Tide curves for three cycles showing “critical”, “normal” and “high” amplitude patterns for Malindi are shown in Fig. 3. “Critical” cycles are considered to be those where maximal heights reached during the spring phase are the lowest within a semi-annual period. The tide tables for 1981 show these in January and August. On the other hand, very high tides had been predicted in April and November-December. The tides in-between these intervals are considered normal.
For Malindi (and perhaps also for Ngomeni) the highest astronomical tide as recorded is 4.0 m, occurring only on 6 and 7 April, 4, 5 and 6 May and 13 and 14 November, or a total of seven days in 1981. High tides for the normal cycles are in the 3.7–3.8 m range; for the critical ones below 3.6 m.
The layout so for proposed comprises the southern section only. Work is being concentrated here. This particular portion of the farm is on high grounds, but minimal excavation is being made to obtain the earth needed for dikes only, the result of which is shallow ponds that could take in water only when rising tides go behond the 3.3 m level.
The eventual impoundment of the area will restrict entry of water only through the main gate. This restriction, plus friction and demands of other ponds situated upstream would inevitably lower the head and reduce the volume of flow into these ponds. To add to this will be the holding action of gate screens.
Further, with the semi-diurnal cycle and even with tides reaching nearly 4.0 m, the rapid rise and fall within a 12-hour period leaves relatively little time for filling elevated ponds and as pond water levels go up. This problem is already felt with the pond now in operation.
Random samples of soil up to 10 cm deep were collected from the stocked pond and prepared for analysis. Samplings at four other sites were also done at 25 cm profiles to 1.0 m below the surface. These would be tested for acid potential at the nearest soils laboratory after the samples have aged for 4 weeks in the dark.
A cursory examination of the individual core samples from the pond showed very little, if any, gelatinous mud formation on the top layer. As the pond is new this is only to be expected. On grounds formerly covered with Rhizophora, however, the first 50 cm column is peaty, the next 25 cm clayey sand, and the last layers of clam shells intermixed with sand. Other poor sections are located in the NW portion where the soil is sandy.
The results of soil analysis and pH tests might reveal deficiencies that will have to be corrected.
On 24 August, waters at different sites were tested for salinity. At the mouth of the lagoon, it registered 35‰. However, at the creek near the main concrete gate, it was 36‰. In the stocked pond, it was 40‰ (these readings were made 7 days after the last day of taking in water). On 27 August a check on water coming again into the area showed salinity at 36‰.
As no regularly scheduled tests for salinity and observations on the amount of precipitations in the immediate vicinity had been conducted and monitored, no information on the salinity patterns was available. However, assuming that there is not much difference between rainfall and evaporation rates at Malindi and Ngomeni, it is quite unlikely that even during the long rainy season (April to June) the average total rainfall of 655 mm could greatly affect salinity in the lagoon itself, considering the sandy nature of the surrounding water shed, the regular rise and ebb of the tide and the absence of any river emptying into the area or immediate vicinity. However, the pond water salinity may go down slightly as a result of direct precipitation and rainfall exceeding evaporation during this period (about 130 percent in May and 25 percent in June).
The growth of P. indicus, the major component of penaeids stocked, was very slow. The largest specimen seen was barely 1 g after almost two months of rearing from the juvenile stage.
There was a notable lack of lab-lab1, appreciable plankton bloom, filamentous algae or even higher submerged aquatic vegetation that would indicate some degree of fertility. It was reported that shortly after filling, lab-lab was seen growing on the pond bottom, but that these rose to the surface and were drifted to the shore where they eventually deteriorated. Thereafter, no succeeding new growths were observed. No chiromid larvae were seen either.
Many factors, aside from food, could be responsible also for the poor growth. Only until pond conditions have stabilized and suitable conditions for growing natural food have been reached, will it be possible to determine which of these are highly limiting and whether they could be overcome to successfully farm shrimps on a commercial basis.
Data so far gathered on the availability of finfish seed that may be available for culture have shown only mullet at the mouth of the Sabaki River, some 20 km south of Ngomeni. Availability, from data collected, appears to be all year round. However, the species have not been identified.
There is no report of siganid fry in collections so far, although rabbit fishes are one of the important commercial catches of fishermen in Malindi.
Finfish culture has not been planned as yet.
