Classification of fish culture on the basis of number of species

Fish culture is classified based on the number of fish species as monoculture and polyculture

Monoculture

This is the culture of single species of fish in a pond or tank. The culture of Clarias only or Oreochromis niloticus or Heterotis or Gymnarchus are typical examples of monoculture.

The advantage of this method of culture is that it enables the farmer to make the feed that will meet the requirement of a specific fish, especially in the intensive culture system. Fish of different ages can be stocked thereby enhancing selective harvesting.

Common practices around the world

• Common carp in East Germany
• Common carp in Japan
• Tilapia nilotica in several countries of Africa
• Rainbow trout (Salmon gairdneri) culture in several countries.
• Channel catfish (Ictalurus punctatus) in U.S.A.
• Catfish, Clarias gariepinus in Africa.

Polyculture

Polyculture is the practice of culturing more than one species of aquatic organism in the same pond.

The motivating principle is that fish production in ponds may be maximised by raising a combination of species having different food habits. The mixture of fish gives better utilisation of available natural food produced in a pond. Polyculture began in China more than 1000 years ago. The practice has spread throughout Southeast Asia, and into other parts of the world.

Ponds that have been enriched through chemical fertilisation, manuring or feeding practices contain abundant natural fish food organisms living at different depths and locations in the water column. Most fish feed predominantly on selected groups of these organisms. Polyculture should combine fish having different feeding habits in proportions that efficiently utilise these natural foods. As a result, higher yields are obtained. Efficient polyculture systems in tropical climates may produce up to 8000 kg of fish/ ha/year.

Fishes used in polyculture

Combinations of three Chinese carps (bighead, silver and grass carp) and the common carp are most common in polyculture. Other species may also be used. While fish may be grouped into broad categories based on their feeding habits, some overlap does occur. Descriptions of the feeding habit categories and examples of fish from each category are as follows.

• Plankton Feeders - Plankton is usually the most plentiful food in a pond, so it is essential to include a plankton-feeding fish in a polyculture system. This group of fish feeds on the tiny, free-floating plants (phytoplankton) and animals (zooplankton) which multiply abundantly in fertilised ponds. Two fish typical of this group are the silver carp, Hypophthalmichthys moliirix, and the bighead carp, Aristichthys nobilis.
• Herbivores - This group of fish feeds on aquatic vegetation. The grass carp, Ctenophanjngodon idella, is most noted for this behaviour and is stocked in ponds for weed control.
• Bottom Feeders - Fish in this group feed primarily at the pond bottom. They consume a variety of decaying organic matter, aquatic organisms such as clams, insects, worms, snails, and bacteria living in or on the sediments. The common carp, Cyprinus carpio, is well noted for this behaviour.
• Piscivorous Fish - These predatory fish feed on other fish and must consume about 5 to 7 g of prey to grow 1 g. They are frequently stocked in ponds to control unwanted reproduction, particularly in tilapia, and other fish that enter the pond with the water supply and compete for food with the stocked fish. Commonly used predator fish include the sea bass, Lates spp.; catfish, Clarius spp. and Silurus spp.; snakeheads, Ophicephalus spp.; cichlids, Cichla spp.; Hemichrotnis fasciatus and Cichlasoma managuense; knife fish, Notopierus spp.; and largemouth bass Micropierus saimoides.

Adding predator fish to a polyculture system increases the average weight of prey species. It is most efficient to use a predator fish that consumes small prey. This prevents the prey from growing large enough to compete for food with larger fish of its species. Use of predator fish in polyculture systems is experimental in most areas of the world. In small ponds, it is almost impossible to stock the exact number of predator fish to achieve the same predator/prey balance occurring in nature. In small-scale aquaculture, predator fish are usually stocked at rates of 5 to 20 fish/100 m² of pond surface area to control reproduction of the prey species completely. Typically, the stocking rate is about 19 fish/100m² for catla, 38 fish/100m² and 6 fish/100m² for mirgal.

Issues in polyculture

Polyculture is an efficient way to maximise benefit from available natural food in a pond. But, pond management becomes more difficult when stocking fish species having specialised feeding habits in the same pond because proper fertilisation and feeding practices must be followed. If inadequate fingerling supply severely limits the choice of species available for polyculture, at least one species should have general rather than specialised feeding behaviour. This will allow more of the available natural food to be utilised.

References

• http://www.unaab.edu.ng
• Introduction to polyculture of fishes. International centre for aquaculture and aquatic environments. Auburn University.