Cages are used as a form of farming in their own right within flowing or large bodies of water and can also be used in small pond fish culture to protect fingerlings in the initial stages of development. Small cages with a capacity of one cubic meter are suitable for fingerling protection. The cage can hold up to 300 fish at a time. People grow fish in their local ponds using a simple fish cage known locally as a “hapa”. A few young fish are put into each “hapa”, which acts as their home, floating just below the surface of the pond.
Cages can be made using a few cheap materials. Bamboo poles form an outer frame that is covered in netting; inside is a “nursery” section for the younger, more delicate fish; and floats are added at the corners.
A cage is a very simple means of restraining fish in one place and it can be easily made using local materials. Cage design must incorporate certain physical properties, including the ability to hold fish securely but also to be within the financial means of the cage operators. The cages presently used are small in size, measuring between 1 and 2m3, inexpensive and simple to construct.
Farmers use both fixed and floating cages. In general, fixed cages are installed in water where the depth is relatively low and bamboo poles can be fixed into the riverbed or substrate. Floating cages do not have this limitation and can be used in deep water. Floating cages tend to be easier to manage but when selecting the type and design the following points should be considered:
- Input availability
- Natural disaster
- Type of water body
- Water depth
- Water current
- Water retention period over one year
- Social problem
- Cage management
Fixed cages are very easy to construct and only require a small amount of capital investment. The materials needed to make fixed cages are netting with an 8mm mesh size, bamboo, rope, twine and sinker. Routine management is difficult for these cages and storms, strong currents, tidal surges and flooding may cause damage if precautionary measures are not taken. Fixed cages are difficult to move from one place to another during water fluctuation.
A top cover is usually provided on cages to reduce the risk of fish escaping, especially in areas prone to flash floods where water levels rise very quickly. A small opening is kept at one edge or in the middle of the top covering for feeding purposes. A feeding platform made of fine mesh is placed on the bottom of each cage to minimize food loss. To fix the cage, four bamboo poles are fixed in the substrate and the four top and bottom corners of the cage are tied to the bamboo poles with nylon rope, allowing the cage netting to stretch.
To minimize installation cost and to reduce daily management labor, cages are sometimes fixed in rows with a narrow space between the adjacent cages.
A. Floating Cages
The size of the cage is usually 1m3. A top net is always used to minimize escapees because the cage is only a few centimeters above the water surface. The top of the cage is on hinges that can be opened to allow feeding, the checking of fish, the removal of waste and harvesting.
A layer of fine mesh net is placed along the bottom of the cage and 10 cm up each side which reduces food loss. However, where water has a high turbidity, the use of fine mesh is not recommended as it clogs up the mesh and causes structural stress on the cage frame. In these areas, feeding trays should be used instead of fine mesh. Buoyancy is achieved by using four plastic floats (buoys) which are tied to the four horizontal frames, approximately 10 cm from the top of the cage.
B. Bamboo Frame Cage
To make a 1m3 bamboo cage, twelve one meter long bamboo sticks (about 2cm in diameter) are required and fixed into the holes of the angles, one angle for each corner, giving a box shape.
Advantages and disadvantages of fixed and floating cages
- Cost per unit is small
- Not damaged by storms
- Easy cage management
- Cage bags spread properly in the water
- Easy to construct
- Water volume remains constant even with a fluctuation in the water level
- Cage can be installed in deep water
- Floating cage with box type frame gives adequate space inside
- In tidal water bodies the effective cage depth is greater
- Water depth inside cage varies with the fluctuation of the water level
- Vulnerable to tidal surge and storm
- Cage cannot be installed in all types of water bodies
- Prone to crab cutting
- Poorer fish growth and higher mortality rate
- Cost per unit is higher than fixed cages
- Algae deposits on cage net affect the water exchange
- Cage management i.e. cleaning, moving and sampling are not easy
- Due to water flow sometimes the cage bag does not spread properly
- Easy to steal fish by lifting the cage
- During flooding may wash away
C. Cage Management
Care of cages – cleaning of aquatic weeds nearby, removal of water hyacinth, cleaning of waste feeds from cages, cleaning of deposited silts from cage, removal of dead fishes, checking cage frames, floats, ties, anchors, feeding trays etc., cage shifting, considering the water level, checking water pollution and guarding.
Care of net – Algae attachment should be cleaned at regular basis to ensure water exchange, net holes must be checked properly and need quick repairing when necessary.
Care of fish – profitability depends on proper attention to the fish growth, regular, adequate and quality feeding is important. Fish health and disease should be monitored during feeding and sampling. The fish must be fed daily using aquatic weed or a mixture of rice bran, oil cakes, kitchen waste, chopped snails or cow dung.
D. Minimizing Risk of Cage Culture
- Appropriate cage design can help minimize failure.
- The use of more than one cage per household greatly reduces the risk of an individual losing all fish.
- Placing many cages together in clusters also reduces the risk of poaching.
- Individual farmers can form groups that can guard the fish and therefore considerably reduce potential poaching threats.
For more information, contact:
Bureau of Fisheries and Aquatic Resources
Dept. of Agriculture
PCA Building, Elliptical Road
Diliman, Quezon City
Tel. Nos. (632) 929-8074 / 929-9597
Email :[email protected]
source: The original article appeared in www.practicalaction.org and was intended for Bangladesh fish farming.