The optimum stocking density of a fishpond is that amount of fish released into the pond at the beginning of the production period, which guarantees the highest possible economic income.
The assessment of the fishpond stocking is one of the most important parameters for making the success of the breeding.
For aquacultural systems, a crop of young fish will likely grow at a near maximum rate until food or other environmental conditions become limiting. This point is termed the critical standing crop (CSC).
Even though growth is reduced at CSC, biomass continues to increase once fish exceed CSC, until the population reaches the carrying capacity (K).
At K, density effects of the population are so strong that growth reaches zero and biomass remains stable. As long as the rate of increase in fish density is higher than the rate of decrease in individual growth rate, yield increases.
But when the decrease in growth rate exceeds the increase in fish density, yield decreases, Figure 2. Schematic presentation of the relationships between the stocking density, the short interval growth rate and the short interval yield per unit area, with (broken line) and without (solid line) supplementary feeding (from Hepher, 1988, redrawn).
For example, in a carp pond fertilized and fed with cereal grain (2200 carp.ha-1 ), CSC will be reached when the fish attain an average weight of 275 g (0.275 x 2000 = 550 kg.ha- 1 ). Below this weight the potential growth rate has been achieved, but above 275 g their growth rate will be less than the possible potential due to lack of food. When the fish attain 1 kg (2000 kg.ha- 1) they will cease growing. If however the same pond is stocked with 4000 carp.ha- 1, they will reach CSC at 137 g and will cease growing at 500 g.
If fish are stocked in ponds at low density and natural food are abundant, they will grow at a maximum rate for that temperature. Addition of supplemental feed at this point has no effect, because food is not limiting. However, once stocking reaches CSC, food becomes limiting. Growth then diminishes unless management is intensified.
If natural food production can be enhanced by fertilization, growth should again increase up to a new CSC, at a higher level. At that point, supplemental feed may be required to increase growth to maximum. Again, another CSC will be reached until food quality or water quality limits fish growth. This relationship between fish stocking and input level can be clearly seen in figures 1 and 2.
The density can be used for regulating the average growth rate of the fish and therefore, the length of the rearing period. As formerly seen, when stocking rate is increased, CSC will be reached at a lower individual fish weight and the growth rate above CSC will be reduced. The average growth for the entire rearing period will thus be lower.
More widely, yield and individual growth rate are respectively positively and negatively correlated to density. In other words, up to a certain level, the lower the density, the quickest the growth and the lowest the yield.
The economic income of a fish farm is not only dependent on the total yield, but also on the selling price of the fish. When the fish is sold at the same price whatever its individual weight, the fish farmer will choose the optimal density in which the fish utilizes the natural food to give the highest yield per unit area.
Larger fish are sold at a better price per kilogram on the market; the fish farmer will have to find a compromise between yield and final individual weight. In semi-intensive culture conditions (use of manure or lo w value agricultural by-products), the main factor under his control is the stocking density.
By using low densities, he may have better growth rate, higher final individual weight but a lower total fish yield. With a higher growth rate, the duration of growing cycles is shortened, which may give a better cash flow. Experiments carried out in Côte d’Ivoire (Figure 3) showed that by using rice bran as unique input, the compromise between yield and average final weight is situated at a density comprised bet ween 4000 and 7000 Nile tilapias.ha-1. There is now a strong tendency for low input aquaculture development to advise African fish
farmers to use lower stocking densities than practiced before (20 000 fish.ha-1).
Figure 3. Yield and average final individual weight of Oreochromis niloticus as a function of stocking density.
Authors:
Lionel Dabbadie and Jerome Lazard
