by
W.L. Chan, B. Tiensongrusmee, S. Pontjoprawiro and I. Soedjarwo1
1. INTRODUCTION
The success of seafarming depends to a great extent upon the proper selection of culture sites. Common to all species groups are probably the importance of the risk and convenience factors. These are discussed below, followed by considerations specific to the three groups of organisms for culture, viz., finfish, mollusc and seaweed.
2. RISK FACTOR
The problems of the risk factor are primarily four-fold. These are briefly discussed.
2.1 Exposure Problem
A suitable culture site should be well-sheltered from the quarters of prevailing or monsoonal strong weather conditions. This is therefore, a consideration of the safety of the culture facilities to be invested and the stocks to be cultivated with a view to reducing the chances of physical loss as far as possible.
Sea conditions should include wave actions and strength, tidal and localised current speed and direction, and other conditions associated with these variables. A representative set of weather records on the site for detailed consideration would be highly desirable.
2.2 Pollution Problem
Domestic sewage is a source of detergents, stable and unstable nutrients, solids, various toxic substances and pathogens causing health hazards to varying degree. It is thus advisable to select a site away from the influence of pollutants from population centers.
Agricultural pollution problems are equally serious, typically involving animal wastes, solids, insecticides, herbicides, and other pollutants. For example, the discharge of animal waste into bays and indented coastlines have been known to cause eutrophication, highly prolific disease infestation of finfish and mollusc, and also parasitic infection in oysters. These are but some of many cases of the adverse effects of agricultural pollutants. Needless to say, the toxicity of insecticides and herbicides is also well-known in causing serious health hazard to human being, and its accumulation in the bivalve can also bring out serious consequence.
Industrial activities offer a diverse variety of pollutants. In areas where the effluents are not treated before discharge as in many developing countries, the siting of culture farms in the proximity of industrial activities is most unwise.
When pollution problems are considered, it would be prudent to ensure that culture investments would not be unduly affected by the adverse effects of pollutants. In this consideration, the located of pollution sources and the pattern of seawater transport should be carefully considered.
1) FAO/UNDP Seafarming Development Project, INS/81/008
2.3 Human Problem
The problem of poaching and intentional sabotage is common in fisheries, perhaps more so in aquaculture. Seafarming will face no exceptional treatment. Being human problems, these are difficult to overcome. If appropriately handled however, perhaps some solutions could be achieved. It would appear, as a rule of thumb, skills and sound experience in handling human problems are the pre-requisites to solving such problems. The need of some meaningful form of seafarming regulatory measures would also appear to be in order.
The availability of skilled labour is another problem to farms greater than the small-scale, or family, size. But skilled labour can be trained, depending on needs.
2.4 Conflicting Problem
These refer to problems arising from conflicting activities of the “common users” of the sea. Proximity of culture farms to navigational channels tends to be exposed to the influence of bunker oils, abrupt wave actions and possibly also other forms of pollutants. On the other hand, industrial activities do tend to have harmful effects on seafarming operations through the introduction of the various industrial wastes. The selection of culture sites should thus emphasise the remoteness from such conflicting activities.
3. CONVENIENCE FACTOR
The problems classified hereunder are actually fairly much the same as those discussed under Section 2. They are reiterated under this Section to emphasise the often subjective approach in site selection.
In this consideration, an operator tends to favour the sea area immediately opposite his place of residence, stressing on the priority need to keep constant watch on his stocks and facilities. This priority has often rated over and above everything else. Ready accessibility to access roads, markets, suppliers of goods, electricity and telephone services, and a variety of a range of otherwise legitimate reasons, are also commonly stressed in site selection.
For small-scale operations such subjective criteria are difficult to overcome as there are in many instances no other options to the operators. In commercial-scale investment however, care must be taken not to create liabilities by emphasising on convenience.
4. FINFISH
The potential species of finfish suitable for culture in netcages are fast-growing percoids capable of thriving under impoundment conditions. Some of the species are : Epinephelus tauvina, E. malabaricus, E. fuscoguttatus, E. kohleri, Promicrops lanceolatus, Plectropomus leopardus, P. maculatus, Lates calcarifer, Lutjanus sebae, L. sanguineus, L. argentimaculatus, L. johni, Caranx sexfasciatus, Carangoides chrysophrys, C. malabaricus, C. armatus, Trachinotus blochii, Ulna mandibularis, Siganus canaliculatus, S. javus, S. guttatus and S. spinus. Under favourable environmental conditions depending on the individual species, these fishes normally attain optimal growth in confinement. The following hydrograpical criteria are important in the selection of culture sites for these species.
4.1 Salinity
Salinity requirements vary from species to species. The seabass can be raised in salinity ranging from 0 to 33 parts-per-thousand ; whereas, the carangids prefer to thrive under oceanic conditions. The estuarine grouper ( Epinephelus tauvina ) and certain rabbitfishes ( Siganus spp. ), tolerate between 15 to 30 parts-per-thousand.
Stratification of water layers due to differences in salinity in the water column, is perhaps one of the most important role of salinity as an environmental variable governing the well-being of the caged fish. This refers to the formation of haloclines. Haloclines prevent the transfer of dissolved oxygen in the water column, and the mixing (vertical) of waters to transfer dissolved oxygen. In more serious cases, the surface low salinity layer during slack tide served as a lid blocking the contact of air with the lower layers of waters. This often results in rapidly rising of ammonia, decline of pH and lowering of dissolved oxygen, a situation similar to the early morning conditions in over-fertilised fishponds.
4.2 Water Transport
Coastal waters are extremely dynamic in their transport, generated normally by winds, tides and coastal currents including some seasonal water masses. From a site selection viewpoint tidal current plays an important role in dictating the design of floating rafts and netcages and the mooring system. Water movements serve to flush out metabolites and replenish the level of dissolved oxygen. The tidal range is thus often emphasised in site selection. Generally speaking the greater the range, the higher the biomass carrying capacity of the water.
Fast flowing water is however, a disadvantage in that it requires considerable stronger facilities, special mooring arrangement, and greater wastage of feeds. Averagely greater than 90 % of the total food intake would be expended for locomotion and other activities in coping with this much vigorous environment.
It is generally recommended that running tidal water speed should be within 20 - 40 cm/sec.
4.3 Turbidity
The amount of particulate materials in the water serves as a suitability indicator in site selection. This refers to the level of turbidity. If it is too high, it implies that the sedimentation of solids on the netcage takes place at a high rate thereby clogging the meshes and reducing the flow-through of tidal waters in the thinning out of the trapped metabolites and in raising the level of dissolved oxygen. It is recommended that a potential site should have suspended solids less than 400 mg/l.
4.4 Nutrients. Dissolved Oxygen and Chlorophyll a
One of the key assessments of the overall water quality of a site is by collective reference to the following parameters : nitrate, phosphate (total), ammonia, chlorophyll a and dissolved oxygen. Chiu et al (MS, a) suggested that the critical value of the various parameters should not exceed or fall below the following critical values;
| Parameter | Critical value | |
|---|---|---|
| Upper | Lower | |
| Nitrate (mg/m3) | 75 | - |
| Phosphate (mg/m3) | 70 | - |
| Ammonia (mg/m3) | 100 | - |
| Dissolved oxygen (%) | - | 55 |
| Chlorophyl a (mg/m3) | 9 | - |
Chiu et al (MS,b) showed that the interactions among these variables were closely associated with the well-being of the confined fish stocks; and during periods of slack tidal movements abrupt rise in ammonia, drop in pH and dissolved oxygen and increase in solid loads had led to mass fish kills.
4.5 Exposure to Wind and Wave
A potential site must be protected from strong winds and waves particularly those influenced by long swells from the open seas. Adequate shelter to protect installed facilities from such adverse conditions is essential.
4.6 Pollution
The problems of pollution are usually diverse and are difficult to generalise. In essence, the following criteria may be used as a yardstick :
4.7 Pest
A potential culture site should be free from all pests. These may include large transient predatory fishes, pufferfishes, otters, birds, jellyfishes and any other organisms which may pose potential threats to the well-being of the confined fish stocks.
4.8. Topography
A potential site should be located in an area which should ideally provide a 5 - meter bottom clearance from the seabed. This serves a number of practical purposes. First, trapped nutrients once released from the seabed may not immediately affect the waters inside the cages, and the dissolved oxygen values are therefore not seriously affected. Second, any chemical processes that take place in the substrate would not be immediately transferred to the cages. Third, it provides a free flow of sub-surface and bottom currents thereby optimising the given oxygen budget.
In waters having unusual short slack tide periods, deviations from the above may be considered in the opinion of experienced staff having a strong background of coastal hydrography.
5. MOLLUSC
Molluscs suitable for seafarming are the cockle (Anadara granosa), the green mussel (Perna viridis) and oysters (Crassostrea belcheri, C. lungubris, C. cuculata, C. commercialis, C. rivularis and C. gigas). As for finfishes each species has specific requirements of environmental conditions for growth and survival. Hydrographical parameters that should be considered are discussed.
5.1 Substrate
For cockle culture the substrate plays a major role in the survival of the species. Soft mud flats normally provide the ideal substrate for the culture of cockles. Although cockles can thrive in sandy mud seabed, they grow better in fine soft mud that enables them to bury themselves down to about 0.6 m. Such type of substrate is often found off mangrove swamps and those stretches of foreshore at or near the mouth of tidal creeks and estuaries.
The green mussel and the oysters thrive in most muddy types of substrate, and the substrate criterion in site selection is not as important to these bivalves. The culture of these latter two groups of molluscs is done mostly off-bottom.
5.2 Water Depth
Cockles and oysters require a water depth at low tide of 1 to 2 meters. The green mussel at low tide needs a water depth of 3 to 10 meters.
5.3 Exposure to Sun
Prolonged exposure to the sun during the slack of low tide is harmful to molluscs. Frequent prolonged exposure causes retarding growth and mortality.
For cockle culture, it is therefore best to select a seabed with a moderate rather than a slight gradient to ensure that exposure would not be unduly too long and that the substrate should be submersed in seawater.
For the culture of the green mussel and oysters using the pole and/or stick methods, care should also be taken to avoid prolonged exposure to the sun. As a rule of thumb, a maximum allowance of 1 hour during slack low tide is recommended. In all cases the tidal range at a site should be well studied before hand.
5.4. Exposure to Wind and Wave
A mollusc culture bed is best sited in a shallow bay so that it is sheltered from strong wave actions. In exposed waters cockles have been known to be washed off, and mussel and oyster sticks and hanging have also been known to be endangered.
5.5 Salinity
Optimal salinity values for the growth of cockles and the green mussel range from 20 to 30 parts-per-thousand. For oysters, it much depends upon the species. Crassostrea rivularis and C. gigas are commonly known to thrive for short periods in near-fresh waters. Such conditions are found adjacent to the estuaries of rivers.
The cockle is thus subjected to very wide salinity fluctuations. Generally speaking, optimal dilution of 50 % of the normal seawaters on a ground should serve as a general guide; a cockle stock would tend to be under stress as dilution exceeds 50 %.
For the oyster Crassostrea lungubris and C. belcheri the optimal salinity values for their survival are 15 – 20 parts-per-thousand; whereas, C. cuculata and C. commercialis prefer much higher values of 28 to 32 parts-per-thousand.
5.6 Plankton
A potential culture site should be free from diatom blooms whose toxic contents could bring about serious consequences to the stocks.
Since molluscs are filter-feeders, any toxic substance accumulated could be tranmitted to man. Depuration or cleansing of molluscs before marketing is therefore, highly desirable.
5.7 Pollution
As in the case of finfish culture, all potential sites should be remote from pollution sources. To indicate presence of pollution, it is recommended to use BOD-5 at 5 mg/l and total bacterial count of 3,000 cell/ml.
5.8 Pest
All potential sites should be free from pests notably the eagle and cow rays, petroscirtid blenny, helminths (as parasites), starfishes and oyster drills.
6. SEAWEED
The identification of a potential Eucheuma seaweed culture site is best done by test planting initially in areas where desirable strains grow naturally. This is to suggest, as for any other groups of culture organisms, that a suitable site is one where the target species naturally thrives and propagates. The development of such sites for economic gain through the domestication of the species then rests upon the choice of the economically and technically practical, realistic culture facility and the backing of a functional management team.
The choice of culture facility must take into consideration of the ecological features of the species; and the management team can only be functional, apart from being skilled, when management is fully meant in its real, practical sense.
These are the salient pre-requisites which should be objectively agreed and implemented by those concerned.
From research and development experiences obtained in the Philippines, it has been established that natural grounds of stocks of utilisable Eucheuma species are invariably found in tidal reef channels having the following characteristics:
When these observations are considered, the siting of Eucheuma farms can be referred.
6.1 Water Depth
At the lowest tide, there should be 40 – 50 cm of seawater covering the site. At several cm below this level are strung firmly Eucheuma cluster. At lowest tide, each Eucheuma hanging should be about 40 cm above the seabed.
6.2 Water Condition
Oceanic waters should frequent the site to provide the necessary nutrients and trace elements for the growth of the plants. Rapid flow of seawater should therefore, be expected, and this can be overcome by the design of special culture facility.
6.3 Substrate
Since reef channels are preferred, the substrate is expected to be hard coral sand covering dead reef flats.
6.4 General
The substrate of a site should support good growth of the seaweed. The clusters attached to monofilament lines should not be exposed to the sun, or too close to the interphase between the air and the sea. Suitable off-bottom culture methods are therefore preferred.
