A fishpond is an ecological environment in which many bacterial, animal and plant organisms coexist and interact. Their impacts on the fish production are major, not only because of their food value but also because of the induced modifications of water parameters (oxygenation, pH).
The contribution of natural feed to the fish yield is essential, even in systems based on the use of artificial feed.
Table 9 shows the respective contributions to growth of feed pellets and natural food for fish, obtained using methods based on the measurement of the 12C–13C ratio in fish flesh and in its potential food components.
Similarities in results allow the determination of consumed items (those with a12C–13C ratio close to that of the fish flesh) and unconsumed ones (those with a 12C–13C ratio quite different from that of the fish flesh). Table 9 shows clearly that in such a pond, most of the growth comes from natural feed (> 50% for carp and > 60% for tilapia).
Table 9. Source of fish (3000 Cyprinus carpio.ha-1, 1000 silver carp.ha-1, 450 grass carp.ha-1and 3500–7000 tilapia.ha-1) and prawn (5000–15000 Macrobrachium rosenbergii.ha-1) growth in polyculture ponds as indicated by 12C–13C analyses in a pond receiving 25% protein content pellets at a daily rate of 2% of the tilapia biomass plus 6% of the common carp biomass reducing to 4% of the carp biomass when carps weight exceeded 150 g. (From Schroeder G.L. (1983). Sources of fish and prawn growth in polyculture ponds as indicated by ?C analysis. Aquaculture 35, pp. 29–42).
The fish production is carried out during cycles at the end of which the ponds are entirely drained and dried. As opposed to the perennial aquatic environments, the ecological colonization of ponds cannot be ignored because its consequences on the characteristics of the ecosystem are major. During this period, which lasts from a few days to a few weeks, many qualitative and quantitative changes contribute to structure the aquatic environment.
The regulation processes are primarily endogenous factors, related to the dissemination capacities of the organisms. C/N ratio could strongly contribute to the aquatic settlement. Indeed, a medium rich in carbon and low in nitrogen might support the organotrophic behavior of bacteria instead of mineralization.
Consequently it might promote the development of the bacteriophagous micro-zooplankton (protozoa, copepods nauplii, and rotifers etc). On the contrary, a nitrogen rich fertilization might support the mineralizing behaviour of bacteria, and by the intermediary of phytoplankton, might stimulate macro-zooplankton.
These steps, very unstable, precede a state of relative balance.
The ecological dynamics of balanced aquatic ecosystems are abundantly described in limnology (cascading interactions theory). In spite of that, there is not any model useful for ponds because these environments are too complex and variable to be correctly represented by simplified food chains. Any way, some information is available. All the energy transfers stemming from the primary production can be called autotrophic pathway.
The planktonic micro-algae develop as a result of the photosynthetic activity. Thereafter, they are consumed by phytoplanktivorous fish (like the tilapia) or filtered by zooplankton, which is then consumed by fish. Actually the autotrophic pathway is much more complex, as the energy transfers are not linear at all. In Nile tilapia ponds, phytoplankton composition has a great importance.
The biological filtration of small-size algae is not energetically interesting for tilapia, which then rather feeds on pond bottom when these algae dominate the aquatic flora.
The nutritive value of these feeds is good, and fish growth is satisfying. Conversely, when large-size algae dominate (mainly cyanobacteria), filtration is energetically more interesting for fish, but the nutritive value of algae do not always meet tilapia’s requirements. Moreover, water quality is frequently bad when cyanobacteria develop, and oxygen content is often very low. As a consequence, tilapia growth is not very high.
Other relations exist, that do not stem from the primary production. It is the heterotrophic pathway, identified using methods based on the measurement of the 12 C– 13C ratio in fish flesh and in its potential feeds (as above). Many works confirmed its significant contribution to the fish yield. The organic matter provides a source of carbon for heterotrophic grazing which then benefits the fish yield.
Table 10 presents the relative contributions of the autotrophic and heterotrophic food chains to the growth of fish and freshwater shrimps in ground ponds receiving a high level of organic fertilization. If the growth of silver carp, a phytoplanktivorous species, relies exclusively on the autotrophic pathway, this is not the case of the omnivorous species like the tilapia, and especially the common carp whose growth relies for more than half on the heterotrophic pathway.
However, the heterotrophic pathway can also be unperceived when all the conditions are met to allow the autotrophic pathway to express its full potential (sunning, carbonates and minerals rich environment). Moreover, many autotrophic algae can have heterotrophic nutrition and the majority of planktonic organisms have a very diversified feeding. The biological mechanisms that contribute to the autotrophic and heterotrophic pathways are thus tangled and badly known.
Table 10. Relative contributions of the autotrophic and heterotrophic food chains to the growth of fish (3000 Cyprinus carpio.ha-1, 1000 silver carp.ha-1, 450 grass carp.ha-1 and 3500-7000 tilapia.ha-1) and freshwater shrimps (5000-15000 Macrobrachium rosenbergii.ha-1) in ground ponds receiving a high level of organic fertilization: 50-200 kg.ha-1.day-1.
(From Schroeder G.L. (1983). Sources of fish and prawn growth in polyculture ponds as indicated by ?C analysis. Aquaculture 35, pp. 29–42). In such a context of partial knowledge, the main objective of the fish farmers is to direct the circulation of energy to the pathways that best benefit the fish. The pond fertilization is the first tool at the disposal of the farmer.
As a matter of fact, it is logical to assume that by stimulating primary production, one can stimulate the production of all other trophic levels and therefore affect fish yield. This can be done by using chemical fertilizers, which supply the minerals required for production of organic matter through photosynthesis.
But when minerals are present in sufficient amounts, the density of phytoplankton increases to such a level that light or dissolved carbon soon become limiting factors. One way to overcome this limitation is stimulating the production of heterotrophic organisms by using organic fertilizers.
Figure 1 shows the relationship between growth rate (g.day- 1) and the average weight (g) of common carp, as determined by two weeks interval samples weighing for four treatments.
The comparison bet ween curve 1 (no fertilization and no feeding) and curve 2 (fertilization but no feeding) shows the impact of fertilization, in prolonging the growth of fish to higher individual weights. As a matter of fact, when fish grow, the predation exerted on natural foods increases and sooner or later, the amount of natural organisms is insufficient to meet the fish nutritional requirements.
The individual growth does not stop immediately, but its rate decreases quickly. In a fertilized pond, the growth evolution is identical, except that, as the natural feeds are more abundant, the critical level occurs at a higher individual fish weight.
Lionel Dabbadie and Jerome Lazard