Scope and opportunities of cage and pen culture in open water systems
- Part 1 -

Pampa Bhattacharjee *

Introduction

Cage culture system offers the scope for raising fish seed and production of table size fish utilizing underutilized vast freshwater resources available in our country. In this system of culture stocking density of reared fish are normally kept much higher than that of pond culture system. Since the fish remains in overcrowded condition and have little chance to get required natural fish food organisms, artificial feed supplementation for the stocked fish is essential to get desired survival and growth.

Unless proper cares of nutritional aspects for stocked fish are taken, cage culture may not be economically viable. In spite of having vast land area and a large number of fish farms, our requirement for fingerlings as stocking material either for reservoirs or food culture of table size fishes is not fulfilled. Average fish production from the Indian reservoirs is about 15kg/ha, if a proper stocking program is taken up in open water bodies, this production can easily increase up to 100kg/ha.

At present country’s requirement for carp seed (spawn and fry only) is 20,000 million, whereas, estimated requirement for stocking open water bodies and aquaculture farms is 1600 crore. For optimum fish production from reservoirs, lakes, ponds beels etc, it is necessary to stock them with healthy fingerlings of above 100 mm size. Thus cage culture offers in situ rearing of fry to fingerling before releasing into the reservoirs, is best option available for enhancement of fish production from large water bodies.

Therefore, the production of fingerlings through cage and pen aquaculture will improve country’s production in two ways by regularly stocking open water bodies like lakes and reservoirs with desired varieties of fingerlings thereby improving fish catch from these water bodies by aquaculture of fish in ponds (for which also we need a large nos. of fingerlings).

The inland fisheries resources of India are noted as much for their heterogeneity in composition as far as their opulent productive potential. India is endowed with a vast expanse of open inland waters in the form of rivers, canals, estuaries, lagoons, reservoirs, lakes, ponds, tanks etc. Although the vast and varied inland fishery resources of India have a rich production potential, this potential has not been achieved.

The production potential of a major river in India, the Ganga, in its lower reaches, is estimated at 198.3 kg/ha/yr, whereas the actual fish yield is 30.0 kg/ha/yr, and thus, only 15.2% of the potential is harvested At the present level of management, the yield from the Indian reservoirs, on average, is about 30 kg/ha, whereas a production of 50-100 kg/ha can be easily achieved from large and medium reservoirs, while the small reservoirs have the potential to yield 100-300 kg/ha.

The floodplain wetlands, commonly called beels in India, have a rather poor yield, 100-300 kg/ha, against a production potential of 1000-1800 kg/ha/hr though scientific management. The present average productivity from aquaculture ponds and tanks is 1830 kg/ha/yr against a much larger potential.

These are the aquatic resources where future enhancement techniques like pen and cage culture are to be adopted. Thus it is relevant to understand the present environmental status of these ecosystems.

What is Pen and Cage ?

Pen and cage are enclosures of any size shape and design and can be made of any suitable material. Pen and cage culture involve holding organisms captive within an enclosed space for short or long duration. There is confusion concerning the terms ‘cage culture’ and ‘pen culture’. Both terms are often used interchangeably, particularly in North America, where ‘pens’ and ‘cages’ describe the same method of culture.

Hence a general term ‘enclosure culture’ is used to describe what more precisely could be defined as cage or pen culture. However still they are two different systems in many ways. A cage is totally enclosed on all or all but the top sides by mesh or netting, while in pen the bottom of the enclosure is formed by the lake or sea bottom (FAO).

Pen has fixed enclosure walls cannot be moved about as in the case of cage. Fish grown in pens have access to benthic organisms, and there is some evidence that certain species grow better in pens than in cages.

Types of cage : Four types of cage are being used for cage aquaculture – fixed, floating, submersible and submerged:

Fixed cage, through very primitive in origin still in vogue, is used in shallow water with water depth 1 to 3 m in those reservoirs where water depth does not fluctuate too much and consisting of net bag fitted in posts and should not touch the basin. Fixed cages are comparatively inexpensive, simpler, and smaller in size & shape and their use is also restricted.

Floating cage, on the other hand, is basically supported by a floating frame, where from net bags are kept hanging in water without touching the basin. It is generally practiced in water bodies with depth of water more than 5 m. Enormous diversity of types- size, shape and design of this cage has been developed to suit the wide purpose of the fish growers in open waters.

The net bags of submersible cages are suspended from surface and with adjustable buoyancy – may be rigid or flexible.

The submerged net bags are fitted in a solid and rugged frame and submerged under water – use is very limited in reservoirs.

Shape of cage : Cage of different shapes is being used depending on the objective of cage culture, Normally, in reservoir, if the objective of cage culture is to rearfry to fingerlings, the shape would be square, rectangular or in some occasion circular type. The circular type cages are normally used in big reservoir with huge structure G.I./HDPE pipe frame.

It is stated that the circular type cages are best suited to the reared seeds so as to have more surface area; the only problem is that the shapes are to be maintained with proper mooring by fixed frame and not with sinkers. The cost effectiveness is to be viewed with priority while designing cages to be placed in open water like reservoirs.

Cage Culture Practices

Site Selection :The selection of site for cage culture is very important and success is in many occasions dependent on selection of proper site. Potential sites vary with the size and shape of the reservoirs where it is to be installed. The critical issues in selecting sites are:
o Depth of water column should be at least 5 m or more
o God water quality and circulation
o In large and medium reservoirs, site should be in protected bays to evade strong wind action. In small reservoirs, the cage should be anchored in the deeper lentic sector avoiding direction of current flow through sluice gates and irrigation channel
o Site should be devoid of local and industrial pollution
o It should be away from frequent disturbance of locals and grazing animals
o Access to land and water transportation
o Devoid of algal blooms with less fouling
o Should be free from aquatic macrophytes, over population of wild fish and oxygen stress
o It should not hinder the navigation if being practiced in the reservoir
o Keep distance from bathing as well as burning Ghats.
o Protected from security aspects.

Installation of cages

Cages are suspended in the wooden frame, provided with floats. Net cages are allowed 0.5 to 1.0 meter above the water level and fixed with ropes to four corners of the poles. Each cage is also covered from the top by using the same material lid.

Bottom periphery of each cage is provided with sinkers so as to keep them stretched. Corners of each cage unit are provided with anchors to keep the cage infrastructure stationary at a definite location in the water body.

Stocking

For raising fingerlings in cases, healthy fish fries of carps at a size group of 12-15 mm or even up to 25 mm suit best for Indian reservoirs.

Advanced fries above 35 mm should be avoided to grow in cages for fingerlings as in most occasions these fries are associated with fungal diseases like seprolegniosis if collected from the nurseries already eutrophied.

Especially IMCs are more prone to such attacks of the fungal diseases. A stocking density of 250- 300 nos carp fries of 12-18 mm size/m3 is most suitable for cages installed in reservoirs of India. Stocking should be done during evening or early night hours with due conditioning (site of procurement) and acclimatization (site of release in cages).

Conditioning is required to transport the fries with empty stomach as the waste generated ammonia and CO2 may be proved lethal to fries during transport. The acclimatization for fries is essential at the site of release in cages as to provide balanced environment to the fries in their new abode especially temperature to be adjusted.

The oxygen packets with fries (1000 nos. in 4 litre water in polyethene packet with 2/3rd filled with oxygen) are kept inside cages as such without release of the fries and at least after one hour the fries are released in to cage water.

Prior to release fries from oxygen packets in to cages they are subjected to some prophylactic measures to prevent them from diseases and ectoparasites. They are dipped in 5-6% salt solution as well as potassium permanganate (5-8%) for 1 to2 minutes and then released in cage water.

To be continued .....