The fertilization is not very effective if stocking is not well carried out. Fish population control must be total, which implies to limit the introduction of wild species (drain at the water entrance) and to control the reproduction inside the pond (monosex breeding, hormonal treatment, or polyculture with a police fish).
Polyculture consists in associating fish species with complementary diets, in order to increase the biomass produced by the pond, and if possible, to benefit from synergic affects between the different fish species.
It is an intermediate situation between monoculture where the radiant energy flux is concentrated on one species and the balanced natural ecosystem where the recipients of flow are very numerous.
Table 13 proposes a classification of the various types of fish farming polyculture practiced around the world.
Table 13.Monoculture And polyculture practiced around the world.
If polyculture originates from Asia, it is no w practiced on all continents. Chinese use two methods. The first one consists in realizing the productive cycle in a series of ponds stocked with different sized fish. In the other, the breeding is carried out in the same pond, until the commercial size is reached. The fish are harvested as soon as they reach the market size and are replaced by fingerlings, at least for a period of the year.
The species and ratios used are presented in Table 14.
Table 14. Stocking density of some polyculture ponds, as a percentage of the different fish species. In traditional polyculture of Eastern Europe, carps remain the dominant species and the other fish hardly represent more than 10%: tench (6–7%), pike (0.9–1.7%), salmon (3.2–5.5%), others (1.2– 4.8%).
Because of their growth performances and their short food chain feeding regime, the herbivorous Chinese carps (Ctenopharyngodon idella, macrophytophagous, and Hypophthalmichthys molitrix, phytoplanktivorous) are frequently included in the polyculture.
Israeli polyculture associates 3000 common carps (Cyprinus carpio) with 1000 silver carps (Hypophthalmichthys molitrix), 500 grass carp (Ctenopharyngodon idella) and 7000 tilapias.
In Africa, the mixed-fish farming associates the Nile tilapia (Oreochromis niloticus) with catfish (Heterobranchus isopterus, Clarias sp.), one osteoglossid (Heterotis niloticus) and the predator Hemichromis fasciatus (to eliminate undesirable fry), according to the ratio 0.03 Heterotis niloticus, 0.04 Heterobranchus isopterus, and 0.2 Hemichromis fasciatus for each tilapia. Under these conditions, the secondary species can increase the total fish yield by more than 40%.
In South America, experiments were led with Colossoma macropomum as main species and Prochilodus sp., Cyprinus carpio and tilapias as secondary ones. However, the South American references describing po werful associations are rare and the practices used are rather the result of the empirical experiment of the fish farmers.
Benefits of polyculture are diverse:
- Better and complete utilization of natural feed, as a fish species, even with a wide food spectrum, does not fully utilize all pond trophic resources
- Avoidance of some trophic deadlocks. When a dense stock of common carp is raised in monoculture, a small crustacean, Bosmina longirostris develops and it is considered a harmful side effect as this crustacean feeds on phytoplankton and is not grazed by common carp. In this way, Bosmina longirostris stands as a competitor for other herbivorous zooplanktonic organisms, which otherwise would be consumed by common carp. But when silver carp is introduced in polyculture with carp, Bosmina longirostris declines as a consequence of the grazing of Hypophthalmichthys molitrix
- Enhancement of natural food. The common carp stirs the pond bottom for feeding purpose and this behavior resuspends and aerates the sediment, oxidizes organic matter and improves the recycling of nutrients that stimulates the production of natural food
- Double fertilization. Dejections of herbivorous fish (H. molitrix, C. idella) are so rich that their fertilizing impact can be compared with that of an associated terrestrial breeding. This effect is sometimes called the “double fertilization” because a chemical fertilization is much more effective when these fish are in the polyculture. This “double fertilization” can improve the carp yield by 14–35% compared with the “normal fertilization” observed in monoculture ponds
- Improvement of water quality. In pond, improvement of oxygenation occurs due to the presence of silver carp or tilapia. Silver carp consume excess algae which otherwise could create an imbalance between production and consumption of oxygen. Tilapia may also improve oxygenation by consuming bottom organic matter that would otherwise have been mineralized by oxygen consuming bacteria
- Control of undesirable organisms. Mollusks control is possible in fishponds by using 75–100 black carps.ha-1 or 200 Heterotis niloticus.ha-1, whereas small wild fish or shrimps proliferation’s can be eliminated by using 200–600 carnivorous fish.ha-1 On the other side, there are some negative effects.
They usually consist in competition among the different species when an imbalance is created. For example, polyculture of Colossoma macropomum with Piaractus brachypomus and/or Brycon sp. lead to poor growth rate, probably because of competition for the same food by the various species.
But, when stocking density is very high, the relative role of pond productivity in the overall nutrition of fish decreases since existing natural food resources have to be divided among more individuals. The gain that can be obtained by polyculture is relatively small, while the extra work involved with sorting the different species of fish at harvest time becomes a burden.
Monoculture is therefore the only method of culture used in running water systems and in cages where the supply of natural food is limited. In ponds, high stocking densities are not common, as oxygenation is usually a limiting factor. But as it is possible to aerate pond water, some methods have been developed.
In a recent paper, a monoculture of Oreochromis niloticus proved to have a production potential higher than a 4:3:3 polyculture of Hypophthalmichthys molitrix, Labeo rohita, and Cirrhinus mrigala at 1 and 3 fish.m-2 . The main explanation for this result was that dissolved oxygen concentration in the monoculture tilapia pond might have allowed an increase of the fertilization, whereas this parameter was critical in the carp polyculture.
Authors:
Lionel Dabbadie and Jerome Lazard
