WORKSHOP ON EAST AFRICAN RESOURCES - POLICY MANAGEMENT IN COASTAL ZONES
By
S. Sivalingam
AQUACULTURIST
Government of Kenya/UNDP/FAO Pilot Project
“Development of Coastal Aquaculture”
P.O. BOX 583, MALINDI
OCTOBER, 1981
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STRUCTURES FOR MARICULTURE OPERATIONS
REQUIREMENTS FOR COASTAL MARICULTURE DEVELOPMENT IN EARTH PONDS
1) Water supply
2) Soil Conditions
3) Seed for Stocking
CULTURE METHODS IN EARTH PONDS
1) Extensive
2) Semi Intensive
3) Intensive
POSSIBLE LOCATIONS FOR MARICULTURE DEVELOPMENT ALONG KENYA COAST
SUMMARY
The paper stresses the need for mariculture to help increase food production from areas that are not used for other purposes at present. Different structures in use in other countries for mariculture purposes are examined and it is suggested that earth ponds are the most suitable in the present stage of development. For earth ponds the requirements namely water supply, soil conditions and seed for stocking are discussed. The three methods of mariculture extensive, semi-intensive and intensive are described and it is recommended that in the initial stages it is preferable to commence with the extensive system and then to progressively move into semi-intensive and then finally into intensive methods.
Considering the environmental factors it is pointed out that mariculture farms are in the inter tidal zones and does not interfere with normal fishing activity and the salt industry. In most of the areas suitable for ponds, mangrove is sparse and shrubby and does not affect mangrove exploitation seriously. It is also shown that collection of juveniles for stocking the ponds in the early stages of mariculture development would not seriously affect the penaeid stock and finally goes to stress that mariculture would help increase food production harnessing solar and tidal energy and create employment in rural areas.
Mariculture activity is new to East African Coast and realising that the potential exists along most parts of the East African Coast this paper examines in detail the possibilities of aquaculture and results of recent biological surveys with particular reference to Kenya Coast.
Mariculture is an activity involving food production for human consumption. It is an activity in which aquatic organisms both plants and animals are cultured in a confined environment in the aquatic medium which may be completely marine or marine mixed to various degrees with freshwater in the brackishwater areas.
It is accepted that with world population increasing, the food available per head is diminishing and it is therefore necessary not only to intensify production from existing areas but also to find additional areas for food population. If the experience in the eastern countries is any guide it will be seen that the coastal swamps can be profitably diverted to food production and in this process gain other side benefits such as creating employment in the rural areas and at least to some extent diminishing the treck to urban areas with its associated problems. It has also been shown that production from a unit area of a controlled environment is much more than from open natural waters.
Four main types of structures are in popular use at present:
- Floating cages
- Net enclosures
- Earth ponds
- Constant water circulation systems
In the open sea or large sheltered bays floating cages are used for culture purposes. Other structures are not practicable because of constant wave action. But for culture in cages because of high cost of cages it is necessary to maintain a large number of individuals inside the cage to make the venture profitable. This in turn necessitates supplementary feeding since the quantity of natural food available in the volume of water inside the cage cannot sustain the large number normally cultured in these cages. For this type of culture practice to get popular it will therefore be necessary to first develop inexpensive cages and also fish feed at reasonable prices and understandably as a result, this method has been slow to develop in developing countries.
Net enclosures barricading off large areas in sheltered bays are being tried on a commercial scale in some countries. But this requires considerable capital outlay and frequent replacement as a result of corrosion. In addition, supplementary feeding is essential which requires higher running costs - but it does pay its way that is why some private farms are existing in developed countries.
For inland brackishwater areas where the tidal range is adequate, experience in developing countries in the east has shown that for mariculture, earth ponds constructed to impound spring tide water is the most suitable structure. This can be done without much difficulty in areas where the land is more or less flat or has slight slope and the nature of the soil is satisfactory. The construction of earth ponds and use of tidal water encourages the use of manual labour for construction and harneses gravitational force in the form of tides for the water supply. It would not only be independent of any foreign imported energy source but also reduce the imported material component to negligible quantities.
Constant water circulation units are popular in some developed countries. These are large cement structures. In addition this requires continuous pumping of water in large quantities and also supplementary feed. Thus this system requires not only a heavy capital outlay but also high recurrent costs. This proves economical where a high price can be obtained for the end products and adequate raw material for feed is available.
The most common source of water supply for coastal ponds is the tide. For this the tidal amplitude should be adequate for frequent changes of water in the ponds. For good results it is best to have 75 cm to 1 m of water above the ground at high tide and at low tide the water level should go down at least 75cm below the existing ground level in order to facilitate complete draining of the ponds. While these are the desirable levels nevertheless successful operations are carried out in areas with much lower tidal amplitude.
It is essential that the water supply should be polution free and preferably of pH ranging from 6.5 to 8. The depth of water required inside the pond would depend on
as to whether it is to be
an extensive system i.e. depending entirely on the natural primary production within the pond for food or
semi intensive using fertilizers or
intensive system where the organisms cultured would be given supplementary feed.
For earth ponds availability of soil of good water retaining capacity is important in order to prevent loss of water by seepage specially when it is proposed to use fertilizers to increase production. It is also essential that the pH is within the desired limits as for the water given above. In addition acid sulphate soils should be avoided.
Availability of adequate quantities of seed of the species to be cultured is an important factor. If adequate quantities are naturally available it would reduce the cost of production considerably. Hatchery production of seed is being successfully carried out for some species in other countries. This may be considered only when the natural supply is inadequate and demand for hatchery produced seed is adequate.
There are three main methods, which also represents gradual development in complexity both in management and financial outlay and of course coupled with higher production. They are:-
This is the simplest form of culture where no fertilizer or supplementary feed is added and the production from the pond depends entirely on the primary production which in turn depends on sunlight and nutrients available in the water and soil. The stocking density will therefore have to be low and the recurrent costs are the lowest. For this method, in the case of mullets, milkfish and other herbivores or penaeids it is essential that the water level be maintained between 35 and 40cm in the general platform area of the pond to encourage algal growth on the pond bottom for the organisms to feed upon. If the water is deeper the light intensity that reaches the bottom is not sufficient to promote algal growth. In the tropics if the water level is reduced below 35 or 30cm then the temperature is likely to increase and may reach lethal limits. In rural areas of developing countries this method is popular mainly due to the low financial outlay required for the operation. This method is also useful in the initial stage of mariculture development in developing countries where it is still new and trained manpower is inadequate. Since an initial training phase is necessary and during this phase improper operations can lead to loss of an entire harvest. Extensive systems because of the low financial outlay can better withstand these shocks than the other two systems. In addition adequate experience with the basic system is very helpful in understanding the more complicated systems and helps in mastering the complicated system faster.
The next step towards increased production is to stock at a slightly higher density and then to add fertilizers (organic or inorganic) and in turn increase the harvest from the pond. The function of the fertilizer is to increase primary production within the pond which starts the chain reaction and ends in increased production from a unit area of the pond. This method not only requires a slightly higher financial operational cost, but also involves additional labour for the collection, storage and application of fertilizer. In developing countries, experienced farmers often fertilize their ponds because of the increased profit margin.
In this method supplementary feed is added to the pond with or without the use of fertilizers. The stocking rates would be much higher and also the depth of water in the pond greater than in the former cases because the food for the cultivated organisms is not dependent entirely on primary production within the pond. However, feed is expensive and as a result the operating costs are much higher. The higher rewards also carry with it greater risks. Any unforseen calamity can wipe out a big financial outlay. For this reason and considering the heavy financial outlay required, at the initial stages where coastal aquaculture is still in its infancy and experience is inadequate, commencing with an extensive system would involve much lower risks and establish greater confidence.
Kenya Coast is blessed with extensive lagoon systems. Those at Mida, Kilifi, Mtwapa and Mombasa are large inland water bodies opening to the sea by a narrow entrance. They are subject to tidal influence and edged with a mangrove belt. The width of the mangrove belt varies from place to place. All these except Mida lagoon have rivers opening into them but the rivers flowing into the Kilifi and Mombasa lagoon are perennial while that at Mtwapa is more or less seasonal. For Mida lagoon, there is hardly any fresh water supply other than the run off water from the surrounding area during the rains. The quantity of water flowing into these lagoons is not large enough to cause wide fluctuations in salinity except in the area where the river joins the lagoon. The fishermen in these areas catch mainly inshore marine varieties like mullets, hemiramphids, few crustacea etc. Mangrove cutting is quite popular and the mangrove timber is used for house construction, firewood and probably for export to the middle eastern countries. Mangrove cutting is regulated by the government. Although oysters are found in some areas of the lagoons there are no established fisheries for the oysters.
Around Lamu area in the northern and Shimoni area in the southern end there are large numbers of creeks and small river estuaries with brackishwater conditions. Fishing is quite popular in these creeks. But between Malindi and the Tana river the lagoon system is formed by a ramification of creeks and open bays opening to the sea by narrow entrances. Fishing is popular like in other lagoons. Mangrove is also present.
The intertidal zone around all these lagoons are potential mariculture areas and needs detailed investigations to confirm their suitability. Recently detailed investigations were carried out in the Mida creek and Ngomeni areas to site a pilot project. In these two areas the tidal range proved adequate and water quality satisfactory. There was no sign of any polution risk and in both cases there is no influx of any fresh water. The water is entirley from the sea. But of the two areas the soil conditions in the Ngomeni area proved to be better and the pilot project to test the feasibility of coastal aquaculture has been sited there. Parallel with the construction activities a biological survey program for the availability of prawn and mullet seed was initiated in the same area. It has been found that juveniles of the three commercially important penaeids are available in the open lagoon area. Of these P. indicus forms about 85% of the penaeids while the jambo prawn P. monodon and the third species M. monoceros form only 15% or less. Of these three, the jambo prawn P. monodon although it grows to a much larger size, it takes a much longer time about nine months or longer to get to the required size. In addition the tendancy for the salinity to increase with high evaporation is not favourable to the growth and survival of this species. But P. indicus can not only be obtained in large quantities but also can withstand higher salinities. In addition, even though the harvested sizes are much smaller than P. monodon, it is harvested in a much shorter time i.e. about 3–3½ months. For these reasons at present, efforts are concentrated on P. indicus and this would probably be the species for culture in the Ngomeni area. A twelve month study has indicated that juveniles are available all the year round. This is not very usual and to be assured that this is generally true, the study is being continued for a second year. Regarding availability of mullet seed, it has been found that they are available throughout the year in the Sabaki river estuary and at least during certain seasons in the Ngomeni lagoon.
The availability of the juveniles of penaeids and mullets throughout the year is a great help in improving the viability of mariculture. This permits a year round operation of the farm. In most other parts of the world penaied culture is seasonal, which reduces the annual output. Taking all factors into consideration Ngomeni area appears very suitable for mariculture purposes. Detailed investigations of other potential areas should be carried out to decide on their suitability.
Mariculture in earth ponds is limited to the intertidal zone and generally to areas where the water depth is not more than a meter at spring tides.
In these areas suitable for mariculture no fishing is carried out and mariculture does not interfere with the normal fishing activity. But inthese areas in most parts of the Kenya coast mangrove vegetation is abundant. The mangrove is of various sizes. In some areas like Mida Creek the mangrove is quite dense and is cut regularly for firewood, housing and fencing purposes. Where the vegetation is extensive and of heavy timber, construction costs of earth ponds are expensive and this should be taken into consideration before selecting the areas for mariculture development. It is yet too early to comment on the comparative revenue per unit area for mariculture and mangrove. The results of the pilot project which is underway would give a reasonable comparison. In other areas for instance north of Malindi, probably as a result of semi arid conditions the mangrove vegetation is sparse and as we go further north from Malindi, gets very shrubby. Because of the poor quality timber mangrove cutting is verylimited near Ngomeni and nil as we go further north. But in areas like the Tana River estuary in the fresh water areas it is mostly tall grass swamps which can be replaced by mariculture farms.
Assuming mariculture production is more profitable it is important to consider the effects of complete removal of mangrove from the area for the construction of ponds. In its normal cycle, the mangrove grows taking in nutrients from the ground and water and produces leaves, stem and roots. The leaves in turn fall into the water and decomposes enriching the water. The stem is often removed by man. The roots like the leaves decompose and enriches the ground and water. When alive, the mangrove leaves transpire and increases the humidity of the air. When the mangrove vegetation is replaced by a mariculture farm, high spring tide water is trapped in earth ponds and retained there to encourage the growth of a carpet of algae on the pond bottom and phytoplankton which in turn increases the production of crustacea, annelids, rotifers etc. Which serve as food for the organisms cultured in the pond. In other words construction of ponds for extensive and semi-intensive systems do trap solar energy which finally ends up as animal protein for human consumption. In addition the water from the ponds continues to evaporate in place of the moisture that transpires from the mangrove leaves.
Other industries that are located near the areas suitable for mariculture are the salterns. But since they depend on full and rapid evaporation for which drier the soil conditions the better, they select areas at a higher elevation than what can be used for maricukture purposes and there is no competition for the same location.
In the early stages of mariculture development it is advisable to depend on the seed available in the natural waters without having to start off with a hatchery. Fears may be expressed that collection of such large quantities of post larvae and juveniles may affect the natural stocks and the normal commercial fishery both in the lagoons and in the open sea. Penaeids like most other marine fishes spawn large numbers of eggs - P. japonicus is known to spawn 300,000 to 400,000 eggs each time and more than once each season. But finally a pair seems to survive to maturity to spawn again. Otherwise their population would be on the increase. By numbers the percentage exploited by the commercial fishery is small compared to what is lost to predators on the early stages of development when they fall an easy prey. The mariculture activity at least during the early stages of development i.e. in its infancy with a small acreage would only be competing with the predators. Perhaps some idea of the degree of predation can be obtained from the fact in a recent experiment in a pond of 1.1 ha area where there were no predators except for about ten young snappers. When this pond was emptied excluding those which could not be collected with 5mm by 5mm mesh hand nets and what could go through 4.5mm by 4.5mm metal screens at the outlet gate an estimated 72,000 four to six weeks old juveniles survived. It is difficult to estimate the number that couldn't be collected or escaped through the outlet gate screen and all these entered the pond with the spring tide during the exchange of water in the pond and survived because there were no predators. But such a large number is very rare in the same area under natural conditions where predators are present. This gives a better picture of how much is lost to predators under open natural conditions.
Finally it may be said that what mariculture practice does is to provide a sanctuary for the wanted species from their predators and to cope with increased survival rate promotes increased primary production to end up as food for these cultured organisms harnessing solar energy in the form of sunlight and gravitational force in the form of tides. Thus mariculture would help increase food production and create employment in rural areas.