Farming of fishes in cages : agri learner

Site Selection
Different criteria must be addressed before site selection for cage culture. The physicochemical
parameters like temperature, salinity, oxygen, waveaction, pollution, algal blooms, water
exchange, etc. that determine whether a species can thrive in an environment. Other criteria which
must be considered for site selection are weather conditions, shelter, depth, substrate, etc. Finally
legal aspects, access, proximity to hatcheries or fishing harbor, security, economic, social and
market considerations etc. are to be taken care.

Cage Size
It is a fact that costs per unit volume decrease with increasing cage size, within the limits of
the materials and construction methods used. CMFRI has developed open sea cages of 6 m dia and
15 m dia for grow out fish culture and 2 m dia HDPE cages for seed rearing. Ideal size for grow out
cage is 6 m due to its easy maneuvering and reduced labour. For fingerling, 2m cages can be used.

Cage frames and nets
Different cage materials can be used for cage farmes. Materials commonly used are High
Density Poly Ethylene (HDPE), Galvanised iron (GI) pipes, PVC pipes, etc. HDPE frames are Small groups and fishermen. GI frames have less life span when compared to HDPE frames.
Nets of varying dimensions and materials were tested for cage culture in India. CMFRI has
used braided and twisted HDPE nets for grow out purpose. It can last for two or more seasons.
Nylon net can be used economically, but since it is light weight, to hold the shape intact more
weight has to be loaded in the ballast pipe. Cost factor has to be taken care while using new netting
materials like sapphire or dyneema materials for net cage. The depth of net ranging from 2 to 5 m is
ideal. For open sea cage culture, predator net to prevent attack by predatory organisms is essential.
Potential species and criteria for selection of species for cage culture The selection of species for cage culture should be based on a number of biological criteria such as omnivore or carnivore, hardiness, fast growing, efficient food conversion ability, availability of quality seeds, disease resistance and market demand.

Stocking
Although stocking densities should be determined by species requirements and
operational considerations, the influence of stocking densities on growth and production has been
determined empirically. The stocking density depends also on the carrying capacity of the cages
and the feeding habits of the cultured species. Optimal stocking density varies with species and size
of fish.

Feeds and feed management
Fresh or frozen trash fish, moist pellet (MP) and floating dry pellets are the commonly
utilized feed for growing fish in cages. Feeding in cages is quite easy compared to that in ponds. The
ration can be divided into equal portions and supplied at regular intervals. Feeding can be done
either by broadcasting or using feeding trays. Feeds must be nutritionally complete and provide the
necessary proteins, carbohydrates, fats, vitamins and minerals needed for growth and health. Feeds
cannot be allowed to deteriorate during storage.

Harvest
Harvest of fish in cages is less labour intensive when compared to that in ponds. Floating
cages can be towed to a convenient place and full or partial harvest can be carried out based on
demand. Marketing of fishes in live conditions as a value addition can also be done.

Cage management
Cage culture management must result in optimizing production at minimum cost. The
management should be so efficient that the cultured fish should grow at the expected rate with
respect to feeding rate and stocking density, minimize loss due to disease and predators, monitor
environmental parameters and maintain efficiency of the technical facilities. Physical maintenance
of cage structures is also of vital importance. The net-cages must be routinely inspected. Necessary
repairs and adjustments to anchor ropes and net-cages should be carried out without any delay.
Monthly exchange of net should also be considered, as this ensures a good water exchange in the
net, thereby washing away faeces, uneaten food and to a certain extent reduce the impact of fouling.

Disease monitoring
Monitoring of fish stock health is essential and early indications can often be observed from changes in
behavior, especially during feeding.

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