by
Mounir M. Ishak and Maher M. Shafik
Institute of Oceanography and Fisheries
Inland Waters and Fish Culture Division
Cairo, Egypt
Fish culture was a very ancient practice in Egypt, and dates as far back as the time of the Pharaohs. The most ancient fish culture pond is engraved as a bas-relief in an ancient Egyptian tomb dating before 2000 B.C., showing Tilapia nilotica, a species common in the Nile, being fished out of an artificial pond (Hickling, 1962; Maar, Mortimer and Van Der Lingen, 1966).
Since the ancient Egyptians, fish culture has not been practiced in Egypt until recently when carp Cyprinus carpio was introduced to the country for the first time in 1934.
Egypt is characterized by the presence of large areas of natural water ponds, vast areas of fresh water, brackish and marine water bodies which are most suitable for fish culture. Moreover, the climate is more or less uniform all the year round, except during the few winter months, which is most suitable for aquacultural practice and high growth rates for the reared fishes.
The coastal line is about 1 500 km long. In the northern part, numerous shallow delta lakes are connected to the Mediterranean Sea as lagoons, as for example, Lake Manzalah, Lake Borollus, Lake Edku, Lake Maruit, Lake Quaroun, the Port-Fouad and the Bardaweel Depressions. Vast areas of low-lying fluviomarine swamps also exist in the coastal delta of the Nile system. Egypt has therefore a great potential for coastal aquaculture, especially in the coastal waste lands, which are not suitable for agriculture, but are most suitable for fish farming.
It is located in the northeast of Egypt and is considered as the largest northern delta lake (285 000 feddans)1. Generally, it is a shallow lake with an average depth of 1.15 m, with some greater depths through the navigation canals. The lake is connected to the Mediterranean Sea and the Suez Canal through Boughaz El-Gamil and El-Abouti, respectively. The lake is also connected with Damietta branch of the Nile through two fresh water canals, El-Ratama and El-Souffara north to Fareskour. Most of the drains of the surrounding cultivated lands open to the southern border of the lake.
It is located in the northern part of the delta region, stretching between the two branches of the Nile, Damietta and Rosetta. It has an area of about 125 000 feddans. The depth of the water varies annually between 0.40 m and 2 m increasing gradually from east to west and from south to north. It is connected to the Mediterranean Sea through a single strait, Boughaz El-Borollus. The main source of the fresh water comes through six drains to the southern part of the lake.
It is the smallest lake (25 000 feddans) of the three shallow Egyptian delta lakes that are connected to the Mediterranean Sea. The depth ranges between 0.50 m and 1.10 m with an average depth of 0.75 m. It is connected with the Mediterranean Sea through Boughaz El-Maadiya at the western extremity in a partially sheltered bay known as Abu-Qir Bay. The western as well as the southern sides show plenty of irregularities being bounded by small islands forming sheltered ponds. The lake receives fresh water from Edku and Barzik drains in the east section.
The fish fauna of these three lakes come from two origins, being connected with the Mediterranean Sea (saline water) and with the drains, more or less fresh water.
The fish of marine origin are found especially at the lake-sea connections. These fish are mainly mullets of which Mugil capito dominates; gilthead bream Chrysophrys aurata, sea-bass Dicentrarchus labrax and D. punctatus, and soles Solea vulgaris.
The freshwater fish are found in large communities in the southern part of the lakes and specially near the drains: Tilapia zillii, T. nilotica, T. gallilea, Lates niloticus, Bagrus bayad, Barbus bynnii, Hydrocyon forskalii and the catfish Clarias lazera.
It is located near Alexandria where it occupies nowadays an area of 20 000 feddans due to land reclamation projects. The depth of the water varies between 0.50 m and 1.15 m with a mean depth of 0.80 m. Not connected to the sea, it receives mostly irrigation water from the surrounding cultivated lands, as well as some sewage at the northern margin. Wahby (1961) has divided the lake ecologically into three regions: a polluted area, a plant area and the open lake. The major freshwater fish inhabiting the lake belong to the Tilapia genus, especially Tilapia zillii. The marine fish, viz, mullets and eels, have to be restocked annually from the Mediterranean Sea.
It is located in the western desert, south-west of Cairo. It has an area of about 53 000 feddans with a maximum depth of 16 m. The lake has undergone pronounced ecological changes from mainly fresh to highly saline conditions affecting its fauna and flora. Since it receives only drainage water on the southern coast, Tilapia zillii is the only freshwater species surviving the increased salinity. To compensate for the loss of freshwater fish, the Institute of Oceanography and Fisheries began to transplant mullet and soles fry from the Mediterranean Sea. Solea vulgaris appears to have established itself successfully in its new environment but mullet fry have to be transplanted annually. In the last few years other marine fish (Chrysophrys aurata, Dicentrarchus labrax and D. punctatus) were also introduced into the lake together with shrimp.
The Port-Fouad Depression is located on the eastern side of the Suez Canal near Port Said. It covers an area of about 30 000 feddans. It is connected with the Mediterranean Sea by a single strait in the north-west side and with the Suez Canal by 24 tubes of 1 m each. The depth varies between 0.70 m and 12 m.
The Bardaweel Depression is located in the northern part of Sinai Peninsula. It covers an area of about 130 000 feddans with an average depth of 6 m. The depression is connected to the sea by a single strait on the northern side.
The gilthead bream predominates the marine fish fauna which provides the main production in these two depressions.
Generally, for all inland and northern lakes the water holding capacity is great and under estuarine conditions the biological productivity is very high. Fish farming is carried out either in ponds or enclosures called ‘howash’, constructed in the low lying grounds. In this method, the fish are trapped and left to grow during the period when the water table in the area is high. Harvest takes place when the water recedes. Some regulations are being reinforced in order to protect the fishery of the lakes where these howash are constructed. This includes minimum size limits of fish and access water ways.
Tang (1977) classified the howash fish farming activities into three types, as follows:
Coastal howash: located along the strips between the Mediterranean Sea and the northern lakes. They are characterized by a great salinity range (from 10 ppt to 30 ppt), the level of salinity being affected by tidal action. The water level is regulated to a certain extent through the tidal cycle.
Lake-shore howash: generally distributed along the shores of Lake Manzalah, Lake Borollus and Lake Edku. They are characterized by a low salinity, less than 5 ppt. Their water level is affected by the amount of water discharged into the lake through the irrigation drains.
Lake-water howash: constructed within a lake and especially abundant in Lake Manzalah. The water depth is about 2 m. The salinity and the quality of the water varies locally, according to the distance from the outlet to the sea. However, this type of howash is considered as illegal since most of the catch is made of undersized fish. Legal howash are now being constructed to improve this system, proper management and stocking programmes being planned.
The species being cultured in Egypt vary according to the locality, the ecological and the chemical characteristics.
In fresh waters: Mugil cephalus, M. capito and M. saliens; Cyprinus carpio; Tilapia nilotica, T. gallilea and T. zillii; Clarias lazera; Lates niloticus.
In brackish and marine waters (salinity 10–39 ppt): Mugil cephalus, M. capito, M. saliens; Chrysophrys aurata; Dicentrarchus labrax and D. punctatus: Tilapia zillii; penaeid shrimp.
The culture of molluscs, oysters and seaweeds is not practiced in Egypt despite the availability of large areas suitable for such purpose.
The area of coastal and howash fish farms under operation totals approximately 12 000 feddans. There is a possibility of expansion in an area of about 40 000 feddans.
(see Note of the Editor, p.29)
Seeds of the fish used for coastal aquaculture originate mainly from their natural habitat, near the eastuarine areas for the brackish and marine water species. The highest catches of mullet fry are obtained one month after the peak flood of the Nile River during January, due to the bloom of microalgae on which the mullet fry feed vigorously. The diatoms appear also in massive quantities in the estuary after the Nile flood (Halim et al., 1974).
Mullet fry are available almost all the year round. Mugil cephalus fry can be collected and transported during ten months of the year (except June and July), due to their longer spawning period which extends from May to November (El-Gammal, 1970). Mugil capito fry are recorded from December until August only, while Mugil saliens fry are available from July until March. The locations of collections differ according to the different species. M. cephalus and M. capito fry are extremely tolerant to very low salinity but M. saliens fry show a preference for areas of relatively high salinity.
Sea-bass fry are available from April to August, especially in areas of lower salinity. Chrysophrys aurata fry are available for collection from early January to the first week of April.
Sole fry when required for transplantation can be available from April to July. Shrim seeds are collected both in estuarine waters and in the Red Sea, with maximum availability in May.
Seeds of freshwater species, such as carp, tilapia, catfish and Nile perch, are obtained from some experimental research stations affiliated to the Institute of Oceanography and Fisheries where natural breeding as well as artificial propogation take place.
It is difficult to obtain reliable data on the actual production and on the economics of coastal aquaculture. Such operations are carried out mainly on a family basis without records. The yield, depending on the management system, varies between 500 and 800 kg per feddan. In freshwater howash farms a production of 1 500 kg/feddan was obtained by giving supplementary feeds such as cotton seed cakes and rice bran.
Under the current research programme relating to coastal aquaculture, experimental work is being carried out on the polyculture of mullets, gilthead bream and sea-bass. An annual production of 500–800 kg/feddan has been obtained.
Penaeid shrimps (Penaeus kerathurus and Metapenaeus monoceros) were cultured successfully in Lake Quaroun and attempts are being now made to raise them in the laboratory
Although the culture of some bivalved molluscs holds great promises for coastal aquaculture, experimental work has not yet started.
Experimental work is being carried out on rearing fish in cages and pens.
The problems can be enumerated as follows:
lack of technical know-how and trained personnel;
mass production of quality seeds of the culturable species;
nursing fry to the proper stage for stocking;
development of suitable artificial feeds;
control of fish diseases;
water pollution;
reclamation of tidal lands and lakes for other purpose.
Note of the Editor: The views expressed in Section 4 - Sources of aquaculture seeds - concerning the seasonal availability of mullet fry, contradict previous results published by S.A. El-Zarka and F. Kamel - in 1967, in their paper entitled "Mullet fry transplantation and its contribution to the fisheries of inland brackish lakes in the United Arab Republic. (Proc.Gen.Fish.Counc.Mediterr., (8):209–226)
El-Gammal, F.E., 1970 Biological studies of the fish fry in the Nile Mouth between Edfina and Rosetta compared with El-Mex area. M.Sc. Thesis, Faculty of Science, University of Alexandria, 161 p.
Halim, Y. et al., 1974 Estuarine plankton of the Nile and the effect of water phytoplankton. Paper presented at the Symposium on the Effect of fresh water on biological processes in fjords and coastal waters, Geilo, Norway, April 1974. (mimeo)
Hickling, C.F., 1962 Fish culture. London, Faber and Faber, 295 p.
Maar, A., M.A.E. Mortimer and I. van der Lingen, 1966 Fish culture in Central East Africa. Rome, FAO, 160 p.
Tang, Y.A., 1977 Report of the Aquaculture Mission in Egypt, 20 October – 28 November 1976. Rome, FAO, 48 p. (mimeo)
USAID Mission, 1977 Egyptian Aquaculture feasibility report. Cairo, USAID
Wahby, S.D., 1961 Chemistry of Lake Mariut. Notes Mem.Inst.Oceanogr.Fish.,Alexandria, (65):25 p.
by
F.M.K. Denyoh
Deputy Director of Fisheries
Inland Fisheries Branch
Accra, Ghana
Several lagoons occur along the coast of Ghana, stretching from Half Assini in the west to Aflao in the east, a distance of approximately 550 km (Fig. 1). There are approximately 50 lagoons with a total surface area of about 40 000 ha, but most of them are small and unimportant in terms of fish production (Mensah, in press).
The latest and the most important lagoons are Keta and Angaw situated on the east, and the Aby Lagoon on the western border between Ghana and the Ivory Coast, the bigger portion being in the Ivory Coast. In between these extremely situated lagoons are some other important lagoons, even though they are smaller in size; these are the Amansuri, Brenu, Benya, Fosu, Amisa, Nakua, Sakumo I, Kpeshie, Sakumo II and Songaw.
All these lagoons may be classified in two categories to correspond to the two types (closed or open) described by Boughey (1957). The first type, narrow and brackish, lies immediately behind a sand barrier and has many widely separated temporary openings. It is usually not associated with large river systems (Kwei, 1976). It was described as “closed” lagoons for the reason that they are closed off from the sea by sand bars for the greater part of the year. In Ghana, most of these types occur in the eastern coastal region where the annual rainfall is relatively low, and examples are the Mukwe and Kpeshie lagoons.
The second type described as “open” lagoons is normally fed by a large river or rivers during most part of the year. In Ghana, examples of this type are the Angaw, fed by the Volta River, the Keta Lagoon fed by the Kplikpa and Aka rivers, the Aby Lagoon fed by the Tano River, and the Sakumo I Lagoon fed by the Densu River. These lagoons also have direct or indirect connection with the sea.
It is not known how much all these lagoons are contributing to the national fish production by way of capture fisheries because no catch statistics have been collected on them. A study carried out by Pauly (1976) on Sakumo II Lagoon, however, indicated a catch of 150 kg/ha/year, with 93 percent of the catch being Tilapia melanotheron/heudeloti. There is a strong indication that lagoons contribute significantly to the natural fish production. This is also confirmed by the large number of fishermen inhabiting the lagoon areas who depend entirely on lagoon fishing for their livelihood.
Most of these coastal lagoons are nursery grounds for marine fish species and shrimps, and often sustain significant fisheries.
Coastlines were described by Gauld and Buchanan (1956) as sandy or rocky. The sandy shores are found at the western and eastern sections of the coastline. The western section extends approximately 93 km from Half Assini to the Ankobra River. The eastern section which extends from the neighbourhood of Ningo to Aflao, covers a distance of some 128 km. Between the Ankobra River and Ningo, the coastline is a mixture of sand and rocks. The extent of purely rocky shore is small. A few sand bays occur at the west, e.g., at Butre and Busua.
Important major permanent rivers are:
the Volta with its extensive estuary on the east, with a connection with the Angaw lagoon;
the Densu which empties into Sakumo I;
the Nakwa which feeds Nakwa Lagoon;
the Pra and Ankobra rivers which empty directly into the sea; and
the Tano River, at the extreme west, which drains into the Aby Lagoon.
Only a few of the coastal lagoons have been studied. In their studies, Pauly (1975) and Kwei (1976) studied the ecological, biological, chemical and hydrological characteristics of Sakumo I, Mukwe and Sakumo II. Temperature, salinity and dissolved oxygen for Mukwe Lagoon are given in Fig. 2. Most of these lagoons are known to be exposed to tidal currents which are semi-diurnal. Mean high tides are 162 cm and mean low tides 34 cm above datum for Accra (Ghana tide table for 1971). The inflow and outflow of these tides incluence the salinity of these lagoons.
(Fig. 3)
This occurs 11 km west of Accra and lies in the Densu Valley. It is about 5 km long and 140 km wide. Its depth during the dry season ranges from 50 cm to 10 m in its lower reaches. Its estuary is fringed by extensive marshes. Fed by the Densu River, it is an example of an ‘open’ lagoon.
(Figs 2 and 4; Table 1)
Lying east of Accra, it is 2 km long and has a surface area of about 4 ha; an example of a ‘closed’ lagoon, separated from the sea for nearly 10 months of the year. During the remaining 2 months of heavy rains, it is opened to the sea. The floodwaters cause the sand bar at the mouth to be broken, allowing lagoon waters to flow into the sea and thus emptying the lagoon of its water. During this period tidal currents flow into and out of the lagoon periodically for a few weeks. It is a shallow lagoon having a depth of 0.50–1.5 m.
(Fig. 4 and Table 1)
It lies some 2 km west of Tema. It is 4.42 km long and has a surface area of about 53 ha. Its maximum depth under tidal influence is 0.90 m (Kwei, 1976). It was originally a ‘closed’ lagoon, but in order to prevent any possible flooding of the Accra-Tema trunk road, the lagoon is made open by two parallel culverts which permit exchange of sea water during high tide. Sakumo II has thus become an ‘open’ lagoon.
(Fig. 5)
This is the largest in Ghana with a surface area of 2 120 ha. There is no reliable information on the tidal effect on the lagoon, neither is there any published information on its ecological, biological, chemical and hydrological characteristics. It has an indirect connection with the Volta estuary and the sea through the Angaw Lagoon. The main source of its water supply is the Aka and the Kplikpa rivers and the Denu Lagoon. It was quite deep in places and navigable by fair-sized motor launches. But in recent years, it has become shallow after the building of the Akosombo Dam. This dam has prevented the flooding of the Volta River, thus checking its floodwaters from entering the lagoon. It has caused the reduction in fishing activities on the lagoon. The water has become very brackish in recent years, resulting in the production of large quantities of salt at the marginal areas. Annual evaporation rate is 1 524 mm. Most of its marginal areas are now dry and could be reclaimed for the construction of fish ponds. The lagoon had a direct connection with the sea through a canal at Kedzi, created by heavy floods in 1963 and 1968. During the period, there was migration of marine fish species and juvenile shrimps from the sea into the lagoon, resulting in a flourishing fishing industry in the lagoon. The fishing industry was badly reduced when the canal was closed by the Public Works Department, aided by the sand bars in order to reconnect the turnk road which was cut off by the canal.
Table 1
Mean daily variation of surface water quality during 24-h observations in 2 lagoons (Kwei, 1976)
A. Mukwe Lagoon
| Salinity (ppt) | Oxygen (mg/l) | Temperature (°C) | |
| January | 3.90 | 1.97 | 5.03 |
| May | 1.32 | 1.27 | 4.60 |
| September | 0.37 | 2.08 | 4.60 |
| December | 3.60 | 2.23 | 4.16 |
| Annual | 2.07 | 1.87 | 4.53 |
B. Sakumo II Lagoon
| Salinity (ppt) | Oxygen (mg/l) | Temperature (°C) | |
| January | 4.32 | 2.56 | 5.32 |
| May | 1.90 | 2.48 | 5.20 |
| September | 1.20 | 2.30 | 4.92 |
| December | 2.20 | 2.76 | 5.50 |
| Annual | 2.38 | 2.52 | 5.23 |
A summary of the hydrological characteristics of the three lagoons, Sakumo I, Mukwe and Sakumo II, can be stated as follows (Kwei, 1976):
rainfall, evaporation and tidal currents have a combined effect on temperature and salinity variation;
the minimum temperatures of the lagoon head waters were found to be lower than those near the sea, while the maximum temperatures were found to be higher for the head waters. During the dry season, the salinity in the ‘closed’ lagoon rises steeply due to continuous evaporation, while in the ‘open’ lagoon it rises gently and remains less due to tidal influence;
the levels of dissolved oxygen concentration are higher in the ‘open’ lagoons than in the ‘closed’ lagoons, due to higher phytoplankton populatio in the ‘open’ lagoon (Pauly, 1975);
there is generally a higher organic productivity in the ‘open’ lagoons than in the ‘closed’ lagoons;
these ideal conditions result in a higher production of fish and crustacean biomass in the ‘open’ lagoons.
(Fig. 5)
This lagoon, situated west of the Keta Lagoon, has a direct connection with the Volta estuary and the sea. It is under the influence of tidal waves which are semi-diurnal and measure approximately l m at a time. It is fringed extensively by swamps and mangroves. This lagoon is known to support a large fishery, mainly of grey mullet and shrimps. Other marine species include Cynoglossus sp., threadfin and barracuda. No detailed studies are known to have been conducted on this Lagoon. It is connected to the Keta Lagoon through a narrow channel under a culvert, over the Srogboe-Dabala Highway.
This lagoon, situated on the extreme west of the country, is the largest next to the Keta Lagoon, but has its larger portion in the Ivory Coast. It is fed by the Tano River. Its depth ranges from 1.5 to 3 m. The water of this lagoon remains fresh during the rains, but becomes brackish during the dry months, when there are tidal effects. A narrow belt of evergreen papyrus is found along the margin with water depths of about 1 m. Along this belt there is relatively high ground which is swampy. The soil of this swampy ground consists mainly of a layer of silt of about l-m thick with a sandy subsoil. Excavation of this soil should be at least l m when it is considered for pond culture.
Most of the described lagoons support quite a significant fishery which is mostly at subsistence level. The cichlids constitute the most significant of all the species present. Those known are Tilapia melanotheron/heudeloti, T. zillii, T. galilaea and Hemichromis fasciatus. Despite the heavy fishing efforts on the lagoons, these cichlids are available in most of the lagoons throughout the year.
Due to their prolific breeding nature, the seeds are naturally produced mainly when the waters are high and at a time when natural food is abundant. This results in rapid growth, making adults available for capture when the waters are at their low level during the dry months. Even though the waters become brackish at this time the cichlids are able to adjust to this kind of condition very well.
Some of the lagoons which have connections with the sea serve as nursery ground for some marine species and shrimps. The commonest marine species is the grey mullet. This species is next to the cichlids in importance. Large scale migrations of the adults of this species are known to occur seasonally to the sea, from the Keta and Angaw lagoons mostly at night. They are reported to migrate through narrow channels from which fishermen in nearby villages catch them in large numbers. The juveniles migrate seasonally from the sea to these lagoons.
Two species of shrimps are known to be present in the Keta and Angaw lagoons. They are Penaeus duorarum and Parapenaeopsis atlantica. The juveniles of these are also known to migrate from the sea into these lagoons. The adults are captured in the lagoons or during their migrations back into the sea. During the 1963 and 1968 floods, when the Keta Lagoon had a direct connection with the sea through the Kedzi Canal, formed as a result of the floods, there was a flourishing shrimp fishery in the lagoon. This fishery died down when the canal was later closed by the Public Works Department and the sand barriers of the sea. This was to reconnect the trunk road from Denu to Keta which was otherwise cut off by the canal. The above species are caught also in Sakumo I, Sakumo II, Kpeshie and Aby lagoons.
Other common freshwater species are Clarias senegalensis, Chrysichthys spp., Pelmatochromis spp. and small quantities of Macrobrachium spp. Other common marine species are Caranx hippos, Ethmalosa fimbriata and the swimming crab, Callinectes latimanus. There are other species of fish and crustaceans which occur in the lagoons, but these are comparatively not very significant. A list of the species of fish and crustaceans, caught in the lagoons and estuaries, is shown in the Appendix.
Most of the fish caught in the lagoons is readily marketed fresh at the landing points. Small quantities of tilapias and shrimps are processed into fried fish, while other marine species are smoked for sale at nearby market or transported to distant markets where there is a great demand for them. During the flourishing shrimp fishery in the Keta Lagoon, there was a factory sited at Keta which processed quantities of shrimps for export. The factory closed down when the shrimp fishery declined.
The heavy fishing effort on most of the lagoons does not allow the species of fish present to grow to a good marketable size or to give them enough time to get to adult size to breed. As a result, fish populations in the lagoons are kept at a very low level. Nevertheless, in view of the heavy demand on fish, the small size fish are easily marketed soon after it has been caught.
The most common methods of exploitation of the coastal lagoons, as well as the estuaries, are by the use of the castnets, longlines (hooks) and many different types of traps. Sometimes, the fish are confined to temporary enclosures from where they are hand-picked by a group of people. This type of fishing is usually referred to as ‘community fishing’. This is commonly observed in the Keta Lagoon. The size of the castnet used ranges from about 1.5 m to 3 m in total length and from 1 m to 2 m in diameter, at the cod end. This castnet may be operated from a canoe or while the fisherman is standing in the water, depending on the depth of water. Species commonly caught with castnets are Tilapia, Ethmalosa fimbriata and Mugil spp.
The gillnets are usually used in the large lagoons in which the depth of water is normally considerable. These are the Keta, Angaw and Aby lagoons. A special type, called ‘ali’, is used in the Aby Lagoon. It is a kind of ring net. This is similar to the ‘ali’ net used in capturing pelagic fish in the sea, except that these are smaller (approximately 5.5 m deep and 1 500 m long - Mensah, M.S.).
A crew of 12–14 operate this net from a dug-out canoe. The net is set in circular form, then all the crew, except 2, jump into the water and start to pull the net slowly into the canoe, the cod end being pulled faster than the float line. This kind of operation continues until the fish caught are concentrated in a small part of the net which is then lifted into the canoe. The whole operation may take from 4 to 6 hours. In shallow waters dragnets are commonly used. In the other large lagoons, a smaller type of ‘ali’ net than the one described above is commonly used. This net is approximately 1 m deep and ranges from 50 m to 300 m in length. This can be operated by a crew of 2 to a maximum of 10, depending on the size. Here, the difference in the operation is that the net is dragged to the beach after the water is disturbed to scare the fish into the net. This net catches, apart from Tilapia, other species like crabs (Callinectes latimanus) and shrimps.
For shrimping in the Keta Lagoon in particular, a special trawl net with a bag is set in the narrow channels against the outgoing tides. The shrimps (Panaeus duorarum) are then caught in the bag. In the Volta estuary, a special shrimp trap is used with its wide opening against the flowing water. The wall may be approximately 7 m long. The actual net in the form of a bag is fixed at the narrow end of the ‘V’ wall. The walls direct the swimming shrimps, as well as other species like Tilapia, into the bag (Fig. 6).
In Sakumo I and II lagoons and in some other smaller lagoons, a type of trawl net, small enough to be dragged by 2 men and therefore called a ‘2-man trawl’ (Fig. 7), is used to catch shrimps and the portunid crab (C. latimanus).
Traps of various designs and sizes are used for catching this same crab usually called the blue-legged lagoon crab. A common crab-trap, used in the Sakumo I and II and Mukwe lagoons, is woven with the thin sticks removed from the leaves of the coconut palm (Fig. 8). This is usually baited and set at the bottom, with two pieces of sticks across to prevent it from being moved by water current. Crabs and sometimes fish are lured by the bait into the trap.
The common crab net, used in almost all lagoons, consists of a net attached around a metal ring about 30–50 cm in diameter (Fig. 8). One end of a thin rope is attached to the metal ring while the other end is attached to a cork to indicate the position of the net. The net is positioned at the bottom with a bait which attracts the crab. As the crab feeds on the bait, it is entangled in the net. The net is lifted up at regular intervals to collect the crabs.
In the Keta Lagoon a straight wall is constructed with mangrove or palm branches across a channel to act as a barrier against the movement of fish and crabs. Several holes are made in the wall at intervals of 1–1.5 m and specially woven traps are set at these holes. As the crabs and fish try to get through these holes, they are caught in these traps. Ordinary set nets of varying sizes are commonly used in most of the lagoons. These nets are set at night and visited a number of times before morning. Species of fish caught by this method include Sphyraena spp., Gerres melanopterus, Caranx hippos and Pomadasys spp.
In the large lagoons, traditional extensive aquaculture is practised in the shallow areas. In the Keta Lagoon, some fishing families claim certain portions which they use for this type of culture. Twigs or branches of trees are dumped in the area which is marked with sticks (Fig. 9). This area may measure between 150 m2 and 300 m2. These twigs and branches with their foliage are left in situ for 3–4 months. The rotten foliage and the algae which grow on the branches attract fish. While the fish feed on these, they remain in the area and grow to attain marketable size. The fishermen surround the area with a net, remove the twigs and branches and harvest the fish thus encircled. This traditional culture method is called ‘achidja’ and this is very popular with fishing families dwelling around the Keta Lagoon.
Some families also build up low barricades to enclose lagoon water. This may be referred to as ‘mini fish ponds’. The barricade may be of soil or made from grass. The fish so trapped in the water are allowed to grow for 3–4 months after which they are harvested with dragnets.
Some commercial aquaculture is being practised in the inland areas of Ghana, especially in the northern areas. This is mostly by government agencies to serve as demonstration to private farmers and other private agencies. Such farms are completely absent in the coastal lagoon areas. Pillay (1962) after conducting a short survey, described the potential of aquaculture in the coastal lagoons as very great and economically viable. Wakuti also conducted studies on the reclamation of the Keta-Avu area for its utilization in agriculture, flood control, irrigation and aquaculture. In his report (unpublished), commercial aquaculture has been highly recommended and he went on to describe the methods of pond construction.
It was indicated that an area will be available for 800 ha of fish ponds where annual production of 3 t/ha was predicted through aquaculture, at least three harvests a year being feasible.
By tradition, most areas of the lagoons are owned by families, especially where these families reside along the lagoon shorelines. Only these families have claim over these particular areas and can fish in them. Any other individual or organization wishing to fish or utilise an area belonging to another, will have to lease or buy the area from the owner. In the case of Government wishing to acquire the area for the purpose of its development, compensation will have to be paid to the owner. In acquiring areas for aquaculture development, other kinds of developments should be considered like in the case of developing the Keta Lagoon area for flood control.
At present the Fisheries Department has a few aquaculture experts and technical officers, with others now in training. They would give extension services to fishermen who would accept the concept of aquaculture in the lagoon areas. It is the policy of the Government to provide the necessary inputs at subsidized rates. At the moment, seeds and technical supervision of pond construction are provided free of charge by the Government. Fish farmers are offered free training in fish farm management methods which will enable them to obtain high yield from their farms.
It is the policy of the Government to integrate aquaculture with every large scale irrigation project. Such aquaculture projects have been planned in the inland areas at Vea (farm completed), Tono (construction to start soon) and Afife (dam under construction).
Similarly, in their reports to the Government, Wakuti and Nathan Consortium for Sector Studies (1970) also recommended commercial aquaculture to go along with other developments like agriculture, flood control, housing and conservation, thus avoiding any possible conflicts with planned aquaculture projects.
Table 2
Annual consumption of fish products in Ghana excluding fish catches from lagoons and rivers (in t)
| 1974 | 1975 | 1976 | 1977 | 1978* | ||
| Domestic catch | ||||||
| Marine | 182 967.3 | 215 548.3 | 198 735.4 | 226 211.2 | (222 088.8) | |
| Volta Lake | 37 300.0 | 41 945.0 | 41 000.0 | 38 500.0 | (37 500.0) | |
| Landings from foreign vessels (frozen fish) | 4 881.0 | 872.0 | 1 261.0 | 1 388.0 | (17 830.7) | |
| Imports | 22 130.2 | 16 973.9 | 52 897.9 | 49 908+ | not available | |
| Total weight | 247 278.5 | 275 339.2 | 283 894.3 | 316 007.2 | (277 419.5) | |
Fish is the major source of protein supply in the Ghanaian dish. Demand for fish has outstripped production from capture fishery in recent years, Ghana being one of the greatest fish consumers in Africa (25 kg per caput per year). It has been observed that while there has not been significant increase in domestic fish catch over the past five years, consumption has increased considerably (by about 30 percent).
It is obvious therefore that demand for fish will ever increase over the years as long as there is population growth. The possibilities of further increasing total fish production targets to meet the estimated requirements lies with production through aquaculture.
The inadequacy of water supply, however, is a serious limiting factor in many inland areas for the operation of freshwater fish farms. The development of brackish-water farms, utilizing the coastal lagoons and estuaries, should therefore be given some serious consideration.
In Ghana, it is not difficult to achieve an annual production of 5 t/ha through intensive aquaculture. If therefore some of the coastal lagoon areas could be converted into fish ponds, this would definitely assist in achieving production targets.
Most of the lagoons described above have around them swamps which serve no useful purpose. Some of these are suitable for reclamation into fish ponds. Some of the small ‘closed’ lagoons are also suitable for conversion into fish farms through proper management.
The Ghana Government has set up banking institutions with the mandate of giving out soft loans to finance agriculture projects, including aquaculture. Three types of loans are granted to farmers: short-term, medium-term and long-term loans, depending on the type of crop to be raised. Aquaculture falls within the medium-term loan, the refund of which is expected to commence after a period of twelve months, after it has been granted, with reasonably low interest rates.
Other private banking institutions also do grant such loans to farmers with similar terms, with the purpose of financing agriculture development projects.
Considerable interest has been aroused in some big organizations which in recent years have been investing heavily in agriculture development projects. This has been the result of the ‘Operation Feed Yourself’ and ‘Operation Feed Your Industries’ campaign, which the Government instituted a few years ago. Aquaculture being a new concept in Ghana, the impact on this has not been felt yet, but there are some indications that aquaculture will soon develop both in the state and private organizations. The Fisheries Department is ever prepared to offer extension services and technical assistance.
Apart from contributing to an increase in fish production, the establishment of industrial aquaculture in the coastal lagoon areas would provide successful employment for a large number of unemployed and underemployed people who inhabit the lagoon areas. It would also assist in checking the soaring price of fish in the country, the present minimum price of fish being N.Cedi 4.00/kg. A successful shrimp industry in the lagoons will earn substantial amount of foreign exchange for Ghana as was the case in the Keta Lagoon between 1963 and 1968, which prompted the establishment of a shrimp processing plant at Keta.
As recommended to the Government in previous reports, it is necessary to set up pilot aquaculture projects in the lagoon areas and to demonstrate their economic viability to the local fishermen who will later take up commercial projects on their own.
There will be no problem with the marketing of the fish as there are ever ready markets.
As mentioned earlier, Ghana's fish consumption per caput is comparatively high in Africa. Despite improvement in fishing methods, it has not been able to increase fish production sufficiently to satisfy the national requirement. Table 2 shows the state of the fishery production as it compares with consumption. The shortage in production has necessitated the importation of fish products every year in order to supplement the domestic catch. While domestic catch fluctuated between 220 000 t and 265 000 t in the past five years fish consumption has increased steadily from 247 000 t to cover over 300 000 t.
It is imperative that there should be a properly planned commercial aquaculture programme since this is the only way of increasing fish production in order to make up for the balance to satisfy the nation's requirement. The programme should set up a target of some 50 000 t of fish to be produced annually by 1985 through aquaculture. This means some 10 000 ha of fish ponds should be completed to be in operation by then. It is therefore necessary to conduct detailed surveys of some selected lagoon areas with a view of finding suitable swamp areas and soils for brackish-water farm.
The Government will have to invest quite substantial capital in setting up pilot projects. The main constraints of implementing such programmes are financing and the lack of trained extension personnel. There is also not enough machinery and equipment available in the country at the moment for the construction of such ponds.
The Government should therefore request some external assistance in this field, most appropriately from the Food and Agriculture Organization of the United Nations.
The two types of lagoons ‘open’ and ‘closed’ which occur in Ghana are found to have conditions suitable for commercial aquaculture. The shallowness of the lagoons, the regular inflow of fresh water (either from the rivers or surface run off) and the tidal effect seem to prevent the depletion of oxygen in the lagoon waters. The oxygen level remains sufficient throughout the year to support fish life. All the lagoons occur in the parts of Ghana where the average rainfall is adequate and where there are two rainy seasons in the year. There is therefore enough water throughout the year for the purpose of commercial aquaculture.
Many of the lagoons have arond them swamp areas which serve no useful purpose and could be converted into fish ponds.
There are enough fish species indigenous to the lagoon areas and which are well adapted to the brackish conditions. There will be no problem with fish seed supply or the marketing of the fish which will be produced.
With the above conditions available, it is considered that the utilization of the coastal lagoons for the purpose of commercial aquaculture development is economically viable and will assist Ghana to overcome the present short supply of her fish requirements.
Boughey, A.S., 1957 Ecological studies of tropical coastlines. I. The Gold Coast, West Africa. J. Ecol., 45:665–687
Gauld, D.T. and J.B. Buchanan, 1956 The fauna of sandy beaches in the Gold Coast. Oikos, 7:293–301
Kwey, E.A., 1976 Biological, chemical and hydrological characters of coastal lagoons of Ghana, West Africa. Hydrobiologia, 56:157–174
Lawson, G.W., 1956 Rocky shore zonation on the Gold Coast. J.Ecol., 44:153–176
Mensah, M.A., The hydrology and fisheries of the lagoons and estuaries of Ghana. 22 p (M.S.) (in press)
Nathan Consortium for Sector Studies, 1970 Flood control in the Avu Keta area. Report prepared for the Ministry of Finance and Economic Planning, Ghana, 26 p
Pauly, D., 1975 On the ecology of a small West African lagoon. Ber.Dtsh.Wiss.Komm.Meeresforschung 24:46–62
Pauly, D., 1976 The biology, fishery and potential for aquaculture of Tilapia melanotheron in a small West African lagoon. Aquaculture 7:33–49
Pillay, T.V.R., 1962 Report to the Government of Ghana. Possibilities of fish culture in lagoons. EPTA/FAO Rep. (1581):9 p
1. Cichlidae
Tilapia melanotheron
T. zillii
T. galilaea
Hemichromis fasciatus
2. Mugilidae
Mugil spp.
3. Clupeidae
Ethmalosa fimbriata
4. Carangidae
Trachinotus goreensis
Caranx hippos
5. Claridae
Clarias senegalensis
6. Gobiidae
Oxyurichthys occidentalis
7. Pomadasydae
Pomadasys jubelini
8. Soleidae
Synaptura punctissima
9. Gerridae
Gerres melanopterus
10. Elopidae
Elops atlanticus
11. Lutjanidae
Lutjanus agennes
L. goreensis
12. Penaeidae
Penaeus duorarum
13. Cynoglossidae
Cynoglossus senegalensis
14. Portunidae
Callinectes latimanus
Fig.2, (From Kwei) - Variation in the values of salinity and temperature in relation to the amount of rainfall in the Mukwe lagoon, 1971
Fig. 3 (From Kwei) - Map of the Sakumo Lagoon Accra
Fig.4, (Kwei) - Map showing the geographical location, and relative positions of Mukwe and Sakumo 2 lagoon
Fig.5 - Map of Keta, Agaw & Songaw lagoons
Fig. 6 (From Mensah) - A “V” shaped shrimp trap
Fig. 7 Two-man Trawl Net
Fig. 8 - (From Mensah)
Fig. 9 - Diagrammatic representation of “Achidja”
method of aquaculture.
(i) Plan (ii) Section a-a
(From Mensah)
| par | ||
| Andriamahaly Louis Rasolofo Chef Division pêche et pisciculture Antananarivo Madagascar | Alexandre Rabelahatra Directeur Projet PNUD/FAO: ‘Developpement des pêches continentales et de l'‘aquaculture’ Chargé des recherches piscicoles Antananarivo Madagascar |
Géomorphologiquement, on peut distinguer brois catégories de côtes dans le relief littoral de Madagascar:
les côtes basses, d'altitude inférieure à 50 mètres que l'on retrouve surtout le long de la côte est (entre Fénérive-Est et Tolanaro). Ce sont des côtes à cordon sableux, parallèles, multiples, avec des séries de lagunes. Le long de la côte ouest par contre (entre le delta de la Mangoky et le cap Saint André), ce sont des côtes à cordons sableux avec des mangroves;
les côtes moyennes, d'altitude comprise entre 50 à 150 mètres que l'on retrouve a l'extrême Nord (baie d'Antsiranana), au Nord-Ouest (entre la presqu'île d'Ampasindava et la baie de Mahajanga), au Nord-Est (au niveau d'Antalaha jusqu'a cap Mascala) et à l'extrême Sud (du cap Sainte Marie jusqu'à Tolanaro);
les côtes rocheuses élevées, d'altitude supérieure à 150 mètres, le cas notamment de la baie d'Antogil.
Les côtes basses et les côtes de moyenne altitude (notamment sur la côte ouest) présentent en outre des zones d'alluvions particulièrement intéressantes. C'est le cas des deltas de la Mahavavy, de la Mahajamba, de la Betsiboka, du littoral au sud du cap Saint André, des deltas de la Tsiribihina, de la Morondava et de la Mangoky.
Au point de vue climatique, on peut distinguer 4 régions climatiques le long des côtes malgaches:
la côte est qui a un climat du type tropical humide avec une précipitation supérieure à 1 500 mm, sans aucun mois sec au cours de l'année; la température la plus basse y est de 15°C;
la côte ouest qui a un climat du type tropical sec avec une précipitation comprise entre 500 et 2 000 mm et présentant 5 à 8 mois secs dans l'année; la température la plus basse y est de 20°C;
la côte extrême nord qui a un climat du type tropical sec avec une précipitation entre 500 et 1 000 mm et présentant 7 à 8 mois secs dans l'année; la température la plus basse y est de 20°C;
la côte sud-ouest et sud qui a un climat du type semi-aride avec une précipitation inférieure à 400 mm et présentant jusqu'à 9 mois de sécheresse dans l'année; la température la plus basse y est de 20°C.
Au point de vue pédologique, la côte ouest est caractérisée par la prédominance, tout au long de ce littoral, de sols salés et de sols de mangrove, tandis que la côte est et la côte extrême sud ont des sols peu évolués dunaires ou alluviaux ou sableux.
Enfin, du point de vue hydrologique, la côte ouest connaît des étiages très faibles mais aussi des crues violentes en période de pluie. Le débit spécifique y est supérieur à 10 000 1/s/km2. La côte est, beaucoup plus arrosée, a des rivières abondantes toute l'année et l'étiage y est de l'ordre de 15 à 30 1/s/km2. Le débit spécifique moyen y est de l'ordre de 20 à 100 1/s/km2. La côte sud a des débits moyens annuels faibles (quelques litres/s/km2). La côte extrême nord a aussi des débits de rivières faibles où l'étiage est de l'ordre de l à 2 1/s/km2.
Il en résulte que par l'interférence de ces divers facteurs, les zones côtières favorables à l'aquaculture sont relativement limitées, à savoir: (a) les zones de mangroves sur la côte ouest et (b) les zones de lagunes sur la côte est.
C'est ainsi qu'on peut recenser le potentiel suivant susceptible d'être mis en valeur par l'aquaculture:
| (i) | zones de mangroves (forêt de palétuviers soumise aux marées) | ||
| Surface (ha) | |||
| Mangrove de la Mahavavy nord (Ambilobe) | 11 400 | ||
| " | de l'Ifasy (baie d'Ambora) | 14 000 | |
| " | de la Sambirano (Ambanja) | 8 000 | |
| " | de la Loza (Analaheva-Antsohihy) | 18 000 | |
| " | de la baie de Mahajamba (Port-Bergé) | 39 400 | |
| " | de la Bombetoka (Betsiboka-Mahajanga) | 46 000 | |
| " | de la Mahavavy sud et Soalala | 34 000 | |
| " | de Besalampy nord et sud | 45 700 | |
| " | de Maintirano | 25 300 | |
| " | de la Manambolo | 9 000 | |
| " | de la Tsiribihina (Belo/Tsiribihina) | 28 000 | |
| " | de la Mangoky (Morombe) | 23 200 | |
| Surface totale | 302 000 | ||
| (ii) | zones de lagunes | ||
| Lagune de la Loza | 15 600 | ||
| " | des Pangalanes | 18 000 | |
| " | de l'Anony | 2 262 | |
| " | de l'Ampahana | 2 175 | |
| " | de Masianaka | 1 329 | |
| Surface totale | 39 366 | ||
Pour la production de poisson, diverses méthodes sont utilisées sur les zones côtières. Ainsi, dans la zone des mangroves (cantonnées essentiellement le long de la côte ouest), la pêche se pratique essentiellement aux changements de marée (montée et descente). Les pêches sont généralement faites à la senne partant de la berge tendue obliquement par rapport à celle-ci, le filet étant orienté de telle façon que les poissons se dirigent vers l'extrémité fixée sur le bord (la position du filet change donc suivant la progression de la marée). On utilise aussi des grandes nasses dormantes fixées aux palétuviers. Quelquefois, les pêcheurs barrent totalement un petit canal de la mangrove avec des gaulettes et des gaulettes et des feuillages, laissant deux ou trois passages derrière lesquels ils mettent en place des nasses coniques et vers lesquels est chassé le poisson.
Sur la côte est, dans les lagunes, l'usage des ‘Vila’ est généralisé. Il s'agit de barrages construits la plupart du temps avec des matériaux de fortune: branches mortes, feuillages. Une nasse est placée à la sortie du barrage qui est généralement ‘pêchant’ lors de la remontée des poissons.
Outre ces ‘Vila’, les principaux engins de pêche sont variés: grands filets de pêche, filets maillants, grandes nasses cubiques ou parallélépipédiques faites en fibre de raphia ou baobas, nasses coniques ou ‘Vovo’, utilisées surtout dans les zones envahies par la végétation et nasses traînantes ou ‘Tandrohotra’.
L'usage des pièges est aussi répandu à Madagascar, notamment sur la côte est, dans la zone des Pangalanes. On peut citer à cet effet le ‘Vovomora’ qui comprend un amas généralement fait de fougères entouré de gaulettes dans lesquelles les poissons et les crevettes viennent chercher asile. C'est en fait un système de piège très rapproché de l'acadja, lequel fait actuellement son apparition dans cette zone sous l'impulsion du Service des eaux forêts.
Une autre méthode très particulière utilisée surtout sur la côte mérite aussi d'être signalée. Il s'agit du ‘Ronkona’ que l'on pratique pendant une certaine période de l'année (juillet à septembre). Il s'agit de surprendre une bande de poissons (en général des mulets) en l'entourant rapidement par un série de pirogues. Les poissons affolés sautent au-dessus de l'eau et une partie retombe dans les pirogues.
Pour clore ce chapitre, il convient de signaler les méthodes de pêche utilisées pour la pêche des crevettes et camarons. Les crevettes, comportant à Madagascar de nombreuses variétés dont plusieurs sont euryhalines, sont capturées aux nasses traînantes souples ou au filet moustiquaire. Aux Pangalanes est, elles sont capturées au ‘Vovomora’.
Les camarons sont pêchés traditionnellement par un petit panier fixé au bout d'un long manche, posé pendant une dizaine de minutes dans les trous du lit de rivière.
(Annexe 1)
En premier lieu vient la famille des Cichlidés avec des espèces autochtones, telles que:
le Saroy (Ptychochromis oligacanthus);
le Masovoatoaka (Paratroplis polyactis);
le Kotso (Paratroplis petiti) sur la côte ouest et les espèces introduites telles que les genres Tilapia (T. melanopleura, T. zillii, T. macrochir, T. nilotica, T. mossambica).
Puis viennent par ordre d'importance:
la famille des mugilidés, dont principalement le Zompona (Liza macrolepis) et le Jebojebo (Mugil robustus);
la famille des anguillidés, dont Madagascar possède quatre espèces, mais trois seulement sont rencontrées fréquemment en zones côtières: Anguilla marmorata, Anguilla mossambica et Anguilla bicolor;
la famille des gobidés: Gobius giuris, G. senefuscus, G. macrorhynchus, sont représentés dans les zones côtières;
la famille des éleotridés: on rencontre dans tous les plans d'eau côtiers l'Eleotris fusca (espèce typiquement euryhaline), ainsi que l'Eleotris macrolepidota.
Dans la famille des chanidés, la région madécasse ne comprend qu'une seule espèce, le Vango (Chanos chanos).
Dans la famille des élépidés, deux espèces euryhalines sont fréquemment recontrées dans nos régions côtières: Elops cyprinoîdes (Bekalana ou Fiafotsy) et Elops saurus sur la côte ouest.
Il est à signaler que dans la famille de cyprinidés, la carpe (Cyprinus carpio) a joué un rôle économique très important spécialement en zones côtières de Mahajanga-Marovoay et Belo-sur-Tsiribihina-Miandrivazo, pendant la période de 1942 à 1957, période pendant laquelle elle dépassait 50 % des pêches. Après 1957, le tilapia a détrôné cette espèce. Ce n'est qu'après des efforts de production d'alevins et de déversement que cette espèce commence à refaire son apparition sur les zones côtières.
Ces zones côtières susceptibles d'être mises en valeur par l'aquaculture paraissent suffisamment vastes et nombreuses dans le pays pour permettre un choix ne soulevant pas de problèmes légaux ou juridiques particuliers, du moins dans l'immédiat. Il s'agit en effet de zones soumises aux marées, impropres à l'agriculture: le seul domaine à préserver est celui de la mangrove (palétuviers) dont l'exploitation doit être réglementée. Il est cependant d'usage, avant de procéder à des aménagements aquicoles, de mener une enquête officielle de vacance de terre, ainsi qu'une enquête commode, in commodo: les avis des collectivités décentralisées sont sollicités. La hiérarchie supérieure de province statue en dernier lieu par le biais de son Conseil populaire.
Cette procédure juridique permet d'éviter tous les risques de conflits qu'on pourrait avoir avec les autres services dans l'établissement d'une exploitation aquicole côtiére.
Depuis la restructuration du Ministère du développement rural et de la réforme agraire qui date du 21 mars 1979 (Décret No 79.078), c'est le Service de la production animale (Division pêche et aquaculture) qui est chargé de la vulgarisation de l'aquaculture en zone côtière. En fait, l'aquaculture intensive est encore pratiquement inexistante à Madagascar.
Comme le programme aquicole côtier a certaines relations avec la mer, une collaboration étroite avec la Division des pêches maritimes du même service est envisagée.
L'aquaculture en zone côtière ne prendra pas tout de suite une très grande extension. Seuls, quelques sites piscicoles pourront être réalisés à court terme du fait que les aménagements et l'élevage nécessitent une reconnaissance minutieuse de terrains et une grande technicité.
Les mêmes remarques que celles figurant au point 4.1 ci-dessus sont applicables. On émet toutefois une réserve sur l'appropriation éventuelle de la mangrove (palétuviers) dont la ceinture littorale doit obligatoirement être protégée; l'exploitation rationnelle de ce domaine devrait être réglementée.
En ce qui concerne la pêche, elle n'a aucun risque de conflit avec l'aquaculture en zone côtiére; les deux activités sont plutôt complémentaires et contribuent à combler le déficit protéinique actuel d'origine animale qui est de l'ordre de 20 000 t par an et qui continue de s'accroître au rythme de 3 % l'an.
L'aquaculture côtière étant encore inexistante, les éléments font défaut pour développe ce point en détail. Toutefois, des essais en cours révèlent de très bons rendements avec Chanos chanos (1 t/ha/an). Ces premiers résultats permettent d'avancer que Madagascar a, dans ce domaine, de très bonnes possibilités de développement pour la production de poisson de consommation (très apprécié) ainsi qu'éventuellement la fourniture d'appâts morts ou vifs pour la pêche au thon et à la bonite. La promotion de ce secteur aura de toute évidence un incidence positive sur le relèvement du niveau de vie des populations côtières, sur l'exode rural, ainsi que sur l'amélioration de la balance alimentaire.
Il est actuellement prématuré d'aborder ce problème. Il semble que la première étape à réaliser doive être la mise en route d'un ou deux centres pilotes aquicoles de type commercial afin d'en démontrer la viabilité. Une fois cette étape franchie, ïl est probable que les organismes de financement étatiques ou autres répondent favorablement à des demandes de crédits pour la promotion de ce secteur.
Il n'existe pas encore de structures fonctionnelles pour le recueil des données statistiques sur la pêche. On procède jusqu'alors par estimation résultant de sondages ou d'enquêtes rapides effectuées sur les plans d'eaux principaux. L'aquaculture côtière inexistante ne figure pas d'ailleurs dans les estimation (1978) de la production de poisson en eau douce:
| - | production des pêches commerciales | 12 000 t |
| - | production des pêches de subsistance (rizipisciculture inclus) | 28 000 t |
| - | production totale | 40 000 t |
A Madagascar, l'aquaculture en zone côtière a un avenir certain. Les zones non boisées adjacentes aux 300 000 ha de mangroves constituent une réserve potentielle importante; la productivité de ce milieu est de l t/ha selon les premiers résultats d'expérimentations qui y ont été réalisées. Il en est de même de l'aménagement des eaux lagunaires et des baie où pourraient être appliqués les techniques de la mariculture.
Evidemment, la mise en route du programme de développement de l'aquaculture en zone côtière nécessite des investissements importants, mais compte tenu du fait que le Gouvernement malgache a inclus dans ses priorités les activités se rapportant à la pêche et à l'aquaculture, il est fort probable que des efforts de financement plus volumineux seront consentis sur les prochains programmes. L'assistance bi- ou multilatérale d'organismes internationaux et privés est la bienvenue pour accélérer la réalisation des aménagements qui seront proposés.
De façon à encadrer techniquement les opérations à entreprendre en matière d'aquaculture en zone côtière, il est indispensable de structurer et de former les personnels de recherche, de conception et d'exécution. Ce qui laisse supposer que des formations techniques et pratiques plus approfondies seront dispensées dans les établissements qui préparent les ingénieurs et les biologistes halieutes. Ces derniers parfairont leurs connaissances par des stages de perfectionnement à l'extérieur.
| Espèces | Biologie | Exigences écologiques | Croissance et reproduction | Régime alimentaire | Aire de répartition | Pêche et importance économique | Intérêt en pisciculture | |
| FAMILLE DES CICHLIDES | ||||||||
| SAROY (Ptychochromis oligacanthis) | Eaux douces et eaux saumâtres; Supporte des salinités assez fortes | Vastes plans d'eau propre à léger courant. Eaux chaudes | Croissance lente (taille limitée à 25 cm). Poids maximum 500 g. Ponte sur fond sablonneux et propre, s'échelonne de novembre à mars. Incubation des oeufs de 8 à 10 jours | Omnivore avec de temps en temps nourriture carnée (petits insectes et crevettes) | Eaux côtières de l'Est, du Sud-Ouest et du Nord-Ouest | Environ 20 à 25 % des prises, sur la côte est | Elevage possible en étangs artificiels. (Essais effectués à Ambila-Lemaitse) | |
| MASOVOATOKA (Paretroplus polyactis) | Eaux douces légèrement saumâtres | Eaux chaudes et propres de grande superficie | Croissance rapide; taille peut atteindre 40 cm. Ponte s'échelonne de novembre à mars, sur une cuvette creusée sous un objet posé sur le fond | Plancton, mollusques et petites crevettes | Zone côtière est | Poisson très recherché et apprécié | Elevage possible en étangs artificiels, mais rendement peu élevé à cause de sa biologie exigeante | |
| KOTSO (Paretroplus petiti) | Typiquement d'eau douce | Zones envahies par une végétation touffue | Croissance rapide mais limitée à 35–50 cm. Très prolifique (femelles donnent en moyenne 2 000 oeufs). Se reproduit en saison chaude et mi-saison | Omnivore: plancton, insectes, petits crustacés | Plaine côtière nord-ouest | Part importante des pêches du lac Kinkony | Non encore essayé en pisciculture à Madagascar | |
| TILAPIA (Tilapia spp.) | Eaux calmes, mais résistent à l'entraînement des courants; T. mossambica est euryhalin, aimant les eaux saumâtres | Croissance assez limitée par rapport aux tilapias de L'Afrique; Poids moyens 700 à 900 g | Omnivore peu exigeant en nourriture naturelle | Répandus à travers tout le pays, notamment T. rendalli et T. macrochir. Pratiquement ont colonisé tous les biotopes | A contribué largement à l'essor de la pêche à Madagascar | Elevage possible de toutes les espèces en zones côtières. Le rendement est de 5 à 6 t/ha/an; T. mossambica et T. nilotica supportent particulièrement une certaine salinité des eaux | ||
| FAMILLE DES MUGILIDES | ||||||||
| ZOMPONA (Liza macrolepis) | Poisson euryhalin catadrome | Jeunesse en eau douce; après maturité, partage sa vie entre la mer et les eaux continentales | Croissance assez rapide. Atteint plus de 25 cm et pèse 300 g env. au bout de l an. A maturité (3 ans): 40 à 50 cm. Très prolifique, se reproduit en saison fraîche en zones marines côtières | Limnivore | Tous les cours d'eau côtiers (sauf en fleuves semi-permanents du Sud) et dans les lagunes | Pêche au Vila et au filet très importante | Possibilité d'élevage dans les zones côtières, à partir d'alevins capturés par banc venant de la mer qui sont très abondants | |
| JEBOJEBO (Mugil rohutus) | Poisson euryhalin | Partage sa vie entre l'habitat marin et les eaux continentales | Croissance assez rapide | Limnivore | Les eaux côtières et lagunes | Pêche au filet maillant très importante, au filet-senne et par barrages | Même condition que le Zompona ci-dessus | |
| FAMILLE DES ANGUILLIDES | ||||||||
| AMALONA (Anguilla marmorata, Anguilla mossambica, Anguilla bicolor) | Les civelles pénètrent dans les estuaires pendant la saison chaude et feraient halte quelques jours ou quelques semaines en zones basses côtières avant de reprendre leur seconde migration vers l'intérieur des terres. Toutefois en ce qui concerne A. bicolore, rien n'est définitivement certain | Très vorace à régime carnivore | Toute l'île | Non essayé en pisciculture | ||||
| - | larves de moustiques pour les très jeunes sujets | |||||||
| - | petits crustacés têtards pour les sujets dépassant 20 cm | |||||||
| - | au-dessus de 50 cm, essentiellement des crustacés et vertébrés de petite taille | |||||||
| FAMILLE DES BAGRIDES | ||||||||
| GOGO (Arius madagascariensis) | Espèce euryhaline mais s'est adaptée en eau douce dans le lac Kinkony. Incubation buccale des oeufs pratiquée par le mâle | Séjour en eau salée nécessaire | Peut atteindre des tailles de 70 cm. Une seule reproduction par an d'octobre à fin novembre. Incubation buccale faite par le mâle | Carnivore | Lacs et fleuves de l'Ouest, région de Sambirano à l'Onilahy | Très importante dans la région de l'Ouest. Se prend à la nasse, au filet et jeu à la ligne | Non encore essayé en pisciculture | |
| FAMILLE DES GOBEIDES | ||||||||
| TOHOFOTSY (Gobius giuris) | Espèce euryhaline fréquente dans les lacs côtiers et lagunes | Eaux chaudes ou tièdes, un peu courantes et propres | Croissance assez rapide. Taille peut atteindre 35 à 40 cm. Reproduction aussi bien dans les eaux douces qu'en mer | Carnassier typique très vorace | Grands Pangalanes-Est et petits Pangalanes; grands lacs de la région centre-ouest et grands fleuves de l'Ouest | Rôle intéressant dans les pêches faites en zones côtières. Pêche à la ligne, à la nasse traînante ou au vovomora | Non encore essayé en pisciculture | |
| TOHOBAKA (Gobius aenofuscus) | Euryhaline en zone côtière, mais totalement adaptée en eau douce | Croissance assez lente. Taille peut atteindre 25 cm et 250 g. Reproduction aussibien dans les eaux douces qu'en mer | Carnassier | Toute la zone côtière et jusqu'à 950 m d'altitude | Pêche le plus souvent aux nasses traînantes par les femmes | Non essayé en pisciculture | ||
| FAMILLE DES ELEOTRIDES | ||||||||
| TOHOMAINTY (Eleotris fuscus) ANICAVY (Eleotris manolepichota) | Espèces typiquement euryhalines | Carnivore | Presque tous les plans d'eau côtiers mais localisé dans la zone côtière de basse altitude | Rôle intéressant mais nécessite pour sa pêche une vaste superficie | ||||
| FAMILLE DES CHANIDES | ||||||||
| VANGO (Chanos chanos) | Espèce euryhaline fréquentant souvent les eaux douces qu'elle quitte pour aller se reproduire en mer | Taille maximum constatée en eau douce et eau saumâtre: 1 m. En mer (du côté de Nossibe), des sujets de 1,30 à 1,50 m sont signales. Ponte en mer, saison encore ignorée. Remontée des larves et alevins dès le début de la saison des pluies et qui semble durer 3 à 4 mois. | Microphage, planctonophage | Côte-ouest. Tous les lacs de la zone côtière centre-ouest | Pêche pratiquee dans l'ouest de Madagascar le plus souvent aux filets maillants (harantovango)spéciaux. Pêche presque tout au long de l'année. Espèce choisie pour le lancement de l'aquaculture côtière | En élevage expérimental à Madagascar. Les premiers résultats ont donné un rendement de 570 kg/ha en 4 mois; deuxième campagne d'élevage semble encore plus favorable | ||
| FAMILLE DES ELOPIDES | ||||||||
| FIAFOTSY (Elops cyprinoïdes) LINGOLINGO (Elops saurus) | Espèce euryhaline pénétrant profondément dans les eaux continentales | Ouest, centre-ouest et est (Pangalanes et lagunes) | Non essayé en pisciculture | |||||
| FAMILLE DES CYPRINIDES | ||||||||
| CARPE (Cyprinus carpio) | Espèce typiquement dulcaquicole mais supportant cependant des eaux saumâtres dont la salinité va jusqu'à 10 p.p. mille | En zone côtière, la reproduction est précoce (facteur to). Toutefois il n'est pas encore défini si la carpe en zone côtière peut s'y reproduire couramment plus d'une fois par mois. Au point de vue croissance, en zone côtière, la carpe croît beaucoup plus vite que sur les hauts-plateaux. De 6 mois à un an, la carpe, en zone côtière, passe de 30 à 150 g. | Répartie entre 1926 et 1930 dans plusieurs régions des hauts-plateaux, elle semblait se localiser à la zone fraîche des hauts-plateaux. Après 1940 cependant, pullulation dans les zones chaudes côtières de Marajunga-Marovoay et de Belosur-Tsiribihina. Mais elle n'a pas colonisé naturellement la côte est (violence des courants, fortes crues, chutes, etc.) | A joué un rôle économique très important en zones côtières de Marajunga-Marovoay et Belosur-Tsiribihina-Miandrivaso | Une production moindre que les tilapias, mais rendement satisfaisant (de l'ordre de 2,5 t/ ha/an en zones de moyenne altitude et sur les plateaux); utilisation en mélange avec tilapias; utilisation en rizipisciculture (rendement env. 600 à 800 kg/ha/an sur les hauts-plateaux); reproduction artificielle et semi-naturelle, en cours de généralisation dans les stations principales de pisciculture | |||
by
J.D. Ardill 1
D.S.O. (F), Ministry of Fisheries
Port Louis, Mauritius
1 Presently: FAO Scientific Officer South West Indian Ocean Project Victoria, Mahé, Seychelles
Mauritius is an island of volcanic origin and the coastline is fairly abrupt, with outcroppings of basaltic rocks appearing at the surface in most places. For one third of the coast of about 240 km, these rocks form cliffs which are washed by the open sea. Around the remainder of the coast, a fringing barrier reef composed of corals and coralline algae protects the beaches. The lagoon formed by this reef is generally less than 1.5 m deep, with a bed of corals, coral sand and algae, but in areas where there are rivers opening into the lagoon, the depth may attain 30 m, and the substrate is mainly alluvial mud.
The lagoon generally has a width of only a few hundred metres, and the beaches there are usually of coral sand interspersed with rock outcroppings. In a few areas, however, the lagoon widens to a maximum of about 5 km. There, the strong currents and wave action, typical of the narrow lagoon, are reduced or absent and the coastline is generally muddy, supporting mangrove growths in inlets. The tidal range in Mauritius is less than 1 m and, as there is no flat land close to the sea, the mangrove belt is very narrow and does not offer the swamp condition appropriate for aquaculture development.
The wider lagoons do however offer the protection needed for coastal aquaculture. To date, this activity has been carried out in 20 barachois ranging in area from 0.5 ha to 45 ha (mean area 13.4 ha; total area 268 ha). These are indentations in the coastline which are closed off from the lagoon by dry-stone walls. The walls are permeable to water movement and usually have several gridded gates which allow enhanced exchange of water between the lagoon and the barachois. This feature has been found necessary in shallow barachois where fish kills are sometimes recorded from lack of oxygenation of the water during the lowest spring tides in hot still weather. Despite the small tidal range, the water exchange between the barachois and lagoon can attain 30–50 percent of the volume of a barachois daily. This not only contributes to the oxygenation of the water, but also makes for little difference between the water of the lagoon and of the barachois.
Freshwater seepage into the sea occurs in most of the wide lagoons. In some areas, springs and small streams further reduce the salinity. With few exceptions, therefore, the water in barachois is brackish, but salinities vary between 0 and 35 ppt in periods of heavy rainfall or of drought. The fresh water entering the lagoon having flowed mainly through basalt beds, carries little nutrients.
Barachois are used for the culture of finfish, crabs and oysters. Management practices are restricted to stocking, some control of predation and harvesting. The absence of any agricultural processing other than those of sugar cane and tea results in the lack of any cheap supplemental feeds, while the high rate of water exchange precludes the use of fertilizers.
Some stocking of barachois occurs naturally, fish fry entering through the walls or gates. Unwanted species (e.g., predators such as Caranx spp. or Sphyraena barracuda) frequently enter the barachois through the gates. Attempts have been made to place funnel traps of small meshed netting at the gates in order to hold the fry passing through, in order to reject unwanted species, but this practice is not generalized.
Natural seeding is rarely sufficient, particularly for the larger barachois, and it is normal practice to fish seed from the lagoon. For finned-fish, fingerlings are caught with small mesh nets in nursery areas and transported to the barachois in wooden boxes towed through the water to the barachois by outboard-powered pirogues. These boxes are perforated to allow water exchange, and mortality of fry is negligible. Stocking rates are variable, depending largely on the availability of fingerlings of the desired species, and optimum rates are in fact not known.
Young crabs are obtained at night by means of dip nets and lights. The supply of seed is generally insufficient. Oyster seed is obtained from mangrove roofs and rocks in natural spawning areas. This practice often results in a high initial mortality, as shells are frequently damaged, but the fecundity of the local oyster is high, and thinning of wild beds may in fact help to promote growth.
Recently, some barachois have been harvested on a regular basis by means of gillnets. This method is species-selective however, and the general practice is to fish once or twice a year with drag-seines. These seines are up to 500 m long, with meshes of 8.5 cm stretched, and are worked in such a way that the whole barachois is swept. Mullet which would normally jump over a seine net, are caught on net maintained horizontally on the water surface by bamboo poles. These nets are tied along the seine nets in the deeper areas where mullet tend to congregate.
A major problem in barachois is that of predation by fish S. barracuda, in particular and some carangids, penetrate into the barachois through the gates or with water percolating through the outer walls. These predators thrive in heavily stocked ponds, and are thought to reduce productivity significantly.
In many cases, poaching is a problem also, particularly as fencing is always necessary, as is the presence of watchmen - both very costly items in this expensive form of aquaculture.
Fish species for culture in barachois are selected with respect to their tolerance of salinity charges, the availability of fry, and their market value. The species normally selected are from the Siganidae and Mugilidae. Natural seeding may account for significant amounts of Lethrinidae and Mullidae.
Siganidae: The main species selected is Siganus abhortani (previously identified as S. oramin or S. canaliculatus). This fish is a macro-algal feeder which usually lives in dense choals. Fry are present along the beaches in vast quantities between November and January, but are very delicate at that stage. Fingerlings for stocking are therefore caught between January and April on weed-beds where they congregate. This species grows to a marketable size of 150–250 g in one year, and may reach a size of over 600 g.
Mugilidae: Two species are stocked, Mugil cephalus and M. saheli, the former being preferred both because of its higher market value, and because of its hardiness. M. cephalus breeds in winter, and fingerlings are available from September to November. M. seheli breeds in summer, and fry is available from March to June. Fingerlings of other species of mullet, and sometimes of milkfish, are usually caught with these species, but are difficult to distinguish apart. Mullet usually atain a weight of 200 g within a year, but are only fished at 2–3 years old when they weigh 1.5–3 kg.
The portunid crab, Scylla serrata, is a swimming crab which lives in deep burrows in sand and mud bottoms. It is a carnivore and generally feeds on detritus, but is reported to crack open oyster shells with its powerful clams. The supply of seed is limited in Mauritius, particularly as the wild stock of adults is submitted to intense fishing effort, so that stocking of barachois is at a low density. Attempts at raising these crabs in small enclosures failed due to cannibalism and predation by eels on freshly moulted crabs, but growth was good (500 g in one year) and the crabs were able to tolerate conditions of very low oxygenation for long periods. As with the juveniles, adult crabs are located at night with lights and fished with dip nets. Production is very small, but this species has a high market value.
The most common oyster in Mauritius is the Bombay oyster Crassostrea cuculata forskalii. It is found in the intertidal zone of all brackish water areas and will tolerate both freshwater and full strength seawater for short periods, but thrives at salinities of about 20 ppt. Spat 2–3 cm long is collected off rocks and mangrove roots and are grown in barachois on wire trays placed, if possible, in the intertidal zone where adventitious growths of algae and sponges are limited by the frequent exposure to air. In some barachois, the grazing of fish helps to keep oysters and trays free of fouling organisms, but in some cases it is necessary to brush the oysters at regular intervals. In the clear water conditions found in Mauritius, the growth of these oysters is very low, and they may take 3–5 years to reach marketable size. This long growing period results in high costs for culture installations, so that only about 2 million cultured oysters are produced annually.
All the barachois in Mauritius were created before 1800. Since that time, under the legal system, all sea areas are considered 'domaine public' - national, as opposed to government property. As such, the creation of a new barachois would require an act of the legislature. There is provision under the law, however, for the granting of exclusive rights for the cultivation of oysters in any specified part of the lagoon.
Of the barachois existent at present, half are privately owned, and half are owned by the state, and leased to private individuals, companies or co-operatives on leases of 30 to 90 years duration. The feature conditioning ownership is the ownership of the land surrounding the barachois, be it state or private. Leases to date are at nominal cost, but all recent leases contain clauses allowing Government to resume possession of a barachois if it is not maintained or operated in a satisfactory fashion.
All barachois are considered to be ‘private waters’, and poaching is punishable by law.
The Ministry of Fisheries runs one barachois where research on different management practices is carried out. In addition, an extension service is available to provide advice and to stock barachois with fish fingerlings at a nominal cost. As a form of assistance to co-operatives, harvesting of some barachois has also been done by Ministry fishermen.
In view of the low economic returns from the operation of barachois, a project in the 1974–79 development plan to recondition barachois for co-operatives has not been carried over to the 1980–85 plan. Areas suitable for barachois development are generally little suited to tourism or housing, but a conflict does exist with artisanal fishermen exploiting the lagoon, so that, unless the productivity of barachois can be increased notably, it does not appear likely that any new areas will be developed.
The reported production of barachois in 1978 was 16 t. This gives a theoretical average production of 60 kg/ha per year. A more realistic figure, however, would be 100–150 kg/ha per year as not all barachois are actively exploited, and not all barachois owners furnish reliable catch statistics.
At these production levels, costs of maintenance and prevention of poaching are only just compensated, leaving no margin to cover depreciation or provide profits. Many barachois are operated by sugar estates, and the catch is sold or distributed among personnel or consumed in associated hotels. Other privately or co-operative owned barachois sell their production to the public.
As stated above, unless the productivity of barachois can be increased substantially, it is unlikely that any further barachois development will take place. Research will continue with respect to stocking rates, fertilization, the reproduction of crabs, and oyster culture, but these are all long-term projects and no rapid development is expected.
