Two areas were initially pinpointed as possible sites for the proposed centres. These are Diego Suarez in the north and Majunga in the northwest coast. Specific swamp areas in these two localities have previously been selected for this survey. On arrival in the country, arrangements and preparations were immediately made to survey the named sites (ANNEX A).
Diego Suarez is a city of about 70 000 people situated at the northern tip of Madagascar island. It has a shrimp processing plant, a drydock shipyard and a French Solar Salt Company with a yearly production of 25 000 tons.
Sites I and II (see location maps, Figs. 1 and 2) are adjacent to each other, hence, the general conditions are the same (microclimatic conditions, water source, area, population, distance from population centre, pH, accessibility, prevailing winds and their velocities, temperature, salinity, etc.). It was noted, however, that soil at Site I has a better load bearing capacity than that at Site II.
Both areas are directly hit by strong winds coming from the east. This being the case, the silting in the area is quite rapid.
Since both areas do not have any natural water inlet, it is necessary to make an opening to the bay, which shall be plagued with natural sand accretion, resulting in costly and continuous reopening and maintenance. The shore at Sites I and II is sandy, silty, and sloping very gradually. This means the intake channel will necessarily be long. The water to be taken in is very turbid and full of silt, necessitating filtration and sedimentation ponds. There is no source of freshwater in the area.
It was further noted that the area is very sparsely inhabited posing a problem for labour availability. The project may not have the needed socio-economic impact hoped for due to its being isolated.
Site III is in an area 2 km from the asphalt highway going to the salt plant from Diego Suarez. The road junction leading to the area is 9 km from Diego Suarez and 3 km before the salt plant.
Of the 2 km route to the area, 800 m are gravelled and can be considered as an all weather road. The remaining 1.2 km need grading and gravelling to be passable to vehicles even during rainy season. An ordinary two-wheel drive vehicle may reach the area during sunny days.
There is a total of about 80 ha in the area. A brief survey revealed a good source of clear water for the project from River Makis. This river is about 90 m wide and the deeper river bed is about 0.8 m below zero of the tidal datum. Results of the preliminary survey convinced the team (consultant, project staff and counterparts) that Site III has better potentials than Sites I and II.
With the Diego Suarez Forestry Team, the area was subjected to topographic survey, soil drilling and testing, survey of the watershed area and also of the surrounding areas. Freshwater was found as surface water running off from adjacent farm lands. The volume of flow at the time of visit was estimated at 200 m3 per hour. This surface water was about 300 m from the Makis River. Farther to the east is another river, Main, which at the time of the survey had an estimated flow volume of 1 350 m3 per hour. A 75-m band of mangrove protects the area from the bay. Because the area is on the southern portion of the bay, it is not subjected to much silting nor wave action like Sites I and II.
Site IV (location map, Fig. 2) is composed of 120 ha across River Main and is east of Site III. Conditions are very similar to Site III, except Site IV is slightly elevated.
Majunga is a city of 45 000 people. There are two shrimp processing plants exporting mainly to Japan.
Site a in Majunga is about 3 km from the heart of the town and is along a main highway. The total swampland in the area is estimated to be about 55 ha. It is a narrow strip transversed by a number of creeks but having a main sizeable river that opens to the sea. A study of the map showed a probable area with enough space for about 18 ha. Preliminary ocular survey was made on the morning following the evening of arrival. The area was subjected to a thorough survey during the next two days with topographical surveys, soil drilling and sampling, pH tests and tide monitoring. The pH was excellent, between 7 and 8, while general ground elevation was 4.7 m. While most of the top soil appeared to be sandy clay, at 0.5 to 1 m from the surface, soil drilling revealed a layer of very coarse dark type of sand with partially decayed roots of mangrove trees, making the area unsuitable for pond bottoms.
After completing the verification survey, an area of only about 2.5 ha was deemed slightly suitable as far as pond bottom was concerned.
Based on tide observations, the general ground level undulated from 4.4 to 5 m. Study of the Majunga tide tables and actual observations (Fig. 3) showed an amplitude of 4.6 m. The highest tides are at 5.3 m while the lowest are 0.7 m.
Like Diego Suarez, Majunga has diurnal tide characteristics, 2 highs and 2 lows within a 24-hour period (Figs. 3 and 4). Because the frequency of the extreme high tides are few within a month's time, a favourable level of pond bottom shall have to be designed no higher than 4 m for Chanos chanos, while 3.5 to 3.8 m will work well with penaeid shrimp, with peripheral and sheltering canals provided.
As with Diego Suarez, the evaporation rate and temperature are on the high side (Table 2). There is much strong dry wind, too. These situations require constant changes of water, otherwise, slinity would rise too rapidly, hence, pond bottom cannot be over 4.2 m, ideally speaking.
Tide characteristics in Majunga being what they are, it is necessary to construct broader and higher dikes than usual to protect the ponds from pressure and height of very high tides. The pond bottom and top of dikes would be designed inversely proportional - that is, the lower the pond bottom the higher the dikes.
Examining the catch of women seining the abandoned salt ponds, the team saw numbers of Chanos juveniles ranging from 7 to 15 cm in length. This shows the presence of this species in this site.
Site B is about 12 km out of town due east, along a big, Red river. Study of the terrain shows a general elevation of about 4.9 m. The soil is generally sandy with little clay. Because of the absence of organized fry gathering in the area, the ponds have to be dependent on “volunteer” fish species. The area was considered unsuitable for the moment because the fry of the Chanos cannot very well run up a river reported to be loaded with silt throughout the year. It was silty during the visit.
Like Site A, Site B was predominantly covered with Avicennia trees indicating elevations higher than mean higher high water (MHHW). The area may also be susceptible to flooding during the rainy season. Both sites were considered unsuitable for demonstration ponds for the time being.
While all sites visited can be engineered for functional ponds, it is recommended that for the demonstration and experimental stages, only the first phase of Site III at Diego Suarez should be constructed. The project can serve as a good training ground for fry catching and handling, initiation of progression method of culture, multi-size culture, pond fertilization and manuring, Tilapia and Mugil culture, penaeid shrimp culture and mixed species culture. Phases II, III, IV and V can be instituted in the selected site (Sites III and IV, Diego Suarez) as the demand increases to have a total complex of about 200 ha. Phase I calls for the development of 11 ha, Phase II for 17 ha, Phase III for 24 ha, Phase IV for 28 ha, and Phase V for 120 ha in Site IV. The whole project can serve as the Madagascar Brackishwater Aquaculture Centre.
The factors leading to the selection of Site III over Sites I, II, IV (Diego Suarez, A and B (Majunga) are:
Water supply - the main seawater supply will come from the Makis River which is 90 m wide and (-) 0.8 m at its deepest portion, running north to south. The water in this river is observed to be silt-free and of lower salinity than that of the bay water. There will be no need for expensive silting pond and filtering devices or for long man-made channels to the bay.
Accessibility - a car can go into the site. It is 2 km from an asphalted main road. Of the 2 km feeder road, 0.8 km of it is gravelled.
Distance - it is less than half the distance from the town of Diego Suarez when compared to Sites I and II. The site is before the salt plant. It will afford a better socio-economic impact.
Facilities - the high probability of electric power and communications (telephone facilities) in the forseable future.
Relation to winds and silting - the area, due to its location in relation to prevalent strong winds would not silt up like Sites I and II.
Adopting the point system (Jamandre and Rabanal, 1975), ratings of the six sites visited are presented in Table 1.
