Cage construction
CAGE CONSTRUCTION
Training manual for Kattakaduwa fishing community
By
N.P.P.Liyanage
S.M.Ruwanpathirana
Dr. S.C.Jayamanne
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Cage construction
Acknowledgements
This booklet was produced for the benefit of the fishing
community of Kattakaduwa Fisheries Society who are
participating in the production of fish in collaboration with
NARA and NAQDA under AECI sponsored AIDA Project.
The author wishes to acknowledge the financial support given
by AIDA and AECI to fulfill this requirement.
The author also wishes to acknowledge the efforts of Hon.
Felix Perera, the Minister of Fisheries and Aquatic Resources,
Mr. Javier Gila, the Director of AIDA, Ms. Priyadarshini
Jayawardena, the consul of Spain in Sri Lanka and Mr.
K.Haputantri, the Chairman of NARA. Dr. Elmo Weerakoon
Director General of NAQDA who geared this project in to
reality. Encouragement and moral support given by Ms.
Avelina Lopez, the consultant of the project and Mr. Ventura
Rodriguez, the coordinator of the project are also highly
appreciated. The author also express his deepest gratitude to
Mr. Vidath Dharmadasa, Senior Aquaculturist of NAQDA for
providing photographs.
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Cage construction
Table of contents
Acknowledgement
Table of content
1.0 Introduction
2.0 Cage construction
2.1 Size and shape
2.2 Materials
2.2 Types
Floating cages
Fixed cages
2.4 Structural designs
3.0 Placement of cages
Factors to consider for cage placement:
4.0 Maintenance
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Cage construction
1.0 Introduction
Cages for fish culture can be constructed from a variety of
materials and in practically every shape and size. Cages can
also be purchased from companies that sell aquaculture
supplies but it will be more economical if construction of cages
can be done by oneself.
The two most important things to remember are:
Cages should be made of sturdy materials. The cage materials
should be strong, durable, and nontoxic.
The cage (mesh size) must be able to retain the fish, yet allow
maximum circulation of water through the cage. It is best to
select the largest mesh possible that will retain your fish.
Adequate water circulation brings oxygen into the cage while
washing wastes away.
2.0 Cage construction
2.1 Size and shape
Cages in a variety of shapes such as rectangular, squire shape,
cylindrical or polygonal, can be easily constructed by the fish
farmer using materials such as bamboo or wooden slats and
wire, nylon and other synthetic meshes or can be purchased
already made from several sources
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Cage construction
Different shapes of small cages
Cages can be made almost any size but most round cages are 4
feet deep and a little less than 4 feet in diameter because rolls
of plastic mesh or plastic coated welded wire are sold in 4 foot
widths. Rectangular cages are usually 8 ft. x 4 ft. x 4 ft. / 8 ft. x
8 ft. x 4 ft or 12 ft. x 6 ft. x 4 ft. (length x width x depth)
2.2 Materials
Cage components consist of frame, nets, floats, weights and
ropes.
Frame
The frame of the cage can be made from wood, plastic,
fiberglass, PVC or metal. Frames made from metals and wood
should be coated with a water resistant paint.
Net materials
Net materials can be plastic coated, welded wire, solid plastic
mesh or nylon netting. Mesh size of the net depends on the
culture species, initial size of the seed, and the culture method.
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Cage construction
If we use 6 - 8 inch1 fingerlings as seed to culture most
suitable mesh size is ½ inch and nets with 1/8, ¼ or 3/8 inch
(0.5 – 1.0 mm) mesh sizes can be used for fry to fingerling
rearing cages.
Different net materials
Floats
For the floating cages, flotation can be provided by waterproof
foam rubber, Styrofoam, sealed PVC pipes, plastic bottles or
barrels, sealed metallic barrels or any other suitable floating
material. For small cages Styrofoam or foam rubber floats are
commonly used and plastic or metal barrels are used for large
cages.
Plastic barrel
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One inch is 25,4 mm
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Metal barrel
Floats
Cage construction
Anchors
On the other hand it should be anchored using stones or cement
or metallic anchors. For fixed cages the cage should be fixed to
the bottom using bamboo, PVC pipes or metallic pipes and
nylon ropes to avoid drift.
Materials used for cage construction should have following
qualities:
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Be durable and strong, but lightweight
Allow complete exchange of water volume every 30
to 60 seconds
Allow free passage of fish wastes
Not stress or injure fish
Be resistant to fouling
Be inexpensive and readily available
2.2 Types
There are two types of cage designs - fixed and floating.
Two types of cages
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Cage construction
Floating cages
Convenient for water depths of more than 5 meters. Usually,
floating net fish cages are made of nylon net meterial, all sides
and corners supported with polyethylene rope fixed by a nylon
twine. Cage is suspending within a rectangular area. Whilst the
top is clamped to bamboo structure which are suspended by
floaters (generally barrels), the bottom is provided with lead,
stone or cement sinkers. Generally, bamboo structure of large
cages clamped to the pond bottom by anchors or weights)
Floating cage
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Cage construction
Fixed cages
Convenient for water depth of 1 to 5 meters. Fixed cages can
be designed with a frame which is used to hold the net material
or without the frame. Using poles, these two types of cages can
be fixed to the pond bottom. For the cages without frame, these
fixing poles also can be used to tide up the net and keep the
shape of the cage.
Fixed cage
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Cage construction
Structural designs
Behavior of the culture species is the most important factor
when designing a fish cage. When less active species like
Tilapia nilotica are under culture, the shape of the cage does
not affect its mobility. However, designing of rectangular
cages makes it easy for assemblage and management.
Any type of cage should have an opening at the top which will
be used to insert seed and give food. In some cases this
opening can be used for other purposes such as get the partial
harvest. Size of this opening can vary and it should be covered
with completely or partially removable lid. This lid can be
made with opaque cover which prevents fish from jumping out
or predatory birds from getting in or can be constructed from
the same type of netting as the rest of the cage. Feeding is
made easier when constructing the lid from netting, because the
feed can throw directly into the cage without opening the lid.
Steps of the construction of large floating cage
Following photographs show the steps followed in construction
of cages. These photographs were taken during active
construction of cages took place at Chandrika wewa.
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Cage construction
Photographs taken by Vidath Dharmadasa
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Cage construction
3.0 Placement of cages
Location of the cage in the water body is a key factor to the
success.
Factors to consider for cage placement:
Placement of the cage is one of the most important
considerations affecting cage culture. Water body must have at
least 1/2 acre or more in surface area. Cage should be placed in
an area where there is at least two feet of water between the
bottom of the cage and the pond or lake bottom. This will help
to keep away the excreta of the fish and excess fish feed fall
through the cage from the immediate area of the cage and
avoid accumulate wastes near the fish cage.
The cage should also be placed where the maximum natural
water circulation present which can move water freely through
and around the cage. This water circulation can create by
prevailing wind action. Even slightest breeze also help to flush
water through and around the cage. It will provide natural feed
and remove the waste away from the cage.
Water circulation can reduce by coves, weed beds and over
heading trees. Therefore cage should be placed away from this
type of places.
Disturbances near the cage are not desirable. It should be taken
in to account whether it has an easy access during all types of
weather conditions when selecting a place to fix the cage.
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Cage construction
An appropriate distance should be kept between cages
otherwise it may reduce Dissolve Oxygen level and cause
adverse effects to the caged fish.
The arrangement of the cages is not a problem if there are only
few of these. However, 8 or more should be arranged
depending upon the direction of the wind. On the other hand its
better if the longest side of the cage is oriented perpendicular to
the direction of the wind.
Placement of cages
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Cage construction
4. Maintenance
Regular maintenance is essential for the all type of cages. It
should be raked off with a stick or broom periodically to avoid
the clogging of mesh due to the bio fouling and accumulation
of suspended particles or bryozoa. This will ensure adequate
water circulation through the cage.
Cage door must be regularly inspected to make sure it is tightly
closed. A strong wind can blow the lid open and allow fish to
escape.
NARA-AIDA Project
2006-2009
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