While on the subject of pen culture it would of interest to look into the ‘acadja’ culture system prevalant in certain parts of Africa and hence of much of local potential application value.
An ‘acadja’ or brush park is a simple fish culture technique used in West African lagoons that can be classified as enclosure culture (see Welcome, 1972; Kepetsky, 1982; Hem, in press). An area of a shallow (1 to 1.5m) lagoon with little tidal fluctuation is delineated by driving large, sturdy sticks into the substrate, and can be over a hectare in size. Bundles and piles of branches are placed inside at about 1 bundle /m2. The acadja serves as a shelter from various predators but is most important as an abundant source of food, principally the periphyton attached to the branches (Hem, pers. coom.).
The acadja is left for 6 months to 1 year, and no stocking is done. The harvest can be from 7 to 20 ton/ha/ year, increasing exponentially the longer the acadja is left undisturbed, and with the density of branches. In West African lagoons, the predominant species harvested is the tilapia, Sarotherodon melanotheron, making up 60 to 80% by weight, although in the open waters around the structure this species constitutes less than 1% of the fishery (Welcome, 1972).
To evaluate the acadja, Hem (in press) used 3 nylon 14 mm mesh pens measuring 25 × 25 m. There was one empty control, one with 100 m2 of floating aquatic weeds placed in the centre, and one with 100 m2 of acadja. Branches were placed at a rate of 1 bundle per m2. All pens were carefully fished with a seine net before the experiment, but it is possible that not all the fish were captured. No stocking was done.
After one year, the empty control pen yielded 11.7kg, the pen with aquatic plants yielded 18.2 kg, and the acadja yielded 640 fish of 13 species (353, S. melanotheron) weighing 80.5 kg. Total density was about 6.5 fish/m2 with an extrapolated production of 8 ton/ha/yr. 79% of the weight was S. melanotheron, mean weight of the females 210 g; males 160 g with a size range of 20 to 560 g.
Fig. 15. Plan of enclosures (pen) at Grand-lahou lagoon in Cote d'Ivoire
Fig. 16. Footrope of netting with wooden pieces attached
The 14 mm mesh restricted movement to only very small fish. A few fish may have been inside the pen at the beginning of the experiment, but it is obvious that most of the production came from fish entering through the mesh or reproduced within the acadja during the year. The proportion of the weight of large, saleable fish in the total harvest is quite high. The growth and high production is confirmed for much larger (1 ha) acadja (Welcome, 1972)
Results were surprisingly good, particularly when one considers that the only input is bundles of branches at the beginning of rearing period. Production of 7 to 20 t/ha/yr is similar or better than well managed freshwater pond culture of tilapia using fertilizers and feeds. The technology is simple. Inconveniences include eutroplication of the lagoon, siltation, interference with fishing, and the enormous quantities of wood that are needed. However, Kapetsky (1982) and Hem (pers. comm.) show that these effects are limited. The nutrient load and siltation caused by the branches is minuscule compared to the effect of other natural sources. The acadja helps recruitment in the wild population, and fishermen are attracted to the area around the acadja inspite of frequent snags on discarded branches.
The replacement rate of the wood depends upon the salinity. In low salinities, it can be less than 100%/yr (36%/yr in Benin) or as high as 300 to 400%/ yr in high salinities where there are high concentrations of ship worm (Teredo spp.) (Kapetsky, 1982) If acadja culture were to become wide-spread, nearby plantations for wood might be possible (Welcome, 1972; Kapetsky, 1882). In most West African lagoons and estuaries, the large surrounding swamps would yield sufficient wood on a sustainable basis (Hem, pers. comm.).
The acadjas system appears to have very high potential in aquaculture as borne out from the production values, given. Further detailed study of inputs, mechanics of production and impact of environment to solve the presently unanswered questions food and energy flow are needed.
