it has no connexion with the sea or the Lagos lagoon.
The habitat of A. senilis in Okrika (Fig. 3) showed no wide seasonal or diurnal fluctuations. The water was brackish all the year round, ranging from 11.2‰ in August to 21.8‰ in February. The maximum diurnal fluctuation was 4.4‰ in August at the peak of the rains. At Isiodun (Andoni Flats), the salinity was above 11.4‰ but below 21‰ all the year round. The diurnal fluctuation ranged from 1 to 2‰ in the dry season to 6.4‰ during the rains.
The salinity of the three Ghanaian lagoons (Figs. 4 and 5) studied was very high during both the rainy and dry seasons. The minimum recorded salinity ranged from 22.9‰ in Sakuma lagoon to 32.7‰ in Princesstown lagoon. During the dry seasons with the seaward openings of these lagoons closed by sandbars, the inflowing rivers dry up and the water becomes hypersaline. The salinity then varied between 30.1 and 38.3‰. The observed diurnal fluctuation during the rains in Sakumo lagoon was 3.8‰, while in Nyanyanu lagoon it ranged between 1 and 2‰. There was no diurnal fluctuation in these lagoons during the dry season.
The lagoon waters at Ikoyi (Fig. 2) was brackish (10–11‰) at all states of the tide during the high salinity months (January to May). Between June and November, the salinity was below 2‰ except during August and September when the salinity rose to 6.7‰. The diurnal fluctuations in the salinity of the lagoon waters was small; less than 4‰ throughout the year.
The salinity at the Boat Club (Fig. 2) during the high salinity season was above 17‰ both at low and high tides. The salinity here dropped rapidly almost to fresh water condition (3‰) in June and July but rose again in August. During October and November the salinity was again low at both states of the tide (below 5‰). The diurnal fluctuation of salinity was small throughout the year except August when it was 9‰.
At the Federal Fisheries, the salinity was above 29.5‰ throughout the year during high tide. During the low salinity season, the diurnal fluctuation was up to 25‰ especially between June and August and in October.
The above results showed that A. senilis occurs naturally in waters with salinity over 10‰ and in which diurnal and seasonal fluctuations are minimal, whereas the salinity of the Lagos harbour and lagoon, where A. senilis is absent, fluctuated widely both diurnally and annually.
2.3 Nature of Bottom Deposits
The deposits in which A. senilis were found ranged from coarse sand to black mud. At Kurano and Onijegi lagoons, three main types of bottom deposits were found. On the seaward side of the lagoons, especially where the beach between the lagoons and the sea is narrow and bare, the deposit consists of 82.5% coarse sand with little silt or fine sand (1.3%). These deposits are derived from the barrier beach. On the northern bank, the deposits contained more silt and fine sand and tended to be blackish. At the western end of Kuramo Water the deposit consists of 60.5% black mud, 2–3 ft (60–90 cm) deep. There was much organic matter (31.6%) in this deposit and a strong smell of hydrogen sulphide. At Baba Egba, mud is about 9 in (23 cm) deep and overlies black coarse sand similar to that found in most parts of Kuramo Water.
At Okrika and Isiodun, the deposits consisted of black mud containing more than 55% silt. The deposit also smelt strongly of hydrogen sulphide.
Sakumo and Nyayanu lagoons consisted of clean coarse beach sand, near their openings into the sea. Away from these areas, the deposits consisted of firm mud containing more silt. In the central water channels and the northern parts of these lagoons, the deposits consisted of rounded pebbles and boulders. A. senilis was absent from these areas.
The bottom deposits of the Princesstown lagoon at the western end consisted of muddy sand with a high silt content. Toward the eastern side of the lagoon, the deposit substratum changed to clean coarse sand derived from the beach. A. senilis was found in both types of deposit.
Bottom deposits from other parts of the Lagos lagoon where A. senilis was absent showed no significant difference from those of its natural habitats. At the Federal Fisheries (Fig. 2) the deposit consisted of 63.6% coarse sand and 22% silt. At Ikoyi, the substratum consisted of sandy mud with much organic silt (14.3%). At the Boat Club, the deposits were of coarse sand containing no organic matter.
2.4 Depth of Water
Kuramo Water and Onijegi lagoon are shallow waters with a maximum depth of about 5 m. At Okrika, Isiodun and Sakumo lagoons, the water rises to a depth of about 2.6 m at high tide. During low tide, the water is restricted to narrow channels which alternate with extensive pools 0.2 to 0.6 m deep. A. senilis was absent from the edges of these lagoons which are completely exposed at low tide, and from the narrow channels with strong seaward currents at ebb tide.
Princesstown and Nyanyanu lagoon have an average depth of about 3 m. Like Kuramo Water, tidal movements do not influence the three lagoons, thus they are full of water at low tide.
The average depth of Lagos lagoon at Ikoyi is about 2 m while the average depth of the Lagos harbour (Boat Club and Federal Fisheries) is about 7 m.
2.5 Water Movements
Kuramo Water is a clean body of water which is little affected by tidal movements. The average daily tidal range is about 4–7 in (10–18 cm). Onijegi, being an enclosed lagoon, has no tidal movements. Both lagoons are subjected to incessent gentle movements caused by wind action.
At Okrika, Isiodun and Sakumo the influence of the sea is pronounced because the lagoons are directly connected with the sea. The water is restricted to narrow channels with strong seaward currents which alternate with extensive pools 0.2–0.6 m deep. A. senilis was absent from the channels with strong currents.
At Nyayanu and Princesstown lagoons the water is calm all year round. The tidal movements of the sea do not influence most part of these lagoons. The tidal currents were weak except near the opening at ebb tide. A. senilis was absent around these openings. During the dry season, when the openings of the lagoons were blocked by sand, the water was more or less static.
The water at the Federal Fisheries and the Boat Club is usually fast during the rains when the current at flood and ebb tides reaches a speed of 3–6 kn. The current is slower during the dry season. At Ikoyi, the tidal current is weak all the year round and the tidal range never exceeds 0.3 m.
2.6 Turbidity and Pollution
The water at Onijegi lagoon is very clean and clear all the year round. The clarity of the water here may be due to the fact that mud brought down by the Ogun river into the Lagos lagoon no longer reaches this part of the lagoon system. Also, there are no fast currents to stir up the silt at the bottom. Kuramo is more turbid than Onijegi lagoon but less turbid than the other parts of the Lagos lagoon system (Fig. 9).
At Isiodun and Okrika, during the rains, the water was muddy with the Secchi disc disappearing at 0.7 m at high tide. In March 1963 the water was clearer; the Secchi disc was visible at a depth of 1.5 m.
The Secchi disc was visible at 3 m during both dry and wet season at the Princesstown lagoon. The water in Sakumo and Nyayanu lagoons was clear and greenish in colour in March but turned turbid in August. At this time (August) the Secchi disc was not visible beyond 0.92 m. During the dry season, however, the disc was visible at the bottom of both the lagoons.
The Lagos lagoon and harbour were turbid during the low salinity season with Ogun river carrying down mud and detritus from the land. The fast bottom currents in the harbour also stir up the mud.
No industrial wastes are discharged into any of the areas studied. The main source of pollution is from domestic wastes and untreated sewage from the village along the creeks and near the lagoon.
2.7 Oxygen Content
The period of lowest oxygen content coincided with the high salinity season at all stations.
The oxygen content of Onijegi lagoon varied between 5.92 cm3 per litre in April and 2.3 cm3 per litre in August. At Baba Egba and the western part of Kuramo Water, the oxygen content at the bottom was low all the year round, being between 1.49 cm3 per litre and 0.25 cm3 per litre. This was perhaps due to the static nature of the water and the occurrence of decaying organic matter.
No monthly determination of oxygen content was made at Okrika and the Andoni Flats. On four occasions when both areas were visited, the oxygen content during low and high tides was between 6.05 and 3.62 cm3 per litre. The oxygen content of the three Ghanaian lagoons where A. senilis was found was about 6.5 cm3 per litre during the rains but fell to between 3.54 and 3.1 cm3 per litre in March at the peak of the dry season.
The oxygen content of the Lagos harbour where A. senilis was absent was high during the rains being about 6.97 cm3 per litre. During the high salinity season, the oxygen content dropped to almost 3.04 cm3 per litre. The oxygen content of Ikoyi, where A. senilis was absent, varied between 2.5 and 6.0 cm3 per litre.
2.8 Associated Macrofauna
This was studied in order to identify the natural enemies and competitors of A. senilis. Covering the rhizophores of the mangroves fringing the lagoons and creeks are colonies of Mercierella enigmatica and the shells of A. senilis sometimes carry colonies of this bivalve. Colonies of Gryphaea gasar on the mangrove stems occur on the soft bottom of the western end of Koramo Water and in Ghanaian lagoons with rocky bottoms. Other common bivalves are Tellina nymphalis and Aloidis sp.
The gastropod Semifusus morio is abundant in the shallow parts of Onijegi lagoon, Kuramo and Princesstown lagoons, while Thais haemostoma is found in the shallow parts of all the localities studied. These gastropods were not observed to prey on A. senilis. Other gastropods found in the shallow estuaries and creeks are Tympanetonus fuscatus, Pachymelania aurita, Neritina glabrata. The hermit crab, Clibanarius africanus occurs in large numbers in the shallow parts of the habitats studied.
Of the 34 rays, Trygon (= Dasyatis) margarita, caught during the investigation, 18 contained shells of spat of A. senilis.
No asteroids were found in any of the habitats studied.
2.9 Discussion and Conclusions
A. senilis occurs only in Kuramo Water and Onijegi lagoon in the Lagos area. It is absent from the harbour, the lagoon and adjacent creeks. In the Niger Delta, it occurs only in some of the numerous creeks. In Ghana, A. senilis is restricted to a few lagoons and estuaries along the coast. To account for this restricted distribution, the ecological factors are discussed in turn below, to determine which ecological factors may be limiting the distribution of the species:
All habitats investigated had a high temperature all the year round, the annual variation being only 5°C (range of 26.7–31.6°C). This agrees with previous findings that temperature is a stable factor in West African waters, Olaniyan (1957), Hill and Webb (1958), Longhurst (1958). Judging by the fact that there were no temperature differences between the localities in which A. senilis occurs and those from which it is absent, temperature does not appear to be a limiting factor in the distribution of the species in the areas studied, although it may account for the failure of the species to spread into waters south of Angola and north of Rio-de-Oro where temperatures are lower (maximum 21°C) and a higher annual range (11°C) exists.
In the Lagos area, A. senilis lives in relatively clean water with little suspended material. In the other habitats, e.g., Sakumo lagoon and the Andoni flats, the waters are turbid, especially during the rains. It would seem then that turbidity is not a limiting factor in the distribution of the species. The sorting mechanism of the gills seems capable of eliminating any excess silt entering the mantle cavity.
The salinity tolerance experiments carried out on specimens from Onijegi lagoon indicated that the range tolerated by the species is between 7.5‰ and 27.5‰. This correlates with the salinity range at Onijegi lagoon from where the experimental animals were obtained, and with that of the two habitats studied in the Niger Delta (annual range 11–21‰). It also correlates with the salinity range (11–28‰) of Buance river in Sierra Leone (Watts, 1958), where the species occurs abundantly (Yonge, 1955; Longhurst, 1958).
In Ghana, the species occurs naturally in hypersaline waters with a salinity of up to 38.3‰. This anomalous occurrence can be explained in either of two ways:
that the Ghanaian specimens belong to a different subspecies, or
that they are physiologically adapted to hypersaline water as a result of gradual accommodation. It seems, then, that like Gryphaea gasar and Balanus pallidus stutsburi (Sandison, 1966), the upper limit of the salinity range tolerated by A. senilis can be raised by acclimatization.
A. senilis occurs naturally in a wide variety of deposits. Experiments, however, show that few larvae settled in Baba Egba deposit which had 64.05% silt and organic content. The successful settlement observed at Okrika in deposits with similar silt but lower organic content suggests that it is the organic matter which is responsible for the poor settlement in Baba Egba deposits.
The lagoons in which A. senilis occurs are slow-moving waters unaffected by waves of the sea. In the tidal creeks and estuaries they are absent from the channels and the estuary mouth where the current is fast at ebb tide. The preference for calm and sheltered waters may be connected with the behaviour of this species. Under laboratory conditions, feeding stops when the aquarium is stirred. Thus their inability to feed in areas where strong bottom currents prevail will account for the observed absence of the species. The cockles are also likely to be smothered by shifting sand caused by swift bottom currents.
The maximum depth at which A. senilis was found during the survey was 5 m. Experiments also showed that the larvae settled preferentially in water less than 3 m deep. These observations suggest that the distribution of A. senilis is influenced by water depth.
A. senilis was found in water with high oxygen content (above 2.7 cm3 per litre) except at Baba Egba where the oxygen content was below 1.5 cm3 per litre all the year round (average 0.8 cm3 per litre). The gonads of the population of A. senilis in this locality remained underdeveloped during the breeding season. No successful settlement was observed at Baba Egba even though the area is in the Onijegi lagoon where larval were abundant. This is probably due to:
avoidance of the deposit which has very high organic content, and
complete mortality of the few that settled owing to the low oxygen content.
Apart from man, the only known predator of A. senilis is Trygon (= Dasyatis) margarita. Predation by T. (= Dasyatis) margarita does not appear to limit the distribution of A. senilis but may affect its abundance in a particular area.
The absence of A. senilis from the Lagos harbour may be due to one or a combination of the following factors: salinity, depth and water movements; though other environmental factors appear favourable.
The work described below was carried out on the population of A. senilis at Onijegi lagoon near Lagos. In order to establish the possible occurrence of sex reversal in A. senilis, the gonads of the 1962 spat were examined at fortnightly intervals during the first twelve months of their life. The gonads of random samples were fixed in Bouins fluid in sea water and embedded in ester wax. Sections were taken at 10 to 12 microns and stained with Ehrlich's haemotoxylin and eosin. The length of each animal was recorded in order to establish the relationship between sex and the size of each animal. The gonads of adults of different sizes were similarly examined.
The breeding cycle was investigated by observing seasonal changes in the gonad and by studying the occurrence and abundance of larvae in the plankton. The gonads of about 200 specimens were examined fortnightly from November 1962 to December 1964. Smears of the gonads of each animal and section of representative specimens were examined in order to assess the state of development.
Plankton samples were collected every two weeks during the same period from Onijegi lagoon using a standard net (No. 12) with a mesh of 125/in2. The samples were preserved in 4% neutral formalin and examined in a counting cell.
The growth of A. senilis was investigated by random sampling of the population at Onijegi lagoon. Between 200 and 400 cockles of various sizes were collected monthly from the south-western part of the lagoon between October 1962 and November 1963. All cockles found in an area of 360 cm2 were collected for measurement. The very shallow parts where regular collection by villagers has resulted in the preponderance of cockles less than two years old were not sampled. The population in the deeper parts of the lagoon (over 2.5 m deep) which were undisturbed by collection and hence could have given more reliable results were not sampled because of lack of the right equipment - a grab and a powered boat.
Length was used as an index of growth, each animal being measured with a sliding caliper. Growth rings were not used to determine the growth rate because the number of rings of individuals of the same age was found to vary; thus they are not produced seasonally.
The larvae occurred in the lagoon in varying numbers almost all the year round. In 1963 larvae were abundant from September to November; in 1964 there were two main periods of abundance; one in February and March and a more conspicuous one from July to October. There were about 80 larvae per plankton sample in March and about 180 in August of the same year.
The population of A. senilis at Onijegi lagoon consists mostly of specimens one to two years old; specimens older than two years were rare. The scarcity of older specimens was caused by collection of the cockles by the villagers. At the end of October 1962, the average length of the newly settled spat was 4 mm (1½ months). In December when they were three months old, the average length was 11.7 mm. The average monthly growth during the first three months was 3.8 mm. Between January and March, growth was slower, being only 2.3 mm per month. The average size of the spat was 18.2 mm. Growth was fast between April and June (average 4.2 mm per month) and at the end of June, the average length was 31.1 mm. The growth rate fell between July and September to about 1.2 per month and so at the end of the first year the spat had grown to about 35.7 mm in length.
The growth of the one year olds during the same period (October 1962 to October 1963) was comparably slower, averaging less than 1.1 mm per month. The average length of the two year old cockles was 46 mm.
In order to determine the rate of growth of spat in other areas of the Lagos lagoon system, batches of about 200 cockles averaging 7.8 mm were collected from Onijegi in November 1962 and sown at Ikoyi. These cockles reached a length of 29.4 mm at the end of May 1963. As in Onijegi lagoon, growth was faster at Ikoyi between April and June.
It seems as if the growth in the tidal waters of Okrika is faster than at Onijegi. The average length of the yearlings was about 40 mm.
In Onijegi lagoon proliferation of gametes took place between December and March and maturation of the gametes occurred between April and June. In December 1962, the water temperature was about 29°C and it rose to a peak 30.8°C in March 1963. A similar pattern was observed in December 1963 when the temperature was 29.5°C and rose to 30.4°C in March 1964.
During the growth and maturation of gametes (April to June) the temperature fell gradually to about 27.5°C in both years. It seems, therefore, that a rising temperature coincides with proliferation of gametes in A. senilis while falling temperature coincides with growth and maturation of the gametes.
The salinity of the water rose gradually from about 12.5‰ to about 22‰ between December and March when proliferation of gametes occurred. Between April and June when the gametes grew and matured the salinity fell to about 16‰. It seems that a rise in salinity coincides with proliferation of gametes and a fall in salinity with their growth and maturation.
The plankton in the Lagos lagoon system shows a clear seasonal distribution (Olaniyan, 1957), being abundant from March to June. Thus, it seems this plankton abundance and associated abundance of organic detritus enhanced the growth and maturation of gametes which occurred during these months.
The rapid growth of A. senilis which occurred between April and June (average increase in length 4.2 mm per month) was not due to either temperature (rose from 28.5°C to 29.0°C) or salinity (fell from about 20‰ to about 15‰). It seems the abundance of plankton during April - June brought about the rapid increase in length. The slow growth during July, August and September (1.2 mm per month) may have been due to the reduction in plankton coupled with the inhibitory effect of the gonads which were then mature (Orton, 1927). The growth rate of the cockies was also slow during spawning (September to November). As suggested by Orton (1927) for Ostrea edulis the slow growth during these months might be due to physiological antagonism between spawning and shell growth. It might also be due to the diversion of food for gametogenesis which in A. senilis starts before the completion of spawning.
A number of oyster species occur around the coasts of Africa. Compared with a species such as Crassostrea virginica, the African species appear small. Perhaps because their ecology has not been well studied, lack of technical knowledge, capital and trained personnel, hardly any of the species is cultivated on the scale that is practised in Europe. This situation had resulted in the impression that these oysters are of little commercial value. However, they provide a good source of cheap protein for the people of the coastal towns and villages. Once the knowledge and other facilities for a large-scale cultivation are available, it is the writer's belief that some of these oysters could assume commercial importance. Where, because of some inherent ecological factor, the native oyster cannot do well, then the introduction of a foreign species that can thrive in that ecological niche should be considered.
This was the case when Korringa (1956) examined the potential for developing oyster cultivation in South Africa. He concluded that the native species Crassostrea margaritifera and Ostrea atherstonei were the most promising for cultivation. He also suggested the introduction of the Portuguese oyster, Crassostrea angulata. He observed that there were a number of coastal lagoons with very high productivity, but Knysna lagoon was the most promising.
FAO fisheries statistics covering African countries from Morocco to South Africa reported production of 800 tons of oysters for Senegal, 100 tons of unnamed mollusc for Sierra Leone and 700 tons of abalones for South Africa. As stated above, large quantities of the molluscs - bivalves and gastropods - are collected by coastal dwellers but the harvests are not recorded. As molluscs assume greater commercial value, production figures will be kept and so become available. Production data would be required for any rational exploitation of the animal resource.
Gryphaea gasar (Syn: Crassostrea gasar) is to be found along the West African coast (Nickles, 1955). It occurs on mangroves between Senegal and Angola, with Lagos cited as a location where numerous specimens were collected. This oyster is much more abundant on the mangroves that line the numerous creeks along the Niger Delta. Preliminary investigations by the writer would seem to indicate that there is a greater potential for the cultivation of Gryphaea gasar in the Niger Delta area in Nigeria than in the Lagos lagoon system.
4.1 Effect of Salinity Fluctuation on the Life Cycle of G. gasar
This study was carried out by E.E. Sandison in Lagos harbour between November 1956 and January 1958. Data on oyster settlement were obtained by using ground-glass plates with an effective area of 169.5 cm2. These were placed below low tide level at each of the four collecting stations (Iddo, Apapa, Dejection Jetty and East Mole) and left submerged for varying periods of time.
Plankton samples were taken fortnightly in the upper, middle and lower regions of the harbour. At each sampling station three vertical hauls were made from just above the bottom to the surface.
Salinity samples were also taken at the four sampling stations mentioned above.
It was discovered from this study that the occurrence of oyster larvae in the Lagos harbour plankton was seasonal. Larvae were present during the high salinity season, but absent during low salinity. Few larvae were present in the harbour at all times between November 1956 and 1957. There was one major increase in April. Between the end of June and end of October, no oyster larvae were found.
Settlement of the oyster spat was confined to the high salinity season. The settlement reached a maximum in the beginning of April and terminated at the end of July. There was no settlement during the low salinity season - beginning of July to end of October. Thus the settlement of G. gasar in Lagos was seasonal, and mainly confined to the upper and middle regions of the harbour. It should be noted that in a similarly tropical condition in Puerto Ricco, Mattox (1949) found that the spat of G. rhizophore settled at all times of the year. Thus the local conditions of the environment in Lagos were precluding settlement during the low salinity season.
Data on spat growth were obtained from series of plates exposed for successive fortnight periods at the four sampling stations, and partly from a series of “succession” plates exposed for varying lengths of time.
Oyster spat growth in Lagos harbour grew almost exclusively during high salinity season. Growth in the upper and middle region of the harbour was faster than elsewhere.
Information on gonadal development of G. gasar was obtained by examining oysters which had settled on plates in Lagos harbour, and whose size-age relationship was known. Oysters of unknown age from Kuramo Creek, whose size only could be measured, and which were in a breeding condition in February and March were also examined.
In Lagos Harbour and Kuramo Creek during the high salinity season, oysters with a shell length of 15–20 mm (6–8 weeks in Lagos) had gonads in the male phase. At 30–35 mm length (3 months old in the harbour), the gonads of both Lagos and Kuramo oysters were in an intermediate phase. At four months old and 40 mm length, the gonads were in the female phase. The real relationship between size, age and maturity applied only to oysters that grew under optimal conditions in Lagos Harbour. Oysters that settled in May and so grew during low salinity, did not reach the male phase even 12 weeks after settlement.
Compared with other species of Gryphaea, gonadal development of G. gasar in Lagos was slow. Paul (1942) found mature gonads in G. madrasensis 21 days after settlement, and Menzel (1951) reported mature gonads in G. virginica at an age of 28 days.
The annual salinity range is 0 to 34‰ in Lagos Harbour (Sandison and Hill, 1966). It would seem that excessively high or low salinity may be responsible for the high mortality rate of oysters in Lagos Harbour.
During the high salinity season a salinity range of 30 to 33‰ appears to correspond to a reduction in the number of oysters at Dejection Jetty and East Mole, whereas when the salinity remained below 30‰ (Apapa and Iddo) mortality occurred, but some oysters were able to live. However, there is an apparent anomaly here, for at Apapa and Iddo oyster mortality occurred in March and April in a salinity range similar to that in which the spat settled. This would seem to imply that either the oyster spat had settled in salinity range unconducive to survival, or that factors other than salinity were contributing to oyster mortality.
To determine the correlation between oyster mortality and low salinity, a controlled experiment on oyster survival under stable salinity conditions was conducted in the laboratory. Oyster survival was observed in a series of constant salinities. It was necessary to conduct this controlled experiment in the laboratory because of the rapid fall in salinity during May and June, making the correlation between oyster mortality and low salinity difficult to determine.
The oysters collected from Kuramo Creek were placed three to six together in one of a series of salinities ranging between 0 and 30‰. Water temperature was maintained at 25–26°C and the water was aerated. At intervals of one to three days the water in each container was changed and replenished with water from Kuramo Creek (salinity 11‰) which was suitably diluted or concentrated by the addition of glass-distilled or harbour water, thereby maintaining as far as possible the living nannoplankton present. Oysters in 0‰ were placed in distilled water to which no food was added. As the variables in the experiment were salinity and the availability of food, differences in oyster survival must have been related to one or both of these.
Oysters living in salinities between 18 and 30‰ died within three to four days. Their death could have been due either to starvation or to high salinity. However, since oysters were able to live for nine days in 0‰ without food, it does not seem that starvation was the primary cause of death in higher salinities. It seems, therefore, that salinity was the factor determing oyster survival; experimental results show that salinities of between 0 and 15‰ were more conducive to survival than the higher salinity range.
It was further observed in the field that in the Lagos area one of the factors contributing to oyster mortality may have been excessive water movement during the low salinity season.
As already stated, cultivation of a particular species of mollusc, essentially implies shifting the ecology parameters in favour of the cultivated species so that it assumes a dominant position over other members of the demersal community. To achieve this requires a through understanding of aspects of the biology that affect the cultivation of the animal.
The studies carried out on A. senilis and G. gasar give some indications of what is required. Thus a knowledge of how temperature and salinity affect cultivable species would enable the culturist to select the correct environment with regard to these parameters.
The same applies in the case of other factors such as the nature of substratum, oxygen tension and pollutants. Aspects of biology of the cultivable species such as gonadal development, life cycle and growth rate are useful when the culturist is considering when and how to collect spat and the rate of growth.
Information about the associated fauna of the cultivable species enables the culturist to identify pests or predators. Mercerella enigmatica, the serpulid worm, is a pest of A. senilis and G. gasar, while Trygon (= Dasyatis) margarita is a predator of A. senilis and and Thais haemostoma is a predator of G. gasar. Since the culturist knows which of the associated fauna is to be eliminated, he can effectively shift the ecological balance in favour of the cultivable species, thereby saving cost and enhancing production.
However, gaps still exist in our knowledge on the biology of cultivable molluscs. Studies carried out on the cultivable bivalve molluscs mentioned above do not exhaust the list of information required for a successful cultivation of these molluscs.
For instance, growth studies have been carried out mainly on the juvenile animals - the spat. Studies are also required on the adult animals growing wild in the field or under culture, so that it may be possible to determine how fast the species can grow to marketable size and when the species can reach the asymptotic point. Market size is different from asymptotic size. The species does not have to reach a symptotic size to be marketable. Food consumption, at the asymptotic point, is essentially a waste from the culturist point of view. Thus it would be quite useful to know from growth studies on the adult animal at what point to harvest for the market.
While considering the cultivable species and its market value, studies on the condition indices of the animal at various stages of its life throughout the season would be very useful. Oyster meat tends to be firm and the gonads creamy white before spawning, whereas the meat becomes watery after spawning. The degree of acceptance of the oyster meat (according to the European palate) depends on the condition of the animal. Thus it would be worthwhile to know when the animal is in the best condition to fetch the best price on the market.
Conditioning cultivable molluscs (especially bivalves) to a physiologically ripe condition, artifically spawning and rearing spat, for planting, are areas where much information is required. Loosanof in America has done much work in this field with the American hard shell clam, Mexcenaria mercenaria, while Walne in England worked on the native oyster, Ostrea edulis. On occasions when bivalves fail to spawn naturally or the spatfall is not as heavy as it should be, the ability to produce seed in the laboratory to supply to the culturist would be very useful.
An extension of studies on the artificial spawning and rearing techniques could also be studies of genetic factors that influence characteristics of cultivable molluscs, especially those characteristics that can be exploited commercially. Results from this type of investigation would make possible selective breeding of the cultivable species. For instance, among the bivalves, some individuals of the same species grow faster than others, some have thin shells while others have relatively thick shells. It would be interesting to find out whether such characteristics are genetically determined and to exploit them.
The diseases of cultivable molluscs also require investigation; very little is known about this. Attention should be paid to the study of diseases that are likely to cause illness in consumers or those likely to dessimate the animals themselves.
The gaps in our knowledge so far mentioned are of a biological nature. Attention should also be given to the technical gaps in knowledge. Cultivation of shellfish on a large scale would pose some problems of harvesting, handling and processing. With special reference to Africa, little or no technical information is available on these aspects.
In order to harvest and handle the large volume of shellfish that would result from cultivation, I presume one can engage human labour. In most parts of Africa at the moment, engaging human labour may be acceptable as it is relatively cheap. However, a stage may be reached when it would be uneconomic to use human labour. In anticipation of such an eventuality, it would be worthwhile to investigate and develop ways of harvesting and handling the product quickly and cheaply.
With successful large-scale cultivation, a surplus is likely to result. In such a situation, it would be necessary to process to preserve the surplus. Various processing techniques are in existence for fish products, but they are not of general application. It is often necessary to study the biochemical composition of a particular fish, to be able to develop an effective processing technique for it. Such information is not available for cultivable shellfish in Africa.
Thus, to eliminate wastage and promote a rational utilization of the surplus that may result from a successful large-scale cultivation of shellfish, it is necessary to investigate the biochemical composition of the species as a basis for developing an effective processing technique.
There is need to identify the cultivable species of shellfish in Africa; this is necessary to avoid wasting time, capital and manpower. Having done that, it would be necessary to draw up a programme of studies, biological and technical in nature, that are relevant to the cultivation exercise. It would also be necessary to set target dates for such studies. For the purpose of the study programme, the continent may be divided into zones. The programme could be termed “cultivable shellfish programme in Africa”.
Research can be expensive in terms of capital and trained personnel. In a particular country, where capital is not the limiting factor, the availability of trained personnel may be a limiting factor. Also, bearing in mind the number of topics that may have to be covered in the study programme, it is possible that trained workers in a particular country may be unable to investigate the problem as quickly as may be desired. In such a situation, and where the countries have the same cultivable species, it may become necessary for workers in the different countries to share the study programme; some workers investigating part of the programme and later pooling the results. The workers can meet periodically to evaluate the progress made.
Where capital cost or trained personnel is a limiting factor to the programme, outside assistance should be sought through FAO as the programme would be yet another effort to increase food (animal protein) on a global level. Since trained personnel cannot be withdrawn from a bank like money, where trained personnel is a limiting factor, then assistance through FAO should be aimed to provide or assist the training programmes of the country in question.
The results emanating from the study programmes can then be utilized to set up pilot projects in suitable areas.
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Nickles, M., 1950 Mollusques testaces marins de la Côte occidentale d'Afrique. Manuels Ouest Africains, Ed. 2, Paris 269 pp.
Olaniyan, C.I.O., 1957 The seasonal variation in plankton in Lagos Harbour, Nigeria, Ph. D. thesis London.
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Reinhart, P.W., 1935 Classification of the Pelecypod Family Arcidae. Bull. Mus. r. Hist. nat. Belg., 11(13):1–67
Sandison, E.E., 1966 The effect of salinity fluctuations on the life cycle of Gryphaea gasar in Lagos Harbour, Nigeria. J. Anim. Ecol., 35:379–89
Sullivan, G.E., 1960 Functional morphology, microanatomy, and histology of the “Sydney cockle”, Anadara trapezia (Deshayes) Lamellibranchia: Arcidae. Aust. J. Zoo., 9:219–57
Thiece, J., 1935 Handbuch de systematischen Weichtierkunde. Dritter Teil. Classic Bivalvia Jena.
Walne, P.R., 1966 Experiments in the large scale Culture of the larvae of Ostrea edulis 1, Fish. Invest. Ser. 11, Vol. xxv, No. 4
Watts, J.C.D., 1958 The hydrography of a tropical West African estuary. Bull. Inst. fr. Afrinoire, Dakar, 19:1020–29
Yoloye, V.L., 1974a The Ecology of the West African “bloody cockle” Anadara (senilia) senilis. (in press)
Yoloye, V.L., 1974b Reproduction and Growth in the West African “bloody cockle” Anadara (senilia) senilis (in press)
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Fig. 1. Areas investigated along the West African coast.
Fig. 2. Lagos harbour and the Lagos lagoon system showing the areas studied.
Fig. 3. Areas studied in the Niger Delta.
Fig. 4. The lagoons along the coast of Ghana.
 |  |
| a. Sakuma Lagoon | c. Princesstown Lagoon |
 |  |
| b. Nyayanu Lagoon |
Fig. 5. The three Ghana lagoons studies.
Fig. 6. Temperature records for the stations indicated (1963).
Fig. 7. Temperature and salinity records for Onijeji lagoon from 1962 to 1964.
Fig. 8. Salinities for the stations indicated (1963).
Fig. 9. Secchi disc readings at the stations indicated (1963)
by
T.T. George
Fisheries and Hydrobiological Research Section
Khartoum, Sudan
Abstract
Published literature, supported by the information obtained through biologists' replies to the questionnaire circulated preparatory to writing this paper, reveals that various attempts have been made across the years to introduce and transplant cultivable species into Africa, the exotic fish and shellfish and the non-endemic fish.
A critical analysis of all available information on the subject from the entire African continent indicated that the objects of deliberate introductions and transplantations are mainly for sport or recreation, to replace or augment the existing pond fish fauna or to produce hybrids for better culture, mainly of tilapia and hence improve the nutrition of rural populations, to control malaria, bilharzia or weeds and to fill niches left vacant by endemic species.
In this paper, therefore, a knowledge of species introduced deliberately, method of transport and acclimatization, reasons for introduction and experimental results achieved, beneficial or otherwise, of particular species are reviewed. Also in this paper, accidental introductions are briefly considered, followed by a detailed discussion on the whole operation with conclusions and recommendations.
It is anticipated that more introductions will take place in the near future, particularly for pond culture which consequently will come to play a role of appreciable importance in Africa as it will develop from a subsistance level activity to a commercial-scale operation. Establishment of an Advisory Committee for fish introductions into Africa is, therefore, recommended.
Résumé
Les publications appuyées par les informations obtenues des réponses des biologistes au questionnaire mis en circulation avant de préparer cette communication révèlent qu'au cours des ans il y a eu de nombreux essais d'introductions et de transplantations d'èspèces pouvant être élevées en Afrique, de poissons et mollusques exotiques et de poissons non-endémiques.
Une analyse critique de toutes les informations existant sur le sujet dans tout le continent africain a indiqué que les objets d'introductions et de transplantations délibérées l'ont été surtout pour le sport ou l'agrément, pour remplacer ou augmenter la faune piscicole de pisciculture ou pour produire des hybrides pour une meilleure culture, principalement des tilapias et ainsi améliorer l'alimentation des populations rurales, pour lutter contre le paludisme, la bilharziose ou la végétation et pour remplir les niches laissées vacantes par les espèces endémiques.
Cette communication passe donc en revue les connaissances sur les espèces introduites délibérément, les méthodes de transport et d'acclimatation, les raisons de l'introduction et les résultats expérimentaux obtenus, avantages ou inconvénients des espèces particulières. Les introductions accidentelles sont aussi étudiées brièvement, avec une discussion détaillée à la suite sur toute l'opération avec des conclusions et recommandations.
On prévoit davantage d'introductions dans le proche avenir, en particulier pour la pisciculture en étang qui, par conséquent, arrivera à jouer un rôle d'une importance appréciable en Afrique car elle se développera du niveau d'une activité de subsistance à celui d'une opération à l'échelle industrielle. Il est donc recommandé de créer un Comité consultatif pour les introductions de poissons en Afrique.
Natural freshwater fisheries have long been practised in Africa (van der Lingen, 1967) but only in comparatively recent years has aquaculture been developed in much of the continent (Meschkat, 1967). Fish culture is believed to have been in existence on the African continent in Egypt as early as 2500 B.C., but its expansion probably occurred between the last two world wars (FAO, 1974; Maar, Mortimer, van der Lingen, 1966). As a matter of fact, fish culture had started before the second world war in the Democratic Republic of Congo and it was not seriously investigated in Africa until after this war (van der Lingen, 1967). Swingle estimated an area of 27 236 ha of fresh and brackish water ponds in Africa out of a world total of 3 446 048 ha (Lennon et al., 1970). However, there are great opportunities to develop the area unutilized for aquaculture in the African countries and consequently increase fish production (FAO, 1972; Odero, 1974; George, in press).
The African freshwater fish fauna is very great and diverse but because fish culture had been introduced into Africa (Meschkat, 1967; Huet, 1970) and hence, there is no native tradition of aquaculture south of the Sahara (Bardach, Ryther, McLarney, 1972), proportionately few species other than Tilapia spp. have been cultured even experimentally. The use of Tilapia for aquaculture had been suggested by Monod in 1925 (Bard, 1962 a and b) and since then this has been the most commonly used pond fish in Africa.
In some African countries, aquaculture has involved almost exclusively Tilapia species (Huet, 1970; Bardach, Ryther, McLarney, 1972). Tilapia species were apparently first tried in ponds in Kenya in the twenties (van der Lingen, 1967) but part-time subsistence cultivation of fish in ponds has developed since 1946–1948 following the first successful raising of tilapia in Katanga (Zaire) and in Cameroon (Huet, 1970; Meschkat, 1967). This was initiated by colonial administrators to improve the nutrition of the natives (Pillay, 1973). The favoured Tilapia species for fish culture in Africa are: T. macrochir, T. nigra, T. nilotica and T. zillii (Meschkat, 1967); T. galilaea and T. heudeloti are considered to be slow growers (Maar, 1960).
However, various attempts have been made across the years to introduce and transplant cultivable species into Africa, the exotic fish and shellfish and the non-endemic fish. The introduction of exotic fish started before the non-endemics (Jackson, 1960) and were first initiated by Europeans who probably for reasons of familiarity, cultivated these species, plus the easy-to-breed members of the genus Tilapia (Bardach, Ryther, McLarney, 1972). Actual production of fish in ponds started in Africa with the introduction of cold-water fish culture (trout and pike) between the two world wars (Chapuis, 1959). The raising of trout in cold water for angling has been practised only since the end of the first world war in North Africa (Morocco) and Kenya, and for a longer period previously in Republic of South Africa (Huet, 1970). But the common carp had been tried in the Republic of South Africa in 1859 (Bard, 1962 c) and in Egypt in 1934 (Koura, El-Bolock, 1960). The goldfish, Carassius auratus, had been acclimatized in Malagasy Republic in 1861 (Hickling, 1962). The grass carp, C. idella was introduced from Hong Kong into Uganda and Egypt in 1965, 1968 respectively and from India into Sudan in 1975 for the control of weeds in ponds and other water bodies.
Among shellfishes, seven consignments of culch-free seed oysters have been introduced into Mauritius from California, U.S.A., during the period from December 1971 to December 1972. These consisted of over one million seed oysters belonging to three species, namely: Crassostrea gigas, C. virginica and Ostrea edulis. They are cultured on rafts at an artificial coastal lagoon (barrachois) of the Fishery Division Farm at Mahebourg. Also, a consignment of 50 000 C. gigas was introduced into Seychelles (FAO, 1973).
Among the African non-endemic fish, T. aurea from Israel, T. hornorum and T. mossambica from Zanzibar and T. nigra and T. melanopleura from Kenya were introduced between 1962–1966 in the ponds of Kajansi Fish Farm in Uganda after crosses between two of the indigenous species, T. leucosticta and T. nilotica, failed to produce hybrids (Pruginin, 1965, 1967). In 1968 T. andersonii was introduced from Zambia into the fish farm at Mayla (about 120 km from Mawanza) and later into ponds at Nyegezi in Tanzania where it was used for hybridization work (Ibrahim et al., 1974). Between 1953–1963 five non-endemic species (T. macrochir, T. rendalli, T. nilotica, Astatoreochromis alluaudii and Heterotis niloticus) were introduced into the Central African Republic (Vincke et al., in preparation). Serranochromis robustus jallae, which is native to the Upper Zambesi river system, was introduced in the Republic of South Africa to replace or augment the existing pond fish fauna (FAO, 1970b). Clarias lazera was imported into the United Republic of Cameroon in 1972 from Central African Republic for pond culture (Hogendoorn, personal communication).
The introduction and transplantation of the exotic and non-endemic fish into Africa was not carried out for pond culture only, but also for stocking inland waters, rivers and lakes. Trout hatcheries, mainly for stocking sport fishing waters, have been established in higher altitudes in Morocco (the first in 1924), Republic of South Africa, Kenya, Reunion and Basutoland (Lesotho) (Meschkat, 1967). Pike hatchery for river and lake stocking started before the second world war in 1939 in Morocco (Chapuis, 1959). The largemouth black bass, Micropterus salmoides had been introduced into the Republic of South Africa, Malagasy Republic and Morocco and occasionally into other countries, essentially for recreation (Huet, 1970). It was introduced in Swaziland in 1933 from the Republic of South Africa and is still widely distributed in all the reservoirs; it is the major predator in the country's water bodies (FAO, 1973). Gambusia affinis halbrooki was introduced into Sudan from Italy through Egypt in 1929, but its use as a larvivore in the irrigation canals of the Gezira Cotton Scheme is still in the experimental stage (Motabar, 1973; George, unpublished). During 1925–1927 and 1954–1955, the African non-endemic species, T. spilurus nigra, T. zillii and unintentionally T. leucosticta were introduced from the Athi river, Lake Albert (via Kidetok dam, Uganda) and the Kisumu fishponds respectively into Lake Naivasha (Kenya) which had no species of Tilapia before 1925 and its native fish fauna dominated by the Cyprinodonts; Aplocheilichthys, Gambusia and Lebistes (FAO, 1970 b). Also fingerlings of Barbus holubi, the small-mouth yellowfish were introduced in Lake Kyle (Rhodesia) in June 1964 and August 1965 from the Republic of South Africa (FAO, 1970 a). Jackson (1960) mentioned some known cases of actual or proposed introductions of this nature (Table I).
TABLE I
Known cases of actual or proposed
introductions of non-endemics (Jackson, 1960)
| Species | Area |
| Limnothrissa and Stolothrissa (clupeids) | Lakes Malawi (29 604 km2 ) and Bangweula (2 072 km2 ) |
| Tilapia mossambica | Lualaba swamps |
| T. macrochir, Serranochromis spp., Limnothrissa, | Lake Kariba |
| Stolothrissa, Lates spp. and Bagrus meridionalis | |
| Tilapia zillii, T. leucosticta and Lates niloticus | Lake Victoria (68 635 km2) |
| Hydrocyon vittatus | Tokwe River (Southern Rhodesia) |
| Barbus holubi | The Gouritz river system (South Africa) |
| Lates niloticus | Lake Kioga (1 970 km2) |
In Mauritius where the inland waters are reported to have only three important endemic species (Agonostomus sp., Nuraera sp., and Dules sp.), the exotic Osphronemus olfax had been introduced since 1761 while between 1944–1961 eight other exotic species and four non-endemics (Lepomis microchirus, Micropterus sp., Carassius auratus, Etroplus sp., Dules rupestris, Agonostomus talfairii, Catla catla, Labeo rohita, Tilapia melanopleura, T. macrochir, T. nilotica and T. zillii) were introduced and transplanted in natural inland waters, irrigation reservoirs of sugar plantations and fish culture ponds (FAO, 1971). Also between 1965–1966 six exotics and two non-endemics (Cyprinus carpio var. specularis, Scardinius erythropthalmus, Tinca tinca, Micropterus salmoides, Salmo giardneri, Esox lucius, Tilapia nilotica and T. mossambica) were introduced into Tunisia for stocking lakes, rivers, irrigation reservoirs and other water bodies because the indigenous freshwater fish fauna is of no commercial value (FAO, 1970 a; Nasfi, personal communication).
In the course of these purposive introductions of species, accidental introductions have also taken place. Heterotis niloticus has been recorded in the Congo river basin and Lake Tumba (550 km upstream from Kinshasa) and in the Kasai river (FAO, 1969). T. melanopleura which was introduced into Mauritius in 1956 for pond culture escaped into water courses and reservoirs (Ardill, personal communication).
While the introduction and transplantation of cold-water species like trout in Africa has been mainly for sport and recreation, culture of warm-water species of fish was introduced to replace or augment the existing pond fish fauna and hence improve the nutrition of rural populations. Other important objects of this operation may be the biological control of malaria, bilharzia, and weeds or to fill the niches which the endemic population in a new lake will leave vacant for a very long time or to produce hybrids for better culture, mainly of Tilapia. These are the objects of deliberate introductions which concern us more directly than the accidental. However, the success or failure of these introduction measures is to be judged by the response of the concerned species to the new habitat, or the transplanted medium in the light of the object with which the effort was made. It thus becomes clear that a critical analysis of all available information on the subject from the entire African continent is necessary. With this aim in view, information on practically all the deliberately introduced cultivable species into the African countries is summarized in the present paper. This includes a knowledge of the species introduced deliberately; methods of transport and acclimitization, reasons for introduction and experimental results achieved, beneficial or otherwise, of particular species. Also in this paper, accidental introductions are briefly considered followed by a detailed discussion on the whole operation with conclusions and recommendations.
The material for this study was gathered from published literature or by correspondence. It is not claimed that this review is complete by any means because there is a number of African countries which have not yet provided information on their efforts in aquaculture or because fish culture, of any sort, has barely been started. It is really hoped that these gaps be filled in the near future through the cooperation of biologists in the African countries.
2.1 Exotics and non-endemics
In this context, exotic species are those species (fish and shellfish) which come from some part of the world remote from Africa while non-endemics are fish of African origin, but for some reason or another, are naturally absent from the lake, river system or other zoogeographical area where the introduction was proposed (Jackson, 1960).
All the principal exotics and non-endemics introduced and transplanted in Africa (deliberately), are listed in Appendix I, while all those introductions made by specific African countries are shown in Appendix II.
2.2 Methods used in transport and acclimatization of introduced species
In the majority of introduction undertaken, the consignments were transported by air and were always put in polythene bags with polysterene insulation or simply in an ordinary tin. The bags were half filled with water and oxygen added. For example, a gift consigment of 10 000 grass carp (3.3–4.7 cm) were flown from India to Sudan (17 h journey), each 200 being put in a polythene bag, filled with six litres of water plus compressed oxygen; each bag was closed in an ordinary tin. In some cases, however, if the journey was too long, the fish were brought to an induced sleep by means of ethyl-carbamate or other chemical in order to lower their rate of respiration (Nasfi, personal communication).
Not much information is available on the acclimatization procedures followed in the concerned African countries. However, in Tunisia, at their arrival, plastic bags were opened, water temperature recorded, bags put in a basin and water at the same temperature slowly added to that of the plastic bag up to the same maximum level before the bag is emptied in the basin. After a three-week period in an aquarium, the fishes were removed to cement ponds in the laboratory where conditioning started. Water temperature was gradually (10°C daily) raised or lowered, and the salinity raised 1 ppm daily up to the temperature and the salinity of the chosen water body for fish stocking. By means of this method, Tilapia was acclimatized to 4 ppm brackish water and carp to 4 ppm, 8 ppm and 11 ppm brackish water respectively (Nasfi, personal communication).
In Sudan, the gift consignment of 10 000 grass carp and 100 common carp was immediately taken to the Experimental Fish Farm at Shagarra after its arrival by Air India via Beirut. The weight of the plastic bag plus contents was noted and after recording the water temperature, about two litres of river water of the same recorded temperature were slowly added to each plastic bag. After about half an hour, the contents of each bag were emptied into a floating “Hapa” placed in a small cement tank (4.25 × 1.50 × 0.60 m) filled with water (White Nile) and provided with four electric aerators. The fish were then given a bath of acriflavine as a hygienic measure and then released in the laboratory aquaria, cement tank andearthen ponds. A specimen sample of 200 fish were preserved in formalin for study, and the length, weight and number of fish noted before being released in their new habitat. The fish were mainly fed on powdered cotton-seed oilcake. No mortality at all occurred either during transport of the consignment or after release of the fish; all could easily survive in the transplant medium.
2.3 Experimental results of some successful introductions
2.3.1 Old introductions
Cyprinus carpio
Egypt: Two varieties of Cyprinus carpio (scale and mirror) have been successfully introduced in Egypt (El-Bolock and Labib, 1967). About 200 fingerlings of scale carp (5–8 cm) were introduced in the Barrage Fish Farm in 1934 from Indonesia (Koura and El-Bolock, 1960; El-Bolock and Labib, 1967). The fish reached 46 cm in two years, spawned during 1936 and fingerlings were distributed to private farms for rearing (El-Bolock and Labib, 1967). Because this variety was not appreciated by the Egyptians, a consignment of 340 mirror carp (5–30 cm) were introduced from France in 1949 into the Barrage Fish Farm where spawning took place in 1950 and the fish attained an average length of 187 mm and an average weight of 123 g (Koura and El-Bolock, 1960). At the Serow Fish Farm, the fish attained a length of 153.4 mm at the end of the first year and 220 mm at the end of the second year (Bishai and Labib, in press a). During the 1st to 6th years of life, the average length of this fish was found to be 153.4, 220.1, 281.2, 344.3, 408.3 and 481.0 mm with a percentage annual increment between 13.3–16.3% for males and 13.9–14.3% for females. The fish showed annual increment in weight during the first year, a rapid increment in the second and the third years of life and more rapid in the 4th to 6th years; it attained 62.1, 184.3, 385.6, 709.1 1 181, 1 931 g during the first to sixth years of life (Bishai and Labib, in press a). Copopods and chironomids form the most important natural food items of carp during all stages of their life (Bishai, A-Malek and Labib, in press f).
When reared alone under natural conditions, production was 377 kg/ha; in combination with adult tilapia, mullet and adult tilapia and indigenous fishes, production was 711 kg/ha, 800 kg/ha and 1 008 kg/ha respectively. When reared with feeding and fertilization and in combination with mullet and adult tilapia, production was 1 218 kg/ha (El-Bolock and Labib, 1967). A diet mixture of 75% rice bran and 25% cotton-seed oilcake gave the highest gain in carp weight, 1 065% as compared to 815%, 996% and 918% for other tested diets. During winter months (Dec.-Feb.), carp did not show any appreciable gain in weight in spite of the abundant supplementary food given to the fish (Bishai, Ishak and Labib, in press e).
When 20–56 g fish were stocked in rice fields at the rate of 750 to 1 250 fish per ha and reared for about two to three months, a total catch of about 200 kg was obtained, the gain in weight being about 88 to 186 g per individual fish (El-Bolock and Labib, 1967).
Male carp matured during February and March when 8–10 months old and average total length 19 cm; ripening took place during May and June when attaining an average length of 22 cm and 12–14 months old. Females matured during April-June when 12–14 months old and average total length 215 cm; they became ripe in July-September at an age of 16–18 months and average length 23 cm. The number of eggs per g of ovary was found to be 929 with standard deviation 146 and standard error ± 25 and the relative fecundity 123 eggs/g of body weight (Bishai and Labib, in press b). Spawning took place when water temperature was 18°C and extended from April to October (Imam and Hashem, 1960). Nursing experiments showed that 75 000 to 100 000 carp fingerlings, about 10 cm in length could be obtained per ha of nursing ponds in about one month (El-Bolock and Labib, 1967).
Wunder (1963) stated that the climatic conditions in Egypt are excellent for the culture of pond fish such as Cyprinus carpio.
Uganda: Difficulties of breeding tilapia in higher altitudes in Uganda (about 200 m) led to the introduction of carp culture in this area (Meschkat, 1967) because carp has a high rate of growth and a low reproductive capacity (Pruginin, 1968). In 1957, fifty-seven specimens of C. carpio (var. specularis) were imported from Israel by the Fisheries Department and introduced in the ponds of Kajansi Fish Farm in Uganda as an alternative species to the indigenous Tilapia species, T. zillii, T. nilotica and T. leucosticta.
The first spawning took place in August 1959 at an average weight of 2 000 g and the number of fish produced by one female was 3 913 (Pruginin, 1965, 1968). A second spawning occurred in February 1960, but the number of fry produced per female of 3–5 kg was much lower, ranging from 100–500 due to predation by water insects (Corixidae and Notonectidae), frogs (Xenopus sp.) and gill-flukes (Dactylogyrus). However, elimination of insects from the ponds by treatment with 1 ppm of 3% BHC and reduction of gill-flukes infestations by removing the spawners as soon as spawning had taken place, fry survival increased to 2 000– 5 000 fry per female (Pruginin, 1965, 1968). The total number of carp fingerlings produced at Kajansi up to the end of 1962 was only about 7 000 but latter experiments resulted in the survival of 40 000 fry per female (Pruginin, 1967). Under a stocking rate of 600 fry to 2 000 kg per ha, the daily gain in weight per fish was between 0.5 g and 3.0 g when food was not added to ponds (Pruginin, 1968) and 1.6 g per fish with a food conversion of 1:5 when fish were fed (Simpson, unpublished). This poor response in growth of carp was due to the high turbidity of the Kajansi carp ponds, which reduced light penetration and hence production of natural food. However, in sandy ponds with sandy bottoms, clear water with high light penetration as are found in Serere, Teso District, the yields were higher, reaching about 1 000 kg per ha per year (Pruginin, 1968).
In sandy bottom ponds the combination of carp and Tilapia nilotica gave satisfactory results. The best results of all multiple stocking was obtained with carp plus hybrids (T. mossambica x T. nilotica). In ponds where this combination was tried, the yields were higher then when the same or smaller ponds were stocked with only one species.
Therefore, it was decided that carp would be a suitable fish primarily in areas where the waters were too cold for grand growth of tilapia while in the rest of Uganda it should be considered a supplemental fish for increasing somewhat the production of tilapia ponds (Pruginin, 1965). Semakula and Biribanwaha (1974), stated that with the envisaged commercial fish farming throughout the country, there is no doubt that the mirror carp will play a big role as a commercial fish.
Nigeria: C. carpio (var. communis) was first introduced to Nigeria in October 1954 from Austria and was stocked in artificial ponds in the Panyam Fish Farm, Jos, Northern Nigeria. In February 1964 fingerlings of C. carpio (220) were introduced to Western Nigeria from Israel. In Northern Nigeria, the fish was initially cultured separately but at the moment it is grown in polyculture of three Tilapia species and Lates niloticus. Using extensive methods, production was 180 kg/ha/year and it is hoped to be 450 kg/ha/year under intensive methods.
In Western Nigeria, carp is cultured separately and in combination with Tilapia species especially T. nilotica and also with grey mullet. Production was 275 kg/ha/year when the pond was manured and supplementary feeding (groundnut cake and yellow maize) added.
This species is very successful and well established; there is a great demand for it in the market (Elloit, Atir, personal communication).
Malagasy Republic: C. carpio has been introduced a long time into Malagasy Republic and is seriously raised in ponds (Huet, 1970). Difficulties of breeding C. carpio in tilapia home-ponds and the low production in the small ponds of the fish culture stations prevented carp culture from becoming important in Malagasy Republic (Meschkat, 1967). Thérézien (1960 and 1963) stressed the need for larger carp culture stations specially designed for the production of fingerlings to stock home-ponds.
In the high plateau of Malagasy Republic, carp has distinct spawning seasons, whereas in the lower altitudes it matures and spawns at any time (Meschkat, 1967). The annual distribution of carp is around 100 000 fingerlings. The present production in rice fields is estimated around 139 000 tons (FAO, 1971). Carp, together with C. auratus and T. mossambica was cultured from fry up to 1 year fish and the production reached 200 to 250 kg/ha/year when manuring and artificial feed were used (Huet, 1972).
Macrobrachium rosenbergii
Mauritius: In 1972 adults of M. rosenbergii were transported from Hawaii to Mauritius by air individually in polythene bags with polystyrene insulation, with oxygen. After the parent stock bred and reproductive stock ensured, experimental separate culture was practised, with fish used occasionally to rectify pond problems such as excessive filamentous algae. Females reached sexual maturity when they were in the range of 9 to 13.5 cm. The present production is 3 000 kg/ha/year (total 20 tons/year) and will expand to 30 tons/year by 1978. Hatchery techniques and management practices being developed, the private sector is investing heavily in hatchery facilities and growing ponds (Ardill, personal communication Ardill et al, 1973).
Tilapia spp.
Uganda: Between 1962–1966, several species of Tilapia were introduced into Kajansi Fish Farm in Uganda from Israel, Zanzibar and Kenya to produce hybrids. Out of the many crosses tried, the following successful crosses produced 100% males in the F1 generation (Pruginin and Kanyike, 1965; Pruginin, 1967):
When the hybrids of cross (a) were stocked in the same pond with T. nilotica, their growth rate was 20% higher in six months; when stocked separately at a density of 1 500 hybrids per ha, the yield was 800 kg/ha (Pruginin, 1968).
A comparative study was carried out of the 100% male hybrids obtained from crosses (b) and (c). Experiments did not reveal any significant differences between the growth rates of the two hybrids. However, it could appear that only the ♂ T. hornorum × ♀ T. nilotica cross should be recommended for large-scale use in fry centres outside the experimental stations, because of the easily determined distinction between the hybrids and T. hornorum; it was difficult to distinguish the hybrids from T. aurea (Pruginin, 1967).
It was observed that the growth rate of Tilapia hybrids in Kigezi was greater than that of other Tilapia spp. in the same ponds (Pruginin, 1967).
Tanzania: Similar succesful results were obtained in Tanzania at the Freshwater Fisheries Institute, Nyegezi when hybridization between ♂ T. andersonii (Zambia, 1968) × ♀ T. zillii produced 100% males (Ibrahim, 1974).
Kenya, Tanzania, Uganda: Tilapia zillii was introduced into Lake Victoria because this species eats grass and other vegetation and would occupy a niche left vacant by the endemic species. This introduction was successful, in the sense that the fish is now established and breeding in the lake (Jackson, 1960).
2.3.2 Recent introductions
Among very recent introductions which have shown great signs of success are the following:
Ctenopharyngodon idella
Sudan: On 1 January 1975, 10 000 fingerlings of grass carp, C. idella, ranging in size and weight between 3.3–4.7 cm and 0.4–1.4 g, were introduced from India to Sudan and released in experimental ponds where their growth and feeding habits were under observation. The whole consignment was successfully acclimatized, no mortalities occurred and by the end of May 1975, individuals reached 23 cm and weighed 160.1 g without significant artificial feeding. Investigations on the efficiency of this fish in controlling aquatic plants are now being conducted.
Cyprinus carpio
Sudan: Also on 1 January 1975, 100 fingerlings of C. carpio (var. communis), ranging in size and weight between 5.1–6.2 cm and 2.8–5.0 g respectively were introduced from India to Sudan when by the end of May 1975, individuals reached 27 cm and weighed 489.4 g.
Crassostrea gigas
The Gambia: A consignment of 1 000 oyster spat were brought to The Gambia in special containers in dry form from North Wales, U.K. and introduced in the experimental environment in January 1975. Two hundred were placed directly on the bottom of a shallow lagoon (salt water) near the mouth of the River Gambia, 800 were equally divided and placed in four reed baskets of 50 cm × 25 cm anchored to the bottom of the lagoon by rope.
The first lot of 200 were eaten by predators (crabs) while 200 spats died in each basket but noticeable growth rings have been observed in the remaining. It is too early to forecast the rate of production but it can be pointed out from the present experimental observations that there will be no obstacle to culture C. gigas on a commercial scale in The Gambia using collective culture (Agr. Senior Fisheries Officer, personal communication).
2.4 Unsuccessful introductions and transplantations
Tilapia mossambica
Egypt: T. mossambica was introduced into Egypt in 1954 from Thailand but due to severe winter season in 1957 all the stock died and thus it was not possible to acclimatize this species in the Egyptian waters (Eisawy, 1972; Koura and El-Bolock, 1958). Wunder (1963) stated that this species cannot survive cold winters in Egypt and should not be reintroduced.
Cyprinus carpio
Egypt: During 1940–1941, two consignments of 5 284 (average length 16 cm) and 26 000 (average length 20 cm) were transplanted in the Nile south of the Barrage Fish Farm and in the Nile near Cairo, Tawifky and Bahary canal respectively. Also during August 1941 and January 1942 more than a million carp were set free in the Nile south of the Barrage and in the vicinity of Cairo. Inspite of these stockings, carp has failed to establish itself in the natural bodies of water (Bishai, A-Malek and Labib, in press f).
Uganda: In Uganda, C. carpio also failed to establish itself as self-sustaining populations when stocked in 1959 in several dams and lakes in Kigezi District although its growth was better under such natural conditions; it attained a weight of 3 kg after the first year. It was observed to spawn in the lakes but commercial fishing of the species declined after an initial high level (Pruginin, 1967).
Although no actual introduction seems to have been made, Heterotis niloticus has appeared in the catches of fishermen in the Congo river basin. It is now established in the Stanley Pool, where it may have come from the Congo (Brazzaville), Djoumouna Fish Culture Station. The possible effect of the species, which is new in this area, on the overall production of this basin is not known, but it is reported to be well liked by the people who are willing to pay a good price for it.
This species also appeared in 1969 in fishermen's catches in Lake Tumba (550 km upstream from Kinshasa) and in the Kasai river. It is possible that these have come from the Bangui Fish Culture Station through the Ubangui river (FAO, 1969).
In 1956 Tilapia melanopleura was intentionally introduced into Mauritius but accidentally it escaped into water courses and reservoirs and has greatly harmed indigenous flora and fauna (Ardill, personal communication). Murno (1967) reported that C. carpio escaped into Lake McIlawaine and may compete with Clarias gariepinus on Chironomid larvae and displace this fish which presents a number of marketing problems and sells at a low price.
As would be realized from the foregoing account, introduction and transplantation of cultivable species into Africa took place either accidentally or deliberately. Accidental introductions concern us only indirectly, as a reminder that any activity even remotely connected with fisheries might introduce fish to water foreign to it and that such fish might well wreck havoc upon local populations (Jackson, 1960).
On the other hand, many African countries have deliberately introduced, from time to time, exotics and non-endemics to their ponds or natural waters for a variety of reasons:
to enrich their local fauna with fish or shellfish that are more valuable or more prolific or otherwise more desirable as sport or food than the endemic species
to control malaria, bilharzia or weeds in reservoirs or irrigation canals
to fill what have appeared to be unoccupied ecological niches
to produce hybrids and thus overcome the cultural problems of certain species.
The principles that guided the introduction measures of exotics are connected with two of the above objectives (a) and (b) and as most of the concerned species are now found to thrive and breed prolifically, their response to the transplanted medium may be considered as satisfactory. The advent of exotics does not appear to have seriously affected the indigenous fish fauna. Even the trout, the carnivorous habits of which are well known, has not been detrimental as it is restricted to the high, cold upland streams, where economically important varieties of indigenous species are few. Among other African countries, trout is well established in the Bale mountain rivers, particularly river Danka (400 km South East of Addis Abeba) where the most severe constraint in transplantation to other rivers and lakes at high altitudes is shortage of supply for the purpose, the long distance of available source from the home base and means of transportation (Meskal, unpublished). As trout introduction is a step of great value (Jackson, 1960), such handicaps should find a solution. However, this fact should not leave us unmindful of the dangers of indiscriminate transplantation.
The tench and perch have had little local impact but the introduction of the basses and, to a lesser extent, the sunfish are on the whole beneficial to their waters (Jackson, 1960). Essentially a cold-water species, the rearing of the tench, Tinca tinca has been reported to be very satisfactory in Tunisia (FAO, 1970 a). Production of the American black-bass (Micropterus salmoides) with the blue-gill (Lepomis) as prey has been tried in the Congo, The Gambia and Rhodesia but has not gained economic importance (Meschkat, 1967). This fish was introduced i n Kenya in 1928 solely for sport fishing but later it was used in Tilapia culture to control excessive breeding. Although it is a good predatcr, it has not been easy to propagate and it was, therefore, thought desirable to introduce the easy to breed bass, Tucunare ocellaris from Hawaii as an alternative (Odero, 1974). In the Atlas mountains of Morocco, propagation of M. salmoides in rivers and reservoirs is not satisfactory (Chapuis, 1962) but it is widely distributed in all the reservoirs of Swaziland and is the major predator in the country's water bodies (FAO, 1973). It is known that this fish does not propagate easily in Africa (Bard, 1962 c; Huet, 1957; Kiener, 1963). However, propagation difficulties seem to be connected with lower altitudes as they are rarely deported in higher altitudes where the fish has acclimatized itself and propagated in natural waters, e.g. Kenya, around Mount Kenya, some areas of Malagasy Republic and the Republic of South Africa (Meschkat, 1967).
The worst perhaps that introductions of trout and basses have done has been to exterminate locally or greatly reduce in numbers population of local fish such as Amphylius hargeri in Rhodesia (Turnbull-Kemp (1959) Rhod., Agr. J., 54, 4), and freshwater mullets (Trachyistoma euronotus), kurpers (Sandelia capensis), etc. in the Republic of South Africa (Jackson, 1960). Micropterus salmoides is reported to have virtually eliminated the native fish fauna in Lake Naivasha in Kenya, which prior to 1925 was dominated by the Cyprinodonts: Aplocheilichthys, Gambusia and Lebistes (FAO, 1970 b). The risk of extermination of any animal species is always a matter of great regret, so this must go into the debit side of the ledger (Jackson, 1960).
According to Jackson (1960), the introduction of carp, Cyprinus carpio, was a disaster mitigated only by the possibility that they might be used as source of protein in those low-altitude rivers where they have become irrevocably established. Jackson (1960), also mentioned that most legislation now forbids any introduction or transplantation of live carp mainly because:
carp have a wide ecological tolerance and can thrive when conditions become minimal for other fish, thus hastening the elimination of other species where they live
they destroy the eggs of other species, destroy vegetation in their habitats, to the extent often of eliminating all vegetal growth and, by constant rooting in the substrate, muddy the water, and cause salt to accumulate, destroying insect and other benthic life.
As a matter of fact, for many years there has been much controversy and great opposition to the introduction of carp in Africa (Lemasson and Bard, 1968). The introduction of carp was banned in most English speaking countries and in Congo, because of the alarming reports of carp spreading in natural waters in North America and in South Africa (Bard, 1962c; Meshkal, 1967); stocks were even destroyed (De Bont, 1949; Coche, 1960; Maar, 1960). In Uganda, objections raised against the introduction of carp included the possibility that they would compete with the endemic species for food and breeding grounds in the event of their getting access to the major lakes, and that it may also ruin sport fishing for trout in the mountain streams (Pruginin, 1968). Despite these objections, carp was eventually introduced into many African countries. Hogendoorn (personal communication) reported that the production of carp has never presented any problem as such in Cameroon and has reached up to 2 000 kg/ha/year, stocking 1 g fish per 3 m of pond space. However, because there is now in Africa a revival of interest in carp culture and because the fears expressed have not materialized, detailed experimental results of carp culture in African countries, particularly Egypt, Uganda, Nigeria and Malagasy Republic have been fully reviewed in this paper for a comparative study of the behaviour of C. carpio in different ecological conditions and under different cultural regimes.
When transplanted to natural waters, carp failed to establish itself as self-sustaining populations although it was observed to spawn therein. This is because carp could not escape predation of Nile perch in Egypt and Uganda and Gymnarchus niloticus, Clarias lazera and Hepsetus odoe in Nigeria. Contrary to this, C. carpio is well established in the Awash system particularly in Lake Akaki and Lake Koka in Ethiopia but no adverse effects on indigenous fauna are reported (Meskal, personal communication). However, Wunder (1963) stated that until more is known about the possible effect of C. carpio on other species, carp should only be used in controllable fish farms and not transplanted to natural waters. Bishai et al. (in press d) also recommended that introduction of carp in natural waters should be prevented because carp have the habit of digging at the bottom, destroying the aquatic vegetation, causing the water to be turbid and will destroy the nests of Tilapia species and other economically important nest-breeding fish. These views should be taken into consideration.
However, the culture of carp in paddy fields should be encouraged. Malagasy Republic is the only African country in which fish cultivation with rice is really developed (Huet, 1970). In Egypt, there are 300 000 ha of rice fields and if carp culture is practised in even half of this area, an amount of about 30 000 metric tons of fish could be obtained yearly; at the same time, the rice crop itself could be increased by 5 to 7% as a result of raising fish in rice fields (El-Bolock and Labib, 1967), Odero (1974) stated that with the introduction of large-scale rice growing in Mwea and Abero irrigation schemes (Kenya), the role of fish culture in the paddy fields and the control of malaria are stressed.
Introduction of fishes for the control of malaria and allied public health problems has been undertaken in Africa. The top Minnow, Gambusia affinis halbrooki had been introduced in various African countries for larvicidal purposes but could not establish itself in habitats where there are predacious fish such as L. niloticus and Clarias sp. (e.g. Sudan). However, until there is more fundamental research on the impact of such fish in particular, great care should be exercised in its introduction into new ecosystems because it may pose a serious danger to the local native fish, both small and large, on chance entry into the local waters (WHO, 1973, James and Fouler, 1970). The young of T. nilotica are now recognized as larv vores (Bardach, Ryther and McLarney, 1972; Sandon and El Tayib, 1953) and this species should be considered in the African countries for the purpose (George, unpublished).
The introduction of shellfishes in Africa is a recent innovation and in view of the success achieved in Mauritius, trials should be made in other African countries.
Considering the non-endemics, members of the genus Tilapia, the breams have been extensively introduced because they are the most important freshwater commercial species in Africa (Jackson, 1960). In 1959, the exploitation of a large stock of Tilapia was begun on a commercial scale in Lake Naivasha in Kenya, which had no species of Tilapia prior to 1925 (FAO, 1970 b). Kanyike (1974) stated that (T. hornorum x T. nilotica) hybrids have shown a number of advantages with regard to fish farming in Uganda. First, the farmer who had been discouraged by the stunted fish can raise Tilapia in ponds with antispection that good results will be obtained. Secondly, most of the natural food in the pond can be utilized. If the hybrids were stocked together with carp, the hybrids would utilize the plankton, while the mirror carp the bottom fauna; the supplementary food induces faster growth in both fish, thus leading to higher yields. Also Ibrahim (1974) stated that the occurrence of 100% males among T. zillii × T. andersonii is a significant finding and has potential bearing in African fish culture. This is perfectly true and these results should be of a great benefit to other countries where aquaculture is based exclusively on Tilapia species.
As far as the introduction of the breams is confined to those lakes or swamps which have no Tilapia species (e.g. Lake Naivasha prior to 1925) there is no risk. But, when introductions are carried out in lakes which have endemic Tilapia species, the possibility that the non-endemic might breed with the endemic is great, and under the usual conditions of lakes and swamps where control is not possible presumably revert eventually to whatever was the parent stock. For example, it has been suggested that T. macrochir introduced into Lake Kariba as an algae-eating species which adopts a relatively open water habitat, might interbreed with the local T. mossambica possibly defeating the object. This is because many species of a genus such as Tilapia are closely related to one another and separated more geographically than genetically owing to the recency of the speciation (Jackson, 1960).
Introduction of species other than Tilapia has been carried out also. The possibilities of using Heterotis for cultivation was suggested in 1956 by Daget and d'Aubenton (Huet, 1970; Lemmason and Bard, 1968) and as it serves a great potential for pond culture in many African countries, particularly French speaking, its introduction as a pond culture fish should be encouraged and a solution for the difficulties to obtain natural spawning in ponds (e.g. in Central African Republic) should be found.
However, the introduction of any exotic species of fish into a country should be for specific purposes only (Alikunhi, 1957). For example, T. mossambica (Zanzibar) when used for hybridization work at Kajansi Farm (Uganda) was successful and served the purpose for which it has been introduced but proved to be of little value when cultured separately, as the species started breeding at a size of 6–7 cm with the weight of individual fish not exceeding 600 g after a year's growth (Pruginin, 1968).
Besides, all introductions are considered unsuccessful when the species not only fails to acclimatize and establish itself in the transplant medium, but also even if it succeeds to do so and fails to serve the purpose for which it has been introduced. For example, T. melanopleura was introduced in 1953 into Sudan for weed control in the Gezira irrigation canals. This species could acclimatize itself but as the experimental observations on weed control in ponds were not encouraging, the stock introduced was eliminated (George, unpublished). This should be the case wherever new introductions are undertaken.
One last point is that great care should be taken to prevent transmission of diseases and parasitics while undertaking introductions. Parasitic Argulus pillucidus was recorded for the first time in Egypt on mirror carp in 1952 and it seems to have been transmitted with the introduced species from France (Koura and El-Bolock, 1960).
