Polyculture: Meaning, Objectives and Drawbacks

In this article we will discuss about Polyculture:- 1. Meaning of Polyculture 2. Principle of Polyculture 3. Objectives 4. Origin 5. Drawbacks.

Meaning of Polyculture:

Polyculture or mixed fish farming or composite fish culture is the culture of fast growing compatible species of fishes of different feeding habits (or dif­ferent weight classes of the same species) in the same pond so as to utilise the various available ecological niches in order to obtain high production per hactare of water body.

A pond according to its depth can be divided into three distinct zones — upper surface zone, middle column zone and bottom zone. A particular species exploits food of a particular zone. For example — Catla catla is a surface feeder, Labeo rohita a column feeder and Cirrhinus mrigala is a bottom feeder.

In case of single species or monospecies or monoculture only one zone will be utilised or exploited while the other zones would remain unutilised. As a result the entire ecological area would not be exploited and the yield or fish production would be less.

Principle of Polyculture:

When different species of fast growing compatible fishes, occupying different ecological niches of a pond or any water body, are cultured together, they most efficiently utilises all the food sources available in the pond for fish production without harming each other.

Objectives of Polyculture:

(1) To obtain maximum yield or fish production.

(2) To utilise all the available niches.

(3) The fishes cultured should not cause any ecological disbalance.

(4) The fish species cultured should not have any serious competition between them but each species may have a beneficial influence on growth and produ­ction of the other. For example, grass carp by consu­ming aquatic vegetation, converts plant tissue into fish flesh but its excreta fertilises the pond which benefits all other species.

(5) Some species of fishes are cultured which have specific roles to play in maintaining water quality in ponds by feeding on wastes accumulated in it. For example common carp and mrigal consume the fae­ces of grass carp and silver carp, which contain large amounts of undigested plant matter.

(6) Recent combination of fish species cultured are based on one or two species as the main ones and the others as subsidiary compatible species which would be utilising those parts of the food resources that would have been wasted.

Origin of Polyculture:

Composite fish farming has its root in ancient China and India. In both these countries poly-culture was adopted in carp farming based on mixed seed stock, collected from natural sources. It was not pos­sible to sort out stocks of different species in their early stages. The farmers, therefore, were forced to rear the different carp species together until they reached fingerling stage.

The farmers observed that there were differences in the feeding habits and different preferences for natural food organisms. This led to the thought of adjusting the stocking densities according to the occurrence of natural food and the niche occupied by each species in the ponds. Thus, it could be beneficial in traditional production systems by culturing appropriate species combinations.

Such a practice could effectively lead to the utilisa­tion of a much larger quantity of food resources and developing a symbiotic relationship between the cultured species in the system. This would lead to higher production at a lower cost and the farm environ­ment can be maintained at required levels.

The combination of species in polyculture system cultured in China consists of:

(1) Grass carp (Ctenopharyngodon idella) which feeds on macro-vegetation.

(2) Silver carp (Hypophthalmichthys molitrix) which feeds on plankton (mainly phytoplank- ton).

(3) Big head (Aristichthys nobilis) which feeds on microplankton.

(4) Black carp (Mylopharyngodon piceus) which feeds mainly on snails and other molluces at the bottom.

(5) Mud carp (Cirrhinus molitorella) which feeds primarily on detritus.

(6) In addition sometimes the omnivorous common carp (Cyprinus carpio) is added that basically serves as a scavenger in farm ponds.

(7) Modern polyculture system has added a number of other species to the above traditional combi­nation, such as tilapia, wuchang fish, crucian carp, red eye, white amur bream, snake head and mariadarin fish.

The combinations practised in Indian polyculture system comprises the following:

(1) The surface feeder, catla (Catla catla) which feeds on planktonic organisms particularly zooplankton. The larvae and young fry feed on planktonic unicellular algae.

(2) The column feeder, rohu (Labeo rohita) con­suming vegetable matter including decaying aquatic plants, algae, etc. Larvae and fry feed on unicellular algae and zooplanktonic orga­nisms.

(3) The bottom feeder, mrigal (Cirrhinus mrigala) preferring decayed plant and animal matter, algae, detritus, organic matter, etc. The larvae and fry feed on planktonic unicellular algae and zooplankton.

(4) The bottom feeder, calbasu (Labeo calbasu), feeds selectively on benthic and epiphytic organisms and organic debris.

(5) Modern culturists and scientists have developed composite carp culture by the addition of exotic carps such as grass carp, silver carp and common carp.

(6) New species combinations for polyculture have been developed through the introduction of benthic-feeding grey mullets (Mugil cepha­lus). The introduction of carnivorous fish, chital (Notopterus chitala), has been done for the control of weed fishes.

Drawbacks in Polyculture System:

1. Generally the lesser number of species combi­nations with clearly understood specific roles are widely accepted. However, in case of larger species combinations, the question has arisen as to the value of some of the species in such combinations.

2. Although there are clear-cut differences in food preferences in natural conditions, the relevance of such feeding differences is yet to be assessed when the stocks are fed with formulated feeds.

3. It is not very easy to adopt supplementary feed­ing in an economical way.

4. Special skilled knowledge and efforts are re­quired by the farmers to produce or purchase the appropriate numbers of seed stocks of the different species selected to maintain optimum balance of the species.

5. Additional labour is required for sorting out the different species after harvest.

6. In many instances, consumer acceptance of the different species varies in most areas. Difficulties have also been experienced in finding markets for certain species like the silver carp.

It has been found and proved that modern mono­culture with proper feeding can be more productive than polyculture. Thus, it is opined that the value of polyculture depends very much on the situation and needs in a particular area. Polyculture probably is not widely applicable as was generally consi­dered.