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

STATUS OF AQUACULTURE IN UGANDA

by

A.R. Biribonwoha
Fisheries Department
Ministry of Agriculture and Animal Resources
Entebbe, Uganda

Abstract

The favourable topography of the country and its permanent streams and rivers present excellent opportunities for fish culture. Fish culture was initiated in 1953 in remote areas in need of protein supplies and is now well established, mainly on a subsistence level. At present there are over 10 000 fish ponds of about 0.1 ha area each with possible production levels of 1 200–2 300 kg/ha/yr. The major species cultivated are Tilapia zilli, T. nilotica, T. leucosticta and hybrids of T. nilotica and T. hornorum; Lates niloticus and Bagrus docmac are used as tilapia predators.

The Government provides fry for stocking free of charge, absorbs 30 percent of the cost of fertilizers and nets and regularly stocks cattle watering ponds and lakes for licensed public fishing. Research is also underway on culture of frogs, crayfish, Chinese carps and crocodiles. There is excellent scope for commercial aquaculture and the Government has given full support and considerable subsidy to fish farm development.

Resumé

L'Ouganda presente d'excellentes opportunités pour le développement de l'aquaculture grâce à la topographie et au caractère permanent de certains fleuves et rivières. La pisciculture se développa à partir de 1953 dans des zones reculées connaissant un déficit d'approvisionnement en protéines et celle-ci est maintenant bien établie, principalement comme élément de subsistance. Actuellement il y a 10 000 étangs d'une surface moyenne de 0.1 ha pouvant atteindre la production de 1 200 à 2 300 kg/ha/an. Les principales espèces cultivées sont Tilapia zilli, T. nilotica, T. leucosticta et le hybrids de T. nilotica et T. hornorum; Lates niloticus et Bagrus docmac sont utilisés comme prédateurs des tilapias.

Le Governement fournit gratuitement les alevins, subventionne 30 pour cent des coûts des engrais et des filets de pêche, et repeuple les étangs et les retenues d'eau destinés au bétail pour la pêche sportive. Des recherches sont en cours sur la culture de la grenouilles, l'écrevisse, la Carpe chinoise et les crocodiles. Les perspectives de développement de l'aquaculture commerciale sont excellentes et le Gouvernement apporte son soutient total et des subsides importants au développement des fermes d'élevages de poissons.

1 INTRODUCTION

Uganda is a country relatively well endowed with water resources. A total area of 35 340 km² is covered with open water or perennial swamps affording the country tremendous fisheries potential. Fish is regarded as a very important source of animal protein in Uganda, current consumption being estimated at 15 kg/per capita, per annum, which is nearly twice that of beef, the next most important source of animal protein.

Fish production in Uganda is now estimated at 165 000 metric tons mostly coming from the lakes, rivers and swamps.

The favourable topography of the country coupled with permanent streams and rivers presents unusual opportunities for successful fish culture practices. Fish culture was hence introduced in Uganda as early as 1953 in order to provide a source of badly needed protein to people in those remote but suitable areas where fresh fish from the lakes could not be readily obtained. Fish culture therefore is well established in the high altitude valleys of Kigezi, Bugisu highlands, Bukedi, part of Northern and Buganda regions which because of the distances involved or the prevailing poor communications, are not well covered by the present fresh fish marketing activities.

Fish farming is still on subsistence level but plans are well advanced to move it to a commercial scale. Just as in any new venture in a developing country, the Government is providing leadership by establishing large commercial fish farms. These farms will be used for demonstration purposes. It is hoped that their performance will provide a practical proof as to the economic viability of large scale fish farming.

2. AREA AND SCOPE

It is now estimated that there are over 10 000 fish ponds in the country, most of which are not much larger than one-tenth of a hectare. Although no reliable returns of production are available, it has been established that in most areas, under intensive fish farming practices, it is possible to get a yield of 1 200 to 2 300 kg of fish per hectare per annum. The dimunitive size of the ponds is a result of the initial official policy which advocated subsistence fish farming. Other contributary factors have been the lack of earth-moving construction machinery and in some cases competitive use of the limited land in relatively over-populated areas for other, better known, forms of agricultural practices.

3. SPECIES CULTIVATED AND CULTURE PRACTICES

Initial programmes in fish culture involved the indigenous and very much prized Tilapia species, namely: Tilapia zilli, Tilapia nilotica and Tilapia leucosticta. The last did not grow to an acceptable table size in ponds. The other two species have been reinforced by the inclusion of mirror carp, Cyprinus carpio, tilapia hybrids (♀ Tilapia nilotica x ♂ Tilapia hornorum), Nile perch, Lates niloticus, and the cat fish (Bagrus docmac); the last two being used as predators to control the population of prolific breeding Tilapia.

For good results it is recommended that the stocking ratio of Nile Perch to Tilapia should be 1:20 to 1:25. In case of Tilapia zilli the two species should be stocked at the same time since by the time the perch changes its feeding habits, i.e. from aquatic insects to fish, Tilapia zilli will already have started breeding. With respect to the relatively late breeder, Tilapia nilotica, Nile Perch should be stocked some six weeks later.

As a result of successful experiments carried out by the Fisheries Department, farmers are now encouraged to carry out multiple stocking of Tilapia hybrids (Tilapia nilotica × Tilapia hornorum) and Cyprinus carpio in the ratio of 1:1. Production realized from such multiple stocking ranges from 1 200 to 2 300 kg/ha/yr. It is recommended that fish fry be stocked in ponds at an average weight of 5 to 10 g and, with proper feeding, cropping should begin after six months when fish would have attained individual weight of 500 g. The stocking rate giving optimum production is 2 500 to 3 000 fish fry per hectare. Where no supplementary feeding is carried out, it is recommended that the stocking rate should be between 600 to 700 fish fry per hectare. No definite feed has been formulated for fish as yet. The development of feed is of high priority on our fish culture programme, especially if commercial fish farming is going to be practised. For the moment, maize bran and meal are used at the feeding rate of 3–5 percent of the body weight.

4. GOVERNMENT INPUTS

Government provides fry for stocking to farmers free of charge. It runs a number of fry centres for this purpose conveniently located in fish farming centres. Under one of its regular programmes, farmers are also provided with 30 percent subsidy on the cost of fertilizers, and pond seine nets. It is hoped that in future it will be possible to provide assistance in the construction of large commercial fish farms.

The Department of Fisheries maintains a number of fish farming experimental stations, the largest of which is Kajansi fish farm. On this farm, apart from the standard supporting research, observations and techniques on farming frogs, crayfish Procambus clarkii, the Chinese grass carp (Ctenopharyngodon idellus) and crocodiles (Crocodilus niloticus) are being undertaken. It is hoped that the results of these investigations will go a long way to ensuring that fish farming in Uganda is based on a sound scientific understanding.

Related to pond fish culture is the stocking of less productive bodies of water with fish fry raised at the fry centres. More than 600 dams and valley tanks have been constructed for the purpose of watering cattle and other industrial uses. For maximum utility, these dams were stocked with fish and people may take out permits to fish them thereby increasing their income and supplying badly needed protein to the rural area. Likewise, crater lakes have been stocked. They are proving to be productive and are contributing to the rural economy. More and more people realize that fish farming on a commercial scale would be beneficial.

As already pointed out Uganda has many areas which are classified as swamps. They are of low agricultural value but are most suitable for fish culture. There is great scope for profitable commercial fish farming in these areas to raise the already popular types of fish now being used in our fish culture programme. It is in full realization of this fact that the Government has given full support to fish farming programmes with considerable subsidies.

RAPPORT SUR LA PISCICULTURE

par

Direction des Eaux et Forêts
Département de l'Agriculture
République du Zaïre

Résumé

Depuis 1943 les autorités gouvernementales ont envisagé la possibilité d'utiliser les pêches intérieures et la pisciculture pour la fourniture de protéines. Un Service des pêches et de la pisciculture a été créé après des études préliminaires. Ce service avait pour but de promouvoir, par tous les moyens, la production et la consommation du poisson.

La production de la pisciculture extensive s'établit entre 100 et 500 kg/ha/an; cette méthode n'implique pas l'alimentation artificielle du poisson. La pisciculture intensive, dans laquelle le poisson est nourri avec des balayures et déchets domestiques, peut produire 5 à 10 fois plus que la méthode extensive. Avec Tilapia melanopleura on a obtenu une production de poisson atteignant jusqu'à 7,5 t/ha/an.

Le pays est couvert par un réseau de 21 centres d'alevinage pour alimenter les étangs. Les principales espèces utilisées sont Tilapia melanopleura et T. macrochir. L'alimentation est surtout basée sur les balayures de moulin.

On a remis en route la pisciculture à Kinshasa avec Tilapia nilotica que l'on nourrit avec des drèches de brasserie; la production se situe entre 3 à 4 t/ha/an.

Abstract

Since 1943 the governmental authorities have considered the possibility of supplying protein through inland fishery and fishculture. After preliminary studies, a Fisheries and Fishculture Service has been established to promote production and consumption of fish by all possible means.

Extensive fishculture production is between 100–500 kg/ha/year and does not involve feeding the fish. Intensive fishculture, that requires artificial feeding of the fish with domestic waste, can produce 5 to 10 times the above-mentioned yield. A yield of up to 7.5 t/ha/year has been obtained with Tilapia melanopleura.

The country is covered by a network of 21 centres producing fry to supply the ponds distributed throughout the country. The main species utilized are Tilapia melanopleura and T. macrochir. Feeding is based mainly on mill waste.

Fishculture has been restored at Kinshasa with T. nilotica, fed with brewery waste, the yield being around 3 to 4 t/ha/year.

1. INTRODUCTION

Devant le déficit constaté dans la production de protéines d'origine animale en Afrique - devant les nombreuses difficultés rencontrées à l'importation - les pouvoirs publics se sont préoccupés, depuis 1943, à combler, du moins en grande partie ce déficit par le développement de la pêche et l'introduction de la pisciculture en milieu rural.

Dans ce domaine, un premier pas fut la création d'une mission piscicole régionale. Cette mission fut chargée d'étudier les possibilités de pisciculture en milieu rural. Elle termina son rôle en 1947, en concluant à la possibilité de l'élevage du poisson.

Poursuivant l'oeuvre régionale commencée, une Mission Piscicole à l'échelon national, qui fonctionnait depuis novembre 1947, résolut les différents problèmes techniques soulevés par la réalisation du programme d'élevage de poisson. Cette Mission recherchait toutes les méthodes propres à conserver, développer et exploiter le poisson.

Elle mettait au point des méthodes de pisciculture dans les milieux ruraux, capables d'y satisfaire les besoins, de même qu'autour des agglomérations importantes, en vue d'y assurer une production suffisante de poisson, pour compléter le produit de la pêche dirigé sur les centres. Elle se préoccupait également de trouver une formule économique de production de poisson, par la pisciculture, autour des grosses industries et des centres importants pour y suppléer à l'insuffisance d'éléments carnés, nécessaires à l'alimentation de la population.

Pour atteindre le but qui lui était assigné, la Mission Piscicole entretenait une Station de Recherches Scientifiques.

Elle procédait à l'étude hydrobiologique de certains lacs et biefs, elle expérimentait certains procédés de pêche et de protection de la faune ichtyologique, elle créait et entretenait, à titre expérimental, des centres d'alevinage, elle créait ou surveillait des essais de pisciculture en milieux ruraux et autres.

Pour la mise en pratique généralisée des méthodes préconisées par la Mission Piscicole, il a été fait appel, par la suite aux services d'exécution des provinces, dans le cadre de l'agriculture. La Mission Piscicole n'avait donc qu'une mission temporaire.

Après une série d'expériences, qui ont eu des résultats positifs, l'application de la pisciculture a été confiée aux services d'exécution (provinciaux).

La réalisation de ce vaste programme a amené la création du Service de la Pêche et de la Pisciculture, dont le but était de promouvoir, par tous les moyens, la production et la consommation du poisson, en vue de combler le plus possible ce déficit en protéines animales. Le Service de la Pêche et de la Pisciculture a favorisé l'amélioration de la technique de la production par le perfectionnement des méthodes existantes, par l'introduction de nouvelles méthodes ayant fait leurs preuves, par l'enseignement de la manière d'élever le poisson, comment construire les étangs et les entretenir. Il a créé des écoles professionnelles pour former de bons pêcheurs et des pisciculteurs avertis. Il a veillé à une production de poisson optimum et continue. Pour atteindre ce but il a provoqué une réglementation adéquate.

Dans son rôle économique, et également social, le Service de la Pêche a contribué à fixer les pêcheurs et pisciculteurs. Sur le plan scientifique, il a récolté des échantillons, des renseignements statistiques, et contribué grandement à la connaissance des meilleurs régimes alimentaires des poissons, leurs moeurs, la fraye, etc.

Après 10 ans d'actions soutenues, on s'est aperçu que les apports de la pisciculture étaient 90 fois inférieurs à ceux de la pêche fluviale et lacustre et ne dépassaient jamais 1% du total produit, sauf dans la province de la capitale, où ils représentaient 2%.

2. SUPERFICIE ET TYPES DE PISCICULTURE

2.1 Pisciculture extensive

Les rendements varient dans les limites de 100 à 500 kg/ha/an, suivant la richesse naturelle des eaux et des conditions locales (t, pH, etc.).

Ci-dessous pour les différents rendements par ha et en tenant compte d'un déchet de 20% sur les prises, les superficies d'étangs à exploiter de façon extensive (sans nourrissage) pour couvrir les déficits:

2.2 Pisciculture intensive

Si on pratique, dans des étangs convenablement aménagés, une pisciculture intensive (nourrissage avec déchets ménagers notamment) on arrive sans peine à multiplier par 5 à 10 les rendements ci-dessus.

Dans ce cas les superficies d'étangs à mettre en eau sont 5 à 10 fois moindre pour convrir les mêmes déficits de 1 000 t à 6 000 t.

Les rendements suivants ont été obtenus dans des centres d'alevinage, grâce à un nourrissage abondant:

Ces rendements ont été obtenus avec une seule espèce: Tilapia melanopleura. Il est très vraisemblable que s'il avait été élevé en association avec T. macrochir, on aurait pu en escompter un rendement supérieur.

En augmentant la propagande, en vue d'un nourrissage intensif des étangs dans les différentes régions on peut entrevoir la contribution de la pisciculture dans la production de protéines animales pour la population.

On constate que si l'on faisait passer le rendement moyen de 300 kg à 1 500 kg par ha on porterait du même coup la production de la pisciculture de 1 200 t à 6 000 t soit, à cette époque-là (1959) l'équivalent de la production de la pêche maritime.

C'est dans cette voie que les efforts se sont portés: étangs de superficie convenable, minimum 1 are, et abondamment nourris. Des améliorations appréciables ont été obtenues en oeuvrant dans cette voie.

3. ORGANISATION

Le pays est couvert d'un réseau de 21 centres d'alevinage principaux (C.A.P.) qui peuvent ravitailler en alevins ou en géniteurs n'importe quel des plus ou moins 400 étangs d'alevinage répartis judicieusement au sein des diverses circonscriptions.

Ces étangs recoivent des souches en provenance des C.A.P. et assurent l'alevinage des pisciculteurs sans devoir recourir aux C.A.P. eux-mêmes.

4. ESPECES DE POISSONS

Le peuplement des étangs s'est principalement réalisé avec une association de Tilapia melanopleura et Tilapia macrochir. Le premier est surtout phytophage, le second planctonaphage.

Le Tilapia melanopleura mange non seulement des plantes aquatiques, mais aussi des feuilles de manioc, de bananier, etc.

On nourrit artificiellement les deux espèces avec des balayures de minoteries. Un certain pourcentage de nourriture naturelle est nécessaire.

La multiplication du plancton dans l'eau est favorisée par des déversements d'engrais locaux, comme balayures de minoteries, coques de graines de coton, tourteaux de tournesol.

Actuellement, le redémarrage de la pisciculture à Kinshasa et aux environs se fait avec Tilapia nilotica, qui semble très adapté pour la pisciculture.

Les nouveaux étangs construits à Kinshasa donnent des rendements de 3 à 4 t/ha. Ils sont nourris avec des drèches de brasseries.

Les principes de l'implantation de nouveaux étangs dans la région maraîchère de Kinshasa sont les suivants:

• le maraîcher pisciculteur s'engage à respecter les instructions techniques, réaliser tous travaux de digues et empoissoner lui-même son étang à partir de la première récolte

• le projet piscicole prend à sa charge la construction du moine et l'ouvrage d'adduction d'eau. Il apporte son appui technique.

• le projet piscicole fournit les alevins nécessaires au premier empoissonnement, au départ de l'alevinage du Centre Piscicole.

Partie sur ces nouvelles bases, l'avenir de la pisciculture à Kinshasa semble prometteuse.

PRELIMINARY STUDIES ON THE POSSIBILITY OF
STARTING AQUACULTURE OF FISH IN ETHIOPIA

by

F.H. Meskal
University of Addis Ababa
Addis Ababa, Ethiopia

Abstract

The possibility of starting aquaculture of fish in Ethiopia is considered for the first time. Climatic differences between different topographic regions of the country are found significant in the choice of fish species for aquaculture. Fish species already well established in natural waters and recommended for aquaculture in the low lands include Tilapia nilotica and Cyprinus carpio. The temperate fish Salmo gairdnerii and C. carpio are suggested for pond culture in the highlands. The cheapest supplementary fish feed developed at present consists of wheat bran, mill sweeping, meat meal and oil cake. The raw material required to produce dry pellets costs U.S. $ 4/100 kg.

Résumé

La possibilité de développement de l'aquiculture de poissons en Ethiopie est évoquée pour la première fois. Les différences climatiques entre certaines régions du pays ont un rôle significatif dans le choix des espèces à élever. Les espèces de poissons déjà bien établies dans les zones naturelles, recommandées pour la culture dans les zones de plaines sont Tilapia nilotica et Cyprinus carpio. Les espèces tempérées Salmo gairdnerii et C. carpio sont suggérées pour l'élevage en étangs dans les régions des plateaux. L'aliment complémentaire le moins cher produit actuellement est composé de paille de blé, de déchets de meunerie, de farine de viande et de tourteaux. La matière première nécessaire pour produire 100 kg d'aliment est évaluée à U.S. $ 4.

1. INTRODUCTION

Aquaculture of fish in Ethiopia is practically non-existent up to the present time. The need to develop this means of fish production is however uncontested although not very well appreciated by concerned agencies. In the absence of any documented data or any active research on practical fisheries in the country an attempt has been made to carry out a preliminary investigation on the possibility of initiating fish culture. Issues considered most important to tackle at first include (a) identification of fish species of economic value that are already well established in the various climatic regions of the country, (b) locating suitable sites for initial supply of appropriate fry, and (c) identification of possible sources of supplementary fish fodder.

The present paper therefore deals mainly with field observations in connexion with the first two items, and labóratory investigation on the last.

2. FISH SPECIES FOR AQUACULTURE

In considering the types of fish that could be profitably used in fish culture, the climate of the country must be taken into consideration. Because of its location in the torrid zone (between 3 and 18°C north of the equator) Ethiopia ought to have a hot and dry climate like its neighbours lying in the same latitude. The torrid climate feature is actually expressed in the eastern and western slops of the country and the Ethiopian Rift Valley. These regions are generally low in altitude ranging from about 100 m below sea level near the Red Sea to about 1 800 m above sea level in the lake district of the Rift Valley. The most densely populated region of the country is however that of the very high plateaus ranging from 1 800 to 3 000 m. The plateaus are towered by sky-scraping mountains that sometime reach over 4 000 m above sea level. This topography has tempered the climate over a large portion of the country (Atkins, undated; Wolde-Mariam, 1972). Thus, while parts of the low lands experience some of the hottest average temperature reaching over 32°C and little rain (less than 50 mm a year), the high plateaus enjoy a much cooler temperature (the average yearly temperature being around 16°C) and abundant rain (over 1 000 mm a year).

Fish species adapted to the low lands may not be suitable for the cool waters of the high plateaus. It was therefore necessary to identify fish species that are successfully established in the various parts of the country. The method adopted consisted of collecting commonly occurring fish in lakes and rivers with the help of local fishermen.

Between 1973 and 1974 eight lakes and one dam in the Rift Valley, six rivers and one lake in the highlands of Bale and Shoa provinces were inspected for their fish fauna. The most commonly occurring fish of economic significance in the low lands include Tilapia nilotica, Clarias mossambicus, Lates niloticus and Cyprinus carpio. The single lake (Science Faculty Report, 1974) and three rivers studied in the highlands revealed no fish of commercial value. Two fish species of temperate origin (Salmo gairdnerii and S. trutta) are found in three rivers in the Bale mountain. More rivers in the same area are reportedly inhabited by rainbow trout. Trout were introduced by a Unesco wildlife expert sometime between 1967 and 1968. Both species are performing so well that the local Fly Fishing Club is presently encouraging its members not to return angled fish back to the water so as to curtail the rapid growth of the population.

Fish species that may be recommended for aquaculture in the low lands therefore include T. nilotica and C. carpio. Rainbow trout and common carp may prove appropriate for pond culture in the highlands (Meskal, 1974).

3. SOURCE OF FISH SEED FOR INITIAL POND CULTURE

Having identified the type of fish that may be used for fish culture at different altitudes the next factor to consider is the location of easily accessible sources of fry. Again the method used for the purpose included closer inspection of rivers and lakes.

Of the Rift Valley lakes inspected, Lake Awasa and Lake Arsedi (Hora Arsedi) alone offer easily accessible nurseries for the fry of T. nilotica. The fry at Awasa are however heavily infested with external parasites including unidentified species of Lernaea, Argulus and Cichlidogyrus. Cursory examination of tilapia fry from Lake Arsedi indicated that this fish is relatively less infected with external parasites. This lake is therefore preferable as a source of fry for initial stocking of ponds. With little modification of the breeding and nursery grounds in this lake an adequate supply of fry could easily be secured throughout the year. The proximity of this lake to the capital city (only 50 km) is another advantage over Lake Awasa which is about 275 km from Addis Ababa.

Mirror carp is found in the Awash river system. The most reliable site for adequate supply of its fry at present is the Akaki river, about 26 km south of Addis Ababa. Although more reliable than the Koka dam where scattered individual fry are present, Akaki river does not offer large supplies of the fry that would be needed for large-scale pond culture. A good number of the fry at Akaki river were detected in the month of November. However, we are not yet sure if this same number of fry may be found throughout the year. The breeding season and nursery grounds for common carp need to be studied thoroughly before the supply of fry can be ascertained. The development of artificial hatcheries might be the most reliable means to secure continuous supply of carp fry for large-scale production of the fish.

Although the Danka river in the Bale mountain is presently the only source of rainbow trout, the number of fry that can be collected from this river for pond culture is limited. Another disadvantage is that the river is too distant from highland settlements where the fish may be needed most. Development of a hatchery at some other suitable site, preferably close to the capital city, has been strongly recommended.

4. SUPPLEMENTARY FISH FEED

Fish fodder that is being developed in the laboratory includes feed for tilapia and common carp. Edibility of the various formulae developed is tested on fish kept in aquaria, concrete cisterns and sinks. The animals are originally maintained on algae and Daphnia grown in concrete tanks and some imported fish pellets. The fish include T. zillii from Uganda, T. nilotica and C. carpio collected from Akaki river. Cheap industrial byproducts like wheat bran, mill sweeping, brewery waste, meat meal, blood meal and oil cakes are mixed with water at various proportions and pellets are made by passing the thick paste through a meat mincer. Criteria used in the choice of a particular recipe include availability of the raw material at low cost, the ease with which pellets are formed and edibility of the pellets to the fish. The best formula developed so far is given in the following table, the current price of each raw material is also shown.

TABLE I
Fish Feed Recipe

Excluding cost of labour and transport, 100 kg of dry pellets of the above formula can be produced for U.S. $ 4.00. It is hoped that further trials with various other raw materials will result in a still cheaper formula for industrial production of the pellets.

5. ACKNOWLEDGEMENTS

My thanks are due to the
necessary funds to carry out the present study.

6. REFERENCES

Atkins, H., undated A geography of Ethiopia, Addis Ababa, Sim Printing Press

Meskal, F.H., 1974 Research report on increasing the freshwater fish stock of Ethiopia. (mimeo) Department of Biology, H.S.I. University, 14 p.

Science Faculty, 1974 Report on expedition to Lake Wonchi (Ethiopia). (mimeo) Hailé Selassié I University, 32 p.

Wolde-Mariam, M., 1972 An introductory geography of Ethiopia. Berhanena Selam H.S.I. Printing Press, Addis Ababa

POSSIBILITIES FOR AQUACULTURE DEVELOPMENT IN TUNISIA

by

T.G. Pillai¹
Fishery Biologist
FAO/UNDP Fishery Survey and Development Project
Tunisia

¹ Present address: FAO/UNDP Integrated Fishery and Fish Culture Development Project Nepal

Abstract

The potential for aquaculture in Tunisia is good, with several culturable species of molluscs, fish and shrimp, as well as about 100 000 ha of lagoons, lakes, irrigation reservoirs and cases ponds. While satisfactory progress is being made on the commercial cultivation of oysters and mussels, fish culture is still in its very early stages.

Experiments carried out on the culture of mullets and carp in ponds have given encouraging results. However, there are some important problems that have to be overcome if fish culture is to be developed on a large scale. One is that of pollution, particularly secondary pollution, and that of Tunis Lagoon proved to be a striking example. Losses due to predation by birds and predatory fishes could be very high, and have to be prevented while, thirdly, the construction of fish culture systems in lagoons and lakes should be strong enough to withstand the effects of rough weather that prevails seasonally.

A strategy for the development of aquaculture in Tunisia would include, among other points, finding more sales outlets for the increasing production of oysters and mussels, and the expansion of fish culture activities. The latter could be carried out by stocking suitable natural and artificial lakes, as well as in artificially-constructed ponds and cages. From the point of view of market demand and abundant availability of fry and fingerlings, mullets have a good potential for culture in ponds as well as stocking in natural and artificial lakes. Eels are also abundantly available in their elver and juvenile stages and can be used for such stocking, and even culture in ponds. The culture of bass (Dicentrarchus labrax) and bream (Chrysophrys aurata) on a large scale would depend largely on the production of their fry and fingerlings by induced breeding. Likewise, a large scale stocking programme with mullet fry and fingerlings captured from nature should eventually be supplemented by, and even replaced by, those produced by induced breeding, if the existing lagoon fisheries are not to be affected. Importation of species for stocking freshwater lakes and reservoirs in Tunisia should best be carried out after establishing the necessity for such introduction of exotic species, and an adequate study of the waters concerned.

Résumé

En Tunisie les conditions sont favorables pour l'aquiculture. On y trouve plusieurs espèces cultivables de mollusques, de poissons et de crevettes ainsi qu'environ 100 000 ha de lagunes, lacs, réservoirs d'irrigation et réservoirs d'oasis. La pisciculture n'en est encore qu'à ses débuts alors quel'on enregistre des progrès satisfaisants de l'ostréiculture et de la mytiliculture commerciales.

Des expériences effectuées sur l'élevage du mulet et de la carpe en étangs ont donné des résultats encourageants. Cependant, il faut résoudre d'importants problèmes pour développer la pisciculture sur une grande échelle. La pollution est un de ces problèmes, en particulier la pollution secondaire, et celle du lac de Tunis en est un exemple frappant. Les pertes provoquées par les oiseaux et poissons prédateurs pourraient être très élevées et il faut s'en protéger. En troisième lieu la construction de systèmes de pisciculture dans les lagunes et les lacs doit être suffisamment solide pour faire face aux effets du mauvais temps qui règne pendant certaines saisons.

Pour le développement de l'aquiculture en Tunisie la strategie doit comprendre entre autres la recherche de nouveaux débouchés commerciaux pour la production croissante d'huîtres et de moules et l'expansion des activitiés de la pisciculture. Cette dernière pourrait être effectuée par le peuplement de lacs naturels et artificiels convenables ainsi que d'étangs et de cages construits artificiellement. Du point de vue de la demande du marché et de la disponibilité abondante de frai et d'alevins les mulets offrent une bonne possibilité d'élevage en étangs ainsi que pour le peuplement de lacs naturels et artificiels. Les civilles et les anguilles aux stades juvéniles existent aussi en abondance et peuvent être utilisées pour le peuplement et même l'élevage en étangs. L'élevage du loup (Dicentrarchus labrax) et de la daurade (Chrysophrys auratus) sur une grande échelle, dépendrait largement de la production de leur frai et de leurs alevins par la reproduction provoquée en employant des injections d'hormones. Ainsi, un vaste programme de peuplement avec du frai et des alevins de mulet capturés dans la nature devrait être complété éventuellement et même remplacé par les produits de la reproduction provoquée si les pêches pratiquées dans les lagunes ne doivent pas être affectées. L'importation d'espèces pour le peuplement de lacs d'eau douce et de réservoirs tunisiens serait effectué dans les meilleures conditions après avoir établi préabablement la nécessité de l'introduction d'espèces exotiques et effectué une étude convenable des eaux en question.

1. INTRODUCTION

Although Tunisia is making satisfactory progress in the commercial cultivation of oysters and mussels, the culture of fish in fresh and salt waters is still in its very early stages. The centre for the commercial cultivation of oysters and mussels is located in the lagoon of Bizerte and it is operated by the Office National des Pêches. The species cultivated are the Portuguese oyster (Crassostrea angulata), the Japanese oyster (Crassostrea gigas), mussels (Mytilus galloprovincialis) and clovisses (Tapes decussatus). The average production of the two species of oysters for the market totals about 180 000 oysters per annum, and production is not being stepped up owing to an apparently limited market for them. An interesting fact is that oyster spat for culture has to be regularly imported from abroad, since the high salinity of the water in Bizerte lagoon is unfavourable for the reproduction of these species.

With regard to mussels, the Mediterranean mussel (Mytilus galloprovincialis) and clovisses (Tapes decussatus) are cultivated, and their total annual production is around 75 tons and 85 tons respectively. A stabilization and cleaning plant for shellfish is available at Gammarth, and a smaller one at INSTOP, Salammbô.

Attempts to introduce fish culture consisted mainly of the establishment of a freshwater fish culture station in Aïn Sellem, Beja, which remained under the control of INSTOP since 1966, and some stocking of water reservoirs and streams with imported species of fish. The station at Beja consists of an artificial pond of about ½ ha constructed by erecting a cement dam across a natural stream arising from a mountain spring, as well as a series of cement ponds for a fish hatchery. The species cultivated is common carp, and until spring 1974, when techniques for its breeding under controlled conditions were successfully introduced, production at this centre depended mainly on natural breeding and was low because of predation by other species of fish and the lack of a trained personnel.

Some stocking with imported species of fish has been carried out with a view to establishing good quality edible fish in several irrigation reservoirs and streams. The species are the mirror variety of common carp (Cyprinus carpio), rainbow trout (Salmo gairdneri) tench (Tinca tinca), black bass (Micropterus salmoides) and tilapia belonging to the species Tilapia nilotica and Tilapia mossambica. Common carp stocked in the Mellegue dam was subsequently found in the catches of Kelbia Lake, and Tilapia nilotica has established itself in freshwater streams in Kebili and other places in the Gouvernorat of Gabes.

2. RESOURCES FOR AQUACULTURE

2.1 Water

From the point of view of available lagoon, lakes and other freshwater areas, the potential for aquaculture in Tunisia is good. There is a total of approximately 80 000 ha of lagoons and slightly brackishwater lakes, and another 20 000 ha of irrigation reservoirs, freshwater lakes, rivers, oasis ponds and streams. An interesting example of a freshwater lake is that of Garât Sidi Mansour in the Gouvernorat of Gafsa which was formed during the floods of 1959 and has persisted. Its surface area varies between 2 500 and 3 500 ha, depending on evaporation during summer and the amount of rainfall in winter, and has now attracted attention with regard to its potential for aquaculture.

The most important lagoons and inland lakes are as follows:

* summer figure
** when lake is drying up

Irrigation reservoirs and other freshwater storage reservoirs are located in Le Kef (Mellegue, La Khmass), Jendouba (Beni Mtir), Beja (Kesseb), Kairouan (Nebhana), Bizerte (Nachrine, Beni Ata, Chaab-Ed-Doud), Cap Bon (Bezirk, Mlabi, Abdel Menam, Masri) Bessbessia, El Habibia, Zaghouan (Zriba), Mjez El Bab, Siliana, Djradon, Maktar and Aïn Draham. Natural and artificial water storage ponds, streams and artesian wells discharging water suitable or too saline for agriculture are found in or near the oases of Gabes, Gafsa, Nefta, Tozeur, Kebili and Douz. Several rivers are present, most of which are only seasonal. The River Medjerda is one of the most important. It originates in the Algerian mountain region, retains water throughout the year, and empties into the sea in the region of PortoFarina.

2.2 Culturable species

Although the fresh waters in Tunisia contain hardly any culturable species of fish, the lagoons and slightly saline lakes contain a number of them. The fresh waters harbour some species of cyprinid fish, especially of the species Barbus barbus and Phoxinellus spp. and although the former are sometimes eaten by the local people, their quality is considered very low. With regard to lagoons and brackishwater lakes, the important species of fish produced, with annual average production figures based on catches for 1968–70 inclusive, are listed below:

Much progress has already been made in different parts of the world with regard to the culture of mullets, and the techniques for the commercial cultivation of sea bass, gilthead seabream and eels are being perfected.

With regard to mullets, there were five known species in Tunisia up to 1972 (Heldt, 1948), namely Mugil cephalus, M. (= Liza) ramada, M. (= Liza) auratus, M. saliens and M. (= Chelon) labrosus. Of these, the first four are the most suitable for aquaculture, in the order mentioned, from the point of view of their adaptability to water of varying salinity and the availability of their fry and fingerlings.

In view of the importance of mullets for aquaculture in Tunisia, a key to the determination of species, prepared after Thong (1969) and Farrugio (1973), is given below:

Key to the determination of Tunisian species of mullet (the number of pyloric caecae and their arrangement is useful for the classification of fry and small fingerlings)

Among the other cultivable species available in Tunisia are the Caramote or royal shrimp, Penaeus kerathurus, the oyster Ostrea edulis and the mussel Mytilus edulis. Studies on the biology and the possibilities for culture of O. edulis have shown that it is less suitable for culture than the Portuguese oyster (Azouz, 1966) and Mytilus galloprovincialis is preferred to M. edulis in the market. Hence, these two species are not at present cultured. However, with regard to shrimps, a centre for breeding and cultivation of Penaeus kerathurus has been established in Porto Farina by a French firm in collaboration with INSTOP, and the first breeding attempts are scheduled to commence in spring 1975.

2.3 Fry and fingerlings of cultivable species

2.3.1 Mullets

Surveys carried out on the availability of natural mullet fry and fingerlings have indicated that Tunisia abounds in this resource. Although variable in abundance with the seasons, they are available throughout the year in the shallower areas of lagoons, lakes which have communication with the sea, estuaries, harbours and calmer regions of bays.

Mullets breed in the sea some kilometres off the coast of Tunisia, and the fry reach the coasts and enter lagoons and estuaries when they are around 15 cm to 30 cm total length, sizes corresponding to an age of one to one and a half months as found by rearing from the egg stage by induced breeding techniques (FAO Aquacult. Bull., 1970). In calm weather they could be seen during the day in shallow water of only 5–25 cm depth in bays, harbours, lagoons and estuaries, swimming in schools of a few hundred to even more than a thousand and feeding actively on the plankton. In rough weather and at night they move into deeper water of a metre or more. At dawn, in calm weather they could be seen in large schools of several thousands, under wooden piers and other sheltered places within lagoons and harbours. As the water warms up they break up into smaller schools and disperse for feeding in shallower water. On attaining a size of 4–6 cm they form smaller schools for feeding during the day and remain in slightly deeper water, making short visits to very shallow water and returning to deeper water at the sight of danger.

Locations where mullet fry and fingerlings can be obtained in great numbers include: two little harbours at the entrance to the navigation canal of Bizerte, Mezel Djemil (Bizerte Lagoon), Hergla Lagoon, Oued L'Akarit (very near the experimental fish farm), the fishery harbour in Gabes, the estuary of Oued de Gabes, the shallow sea area in the vicinity of Monastir Lagoon and Tunis Lagoon. There are many others but those already mentioned are sufficient to cater for a regular programme of mullet culture development for some years to come.

2.3.1.1 Methods of capture

The methods of capture employed with success are as follows:

Handnets provided with a metal rectangular frame, a wooden roller along the front edge and a handle at the other. Like the other nets used for fry and fingerling capture, they were fitted with mosquito netting. The method of capture consisted of locating a school of mullet fry in shallow water of about 10–50 cm depth, approaching it quietly with the net held in position to a distance of about 50 cm or even less, and suddenly thrusting it under the school by letting it run on the roller along the bottom. Once this technique begins to operate properly, very high catches can be obtained, up to 400 or 500 individuals at a time, depending on the size of the schools. Three men, two using a net each and the third assisting in the removal of fish can collect between 3 000 and 5 000 fry per hour. This method is, however, most suitable for sandy bottoms, on which the roller could operate without obstruction.

The second method consisted of using small dragnets of mosquito netting, approximately 2.0 to 3.0 metres long, 1.5 metre high and fitted with a simple head rope and a lead-weighted foot rope. Each net is operated by two men. Once a school is located, the men enter with the net poised above the water, approach it quietly to an operational distance of about one to two metres, lower the net suddenly and drag it vertically and quickly toward the school. Capture of schools by this method was often 100 percent, and best results were obtained when the schools were positioned between the net and the shore before final capture. The rate of capture using this method during late winter/early spring in the canal of Zarzouna at Sidi Salem (near the bridge) in Bizerte was often more than 20 000 fry an hour, including the time spent in the process of retrieving the fish alive from the net after capture. During winter and spring fry and fingerlings seek refuge in the calm waters of this canal by the thousands. On sunny days, as the water gets warmer they move out in schools into the adjoining harbour but return later in the day or in windy weather.

In the third method, shallow areas frequented by mullet fry, such as the vicinity of the oyster farm in Bizerte Lagoon, were partly encircled with staked small-meshed plastic netting, and when a significant number of fry had entered the trap it was closed and fish were collected with fine-meshed handnets.

The fourth method utilized fine-meshed liftnets. Although this technique sometimes gave good results, up to 400 fry per catch, the return per effort spent was not as good as in the other methods.

2.3.1.2 Seasonal availability of mullet seed

Periods when fry of a particular species are available naturally follow the breeding period of that species, with a certain amount of overlap between species with succeeding breeding periods. According to Heldt (1948) the periods of sexual maturity of mullets in Tunisia are as follows:

According to observations made by the author in Ichkeul Lake in 1972, sexually mature M. cephalus began to be caught in the bordigue on their seaward migration from 21 July and continued until 4 October, while M. capito commenced on 3 October and diminished to insignificant numbers on 10 November. This indicates there could be some annual variations in the commencement and duration of the breeding period of each species.

2.3.2 Eels

Surveys for young eels, both elvers and later stages, proved very encouraging. They were found in practically every lagoon and estuary surveyed, and at the entrances to small streams and rivers leading into lagoons; for example, those leading to Bizerte Lagoon along the route Tunis-Menzel Borguiba, during the period January through early spring. In day-time they remain hidden among masses of filamentous algae, soft bottom mud and detritus and actively move and migrate at night. They could be captured in considerable numbers using the gear described for the second method of capturing mullet fry in the preceding section, ensuring that the footrope passes under the masses of algae and disturbs the bottom mud. Although the first two or three trials in any area may sometimes prove negative, subsequent hauls often bring out the elvers after they have been disturbed from their hiding places.

With more importance being given to the commercial exploitation of eels in Tunisia, especially for export, further research and regular surveys need to be carried out to determine exactly the period when elvers reach the coasts of Tunisia each year and the best locations where they could be caught in large numbers during their nocturnal migrations into inland waters. As stated by Deelder (1970), after their long trans-Atlantic journey by current transportation the larvae arrive near the European coasts in autumn, remain around the 1 000 m depth line until metamorphosis into elvers, which takes a period of some months, and resume their landward migration in winter. Deelder also states the period when elvers strike land is strongly influenced by water temperature, and that in mild winters they arrive in Holland as early as January.

Once the period and locations where elvers strike land are reasonably well determined, special techniques could be used for their capture. According to Deelder (1970) the elvers, in their early stages of arrival in the vicinity of an estuary are repelled by strong light, but attracted by weak light. A faint kerosene light used by fishermen induces elvers to congregate at a short distance below the surface of the water where they are easily collected. The negative reaction to strong light could also be used to deflect elvers from their path of migration into fine-meshed nets waiting for them.

2.3.3 Sea bass (loup) and gilthead seabream (daurade)

With regard to fry and fingerlings of the culturable species Dicentrarchus labrax and Chrysophrys (= Sparus) auratus, they are not normally found in locations where mullet fry could be captured. They occur in deeper water, of around a metre or more, and are not found in large numbers. Specimens of both these species which are too small for the market are sometimes found in considerable numbers in the bordigue catches of Tunis and other lagoons, and they could be cultured to marketable size.

It could be said, however, that any large-scale culture of these two species would have to await the production of fry and fingerlings in steady quantities in special hatcheries, with the use of hormone techniques for induced breeding.

2.3.4 Culturable species of freshwater fish introduced to Tunisia

The species of imported freshwater fish which are definitely known to have established themselves in Tunisia are: the mirror variety of common carp (Cyprinus carpio) which is available at the freshwater fish culture station in Aïn Sellem (Beja) and certain lakes and reservoirs (e.g., Mellegue, El Habibia, Belverdiere Park) and two species of tilapia (T. nilotica and T. mossambica) in the oases and streams of the south of Tunisia. Of doubtful occurrence are brochet (Esox lucius) and black bass (Micropterus salmoides) in Bessbessia, tench (Tinca tinca) in El Habibia, rotengle (Scardinius erythropthalmus) in El Habibia and rainbow trout (Salmo gairdneri) in Aïn Draham.

With regard to carp and tilapia, techniques for the controlled breeding of the former have been successfully demonstrated in Aïn Sellem, and a very high production of Tilapia nilotica fingerlings has already been obtained in 1974 at the experimental centre in Kebili. The pond area and production in both these centres have to be progressively increased to keep pace with the growing need for fingerlings, both for aquaculture in ponds and a programme of intensive stocking reservoirs and certain lakes.

2.3.5 Shrimps

The sea off Tunisia is rich in shrimps of the culturable species Penaeus kerathurus, production of which rose from an annual average of about 400 tons to 1 000 tons in 1974. Unlike certain Asian species of shrimps which migrate in large numbers into estuaries and lagoons, the caramote, as the Tunisian species is often called, is mainly marine. Occasional specimens are captured in lagoons such as Bibans, Bizerte and even Tunis and Ichkeul Lake, but fry cannot be captured in sufficient quantities in these waters for purposes of aquaculture, and would therefore have to await production from special hatcheries, as the one that has been established in Porto Farina and is due to commence trials in 1975.

3. EXPERIMENTS ON FISH CULTURE IN PONDS

As fish culture in ponds is still in its very early stages in Tunisia, it is necessary that experiments be carried out on a small scale before embarking on any large-scale ventures, not only to determine its feasibility both from a technical and economical point of view, but also to learn the various problems that could arise, including those of management. The period could also serve for training technical and labour personnel in the various aspects of fish culture.

With these main aims in view, three different types of sites were selected for experiments, one using water from an artesian well which was too saline for agriculture, the second using water from natural springs and the third in Tunis Lake, an example of a lake with a high degree of sewage pollution.

3.1 Experimental centre at Oued L'Akarit in Gabes

This centre was established in 1973 and uses water discharged continuously from an artesian well. An artificially constructed pond of 1 ha and average depth 1 m was appropriately stocked with acclimatized mullet fry of the species M. cephalus and M. ramada, as well as common carp in June 1973. A second stocking was made in December the same year with acclimatized M. auratus fry.

The pond was fertilized with animal manure from dairy farms and supplementary feeds consisted of ground locally available grain, bread waste and fish meal. Ducks were raised in combination with the fish. There were no predatory fish in the pond as the only sources of water were the artesian well and some rain water in winter.

Of the three species of mullet stocked, M. cephalus showed the most rapid growth rate. Starting with a size of 2–4 cm total length, this species gained a maximum weight of 365 g in 15½ months, while M. ramada gained 260 g in the same period. In the case of M. auratus the weight gain was 170 g in 11 months. Carps reached weights of 1.5 to 2.5 kg in 15½ months, starting from 6–15 cm total length. The growth of all four species was found to be satisfactory, considering the fact that there was one winter season involved.

The production of mullet at this experimental station was estimated at 1 012 kg/ha/15½ months. In polyculture with carps, the total production was estimated at over 2 tons/ha/y. It was found that mullet would need a 24-month rearing period from the fry stage to attain a good marketable average size of 450–500 g.

In order, therefore, to obtain a regular annual harvest of fish from a pond system, one of two practices could be adopted. The first is of multiple stocking and selective harvesting. It consists of stocking the ponds each year with appropriate numbers of fry and small fingerlings to compensate for two-year old fish harvested annually with gillnets of determined mesh size. The second consists of a single stocking of ponds specially meant for rearing fish to harvestable size, with fingerlings of 10–15 cm total length and harvesting them completely at the end of a 12-month rearing period. This would either involve the capture of fingerlings of the required size from natural waters, or rearing them in special fingerling ponds for a period of 12 months from the fry stage.

The results of this experiment favour the expansion of this centre into a pilot-scale commercial venture of 5–10 ha to serve as a model for the utilization of other similar sources of water profitably for fish-cum-duck farming. In areas like these, measures would have to be taken to minimize seepage of water into the soil so that more water would be available for the increased pond area.

It is worth noting here that in the River L'Akarit, which lies a short distance away from the experimental centre and opens to the sea nearby, harbours literally hundreds of thousands of mullet fry and fingerlings during certain periods of the year, especially during winter months when there is not much flow of water in the river. Although they are seen at the surface of the water on calm days, they seek deeper water in windy weather. Special traps and liftnets could be used for capturing them for culture in the ponds.

3.2 Experimental centres at Kebili and Douz

Both these experimental centres were started in 1973; the ponds are fed with nearly fresh water from natural oasis springs. The centre at Kebili consists of four ponds totalling about one ha in area, while that at Douz, in the Tunisian Sahara consists of a pond of approximately ½ hectare. The ponds in both stations were stocked with carp of 6–15 cm in June 1973, and acclimatized mullet of the species M. auratus in December 1973. In addition, Tilapia nilotica was cultured in the centre at Kebili with the aim of producing fingerlings for stocking natural waters and ponds, not only in the Gouvernorat of Gabes but also in other parts of Tunisia.

The growth of fish in these two centres was found to be as satisfactory as in L'Akarit. In addition the carp stocked in both centres had bred naturally during spring 1974 providing a large number of fingerlings by the end of the year, and breeding of tilapia at Kebili resulted in several thousands of fingerlings.

Several artificial and natural ponds are found in the oases of Kebili (Bechri, Negga and Mansoura, for example), Gabes, Gafsa and Nefta, and they could be used as community or subsistence fish ponds for the culture of mullets, carp and tilapia. However, they would have to be regularly supplied with fry and fingerlings; the centre at Kebili could be improved and expanded and placed under the direct control of a trained fish culture technician for this purpose.

3.3 Experimental centre in Tunis Lagoon

3.3.1 Experiments on mullet culture in ponds

A small-scale experiment was carried out between spring 1973 and summer 1974 at La Goulette in the eastern part of Tunis Lake, where the water was less polluted than in most other parts of Tunis Lake (N.), in order to determine the suitability of the lake for pond fish culture. An area of water already separated from the rest of the lagoon was partitioned into two ponds of 1½ ha and ⅓ ha by the construction of two dikes in which sluices with wire screens were fitted to facilitate the interchange of water with the tides. The depth of water in the ponds varied between 0.5 and 1.25 m.

The pond was cleared of predators using rotenone and in spring 1973 stocked with mullet fry and fingerlings. The mullet grew satisfactorily, attaining total lengths of 6 cm in two months, 12 cm in four months and 30 cm in 15 months, from an original stocking size of 3–4 cm. Supplementary feeding consisted of bread waste mixed with fish meal.

It was found that the alga Ulva lactuca thrived and multiplied faster in the ponds than in the adjoining Tunis Lagoon (40 t/ha/y). Even with the help of four workmen with rakes and dragnets it was not possible to keep pace with the rate of production of the alga. During the summers of 1973 and 1974 Tunis Lagoon turned red and there were mass fish mortalities, whereas the water of the ponds remained clear. Towards the end of both these summers, however, water temperatures were high and adversely affected the solubility of oxygen in water; during windless nights considerable mortalities of mullet occurred in the ponds.

As stated earlier, well fed mullet take at least two full years to attain marketable size, starting from the fry stage. Taking the breeding seasons of the more important species of mullet into account, fish ponds have to be stocked during one winter and harvested two winters later. It could be concluded from these experiments that it would be risky to attempt any large-scale culture of mullet in ponds or even daurade or loups which are more sensitive to low oxygen levels, under the conditions which now prevail in Tunis Lake.

The other species of fish observed to succumb to low oxygen levels in the ponds in late summer was Cyprinodon fasciatus, a small hardy fish which multiplies very rapidly, competes for food and oxygen with the cultured species in ponds, and could therefore be considered as a pond pest in Tunisia. Application of rotenone to these ponds in December 1974, however, yielded several eels of sizes varying from 8–50 cm length which had entered the ponds at various times during the culture period, through crevices among the stones forming the dikes. There was no mortality of eels in these ponds during the adverse conditions which prevailed during the two summers under consideration, indicating that they were able to survive the temperature and oxygen conditions which were too unfavourable for mullet, and the possibility that these ponds could be experimented with for eel culture. For this purpose, however, the ponds have to be deepened to 1.75–2.0 m to minimize predation by seagulls, and the dikes have to be made escape-proof for the stocked eels.

3.3.2 The problem of Tunis Lagoon

3.3.2.1 Brief review of the problem

This is a lagoon which is constantly enriched with nutritive elements arriving with sewage discharged from the city of Tunis. It has been estimated that about 150 tons of dissolved salts and organic material is discharged into the lagoon daily with about 150 000 m³ of water. The lagoon has a total area of about 4 200 ha, but is divided into a northern part of 2 950 ha and a southern part of 1 220 ha (Björk, 1972) with a shipping canal running between them. Interchange of water between the sea and the two parts of the lagoon during tides is by two means: firstly, through a series of openings between the shipping canal and each part of the lagoon and, secondly, by an opening from each of the latter directly into the Bay of Tunis. The northern part of the lagoon receives the sewage on its western side; its direct communication with the Bay of Tunis is on the opposite side. Thus there is a decreasing gradient in the concentration of nutrients from west to east in the northern part of the lagoon. The southern part receives certain industrial pollutants on its western side. It is also indirectly polluted by sewage from the northern part of Tunis Lagoon via the navigation canal, especially from the west. There is a similar gradient in the concentration of pollutants from west to east.

Sea water generally contains small concentrations of nutrients, such as phosphates and nitrates, which have a limiting effect on primary production. According to O'Sullivan (1971) an increase of the phosphorus content by 1.0 mg results in the additional production of 75 mg of organic matter. Domestic sewage is rich in these nutrients, the daily contribution by man, according to Painter and Viney (1959) being about 2.0 g of phosphorus and 9.0 g of nitrogen per person (or 0.79 kg of phosphorus and 3.55 kg of nitrogen per person per annum). Although most of the phosphate and nitrogen in sewage originates directly from man, there is a high percentage of the complex sodium polyphosphates from synthetic detergents and nitrogen from storm water in the same sewage (O'Sullivan, 1971). The table below gives figures for phosphorus and nitrogen content of water in Tunis Lagoon as found by Björk (1972).

Phosphorus and nitrogen content of Tunis Lagoon water

Tunis Lagoon (north)

Tunis Lagoon (south)

Source: Björk, 1972

As expected, the discharge of sewage into the shallow Tunis Lagoon results in a tremendous increase in primary production which, in turn, presents opportunities for increased production through associated animal biological cycles. This eutrophication of the lagoon has its beneficial effects from autumn through early summer, with blooms corresponding to those of temperate seas during autumn and spring/summer. Unfortunately, this also leads to undesirable effects. The accumulation of surplus quantities of undecomposed organic matter within the lagoon, not only from increased bioproductivity, but also from the sewage that continues to be discharged, leads to secondary pollution. The decomposition of this organic matter, which becomes more intensive during summer, results in the periodic utilization of oxygen available in the water and liberation of hydrogen sulphide, to the extent of causing mass mortality of fish. An idea of the amount of oxygen utilized for such decomposition could be had from the fact that “for the complete oxidation or organic matter containing one atom of phosphorus, at least 150 molecules of oxygen are required” (Stum, 1962 In O'Sullivan, 1971).

The increase in primary production as a result of the high concentration of nutrients in Tunis Lagoon is brought about not only by phytoplankton, but also algae, especially Ulva lactuca and Enteromorpha. The phytoplankton forms blooms in autumn and spring, while Ulva, Enteromorpha and even Gracillaria reach their peak of production from spring to summer. During a good part of this cycle, oxygen produced by photosynthesis appears to be adequate for the biological requirements of the lake. The low temperature of the water also favours both the retention of oxygen produced by photosynthesis and that obtained from the atmosphere, the latter taking place more effectively when the water is agitated by wind. During calm sunny days Ulva and Enteromorpha are buoyed up to the surface of the water by bubbles of oxygen produced during photosynthesis, but in windy weather they are dislodged from under the algae and the latter therefore fall to the bottom. The wind also wafts the floating algae and concentrates them in certain parts of the lake leaving the areas originally occupied by them free for the production of more algae. Björk (1972) estimated the presence of floating algae (Ulva and Enteromorpha) in certain parts of the lagoon at 7 kg/m². In summer 1974 it was observed that about 1/5 of the total area of 4 220 ha of the lagoon had this production, another 1/5 had about 3 kg/m², 2/5 had about 1 kg/m² and about 1/5 had about 0.5 kg/m². This amounts to a total estimated production of 104 500 tons/y for the whole lake, or an average of about 2.47 kg/m² (24.7 tons per ha per annum).

During summer much of these algae reach sexual maturity and reproduce by means of sexual and asexual spores. This is also the period when the thalli normally begin to degenerate. In the meantime, the production of algae has also been increasing to a point where more oxygen becomes necessary for respiration and other biological processes than is produced by photosynthesis; the situation is aggravated by a gradual rise in water temperature and a consequent decrease in the solubility of oxygen. As the degeneration of algae increases anaerobic decomposition processes also increase, and varying extents of the lake turn red, depending on the intensity of the summer in any particular year. The accompanying anoxic conditions often lead to mass mortality of fish, estimes of which have been put at 75–80 tons worth about T.Din. 20 000. This catastrophe occurs each year. According to observations made by the author during the summers of 1972 to 1973, a fish kill need not take place even when most of the lake turns red, provided the days and nights are somewhat breezy. This is because oxygen from the atmosphere dissolved by wind action on water is sufficient for the fish to survive. The biological disaster, however, occurs following hot days and windless nights during pre-dawn hours.

Sawyer (1965) reports the occurrence of secondary pollution as a result of excessive growth of Ulva lactuca in the sewage-enriched waters of Boston Harbour, while Hanks (1966) gives an instance of fish mortality in Chesapeake Bay following the decomposition of excessive growths of the same alga in areas receiving domestic sewage. However, as stated by Burrows (1971), the mere presence of Ulva cannot be used as an indicator of sewage pollution since it can occur in large quantities in locations where there is no sewage pollution. Nevertheless, as in the case of Tunis Lagoon, Ulva does respond to increased amounts of nutrients from domestic sewage pollution, by growing in much greater abundance. Besides the presence of sufficient quantities of nutrients, conditions which favour the rapid growth of Ulva are: shallow depth of up to about one metre, favourable temperatures as normally prevailing from spring through early summer, and calm water of moderate salinity.

3.3.2.2 Measures currently adopted to solve the problem

Among the measures taken since 1974 to tackle the problem of Tunis Lagoon are:

1. mechanical removal of algae

2. pumping out of bottom sediments with suction dredgers, return of water to the lagoon after reduction of nutrients and, during periods of low oxygen levels, after aeration

3. treatment of sewage and circulation of the treated water in a one-way system which, with the operation of a system of sluice gates, permits water to enter the Tunis Lagoon from its eastern end and pass out from the western end, flushing out the treated water with it.

It was observed in 1974 that an amphibious mechanical collector for algae could not keep pace with the growth of algae within about 50 ha of lagoon area close to Tunis Marine. It is evident, therefore, that some more effective and rapid method of removing the algae will have to be found. Secondly, as pointed out by O'Sullivan (1971), even when sewage is treated before discharge, it is in most cases only to remove settlable solids. Even where full biological treatment is involved, it “merely oxidizes the organic matter and does little to remove phosphates or nitrates from the effluent”. Efficient one-way circulation and flushing out of this nutrient-rich effluent would undoubtedly diminish the production of Ulva and Enteromorpha, but their production in significant quantities is nevertheless likely to continue. The critical period for fish, with regard to oxygen, could arrive any time from about mid-July to September depending on variations in summer conditions from year to year. Circulation of oxygen-bearing water from the sea during this period would go a long way to avoid or diminish fish mortalities.

3.3.2.3 An experiment on the control of Ulva and Enteromorpha by alteration of environmental conditions

Although too early to draw a conclusion of wider significance, it is worthwhile describing an experiment carried out to control Ulva by altering salinity conditions. The principle employed was to bring about firstly the elimination of Ulva and Enteromorpha by a process similar to that seen in some enclosed saline lakes which have luxuriant growths of aquatic plants during favourable periods of the year, but which are eliminated as salinity rises with evaporation of water in summer and, secondly, to grow more desirable flora for fish culture.

While carrying out the mullet culture experiments in ponds in Tunis Lagoon, as described earlier, an adjoining area of water of approximately 20 ha was completely closed off from the lagoon by constructing dikes across the two small existing connexions. During summer 1973 evaporation of water raised the salinity to about 80 ppt and destroyed the existing Ulva and Enteromorpha and apparently their spores as well. With the arrival of the winter rains the same year, the salinity gradually returned to normal and a luxuriant growth of phytoplankton (mainly nannoplankton) was produced, but no Ulva or Enteromorpha.

The pond was stocked with mullet fry on 2.5 to 3.0 cm length in early 1974, and their progress was observed in comparison with mullet in the two experimental ponds which had Ulva in them. From spring through summer the phytoplankton continued to bloom in the 20 ha pond, giving the water a rich greenish colour, but there were no signs of the larger algae. Meanwhile, the salinity had been gradually rising to about 65 ppt in early summer, but the mullet appeared to be in good condition, having reached an average total length of 8–10 cm in 6 months. In order to keep the salinity favourable for the fish, a canal of about 3.5 m width and 0.75 m depth was dug between this pond and the adjoining lagoon, and fitted with plastic screens to prevent the escape of fish. For about a fortnight the flow of water was only into this pond to make up for water lost by evaporation, after which levels were equalized and interchange of water with the tides was established.

The comparison of dissolved oxygen values in this pond with those of the other two experimental ponds during summer proved very interesting. While in the latter they dropped to zero at dawn during certain days of late summer and resulted in mortality of mullet, values dropped only to about 6 ppm in the former at the same hours, and the fish remained quite normal. In daytime, during the same period, oxygen values were around 14 ppm in the 20 ha pond, while in the other two they rose only to about 5 ppm.

Ulva and Enteromorpha had not established themselves in the 20 ha pond upto February 1975; this is apparently because their spores or fragments of thalli initially fix themselves or rest on the bottom after they enter the pond with the water supply, but cannot grow further, owing to the plankton bloom above them which adversely limits light penetration to the bottom. The larger algae cannot gain a foothold in the pond once dominated by the phytoplankton bloom.

It would be interesting to follow up the experiment for a few more years, in the meantime stocking the pond with more mullet. Should the larger algae tend to re-establish themselves in appreciable quantities, it would be possible to eliminate them by closing up the connexion between the pond and the lagoon and letting the salinity rise during summer.

4. PROBLEMS RELATED TO POND AQUACULTURE

Although from the point of view of available fresh and brackish waters there is a good potential for aquaculture in Tunisia, there are some important problems that have to be taken into account before embarking on such a venture.

4.1 Excessive growth of Ulva and similar algae

The problem of Ulva and other algae in Tunis Lagoon has been adequately described in the preceding sections, but it is worth keeping in mind the fact that they could cause similar problems in other lagoons such as Bizerte, Porto Farina, certain parts of Bibans, and even parts of bays, given the calm and relatively shallow environment of a fish pond.

A way to avoid this problem is to give preference to pond fish culture near lakes where the salinity is too low for the growth of these algae, as in Ichkeul Lake, for example. Mullets and eels can be cultured in fresh water, while common carp can be cultured in waters up to about 10–12 ppt salinity. As in Oued L'Akarit, Kebili and Douz, ponds for these species could be fed by fresh or brackish water from artesian wells and natural springs. There is also scope for construction of ponds fed with water from freshwater reservoirs.

4.2 Insufficient tidal range for completely draining ponds built in lagoons

It would be desirable if ponds constructed along the borders of lagoons could be completely drained at low tide, to facilitate complete harvesting of fish, eradication of predatory fish and treatment of the pond bottom with fertilizers for improving production. The tidal range in the lagoons of Tunisia, however, is too limited to permit this. The bottoms of ponds would therefore be around 1.5–1.75 below the mean tidal range in the area, and once filled cannot be emptied except by a costly process of pumping. Hence harvesting of fish will have to be by the use of appropriate nets and eradication of predatory fish will have to be done by netting or use of fish poisons.

4.3 Predators

Predation is an important problem that has to be taken into account with regard to pond fish culture. There are several species of migratory birds which seasonally invade lagoons, lakes and reservoirs by the thousands and take their toll of fish. Memmi (1970) gives estimates of the most abundant populations of birds visiting Tunis Lagoon, and an instance of over 100 juvenile mullet being consumed by a single individual in a day. Although most of the species listed would not normally visit fish ponds in the presence of an active guardian, seagulls and wild ducks could be quite persistent. The author has observed seagulls repeatedly feeding on eels 15–20 cm long in Tunis Lagoon. Cormorants are present in Tunisia and could visit fish ponds. According to Deelder (1970) they may take a heavy toll of eels as they are able to dive, the sizes caught ranging between 18 and 60 cm. Effective measures will have to be used to prevent or reduce predation by these birds to a minimum if a good harvest of fish is desired.

There are also predatory fish such as sea bass and eels that could enter the ponds and feed on cultured species such as mullet, especially when the latter are in their fry and fingerling stages. Although the sea bass could be avoided by efficient screening of water before it enters the pond, it is not always easy to prevent a more important predator, the common eel, from entering. Where the dikes are made of stones, they could enter at any size from the adjoining lagoon through crevices between the stones. They could also enter through spaces between the sluices and the dike, if the former is not properly installed. It was possible to make direct observations on predation of mullet by eels in the experimental ponds in Tunis Lagoon. It was found that adult eels take 6 to 18 hours longer than mullet to be killed by rotenone, depending on water temperature and the concentration of the chemical. Mullet fingerlings of 5–10 cm length, which were stunned by the chemical and swimming in an abnormal manner, were preyed on by eels of 40–50 cm length. The effect of predation by eels in fish ponds is illustrated by the observation that 10 adult eels in a pond stocked with 5 000 fry or small fingerlings would leave hardly any fish after 50 days; the rearing period for mullet from this size to harvestable size is about two years.

In order to prevent the entry of eels into fish ponds, the construction of dikes and installation of sluices must be such that there are no spaces and crevices which could permit their entry. Secondly, piscicides must be used to eliminate predators before stocking a pond with fry or small fingerlings. Thirdly, the water must be well screened before entry into the ponds. A method of minimizing predation is to stock ponds with mullet fingerlings of over 10 cm length, these in turn having been reared in ponds free from predators.

With regard to piscicides, although rotenone has been tried successfully it has to be imported. It would seem more economical to use other piscicides, such as locally available tobacco dust instead. After its initial action as a piscicide it acts as a pond fertilizer.

4.4 Damage possible during rough weather

Another important factor to contend with in fish culture in ponds and cages is the very rough weather which could sometimes prevail in lagoons, lakes and water reservoirs, especially during autumn and winter. Unless dikes, wire screens and floating cages are of sturdy construction, they are likely to be damaged sooner or later resulting in the escape of stocked fish.