SECTION I: PRESENT STATUS OF AQUACULTURE AND POTENTIAL FOR DEVELOPMENT (contd.)

5. STRATEGY FOR THE DEVELOPMENT OF AQUACULTURE IN TUNISIA

With a background of steady progress in the culture of molluscs, the results of experiments recently carried out on fish culture, and a knowledge of the more important problems that could arise, it is possible to work out a strategy for gradually exploiting the potential for aquaculture in Tunisia. As a programme of development gets under way, appropriate modifications, based on the results obtained, could be made for the future.

5.1 Culture of molluscs

In this respect, Tunisia is already making satisfactory progress, with a centre for the commercial cultivation of oysters, mussels and other molluscs in Bizerte Lagoon, and centres for their cleansing in Gammarth and Salammbô. Although all the Mytilus and Tapes cultured at present are easily sold, oysters appear to have a limited local market and an export market for them has yet to be found. In 1974, oysters to the value of approximately T.Din. 20 000 could not be sold. The culture of oysters is at present based on regular importation of spat from abroad, since the high salinity conditions in Bizerte Lagoon during the breeding period are not favourable for reproduction and successful spatfall (Azouz, 1966). However, there is no such problem with regard to Mytilus and Tapes.

With regard to expansion of the culture of Mytilus and Tapes, this could, in the first instance, be increased severalfold in Bizerte Lagoon itself. Small trials have been carried out by the Office National des Pêches in Oued Tindja (Ichkeul Lake), and the lagoons of Porto Farina and Bibans. Further trials would have to be carried out to obtain conclusive results. Other lagoons worth investigating in this regard are Khenis (Monastir), as suggested by Raimbault (1973) and Hergla.

Further expansion of oyster culture would depend on developing sales outlets for oysters locally, either by making them available at lower prices as production increases, or by organizing supplies to the numerous tourist hotels, especially during the tourist seasons.

The establishment of a hatchery for breeding of oysters under controlled conditions has been under consideration for some time. With budgetary provisions now made by the Government and a site already selected in Bizerte, the hatchery is likely to be established during the course of 1976. Once this starts functioning, it would be possible to produce the spat needed for culture locally.

5.2 Fish culture

In its most familiar form, fish culture is practised in systems for confining and rearing large, but often determined, numbers of fish in limited volumes of water. Examples are artificial fish ponds for species such as carp, mullets and other species, cages suspended in water and sections of lakes, lagoons or even calm regions of bays partitioned off with nets. Fish culture in ponds, cages and other limited systems involves the use to varying extents of specialized techniques for supplementary or complete artificial feeding, manipulation of stocks, hatcheries for breeding if juvenile fish cannot be easily obtained from natural waters or there is no market from which they could be bought, fertilization of ponds with organic or chemical manures, eradication of pests and predators, and prevention and cure of fish diseases.

Although the return per hectare per annum is frequently much less than in confined systems, stocking of natural waters such as lakes, certain lagoons, rivers and streams, and mad-made reservoirs with fish generally destined for human consumption is also a form of fish culture. However, the total production per annum from large lakes and reservoirs could be several times higher than that from limited areas of fish ponds, depending on the natural productivity of the former and management of stocks.

Keeping in view the problems listed in section 1.3.4, it is possible to establish fish ponds in many lakes, reservoirs and lagoons in Tunisia. It is, however, advisable to phase large-scale projects over a period of some years preceded by small-scale pilot trials. The latter would serve not only to identify modifications that have to be made to suit local conditions, but also for training personnel.

5.2.1 Mullet culture

Owing to the good price and demand they have in the market, as well as the abundant availability of their fry and fingerlings, mullet rates first among the species suited to fish culture in Tunisia. The more important species in the order of decreasing market demand, are as follows: Mugil cephalus (bourri), M. ramada (bitoum), M. auratus (safratouzen) and M. saliens (cigare); the local names are given in brackets.

5.2.1.1 Acclimatization to various salinities

Mullet can be cultured in both brackish and fresh water but when introducing them from sea water to water of salinity lower than 15–20 ppt, they should be gradually acclimatized to the new salinity. It was observed during laboratory and large-scale trials that the rate at which mullet fry and fingerlings could be acclimatized to water of very low salinities varied directly with the temperature of the water. While they could be acclimatized to live in fresh water within two to three days at temperatures of 20–25°C, it would take longer at lower temperatures, as much as one to two weeks at temperatures of 7–15°C. It was also observed that the rate at which they could be acclimatized was faster as the size of the individuals increased. In general, the change from sea water to 15 or 20 ppt could be carried out in a matter of a few hours to a day, depending on the temperature, but further acclimatization has to be at a slower and steadier rate. It would be seen, therefore, that large-scale and rapid acclimatization of mullet fry for low salinities during winter could be carried out faster in appropriately heated indoor tanks than in open air tanks.

5.2.1.2 Transport of mullet fry and fingerlings

From late autumn through spring mullet fry and fingerlings could be transported in considerable numbers in open plastic or other containers, up to a distance of 60–100 km or more, provided there is constant agitation of the water. In summer the numbers per container have to be considerably reduced due to the higher water temperature.

Fry and fingerlings could be transported at any time of the year over very long distances by road with appropriate numbers in plastic bags, as was experimentally done in 1973 from Tunis to Douz, involving a distance of approximately 550 km and a duration of 36 hours. A precaution to be taken in hot summer is to cover the bags with sacks or cloth moistened with water, preferably with some pieces of ice to help the bags cool.

The third method is to transport them in a vehicle fitted with special aeration or water circulating systems, as has been recently acquired by INSTOP. Here too the precautions mentioned in the preceding paragraph with regard to hot summers would have to be taken. The best time for transportation of fry and fingerlings during summer is between 16.00 h and 17.00 h in the evening until about 8.00 h or 9.00 h in the morning.

5.2.1.3 Locations of ponds for mullet culture

As already seen in Section 1.3, ponds for mullet culture could be constructed (a) along the borders of lakes, reservoirs and lagoons, where there is not likely to be any serious pollution problem, (b) using artesian water too saline for agriculture produced in south Tunisia, (c) using water from freshwater springs, and (d) natural and artificial village ponds. In the three latter cases, special techniques may have to be used to minimize loss of water through infiltration into sandy soil, such as plastic film on the pond bottom, or even the construction of a cement bottom if it could be done at reasonable cost.

As demonstrated in L'Akarit, Kebili and Douz, mullet could be cultured in monoculture systems, where only a single species of mullet is reared or in polyculture systems where two or more species of mullet or mullet plus common carp and Tilapia nilotica are cultured together.

5.2.1.4 Feeds for mullet

Some locally available feeds have been tried with satisfactory results during the past three years in Tunisia. Potatoes, discarded vegetable leaves from the market, carrots and eggs all boiled and mashed together into a fine paste were readily consumed by mullet fry. The addition of squid, cuttlefish or octopus improved uptake. The vegetable leaf component was also replaced with very finely shredded Ulva, Enteromorpha and Chaetomorpha with success, being very effective with the addition of cephalopod molluscs. A simple method of making pelleted feed using such components - a mincing machine and sun drying - has been demonstrated. Finely sifted fish meal was taken when either spread on the surface of the water, made into a paste with water, or boiled with potatoes, vegetables and ground to a paste. Fish meal alone was used when temporarily storing mullet fry and fingerlings in floating cages following capture. A very interesting feed, which was very successful, consisted of dried and ground amphipods from Tunis Lagoon. During spring and early summer the lagoon teems with amphipods, which are washed ashore in large quantities in bad weather. These were sun dried, ground into a powder and used to feed mullet fry and fingerlings.

Once introduced into ponds, mullet learned to take food from fixed positions in the pond, and even responded to meal calls, such as the tapping of a can or bottle under water before feeding. Feeds for large fingerlings and older fish consisted of bread waste mixed with fish meal.

5.2.2 Eel culture

Eels can be cultured in both brackish and freshwater ponds. Raimbault (1973) recommends the culture of eels of non-commercial sizes caught from various lagoons, in ponds especially constructed for them in Monastir (Khenis) Lagoon. This has the advantage that their growing period in the ponds could be considerably shorter than if they were to be reared from the elver stage.

With regard to growth in ponds, Dahl (1967) obtained high growth rates of 17.5 cm, 28.9 cm and 43.9 cm, respectively, during the first three years after stocking from the elver stage. The author obtained sizes of 20 cm from elvers of average size, and 10 cm in aquarium tanks in six months. Food consisted of Artemia nauplii, fish meal and Daphnia, Koops (1966) found that the average weight of individuals from 15–35 cm doubled in seven months when cultured in fish ponds. There is evidence that the growth of eels is much faster in warmer waters than in cold waters. According to experiments carried out at the Max-Planck Institute, near Hamburg, eels grown in tanks at 23°C reached maximum sizes of 120 g in six months from the transparent elver stage, whereas those reared in outdoor ponds reached maximum sizes of only 20 g for the same period. The tank-cultured eels attained maximum sizes of 275 g in 12 months and 320 in 19 months.

With regard to feeds, Koops (1966) mentions minced fish, shrimps or fresh meat for elvers and fingerlings. In the case of eel culture in ponds in Tunisia, trash fish from fishing vessels could be used as feed. Another method of rearing eels is to stock them into ponds in which the fast breeding Cyprinodon fasciatus is introduced. The latter species of fish forms a part of the diet of eels in Tunis Lagoon.

5.2.3 Culture of sea bass (loup)

This species (Dicentrarchus labrax) could be cultured in sea water or waters of low salinity, but the main constraint is the difficulty of obtaining sufficient numbers of fry and fingerlings. Young sea bass of 10–15 cm total length are periodically caught in the bordigues operated in Tunis Lagoon. They could be cultured in ponds or cages, but the extent to which this could be done is limited. Any large-scale culture of this species would have to await the establishment of hatcheries for the production of fingerlings through controlled breeding. Arcarese et al. (1972) and Barnabe and René (1972) give accounts of the controlled reproduction of this species using carp hypophyses and synahorin.

5.2.4 Culture of sea bream (daurade)

Unlike the species hitherto discussed, sea bream, Chrysophrys (= Sparus) auratus, cannot be cultured in water of very low salinity. Like the sea bass its culture would have to await the production of fry and fingerlings through controlled reproduction in special hatcheries. Barnabe and René (1973) use the same hormone they used for sea bass for this purpose.

Like the sea bass, the sea bream is sensitive to high temperatures, and ponds meant for them would therefore have to be at least 1.5 to 1.75 m deep with an efficient exchange of water.

5.2.5 Culture of carp

The common carp, Cyprinus carpio, is generally considered to be the fish“par excellence” for culture in ponds. It has a good growth rate, reaches fairly large maximum sizes, can be easily reproduced in ponds and has a number of varieties produced by genetic selection.

Carp production in ponds varies with the fertility of the ponds and on the culture techniques employed. Although it is not unusual to attain production of 1 000 kg/ha/y in well managed ponds, this could be significantly higher with the use of advanced techniques. For instance, a two-hectare pond stocked with 4 000 yearlings and fed with 28 percent protein feed yielded a gross production of 5 412 kg and net gain in weight of 4 434 kg (FAO Aquacult. Bull. 6 (2–3), 1974).

The culture of carps in cages placed in slowly running water, ponds, lakes and reservoirs, with artificial feeding, has given production rates of 150–200 kg/m² with feeds of high nutritive value (Kowamoto, 1957). Kuronoma (1966) gives a figure of 1 034 kg/49 m² in 156 days, starting with 2 405 fingerlings of 148 kg total weight. The fish were fed with 1 799 kg of introduced feed during the growing period. Gribanov et al. (1966) obtained a production of 100 kg/m² in floating cages placed in thermal waters.

The experiments carried out during 1973–74 indicate that there is a good possibility for carp culture in Tunisia. However, in order to carry out an effective programme of development, it is necessary that the freshwater fish culture station at Aïn Sellem, Beja, be expanded and made to function at maximum capacity for production of the necessary fry and fingerlings. In 1974 techniques for the controlled reproduction of carp, care of eggs and young and feeding were demonstrated with significantly higher production than in previous years. For the south of Tunisia, it would help to speed up carp culture development. The centre at Kebili should be improved and expanded to function as a fingerling production and fish culture demonstration centre. For this purpose a number of cement ponds and a small building with space for office, storage of equipment and guardians have to be constructed.

As a programme of carp culture in ponds gets underway, there is likely to be a growing problem with regard to fish feeds. Blood from slaughter houses was tried out with much success at the freshwater fish culture centre in Aïn Sellem, Beja, during 1974. Mirror carp fry fed with a mixture of boiled blood and bread waste attained sizes of 12–15 cm total length 5½ months after hatching. At present, blood from the numerous slaughter houses for cattle, sheep and goats is unutilized, and may very well serve as a good source of protein, not only as feed for carp, but also for other species of fish. Experiments have to be carried out on its use in combination with various proportions of other locally available feed components such as wheat bran, brewery and wine manufacturers' waste, dates, etc., as well as the production of pelleted feeds.

An important point for attention in a carp culture development programme is that of popularizing methods of cooking this fish which, by trial, will prove to be appreciated according to local taste. For instance, one method of preparation which was greatly relished in a trial was grilling carp after garnishing it with salt and Tunisian spices. Other successful methods were a preparation locally called “kemounia” or fried in small pieces after allowing to remain in mixed Tunisian spices, garlic juice and salt.

5.2.6 Culture of tilapia

Two species of Tilapia have already been introduced into Tunisia, namely, T. nilotica into the oasis of Kebili and T. mossambica into the oases of Tozeur and Nefta, both in 1966. While the former has certainly established itself in Kebili, evidence is wanting with regard to whether T. mossambica has been successful or not, although the latter is the hardier of the two.

Tilapia can be cultured alone or used in combination with carp and mullet. Mortimer (1966) describes family ponds in Zambia, stocked with tilapia free of charge by the Government, producing 224–336 kg/ha/y. El-Bolock (1966) records production of 800 kg/ha/y in mixed culture of tilapia, carp and mullets, and 1 218 kg/ha/y using one-year-old carp, mullet and tilapia, with supplementary feeding and fertilizers.

A very interesting fact about the species T. nilotica and T. mossambica is that they could be used to produce monosex hybrids for culture. The advantage of monosex culture of Tilapia is that, without the opposite sex for reproduction, the cultured fish could use the food resources in the pond to achieve maximum growth without having competition problems with the numerous progeny that would otherwise be produced. The principle used in the method is that reproduction between T. mossambica males and T. nilotica females results in nearly 100 percent male progeny, which could then be used for monosex culture. According to Pruginin (1966) the culture of such hybrids, stocked at 1 500 per hectare, yielded 800 kg/ha/y in Uganda. Denyoh (1966) reports that the culture of monosex hybrids in Ghana has yielded up to 1 000 kg/ha/y with little or no feeding while, in contrast, the culture of wild tilapia populations with supplementary feeding used to yield only 300 kg/ha/y.

5.3 The culture of ducks with fish

The culture of ducks with fish is practised with profit in many countries of the world, especially in the Indo-Pacific Region. Experiments carried out in Oued L'Akarit during 1973/74 proved successful. According to Van der Lingen (1966) ducks when kept on or near fish ponds at the rate of 1 000 per hectare helped raise fish production to 4 000 kg/ha in Rhodesia, due to the fertilizing effect the ducks had on the ponds.

6. STOCKING OF NATURAL WATERS, MAN-MADE LAKES AND RESERVOIRS

All of Tunisia's irrigation reservoirs and some of the lakes are poor in fish and are suitable for aquaculture by repopulation with suitable species. The species of fish presently found in most of these reservoirs are barbeau, especially Barbus barbus and Phoxinellus spp., which, although not of high quality, are consumed by people living nearby. Section 1 of this paper gives a list of the species of fish introduced into such waters in the past. It is necessary to study the extent to which they have been successful in establishing themselves where stocked, since the information obtained would be most useful for planning future stocking programmes.

The reservoirs and lakes that do not dry up annually during summer are, naturally, the most suitable for such a stocking programme. With regard to species suitable for stocking, this would depend on temperature, salinity and other conditions of each body of water. In order to ensure a higher survival rate from predators, it is better to stock with fingerling sizes rather than with fry.

6.1 Stocking with carp

Carp can be stocked in any reservoir or lake of low salinity. A programme of stocking reservoirs in Bizerte was started in 1974/75, and is expected to be extended to reservoirs in the other Gouvernorats in due course. In low salinity lakes, fish are best stocked after being acclimatized to the respective salinities. An effective programme of stocking such waters would depend on the intensive production of young carp at the freshwater fish culture centres in Aïn Sellem (Beja) and Kebili. The techniques for breeding and culture of carp are no longer a problem and much of the fish needed for stocking could be produced as the plans for expanding and improving the two centres in 1975/76 are realized.

6.2 Stocking with tilapia

Tilapia nilotica is suitable for stocking reservoirs and lakes in the southern regions of Tunisia. A lake worth mentioning is Garat Sidi Mansour in the Gouvernorat of Gafsa. Juvenile tilapia for a stocking programme would have to be produced at the fish culture centre in Kebili.

Tilapia stocked in irrigation reservoirs and water conservation dams across streams in Zambia yield 112 kg/ha/y (Mortimer, 1966) while in Ghana a figure of 143.5 kg/ha/y has been obtained (Denyoh, 1966).

6.3 Tench (tanche) and rotengle

Tench (Tinca tinca) and rotengle (Scardinius erythrophthalmus) were introduced into Tunisia during the period 1964–67. A carful study of the progress made by these species has to be commenced in order to serve as a basis for future policy with regard to them.

Tench reaches a size of 25–30 cm and weight of 250 g in 3–4 years, and rarely exceeds 50 cm or 2 kg. It is a bottom fish which lives in waters rich in vegetation, and feeds mainly on insect larvae and molluscs. The female deposits its eggs in groups, usually on bottom plants. The rotengle grows to similar sizes, but slower. It feeds on aquatic vegetation, besides insects, molluscs and fish eggs.

6.4 Black bass

According to the records, black bass (Micropterus salmoides) was introduced to the reservoir at Bessbessia in 1966. It prefers stagnant or slowly flowing water and reaches sizes of 40–60 cm and maximum weight of over 10 kg. It takes about 4–5 years to reach a weight of about 2 kg. It is a carnivore, feeding on other fish, crustacea, insects and fish eggs.

6.5 Brochets

Brochets (Esox lucius) were stocked in Bessbessia in 1966. The species lives in fresh or slightly brackish water, and reaches a size of 30–50 cm and weight of about 0.5 to 1 kg in 3–5 years. It is carnivorous, feeding mainly on other species of fish.

6.6 Rainbow trout (Truite arc-en-ciel)

Rainbow trout (Salmo gairdneri) were imported in 1967 and stocked in reservoirs and streams in Aïn Draham, Makthar, Aïn Sellem, Nebhana and Siliana. No positive evidence has yet been provided that breeding populations have been established.

According to Lavrosky (1966) trout grow best at temperatures between 16–18°C, continue to feed up to 24°C and can stand rises of up to 28°C for short periods. With regard to chemical characteristics of the water, upper limits for iron, sulphates and phosphoric acid are 0.9 mg/l, 48 mg/l and 0.19 mg/l, respectively. According to Arrignon (1970) calcium contents of 40–120 mg/l and pH values between 6.5 and 8.5 are good for trout. With regard to dissolved oxygen, minimum values are 5–6 mg/l but values for optimum growth are 8–9 mg/l.

A programme for stocking waters with trout is best based on studies of the physico-chemical conditions referred to above, temperature variations throughout the year and experimental rearing in floating cages in the waters concerned. A regular stocking programme would normally involve the establishment of a hatchery as found in the high altitude regions of Morocco, Réunion, Kenya, Basutoland and South Africa (Meschkat, 1966).

6.7 Eels

Certain lakes, such as Kelbia and Garat Sidi Mansour are suitable for stocking with eels. If, for example, Kelbia lake is stocked with 500 000 elvers (= approximately 200 kg, see section 4.2.2) per annum, a yield of about 100 to 150 tons of marketable eels could be obtained per annum, after an initial growth period of 2–3 years. A programme of stocking the lake with juvenile eels from Tunis Lagoon was commenced in January 1975.

6.8 Mullet

Mullet can be stocked in waters of different salinities and even in freshwater lakes and reservoirs, but they need acclimatization if the salinities in which they are destined to be stocked are considerably different from those in which they are captured. Acclimatization and transport of mullet fry and fingerlings have already been dealt with in sections 5.2.1.1 and 5.2.1.2. However, as intensive stocking programmes get underway, it would become eventually necessary to produce the fry and fingerlings necessary in special hatcheries by induced breeding techniques.

The first report of induced spawning of mullets with the use of hormone treatment was by Tang (1964). Since much progress has been made toward perfecting techniques for large-scale production of mullet fry and fingerlings, notably by Liao et al. (1969, 1971a, 1971b), Shehadeh et al. (1973a, 1973b, 1973c), Kuo et al. (1973a, 1973b) and others.

The technique of induced spawning of mullet using pituitary hormone was demonstrated in 1973 in Tunisia, using M. capito. The scope for increasing production by this method in Tunisia is very high. Large numbers of mature mullets are caught in the bordigue of Ichkeul Lake (Tindja), while on their seaward migration for reproduction. For instance, 11 500 mature female Mugil cephalus were captured from 1 to 10 September 1972. A hatchery for commencing experiments on induced breeding of mullet is due to be established by INSTOP in Bizerte in 1975. Once this becomes functional for producing much of the fry and fingerlings required for a stocking programme, the latter could be carried out without serious negative effects on the stocking which takes place naturally from the sea into the important coastal lagoons and lakes.

7. ACKNOWLEDGEMENTS

The present paper is based on studies and work carried out in collaboration between the author and officials of the Fisheries Department in Tunisia, during a three year assignment with the FAO/UNDP Fishery Survey and Development Project. It is a shorter version of a paper on Aquaculture and Lake Fishery Development in Tunisia submitted elsewhere and presented here because of its relevance to the CIFA Symposium.

It is my pleasant duty to express my thanks to Mr. Zachariah Ben Mustapha, former Director of Fisheries, Mr. Med. Ben Khedder, Director of Fisheries, Mr. Ben Alaya, Director of the Project and the Governors and Commissioners of Agriculture in Gabes, Beja and Bizerte for the support given; to Dr. A. Azouz, Director of INSTOP and Engineers H.B. Nasfi and A. Rhouma, colleagues with whom it was a pleasure to work; the research staff of INSTOP and the Project for their assistance and, finally, to Fish Culture Technicians Messrs. H. Abdelhafidh, C. Nourri, A. Bousfida, H. Kamel, M. Laroussi and H. Wahid for their untiring participation and efforts in the aquaculture programme of the project.

8. REFERENCES

Acarese, G., G. Ravagnan and P. Ghittino, 1972 Primi risultati positivi di fecondazione artificiale nel branzino (Dicentrarchus labrax) su vasta scala. Riv.it.piscic. ITITIOP, A.VII, N2:27–33

Arrignon, J., 1970 Aménagement piscicole des eaux intérieures. Paris

Azouz, A., 1966 Etude des peuplements et des possibilités d'ostreiculture du Lac de Bizerte. Annales de l'INSTOP, Salammbô (15):1–69

Baruabe, G. and F. René, 1972 Reproduction contrôlée du loup Dicentrarchus labrax et production en masse d'alevins. C.R.Acad.Sci.,Paris, t.275 ser.D:2741–4

Barnabe, G. and F. René, 1973 Reproduction contrôlée et production d'alevins chez la dorade Sparus auratus Linné 1758. C.R.Acad.Sc.Paris, 276(D):1621–4

Björk, S., 1972 Projet de restauration du lac de Tunis. (Etude exécutée sur la demande du Ministère de l'Agriculture, Tunis). Institut. de limonologie, Univ.de Lund, Suède, pp. 1–29

Burrows, E.M., 1971 Assessment of pollution effects by the use of algae. Proc.Royal Soc.Lond. B.177:295–306

Dahl, J., 1967 Some recent observations on the age and growth of eels. B.Coarse Fish.Conf. (3):48–52

Deelder, C.L., 1970 Synopsis of biological data on the eel Anguilla anguilla (Linnaeus) 1758. FAO Fisheries Synopsis 80:I-I:8

Denyoh, F.M.K., 1966 Pond fish culture development in Ghana. FAO Fisheries Rep. No. 44(6):223–9

Djajadiredja, R. and S. Natawira, 1965 The use of urea as fertilizer and stimulant for the production of benthic algae in brackishwater ponds. Pac.Sci.Congr.I

Djajadiredja, R. and A. Poernomo, 1972 Requirements for successful fertilization to increase milkfish production. In Coastal Aquaculture in the Indo-Pacific Region. Fishing News (Books) Ltd., pp.398–409

El Bolock, A.R. and W. Labib, 1966 Carp culture in the U.A.R. FAO Fisheries Rep., No. 44:165–74

FAO Aquaculture Bulletin, 1970 Vol. 2. No. 3

FAO Aquaculture Bulletin, 1974 Vol. 6. No. 2–3

Farrugio, H., 1973 Les muges de Tunisie: systematique et taxonomie. Recherches sur la biologie de Mugil (= Liza) ramada Risso 1826, et Mugil (= Chelon) labrosus Risso 1826. (Poissons téléostéens). Univ.de Tunis, Fac.des Sci., Diplôme d'études approfondies de biologie marine et d'océanographie. Rapport de stage: 1–115

Gribanov, L.V., A.N. Korneev and L.A. Korneeva, 1966 Use of thermal waters for commercial production of carps in floats in U.S.S.R. FAO Fish.Rep., No. 44:218–26

Hanks, R.W., 1966 Observations on “milky water” in Chesapeake Bay. Chesapeake Sci., (7):175–6

Heldt, H., 1948 Contribution à l'étude de la biologie des muges des lacs tunisiens. Bull.Stn. Océan.Salammbô, (41):35 p.

Huet, M., 1960 Traité de Pisciculture. Bruxelles

Koops, H., 1966 Feeding of eels (Anguilla anguilla) in ponds. FAO Fish.Rep., 44:359–64

Kowamoto, N.I., 1957 Production during intensive carp culture in Japan. Prog.Fish Culturist (19)

Kuo, C.-M., Z.H. Shehadeh and K.K. Milisen, 1973 A preliminary report on the development, growth and survival of laboratory reared larvae of the grey mullet Mugil cephalus L. J.Fish.Biol., 5(5):459–70

Kuronuma, K., 1966 New systems and new fishes for culture in the far east. FAO Fish.Rep. 44: 123–41

Lavrovsky, V.V., 1966 Raising rainbow trout (Salmo gairdneri) together with carp (Cyprinus carpio) and other fishes. FAO Fish.Rep. 44:213–43

Liao, I-C. 1969 et al., Artificial propagation of grey mullet - Mugil cephalus Linnaeus. Rep. Fish Cult. Res. supported by Rockefeller Foundation, JCRR Fish.Ser., (8):10–20

Liao, I-C., 1971 Experiments on induced breeding of grey mullet, M. cephalus Linnaeus. JCRR Fisheries Rep.Fish Cult.Res., pp. 1–29

Lin, S.Y., 1968 Milkfish farming in Taiwan. Fish.Cult.Rep.Taiwan Fish.Res.Inst. (3):43–8

Memmi, L., 1970 Observations biologiques sur les oiseaux du lac de Tunis. D.E.A. d'océanographie Fac.Sci., Tunis, 35 p.

Meschkat, A., 1966 The status of warmwater fish culture in Africa. FAO Fish.Rep. 44:88–108

Mortimer, M.A.E., 1966 A rational approach to fish culture in rural areas of Zambia. FAO Fish. Rep. 44:143–53

O'Sullivan, A.J., 1971 Ecological effects of sewage discharge in the marine environment. Proc. Roy.Soc.Lond., (B) 177:331–51

Painter, H.A. and M. Viney, 1959 Composition of domestic sewage. J.Biochem.Microbiol.Technol. Eng., (1):143–61

Pillai, T.G., 1972 Pest and predators in coastal aquaculture systems in the Indo-Pacific Region. In Coastal Aquaculture in the Indo-Pacific Region. Fishing News (Books) Ltd., London. pp. 456–70

Prowse, G.A., 1962 The use of fertilizers in fish culture. Proc.Indo-pacific Fish.Coun. 9(11): 73–5

Pruginin, Y., 1966 The culture of carp and tilapia hybrids in Uganda. FAO Fish.Rep. 44:223–9

Quignard, J.P. and A. Raibaut, 1971 Presence du Mugil (Oedalechilus) labeo Cuvier 1829 (Poissons téléosteans, Mugilidés) dans les eaux tunisiennes. Bull.INSTOP Salammbô (2):163–8

Raimbault, R., 1973 Le lac de Khenis et les améliorations de production pouvant resulter de divers travaux d'aménagements. Etude d'aménagement de lagune de Khenis. Rapport ISTPM Ingemar, Paris, 9 p.

San Feliu, J.-M., F. Muñoz et M. Alcarez, 1973 Techniques d'élevage artificiel des crustaces. Etud.Rev. (CGPM) (52):107–23

Sawyer, C.N., 1965 The sea lettuce problem in Boston Harbour. J.Wash.Pollut.Control Fed. (37): 1122–33

Schäperclaus, W., 1962 Traité de pisciculture en étang. Vigot Frères Editeurs, Paris, pp. 1–620

Shehadeh, Z.H., C.-M. Kuo and K.K. Milisen, 1973a Induced spawning of grey mullet, M. cephalus L. with fractionated salmon pituitary extract. J.Fish.Riol. (5):471–8

Shehadeh, 1973b Validation of an in vivo method for monitoring ovarian development in the grey mullet (Mugil cephalus L.). J.Fish.Biol. (5):489–96

Shehadeh, Z.H. et al., 1973c The effect of exogenous hormone treatment on spermiation and vitellogenesis in the grey mullet M. cephalus L. J.Fish.Biol., (5):479–87

Tang, Y.A. et al., 1964 Induced spawning of striped mullet by hormone injection. Jap.J.Ichthyol. 12(1/2):23–8

Tang, Y.A. and S.H. Chen, 1967 A survey of the algal pasture soils of milkfish ponds in Taiwan. FAO Fish.Rep., (44) vol.3:198–209

Thong, L.H., 1969 Contribution à l'étude de la biologie des mugilides (poissons téléosteans) des côtes du Massif Armoricain. Trav.Sci.Rennes Ser.Océanogr.Biol., (2):55–181

Van der Lingen, M.I., 1966 Fertilization in warmwater pond fish culture in Africa. FAO Fish.Rep. (44):43–53

OBSERVATIONS ON THE PRESENT STATUS AND PROBLEMS OF
INLAND FISH CULTURE IN SOME NORTHERN STATES OF NIGERIA
AND PRELIMINARY RESULTS OF CAGE CULTURE EXPERIMENTS IN
KAINJI LAKE, NIGERIA

by

E.O. Ita
Kainji Lake Research Project
New Bussa, Nigeria

Abstract

A survey of inland fish culture activities in some Northern States of Nigeria reveals widespread interest in fish culture and great demand for cultured fish. Shortage of personnel in the various establishments limits expansion in pond construction and hence, in fish production, with exception of Panyam fish farms where maximum expansion had been achieved. A proposal is made for increasing the number of personnel to allow research activities to go hand in hand with production.

Preliminary results of cage culture experiments using multispecies and single species combination in two cages reveal that growth rate is higher when individuals of the same species are fed in a cage than for a combination species. Food wastage in cages is a limiting factor worth serious consideration.

Résumé

Une revue des activités de la pisciculture dans quelques Etats du Nord du Nigeria révèle un intérêt largement répandu pour cette activité et une grande demande de poisson d'élevage. Le manque de personnel dans les divers établissements limite le développement de la construction des étangs et ainsi de la production de poisson, avec l'exception de la pisciculture de Panyam qui a atteint son maximum d'expansion. On a proposé une augmentation numérique du personnel pour permettre aux activités de recherche de s'exercer conjointement avec la production.

Les résultats préliminaires d'expériences d'élevage en cage en utilisant la combinaison de plusieurs espèces et d'une seule espèce isolée dans deux cages révèlent que le taux de croissance est plus élevé quand des individus de la même espèce sont nourris dans une cage que lorsqu'il y a plusieurs espèces. Le gaspillage de nourriture dans les cages est un facteur méritant d'être considéré sérieusement.

1. INTRODUCTION

Cage culture experiments in Kainji Lake, Nigeria, was started in June, 1974 by Dr H. Konikoff (FAO consultant on aquaculture). The sharp decline in the Lake's commercial catches from 28,638 metric tons in 1970/71 to 10,905 metric tons in 1973 (Bazigos, 1974) had given rise to much concern. The projected estimate by the same author shows a predicted maximum catch of 6,048 metric tons for 1975. The cage culture experiments were initiated as one of the ways of finding a possible solution to the decline of fish production in the lake. The aim was to concentrate fish in cages and feed them intensively to achieve high production rate within a short period of time at minimal cost.

Reports from South Vietnam (Tal, 1974) reveal that cages placed on the Mekong River have very high yields; a cage of 60 m² produces up to 18 tons of fish per annum.

Following reports of similar activities in other parts of Northern Nigeria, a study tour was conducted between November and December 1974 for comparative purposes. Time was taken off to visit several fish farms and inland lakes for the purpose of studying the problems and the present status of inland culture. Among the places visited were Bagauda Fish Farm in Kano State; Kware Fish Farm in North Western State; Wamako Fish Farm in North Western State; Kalmalo Lake in North Western State and Panyam Fish Farm in Benue Plateau State. In all the places visited, much progress and interest was shown in inland culture but some problems were also apparent.

2. INLAND FISH CULTURE ACTIVITIES IN NORTHERN NIGERIA

Table 1 shows the type of activities in progress at the various establishments visited. Other man-made lakes visited but not included in the Table are: Kware; Tiga, Bagauda, Natu and Wurno lakes all formed by irrigation dams. Kalmalo lake harbours over 150 fishermen operating different type of gear. Control of fishing activity is practicable because of occasional supply of nets to local fishermen by the Fisheries Department. Emphasis is, however, on irrigation both at Kalmalo and other smaller lakes.

Panyam Fish Farm is one of the oldest farms in the Northern States. Construction was started in 1952 and completed in 1954. The total pond area is about 250 acres (100 ha) (Gyang, 1968). After completion, Cyprinus carpio (imported from Europe) and tilapia were introduced during the later part of 1954. Other species such as Lates niloticus, Gymnarchus niloticus, Clarias sp., Heterotis niloticus and Citharinus sp. were also tried, but the first two species were the most successful. In spite of the magnitude of this farm and the encouraging yields, demand for fish from Jos, Bukuru and other nearby towns has not yet been met.

3. PROBLEMS

Table 1 shows that carp is being cultured in all fish farms visited, sometimes at the expense of the indigenous species. Carps are omnivorous feeders with rapid growth rate and are often cropped before they start breeding. Fry are readily obtained by allowing some to breed at a very early age (within the first year of growth). Although the growth rate is rapid, the ponds readily become overpopulated with juvenile tilapia resulting in a reduction of the growth of the original stock. Fisheries Officers in some of the farms visited have attempted solving this problem by culturing tilapia with some Lates niloticus in order to control the tilapia population. The results so far have been very successful and encouraging.

Personal communication with the Officers in charge revealed that the demand for fish in all the areas visited was higher than the supply. This is to be anticipated as evidenced from the limitation in the number of personnel in charge of the farms. The heavy administrative load on the Officers leaves very little time for adequate farm management let alone data collection. In all the countries where fish culture has proved to be a success, fish production is backed up by intensive research. Fish culture research would be worthwhile in Nigeria since fish are a delicacy all over the country even among coastal dwellers.

Table 1
Summary of Activities in Inland Cultures in Northern Nigeria

4. CAGE CULTURE IN KAINJI LAKE

4.1 Materials and Methods

Six wooden framed, poultry mesh cages measuring 1 m³ were used (Fig. 1). The feeding tube at the corner leads into a metal tray into which the feed settles. Only two of the six cages were stocked with sufficient number of fish for observation. The first cage was stocked with specimens of Tilapia galilaea, T. zillii and T. nilotica; the second cage contained only T. galilaea.

The cages were suspended from nylon ropes into the water and the ropes tied to the float of a landing pier a few meters from Kainji Dam.

Fish were fed daily with food pellets prepared by mixing dried clupeid fish, roasted or fresh groundnuts, guinea-corn bran, yam flour and commercial vitamin premix or blood meal together in the following proportions:

The mixture was pressed through a meat grinder and allowed to dry in an oven or under the sun. Table 2 shows the quantity of pellets supplied to the fish in the different cages monthly.

Table 2
Monthly ration of pellets (kg) supplied to fish in the two cages

Although the Table shows regular monthly increment in feed ration (except December which shows the ration for only twelve days) no accurate estimate was made of the corresponding increase in fish weight.

Fish mortality records were kept until no more dead fish were observed on the surface. Table 3 shows the fish mortality recorded out of a total of 256 and 329 fishes introduced into cages I and II respectively.

Table 3
Number and weight of fish found dead in the cages

4.2 Results

Table 4 summarizes the results obtained after feeding the fish for 171 and 164 days in cages I and II respectively. The increment in mean weight for fishes in cages I and II during the experiment were 23.85 g and 35.33 g respectively. The mean weight figure at the beginning of the experiment shows that fishes in cage I were much larger than those in Cage II. However, at the end of the experiment, the increase in mean weight for cage II was higher than cage I.

4.3 Observations and Conclusions

The original aim was to feed the fish at a certain percentage of the body weight as recommended by Shehadeh (1974), but since regular measurements of increase in weight were not made, it was not possible to estimate the proportion of feed required. The monthly estimate of food ration (Table 2) shows that the fish were fed much below ten percent of their body weight, particularly those in cage II. It was observed that fishes in cage I, consisting of three species of Tilapia, were more active than those in cage II with only Tilapia galilaea. A test sample of the feed was always placed on the surface of the water within the cages but only fishes in cage I came up to the surface to feed. Tilapia galilaea possibly confined themselves to the bottom of the cage since they are by nature detritus feeders (Ita, 1971).

No attempt was made to estimate the carrying capacity of the cages but since no serious mortality was observed after the first fifteen days of the experiment it is believed that competition for space was minimal.

Table 4
Number and weight of fish at the beginning and end of the experiment

Table 2 shows that fishes in cage II with 329 individuals received less quantity of feed (28.55 kg) than those in cage I with 256 individuals fed with 47.57 kg weight of feed. In spite of this, and the fact that the experiment lasted only 164 days for cage II as against 171 days for cage I, fishes in cage II grew much faster than those in cage I as seen from the mean weight differences (Table 4). A possible explanation for this is the absence of inter-species competition in cage II with only T. galilaea. Nozawa (M.S.) reports that it is necessary to increase the amount of food by about 30–40 percent more than estimated to allow for the wastage caused by water currents created within the cages. It was not possible, however, to achieve this since the food items used for the experiment were bought at high retail prices. Further experiments have been designed for comparing different feed combinations at minimal cost appropriate for mass production of fish in cages. Also, modifications of the food tray have been carried out in order to cut down on food wastage and, therefore, step up the food conversion ratio. If this can be achieved, then attempts will be made to reduce the cost of cage construction either by using net-cages or by construction of woven reed cages utilizing materials available locally.

5. ACKNOWLEDGEMENTS

I would like to acknowledge the part played by Dr M. Konikoff (FAO consultant on aquaculture to Kainji Lake) in initiating the programme and designing the first set of experiments.

Thanks are also due to the following individuals for their assistance: Mr A. Abubakar, Fisheries Superintendent, Kainji Lake Research Project; Mr H.A. Adeniji and Dr E.O. Adekolu-John, both of Kainji Lake Research Project; and Mr V.O. Sagua, Director of the Kainji Lake Research project.

6. REFERENCES

Bazigos, G.P., 1974 Recent trends in the yield pattern of Kainji Lake, Nigeria. FAO/UNDP/SF/NIR/24

Gyang, M.P., 1968 Fisheries report to the government of Benue Plateau State

Ita, E.O., 1971 Food and feeding relationships of fish in a tropical fish pond. MSc. Thesis, University of Ibadan

Nozawa, T., 1972 Manuscript report on freshwater fisheries to the Freshwater Fisheries Research Institute, Nyegezi, Tanzania

Shehadeh, Z.H., 1974 A preliminary evaluation of the potential for cage culture of fish in Lake Kossou, Ivory Coast. FAO report, FI:DPIVC/71/526/6

Tal, S., 1974 Preliminary observation on new methods of fish culture in Israel. Bamidgeh: Vol.26, No. 3

Figure 1 Wooden frame poultry wire fish cage 1 m³

EXTENSIVE AQUACULTURE PRACTICES ON AFRICAN FLOODPLAINS

by

Robin L. Welcomme
Fishery Resources Officer
Fishery Resources and Environment Division
Department of Fisheries, F.A.O.

Abstract

Management of African floodplains for fish production by extensive aquaculture is becoming increasingly important in view of the use of such areas for other purposes. Three techniques are currently being tried in various parts of the world; pools (whedos) dug into the floodplain, dams which block the drainage channels and dykes (modulos) which enclose areas of the plains. All these methods increase the area of the plain remaining under water during the dry season.

Resumé

L'aménagement des plaines inondables d'Afrique pour la production du poisson par l'aquiculture extensive devient de plus en plus important en vue des autres utilisations de telles zones. Trois techniques sont à l'experimentation à l'heure actuelle: étangs (whedos) creusés dans la plaine, barrages qui bloquent les chenaux de drainage et digues qui enferment des superficies de la plaine. Toutes ces méthodes augmentent la superficie restant en eau pendant la saison sèche.

1. INTRODUCTION

Floodplains are a natural developmental feature of rivers, which in tropical and sub-tropical areas are subject to an annual cycle of flooding and dessication which regulates the ecology of most of the riverine fish species (Welcomme, 1975). The burst of biological productivity which occurs during the initial phase of the floods is associated with migration, reproduction, feeding and growth of fish. This gives rise to a considerable ichthyomass which forms the basis for some of the richest of the world's inland fisheries and traditional fishing practices exploit the pools left on the floodplain by the retreating waters. Fish populations are however limited by the area of water remaining in the system during the dry season and production from floodplains can be improved under natural conditions by increasing the area of water available to the fish at this time.

Because of the richness of their soils and the abundance of water, floodplains are also in demand for a variety of uses other than fisheries. These include types of agriculture which require drainage and irrigation of the soils; techniques which imply control of the flood regime. As fish populations are highly dependent on the flood, they are extremely sensitive to changes in regime, and usually developments of this type are accompanied by a reduction in the fish catch. Already areas of floodplains have been lost below dams with a consequent disappearance of most of the fish population. Maintenance of fish production under conditions of reduced and controlled flow will also depend largely on aquaculture, possibly using methods similar to those detailed below.

2. “WHEDOS”

The Ouémé River has an extensive deltaic floodplain which covers 1 000 km² of the low lying coastal region of Dahomey. The plain is flooded for at least four months each year and both plain and river support fisheries based upon the natural stocks. During the dry season the plain is left dry and is used for agriculture and grazing cattle. The fish population is confined to the main river channel and to a series of artificial ponds known locally as “whedos”. These generally take the form of channels, often exceeding a kilometer in length, but rarely more than 4 m wide, although the numerous natural pools and swamps which lie on the floodplain are also artificially enlarged to fall into the same classification. The bottom of the whedo lies about 1.5 m below the dry season water table, so the pond is filled at all times.

The whedo tends to be overgrown with floating vegetation during the dry season (Fig. 1a) and the water becomes deoxygenated below this cover in the dry season especially in the narrower and smaller whedos. For this reason there is a difference in the species composition of whedos of different sizes. The smaller, and least oxygenated of waters favouring only those species with auxilliary air breathing organs and the larger waters those species requiring higher dissolved oxygen tensions (Table I).

Fishing is carried out by isolating a segment of the whedo with fences made of split palm stems (Fig. 1b). The vegetation is then removed from the enclosed segment and one fence is advanced toward the other until the fish are confined in a small area from which they can easily be removed with hand nets or baskets. After fishing, the whedo is cleaned of excess mud and vegetation and the banks are squared off (Fig. 1c).

Catches from whedos on the Ouémé floodplain have been fairly consistant for a number of years 1955–1958: 2.12 ± 1.18 t/ha although more recently have fallen to 1.57±0.41 t/ha (Welcomme, 1971) possibly due to overfishing of the stock by other methods. A scatter plot of the yield for whedos of various areas is shown in Fig. 2.

Whedos are concentrated in particularly low lying areas of the Ouémé floodplain where they occupy approximately 3 percent of the area. The total area of water equals 1 095 ha, which with a yield of 1.57 t/ha represents a total production of 1 719 t/yr (FAO, 1971).

A fishery of this scale requires a certain infrastructure and the whedo fisheries are fully integrated into the traditional life of the people of the Ouémé valley.

The success of this method is based on the drain-in principle whereby fish are concentrated into the depressions of the floodplain as the flood waters receed. The provision of pools in these areas retain fish that would otherwise escape to the river where there would be inadequate living space for them or would die through the eventual dessication of the pool.

Whedo pools also serve to dry out many of the permanently swampy areas of the plain, where their raised banks are suitable for various forms of vegetable gardening, and their use may be considered for similar areas on floodplains other than that of the Ouémé. More effective management techniques for whedo type pools might include artificial stocking with juvenile fish at the end of the floods, or supplementary feeding during the dry season. These have not yet been investigated but there are possibilities for further study.

3. RETENTION DAMS

The use of primitive dams to retain water in the depressions of the floodplain is known from several parts of Africa. Such retention dams may be used as fishing wiers as in the Gambia River (Svensson, 1933; Johnels, 1954) or for retaining water for some period after the floods have subsided as on the Niger (FAO/UN, 1969). Whilst these techniques are undoubtedly effective for the capture of fish, Reed (FAO, 1969) proposed that more permanent earth dams with sluice gates would be more effective. By retaining larger areas of water additional food and living space would be available for the fish and the reproduction period of species such as Tilapia might be extended. The delayed harvesting made possible by this method would permit fish to grow for a longer period. In fact five dams were built by Reed who obtained about 15 t from 87 ha (172 kg/ha); about double the normally expected catch.

A similar system is under investigation in the Senegal River where Reizer (1974) found that the construction of a small dam across the channel draining a depression-lake of the floodplain assured an increase of the productive surface by a factor of 8. By semi-intensive techniques of fertilization, feeding and stocking it is estimated that a yield of 440/kg/ha/yr should easily be obtainable from some 16 000 ha. In this way the deficit in production which will arise after the construction of up-river dams on the river would be compensated for. Use of impounded floodplain pools is also being considered in Colombia, where some of the “cienagas” of the Magdalena River floodplain are to be dammed. Here the intention is to stock the waters with mullets or with Prochilodus spp. for controlled rearing.

That such artificial floodplain lagoons could function with a determined minimal release of water is indicated by the observations made by Phelines, Coke and Nicol (1973) on the Pongolo River. Here controlled release of water from the Strydom dam in fact inundated the floodplain pools sufficiently to allow fish to breed and grow. This aspect of floodplain fisheries management however needs considerably more research and must be considered as basic to the future of such areas in the context of multi-purpose use.

4. ‘MODULOS’

A further potential method of floodplain management for aquaculture is to be found in South America, but has great possibilities for some areas of Africa. In South America the “llanos” or plains are used almost entirely for cattle pastures. These areas become almost dessicated during the summer, whereas during the spring they may be flooded to a depth of several meters by overflow from the main river systems (Orinoco and Amazon and their tributaries) or by rain water. In an attempt to conserve such water for a longer period, Venezuela has installed an extensive system of dykes termed ‘modulos’. At present these enclose some 30 000 ha of floodplains and retain water during the dry season (Woynarovich, pers. comm.) which dries slowly maintaining an area of fresh pasture at the edge of the retreating water. That such enclosures can also support fish is amply demonstrated by the large concentrations of fish that appear within them. Results to date have, however, been erratic as some enclosures quickly become populated but others remain almost empty. The factors controlling the movements of the Latin American species on to the floodplain are imperfectly understood and a current FAO project is investigating the possibilities for artificially stocking such waters and for constructing suitable drain-in pools to better exploit their potential for fish production.

5. CONCLUSIONS

The management of floodplain for increased fish production is still at a very early stage of development. Present methods are simple and as yet inadequately elaborated and tested. However, there is no doubt that great possibilities exist for the expansion of this kind of extensive fish culture in conjunction with the general management of floodplain areas for the whole spectrum of human activities.

6. REFERENCES

FAO/UN, 1969 Report to the Government of Nigeria on fishing technology relating to river and swamp fisheries in Northern Nigeria. Based on the work of W. Reed, FAO/TA Fishery Technologist. Rep.FAO/UNDP(TA), (2711):90 p.

FAO, 1971 Rapport au Gouvernement du Dahomey sur l'évaluation de la pêche intérieure, son état actuel et ses possibilités, établie sur la base des travaux de R.L. Welcomme, Specialiste de la pêche. Rep.FAO/UNDP(TA), (2938):97 p.

Johnels, A.G., 1954 Notes on fishes from the Gambia River. Arkiv.Zool., 6:327–411

Phelines, R.F., M. Coke and S.M. Nicol, 1973 Some biological consequences of the damming of the Pongola river. In Proc. llème Congrès des Grands Barrages - Commission International des Grands Barrages, pp. 175–90

Reizer, C., 1974 Définition d'une politique d'aménagement des ressources halieutiques d'un ecosystème aquatique complexe par l'étude de son environnement abiotique, biologique et autrophique. Le Fleuve Sénégal Moyen et Inférieur. Docteur en Sciences de l'environnement. Dissertation. Arlon, Fondation Universitaire Luxembourgeoises, 4 Vols. 525 p.

Svensson, G.S.O., 1933 Freshwater fishes from the Gambia River (British West Africa). Results of the Swedish Expedition, 1931. K. Svenska Vetenskskapsakad. Handl. 12:13 p.

Welcomme, R.L., 1971 A description of certain indigenous fishing methods from southern Dahomey. Afr.J.Trop.Hydrobiol.Fish., (1):129–40

Welcomme, R.L., 1975 The fisheries ecology of African floodplains. CIFA Tech.Pap., (3):51 p.

TABLE I
Differences in species composition of catches from whedos of different areas

¹ P = present at less than 0.1%

	Fig. 1. Views of a typical whedo
	(a) before fishing
	(b) during fishing
	(c) after fishing

Fig. 2. Scatter plot of yield for whedos of various areas