Important factors to consider in selecting a suitable site for fishpen demonstration in sandy shallow bottom lagoons are:
normal average water depth should be from 1 to 2 m deep, with maximum allowable highest tide water of up to 2.5 m;
bottom soil should be flat, level and without big rocks or depressions;
there should be a thorough free movement of water current/circulation inside the lagoon;
the site should be far from any source of effluents and pollutants which may cause mass fish kills in the pens;
it must be free from all established navigation routes.
After selecting a suitable fishpen site, designing the structure to be constructed needs careful study and consideration in order to assure success in the operation and maintenance of the fishpen. Lay out the design to ensure the best use of the available surface at the least cost in materials, i.e., larger pens. The fishpen generally should be rectangular in size, with the longest side of the pen perpendicular to the water current/tidal movements.
In place of wooden poles used as posts to support the nets, strong and thick quality high pressure PVC pipes with iron bars and marine concrete cement mixture are recommended. Three designs for PVC poles were tried and it was found out that only two designs are effective to use as main corner posts (Fig. 4-A) and side posts (Fig. 4-B). For specifications on the design of these poles, see Fig. 4 and 5.
For demonstration purposes and in the absence of locally available netting materials, it is suggested that the same kind of fishpens used in the lagoon in Ivory Coast also be used in the Fish Farming Centre project in Jeddah. The characteristic of the main pen net is 210/60 – 14 mm mesh; the depth of the net is about 3 m, corresponding to 200 mesh deep (Fig.6). The top and bottom of the nets are attached to 10 mm polypropylene rope. The sinkers are made of 2 in diameter PVC, filled with concrete and are attached to the bottom part of the nets. (See Fig. 7).
To install the poles, two men drive the pole down to the soft sandy bottom by a slight up and down movement of the pole until the pole settles to the desired depth under the sand, about 0.5 to 0.7 m deep. After all the poles are installed, the 2 in diameter PVC high pressure pipes are connected to the different T-joints on top of every pole. See Fig. 4-C.
Fishpen nets with attached sinkers are laid out first along the boundaries of the poles inside the pen area. Tie the top of the nets temporarily to the braces above the water surface to keep them in place before driving the bottom of the net down into the sand. To drive the bottom of the nets into the sand, a 2 HP water pump with a 2 in diameter hose about 10 m long has been used. In the process of driving down the nets, the water pressure from the pump hose is guided to the correct place for the bottom of the net. This will dig a furrow into which the sinkers attached to the bottom of the neeting are placed to a depth of 0.5 m. After the bottom of the pen nets are installed, the top of the net is carefully fixed and tied. The nets are also attached to each of the poles at 10 cm intervals. (Fig.9)
For the purpose of demonstrating the method of installing and operating fishpen culture technique, a two-unit fishpen module of 25 × 50 m was to be constructed at the project by the consultant during his second trip in the middle of April 1985 when all the materials would be ready and prepared. The two fishpen module units will be protected by common barrier net using the same materials. (Fig.3)