A review of pen culture in various parts of the world has already been made. We made only brief reference to pen culture in Africa — the practice is being experimented in Ivory Coast mainly and also in Benin Republic. Hereunder we shall briefly describe the pen culture technique adopted in Ivory Coast as described by Hem (1979)*
Hem points out that the sophisticated advanced pen culture technology developed in many developing countries can only be practised in rearing high-cost fishes, where a high price of product is assured. Further this practice cannot be directly applied to developing countries. Hem points out that the low-cost rural technology developed in South East Asia, especially Cambodia, can be more easily applied to Africa and therefore the latter has been adapted in the present trials in Ivory Coast.
In Cambodia two types of pen culture is practised, notably in Lake Tonle Sap, one, adopting intensive stocking (8–10 fish/M2) in swift running water in enclosure units of size 1/8 - ½ ha; and the other adopting extensive stocking (1–2 fish/M2) in enclosure units of 2–3 hectares.
In the extensive stocking no supplementary feeds are given. In Ivory Coast Hem simulated the intensive stocking conditions of Cambodia.
Hem refers to all advantages of pen culture (already referred to) and is of opinion that the cost of pond construction is much higher than for pen culture, especially if culturing the Catfish Chrysichthys spp. is considered.
Site Selection is very important in that water with a good flow should be selected at reasonable depth (0.7m average in his trials). He calculates water renewal rates inside the pen adopting a formula:
| where | fv = | Rate of daily renewals in pen |
| R = | a co-efficient of reduction of water flow through pen due to obstructions in the net enclosure | |
| V = | average water velocity on metre per hour outside the net enclosure | |
| l = | length of shore in metres parallell to the direction of the principal current. | |
| (See Hem's paper for derivations of this relation) |
It has been shown in Hem's trials in Cote d ivore (Grand -Lahon lagoon) that the velocity of water flow (vt) inside pen is reduced i.e.
vt = R.V, ‘R’ being always less than Unity.
In the net material used by Hem the value of ‘R’ was 0.8 (Net type: 210/48, mesh size 14mm).
Under these conditions intensive stocking of catfish (Chrysichthys nigrodigittatus and C. walkeri) can be 8–10 fish/ M2 -stocking size 80 grams. At higher stocking rates oxygen depletion would take place - Hem stocked 5000 fish in enclosure of size 25m × 25m without serious D.O depletion, which otherwise happened in stagnant earthen ponds especially in cloudy days. Accumulation of H2S also was a problem in earthen ponds. Running water through the pens took care of these aspects.
Feeding in pens - was done using supplementary artificial feeds, but the fish stocked had benefits of natural feed available within the pen as well.
As pointed out enclosures of 25m × 25m were made in a group of 8, occupying about ½ ha area in the lagoon Grand-Lahon. The details of construction are given in figures (15–17), taken from Hem (1979).
Nylon netting of type 210/48 and mesh size 14mm (1.6m depth) was used supported by wooden piles, height 2 metres and diameter 10–15cm (Fig. 16–17). These were driven down, in water depth of 0.7m, by using a hydraulic pump, the jet action of water flowing through nozzle removing sand and mud to accommodate the sinking piles (See fig.) Similarly the bottom border of the netting were tied to wooden pieces (20cm long and 3 – 4 an diameter)/ which were also pushed inside / Fig. 16) the substratum, using the same jet water flow device. The whole construction of 8 units was floating on a pontoon, surrounded by an outer barrier net for protection. Catwalk for feeding and inspection (Fig. 15).
