As discussed at an earlier part, the survey focussed on two of the most basic factors for shrimp pond development: soil type and capability of being watered to a desired depth, at the least cost. The type of soil acceptable for pond developmenet is a general factor that need not be unique to Kenya and in this we are limited to six basic types which are acceptable as dyke material: loam, clay loam, sandy clay or clay, sandy clay loam, silty clay loam. The question of potential acidity of the soil was no longer considered since all the areas examined can be considered as equal in this regard. Furthermore, techniques have been developed to manage such acid problem in newly constructed ponds (see Appendix III).
The capability of sea-water exchange in coastal aquaculture is governed by ground elevation and tidal characteristics. Since the tidal range varies from one locality to another, the range of ground elevation deemed suitable for a specific culture system has to be established for each locality. The optimal ranges for various culture systems are shown in Figure 9. Based upon the tidal characteristics of Kenyan waters described earlier, we have established that the optimum ground elevation that will be suitable for an extensive system relying completely on the tide, would range from 0.8 to 2.3 m above datum, if the tide is to be capable of maintaining a minimum depth of 0.75 m during at least 50 percent of all high tides. This in effect means at least 15 days out of each month. An elevation between 2.4 to 2.8 m may be classified as marginally acceptable, but would have severely limited water charge capability and a high tendency to become hypersaline. Within these elevations a pump would be necessary to maintain depth and acceptable salinity conditions.
Due to the additional costs that would be involved in pumping this would most often mean some intensification in the culture system in order to recover the energy cost. The upper limit of 4.0 m is dictated by the effective head of the pump that would most likely be used: for raising a large volume of water up to a height of 1.0 m, a propeller-driven push pump would be the most efficient. Beyond 1.0 m, an axial-flow pump would have to be used. Axial-flow pumps are most effective for raising water up to 3.0 m. In fact axial-flow columns are often manufactured in two standard versions: 10 ft (3.05 m) and 8 ft (2.44 m).
The lower limit of elevation (0.8 m) is dictated by the need to have the capability of draining the ponds without having to wait for an extremely low tide situation. This capability is important if an operator is to have some degree of flexibility in scheduling harvests without having to pump the water out.
Suitability in this report should not be interpreted to mean feasibility. While possible obstacles to shrimp culture development (such as lack of accessibility) in the area deemed suitable are also discussed, a more detailed survey should still be undertaken for any area that may be chosen as a specific site for a shrimp farm. Such a detailed survey would be necessary for feasibility studies and detailed engineering design preparation.
The classification of a mangrove area as suitable for shrimp farm development has been based solely on the physicl attributes. It should not, however, be considered as a blanket recommendation for its destruction. A strip of mangrove forest left intact along the seaward margin would in fact be advisable in order to serve as a breakwater or as windbreaker. Such an undisturbed strip would of course continue to serve its role in the ecosystem.
In the Philippines a 50-m wide strip of mangrove is required by law for conservation purposes. Needless to say a wider strip would even be more advisable as this would restrict development only to the higher grounds, which are infrequently flooded by tide and thus minimize environmental damage. The actual width to be required, and for that matter the whole question of whether to allow selective clearing of mangrove forests in Kenya for aquaculture development, would be a matter of policy which could only be resolved internally within the Government of Kenya, among the various agencies involved.
When the two basic criteria for site suitability are applied against the characteristics of each of the coastal area examined, as summarized in Table 9, only Tana, Kilifi, and Mombasa areas could be deemed suitable for shrimp pond development. The unfortunate fact is that the vast mangrove and tidal flat areas in Lamu as well as that of the Shimoni-Vanaga area will not, literally and figuratively, hold water as far as shrimp pond development is concerned, due to the largely sandy nature of the subtrate. Typical soil profiles in the various areas are shown in Figure 10.
It appears that mangrove poles will continue to be one of Lamu's principal products for many years to come. While pockets of clay may be found within the Shimoni-Vanga area such as in Majoreni and Bodo, it would require a very detailed and painstaking survey to identify specific sites which could be viable. Even then there is no guarantee that the pockets of clay will be large enough in extent to make any development worthwhile and economically viable.
The mangrove areas at Port Tudor and Port Reitz are low enough in elevation to be used for a tide-dependent culture system. However, many of the areas are too narrow for development, so that out of a total of 3 000 ha of mangrove areas available, no more than 650 ha could be targeted for development (see Figure 11).
We have classified the Port Tudor and Port Reitz areas as fair rather than good. There are two reasons for this: one is the fact that not all of the mangrove areas within the two lagoons are suitable. Some, especially the 800 ha of mangrove at the western finger of Port Tudor, are too silty for pond development. Before any development is to take place, a more detailed survey should be undertaken to delineate the exact areas that can be developed.
The second reason is the possibility of future growth of industrial activities within the Mombasa area which might threaten the marine environment. At present the existing industries: meat processing, oil refining and shipping, do not seem to pose any threat. But as Kenya's population grows and the market for consumer goods expands, certain industries not now considered viable may suddenly be considered economically feasible. Mombasa as a commercial and industrial centre with excellent port facilities will be a natural choice for such future activities.
The tidal swamps and flats within the Ngomeni to Kurawa area are mostly occupied by saltworks. An estimated 1 000 ha of mangrove areas remain undeveloped outside the perimeter of the actual saltworks (shown shaded areas on Figure 8). However, these mangrove areas appear to be within the concessions of the saltworks companies and may no longer be available for other parties, or even if available will have problem of access once developed. These are also within the 2.5 to 3.8 m elevation range and are therefore largely unsuitable for tide-dependent, extensive systems. They should still be considered potential areas for development since the saltworks companies themselves might develop them once convinced of the profitability of shrimp farming, or some suitable arrangements might be made with interested parties.
Another area which cannot be discounted comprises the first reservoirs of the saltworks companies. Already many of them are producing shrimps unintentionally since the fry are drawn in by the pumps. Both P. indicus and P. monodon can be found in these reservoirs. The aggregate area of all the reservoirs is estimated to reach 800 ha, as shown in Table 10.
The most interesting areas for shrimp farm development are the tidal swamps along the Tana River bank, specifically the northern bank (see Figure 12). The soil here is clay or sandy clay. A wide range of salinity is available by taking advantage of the tide and natural stratification, making the site ideal for P. monodon. Most of all, they are still unoccupied and being within the high water line, is public land under Kenya laws. The area's elevation which ranges from 3.0 to 4.0 m, precludes its use for a tide-dependent culture system, limiting its development to semi-intensive or intensive systems.
The shaded area in Figure 12 covers some 1 200 ha. The net area that can be programmed for development would be within 1 000 ha. This would allow for a 200 m belt of mangrove forest along the river edge, intact for conservation purposes since hippos appear to forage close to the river edge. The trees would also protect the perimeter dykes from being damaged by wave action or erosion by the river current. Also excluded from the 1 000 ha are the sand ridges which may be too costly to remove. Some 1 000 ha of additional swamp along the Tana have distinct possibilities of being suitable. This would depend largely on a more thorough study of the river's salinity structure at different times of the year and at different tidal conditions.
The second area of interest is the abandoned Bellazoni estate dicussed earlier. Although we were not successful in arranging for a visit to the said estate, secondary data from the Kenya Soil Survey give us a high level of confidence in classifying a portion of the estate suitable for shrimp farm development. About 500 ha of the estate's 4 000 ha is classified as swamp lands, as shown in Figure 13. Furthermore the swamp land within the abandoned estate is contiguous with the swamp land in Kurawa where the Tana Saltworks is located, and which we have examined and found to be suitable. Again its high elevation would limit development to pump-assisted systems.
As far as seawater quality is concerned there are many places along the Kenya coast which could be used for a penaeid shrimp hatchery. We have visited Watamu and Kilifi and found areas there which would be perfect sites for a hatchery. Both places have very clear water, sandy beach, relative shelter from strong wave action, electricity, and are highly accessible (see Figure 14).
Indeed there must be many more places along the Kenya coastline which would have the same attributes except for the fact that they are already being used as beach resorts or as residential lots. Undoubtedly the land cost along the beach front must be very high. Fortunately a very large area is not necessary. A medium-size hatchery can be constructed on a 1 000 m2 lot. Alternatively an area immediately behind a beach front property may be explored. A beach front is not absolutely essential as long as a right of way for a seawater pipeline can be obtained. This would only entail the installation of a longer seawater line. However, the additional cost or an increased pipe length would most likely be minimal compared to the additional cost that would be entailed in acquiring a beach front property.
