Pond and raceway farming consists of fattening high value fishes in artificial impoundments. Establishment of such on-shore mariculture production units, mainly in the form of intensive pond culture, is the principal focus of current official planning for Libyan national aquaculture development.
Small-scale pond operations can be conducted on sites of less than one hectare (see Figures 1–3, Annex 4). In large-scale operations each pond can be up to one hectare and the total farm complex can have an area of more than 100 ha. (Figure 4, Annex 4). The stocking rate of ponds is about 2 fishes/m2. Thus some two million fingerlings would be needed per stocking cycle for a 100 ha pond complex -- a quantity that might begin to make the installation of an independent hatchery facility something to consider.
Raceway culture does not need extensive areas of land, (see Figures 5–7, Annex 4). Even large-scale raceway complexes can be accommodated on five to ten hectare sites. The limited size of raceway fattening systems is compensated by a continuous water circulation which allows stocking densities of 15 kg/m3 by the end of the production cycle, and also ensures automatic maintenance of oxygen levels and evacuation of solid and dissolved wastes. In intensive raceway culture water may be exchanged at rates of once every hour or less, and also receive conditioning with aeration devices.
The main advantage of large farms is to reduce costs per unit of production through economies of scale in investment for infrastructure (e.g. sea water supply) or in operational costs (e.g. hiring specialised staff, purchase of feed and fingerlings). But pond and raceway farming at whatever level of magnitude requires very close and careful management in order to be profitable.
4.1.1 Water supply
Direct pumping from the sea is the most common way to obtain water in proper quantity for pond and raceway culture systems. The cost of installing and running a suitable water supply system is considerable in itself and requires very carefully planning to avoid inflating investment costs even more. Placing a suction pipe at sea may require the assistance of a boat and divers to manoeuvre heavy block anchors, and contractors to build the pump house and fix the machinery.
It is a generally recommended practice to have a complementary supply of water from a borehole or very deep well situated close to the shore. This requires additional investment for drilling and installation of submersible pumps, and may be subject to limitations in terms of yield and salinity levels different from seawater. It offers some important advantages, however, in that the quality of water is stable all year round (low turbidity, constant salinity and temperature). Availability of a complementary water supply may be particularly advantageous for hatchery operations.
Borehole water tends to be steady in the range of 20 – 22°C. This is extremely important in aquaculture because the growth of fish/shrimp depends to a great extent on the water temperature. The optimum is around 22 – 25°C. Below 18°C, the rate of growth is reduced, whilst high temperatures (28–30°C) may provoke problems linked with oxygen consumption or diseases. In ponds, it is difficult to control temperature because water is added only to compensate evaporation and seepages. But in raceways, because water is always circulating, temperature remains close to that of incoming flow. If borehole water is available, temperature inside the raceways can be adjusted close to the optimum during the entire fattening cycle. As a consequence, fattening sea-basses or breams to a commercial size (300/400 g) would require around 18 months, whereas it would take about 30 months in ponds with uncontrolled temperature.
Other points to take into account with regard to water supply include the following.
Sea water is a corrosive liquid and all equipment and piping exposed to it should thus be made of resistant material. Furthermore, metallic salts produced through corrosion of equipment are toxic for marine animals.
The sea is often rough, and suction pipes and pumps should therefore be protected from the waves. Intakes should be located in places without influence from waves, so that turbid water is not sent to the farm. Because surface water is liable to extreme changes of temperature which can affect fish, it is recommended that water be pumped from places below the thermocline.
Biological purification occurs in ponds and there is no need for constant replacement, but the level should be adjusted every day to compensate for seepage and evaporation. A rate of evaporation of one cm/day represents a volume of 100 m3/ha which should be replaced to avoid an increase of salinity.
Drainage water could be used to operate salt cum Artemia production units to improve the economical productivity of the farm (cf. Medina Pizzali et al. 1994).
In raceways, water must be circulated and renewed (once every hour in intensive systems) in order to eliminate wastes and provide oxygen.
Current patterns of land use and ownership, topography, and soil types are all crucial points to bear in mind when deciding on the location of shore-based aquafarms.
Availability. Is land actually available, and if so what is the form of ownership? Sites for fish farms ideally should be selected in such a way that there is no competition with agriculture. It should be stressed that marine aquaculture cannot be conducted near cropland if destruction of soil and pollution of ground water by salt is to be avoided. Aquaculture ponds or raceways preferably should be sited only on marginal land not suitable for crops. Furthermore, if suitable sites exist, are they on public or private lands?
Topography. What is the terrain for a proposed site? Aquaculture pond and raceway sites should be flat in order to avoid heavy development costs for levelling. In this respect it should be remembered that ponds and raceways are generally built up as dyked structures rather than dug out as holes in the ground. It is essential that water is fully controlled (inlet and drainage) during the entire rearing cycle. This is usually achieved through pumped water charge and gravity discharge. Drainage flow by gravity is obviously less costly than by pump.
Soil. What is the soil profile? Sites selected for constructing large ponds should have sandy-clay soil which retains water and can be compacted for constructing dykes. Sandy soil is suitable for small ponds with liners and for concrete tanks. Porous sandy soils do not present obstacles to raceway installations since these are lined with plastic or concrete. In some cases aquafarming could represent an alternative use for agriculture land that has been wasted due to salt accumulations caused by overpumping of groundwater and seawater incursion into the water table.
Lagoons constitute a rather loosely defined category of shallow coastal embayments-enclosed or semi-enclosed marine water bodies - that lie adjacent to the sea behind barriers of sand or other sediment. Connections to the sea are restricted, and may not always be regular or permanent. In some cases interior-exterior water exchange may occur only under specific conditions of wind or tide (cf. Ardizzone et al. 1988; Miller et al. 1990).
The Libyan coast features only a few sites that can be considered as lagoons having any significant (> 50 ha) surface area (Kerambrun 1986). These are Farwa (close to the Tunisian border in the west), Ain Zayanah (just east of Benghazi), Khalij Bumba (ca. 60 km by road east of Darnah), and Ain El Ghazala (ca. 110 km by road east of Darnah). Smaller ‘estuarine lagoons’ or ‘estuarine ponds’6 are found impounded behind sand bars at the mouths and lower reaches of a number of narrow wadi channels along the coastline, particularly in the eastern zones. Most are of a very limited size - usually not more than a few hectares in area.
4.2.1 Traditional lagoon fisheries
Traditional fisheries exploitation of lagoons is associated with two major types of activities -- viz.:
exploitation of inside waters for finfish (gillnets, hooks and lines, traps), eel and octopi (trapping), and shellfish (collection); and
exploitation of waters at lagoon mouths or immediately outside for fish and shrimp migrating between the sea and the inside waters
4.2.2 Production enhancement
Under certain conditions the natural production of lagoons can be improved through environmental modification, but extreme care must be exercised in deciding on whether such modification would be appropriate or not. (Kapetsky and Lasserre 1984; Kapetsky 1985; Ardizzone et al. 1988; Miller et al. 1990). Various techniques are possible.
Extensive management entails minimum intervention and manipulation of fish stocks: the natural food chain is not altered, but there may be control of human activities around the lagoon, stocking of particular species, selective catches (increased fishing effort on predators, protection of small fishes), etc.;
Hydraulic controls and traps: improvement of water flows and levels within lagoon areas on the Italian ‘Valliculture’ model, or construction of traps (‘bordigues’ or drina as common to Tunisian lagoon fisheries;
Conchyliculture: cultivation of shellfish can be done directly on the bottom of the lagoon in shallow places or using poles or rafts at deeper sites; and
Cage culture: cages can be used to stock small fishes obtained through capture or from hatcheries, which are then fed on a balanced diet until they grow to a marketable size.
Interventions of one form or another have already been made at all three of the main Libyan lagoon sites. A sluice gate has been installed to control water flows at Ain Ziana, cage culture production units are under trial at Ain El Ghazala, and mussel cultivation experiments have taken place at both Ain El Ghazala and Farwa.
Evaluation of the carrying capacity of a lagoon being considered for production enhancement activities is a first requirement. It is essential to know the limits of the ecosystem in terms of the total biomass that can be managed over sustained periods of time. In principle operators would seek to maximise production of marketable animals to increase their profits. But, if too many fishes or shrimps are stocked in a lagoon without being able to migrate to the sea, growth can be retarded due to limited availability of food. Another hazard is eutrophication, which can arise under conditions of high nutrient load coupled with warm temperatures and which leads to planktonic blooms followed by mass mortality, increase of bacterial activity, deoxygenation, and eventual total mortality of cultivated animals.
Important determinants of carrying capacity include size, depth, and water circulation.
Lagoons with surface areas of less than 500 ha are generally not economically viable as production does not offset development and operation costs. The latter are mainly associated with opening and maintaining passes to the sea.
Depths of at least one metre are recommended. Physico-chemical parameters are extreme in lagoons. In very shallow places, conditions of salinity and temperature are not suitable for most fish species. Depth should be sufficient to keep these parameters within acceptable range.
Open communication with the sea (already existing or potential) must also be taken into account. This is essential for the feeding and/or breeding migrations of aquatic animals being exploited, and also allows them to escape periods of extreme physico-chemical conditions (salinity, temperature, dissolved oxygen, etc.) in lagoon waters. Passages to the sea also are necessary for water circulation. The flushing actions of currents keep physico-chemical parameters of inside waters within suitable tolerances for animal life.
Wetlands and marshes, by definition, are not drainable. This poses a major obstacle insofar as aquaculture development is concerned. For this reason alone such sites are generally not suitable for fish farming. Natural temperature, salinity, and pH conditions in these environments tend to be extreme, and the investment costs for installations to allow for adequate water exchange and control would likely far outweigh any production earnings.
4.3.1 Feasibility and conservation considerations
Although it may not amount to aquaculture in the strict sense, in theory it is possible under certain circumstances to improve access for breeding or feeding migrations of fishes in and out of marsh and wetland sites by modifying or enlarging links to the sea. The aim would not be to allow fishing inside marshes but to promote a gradual improvement of marine fish catches in adjacent open waters through increased recruitment of juveniles. From a practical standpoint it is highly doubtful that measurable gains could be achieved through such actions in the Libyan context, where true estuarine and wetland zones are extremely limited. Furthermore great care would have to be taken in order to avoid damage to the extremely sensitive environments involved. Otherwise, any effect on fish productivity would be purely negative.
A variation of the above approach would be to open links to the sea through the sand and sediment barriers at the mouths of selected wadis, particularly of the sort that are common east of Darnah. Such interventions could conceivably be of some minor value during the winter breeding migration periods of mullets, but would probably be very difficult to implement because the work of keeping open passages to the sea would require constant attention and expense.
In many cases the ‘best use’ of salt marsh and wetland sites from an economic and environmental point of view is their ‘natural use.’ That is, it is better to protect them as wildlife sanctuaries for conservation, scientific, tourist attraction, or similar ‘non-consumptive’ purposes, rather than ‘improving’ them with low-yielding and costly aquatic farming developments. Marsh or wetland sites along the Libyan coast which have already been officially designated or warrant consideration for designation as coastal reserve areas are indicated on Maps 2 – 4 in Annex 1.
4.3.2 Sites below sea level
Three major areas of geographical depression (land below sea level) exist around the Mediterranean Sea, namely: the Dead Sea, the Short El Djerid (Tunisia), and various sabkhat along the coastline of the Gulf of Sirt.
An apparent immediate cost advantage that such sites offer for aquafarming purposes is the availability of water by gravity, assuming that a link can be opened with the sea. No pumping would be required in such circumstances, though drainage and salt concentration problems of course remain another matter.
Whilst several large-scale aquaculture projects have been proposed for such depression areas, none have yet been implemented. Nor is there any reason, in the absence of proper evaluations of the ecological consequences of large scale development interventions, why they should be.
4.3.3 Selective sabkha development
The suggestion is sometimes made that aquafarming could be practised in one fashion or another by artificially flooding portions of large sabkhat along the Libyan coast with seawater impounded by levees of soil. It is highly doubtful that any such approach would be economical due to the high quantity of water needed to keep the salinity at a level compatible with life of marine animals (maximum 40 grammes/litre). Productivity would tend to be very low due to extreme physico-chemical conditions.
Although for obvious reasons it is not possible to develop aquaculture in a sabkha just by flooding, and keeping in mind the critical conservation and economic considerations just referred to, the feasibility of aquafarming in certain sabkha settings still warrants investigation. Clearly sites would have to be examined on a case-by-case basis, but operations combining sea water supply and circulation systems feeding ponds built on higher ground in or around sabhkat areas, which would in turn drain into lower stretches of ground where enhanced production of Artemia brine shrimp might be carried out, could at least be considered for implementation on a trial research basis, (see also Medina Pizzali et al. 1994; IRC 1994).
Fish farming cages can be of the inshore or sheltered type or the open sea or offshore type. Cage culture in the protected waters of bays, gulfs, or lagoons has been widely practised for several decades. Technological developments in recent years have made it increasingly more feasible and advantageous to conduct operations in more exposed locations and the move towards offshore cages has also been encouraged by the growing realisation that fish culture in protected and shallower inshore waters involves costs of disease risk and marine environmental damage. Aquafarming using pen culture can be an alternative to inshore cage systems in appropriate shallow water (≈ 3 m) locations of lagoons and bays. Pen units are comprised of a framework of posts and stakes anchored in the seabed, to which barrier netting and inner liner netting is secured. Although pen culture has been extensively used in places like the Philippines, installation and maintenance procedures are generally more involved and labour intensive than for inshore cage operations. The pen method is also less flexible in terms of deployment possibilities.
4.4.1 Inshore (Sheltered) cages
Commercial sheltered cage fish farming systems are available in a range of sizes and possible configurations. Standard commercial production cages run about 1000 m3 in capacity and are usually designed as modular units for various applications, whether simple or complex, small-scale or large-scale. Example installations arc shown in Figures 8 and 9, Annex 4. Costs of inshore cages (ca. 14m diametre, 7 m depth) arc roughly $ 15,000 per unit (floats, cages, net and mooring systems together). Units have a normal working life of 7–8 years. A 1000 m3 unit would usually be stocked with around 60,000 fingerlings at the beginning of each production cycle. Operation costs can be reckoned to be roughly $ 100,000 per cycle ($ 50,000 for seed stock, $ 30,000 for feed, and $ 20,000 for labour and support activities).
Installation configurations are of course dependent on the characteristics of specific sites (extent of shelter, water depths and circulation, etc.) and the operational investment being considered. Sites should in any event be at least two times deeper than the depth of the cages so that wastes from the fishes and feed residues can be spread away by the currents. It is essential also that cage operations be established in a way that does not unduly conflict with other actual or potential site user interests, such as those of traditional fishing, marine commerce, or recreation.
As previously noted, cage culture operations have already been mounted at several places within Libya. In addition to the trial units at Ain El Ghazala lagoon, cages have been placed in the brackish waters at Ain Kaam and in the freshwater lake at Abou Dzira. At first glance many other possible sites for sheltered cage farming exist in bays or inlets located at the mouths of wadis, particularly along the eastern stretches of coastline. A number of these wadi mouths appear to have sufficient depth and adequate protection from heavy seas and winds to permit the anchoring of cages. It must be remembered however that saharan wadis are subject to intermittent and unpredictable flash flooding, when heavy torrents of silty and debris-laden fresh water are forced down their narrow courses and out to sea. Any aquaculture installation standing or floating in the path of such torrents would very likely be swept away. Also, it is highly likely that any caged fish stock exposed to sudden rushes of silt-choked fresh water would perish. These circumstances tend to rule out wadi mouths as suitable cage culture sites.
4.4.2 Offshore cages
Offshore cage units7 are large, with volumes of from two to five thousand cubic metres. With an expected working life of about 10 years, they are of robust construction intended to withstand wind and wave conditions found in the open sea. Commercial models are of several different designs, examples of which are shown as Figures 10 and 11, Annex 4. The most sophisticated ones can be semi-submersible and are equipped with solar power panels and automatic feeders controlled by computer. The largest cages have a 4,500 m3 rearing volume and are stocked with about 350,000 fingerlings per production cycle. Some 170 tonnes of feed would be needed to yield about 85 tonnes of table size (250/350 g each), premium quality fishes (sea bass or sea bream). Allowing for some losses through escapes, diseases, etc., this represents a potential current market value of perhaps $ 700,000 (at $8/kg).
Whatever the model and application utilised, offshore cage culture involves substantial initial investment (≈ $ 0.5 million for the largest units) and operational expense (≈ $ 400,000 running costs per production cycle, depending on programme). The utmost care is therefore required in planning for such installations.
It is understood that GADA presently is planning for the eventual installation of four offshore cage units to be operated on a trial basis, though precise locations have yet to be determined. Several fundamental points should be kept in mind when selecting suitable offshore sites.
Maritime traffic risks. Cages must be located in places that are free of heavy maritime traffic - i.e. well away from harbour entrances, shipping lanes, etc.
6 ‘Estuarine’ in this context is intended to apply to the final stretch of wadi channels at the point where they disgorge to the sea, rather than to actual tidal waters - i.e. estuaries in the strict sense. Most wadis along the Libyan coastline do not carry permanent streams of water in their channels, and only flow directly into the sea during times of winter rains or flash flooding.
7 See Dahle (1995) for an up-to-date brief review of offshore farming systems.