Scope and opportunities of cage and pen culture in open water systems
- Part 2 -

Pampa Bhattacharjee *

Feeding the fish in cages

In cage culture system fishes are normally fed with a mixture of rice bran/rice polish and groundnut oil cake/mustard oil cake in 1:1 ratio. For higher growth and survival, a nutritionally balanced feed in the form of pellet is given. Pellets may be of slow sinking and floating types, the latter being most preferred for caged fishes due to its least chance of wastage. For feeding smaller sized fish/fry, feed are supplied in powdered form.

As the fish grow feed are given in the form of water-soaked dough ball or in pellet form either slow sinking or floating types. In the design and construction f cages, consideration should be given to the type of feeding arrangement inside cages for providing different forms of feed such as powdered, dough, slow sinking or floating pellets, as the case may be. Powdered feed or floating pellets are given in feeding ring to avoid the chance of flowing away from the cages through net meshes by water current or wave action.

Feeding ring may be circular or square r rectangular. It is normally constructed by airtight FRP tubes with an attachment of FRP sheet encircling the entire area of the ring so that the sheet extends 4-6 inches into the water to avoid the chance of feed escape below the ring tubes.

Dough or slow sinking pellet feed are given in plastic/aluminium try hanged by rope inside the cages. Some weights are tied at the bottom of the trays so that the trays do not tilt or upside-down during feeding. Regular cleaning and washing of feeding ring and trays are necessary to avoid bacterial or fungal infection.

Feeding the fish once a day is not advisable since they are solely dependent on artificial feed. If they are fed 2-3 times a day better survival and growth of fish is expected. Increasing the frequency of feeding ensures almost fuller utilization of feed; thus, feed wastage can be minimized. For different growth phases of fish, feeding rate is different. As the fish grows the feeding rate is reduced.

The feeding rates for fry to early fingerlings, early fingerlings to advanced fingerlings are respectively 20-15% and 15-10% of the body weight. For grow out cage culture where advanced fingerlings are grown to table size fish feeding rate are kept 5-4% initially and later reduced to 3-2% of the body weight.

Grow out period

Raising carp fingerlings in cages requires 60 to 120 days generally depending on the natural lake productivity as well as quality of supplementary feeds.

Cage and stock maintenance

· Monitoring of water quality- Regular monitoring of some of the water quality parameters viz., dissolved oxygen, pH, free ammonia, phosphate -P in side ages is necessary. Normally, in Indian reservoirs with the objective of raising fingerlings from cage culture, the mentioned water parameters are in conducive condition for fish health and in very rare occasion, with load of algal bloom, some of the parameters show some sign of stress condition for fish survival.

o Cleaning of netlon cages from biofouling -Cages should be cleaned with soft coir brush fortnightly to erase biofouling organisms like algae, sponges, debris etc.

o Routine checking-Loose twine, torn meshes by predators, anchor, sinkers, etc. are to checked in routine manner and immediately need based management is applied.

o Fish stock monitoring – A routine check up of fish health is mandatory to overcome any untoward incident relating to mass mortality of fries.

o Monitoring of growth rate – Samples of fish should be taken at a regular interval to assess length-weight of fries so as to monitor growth performance of stocked fish The information is important in the light of fish health, feeding policy as well as harvesting schedule.

Removal and release

Removal of fingerlings are made batch wise. As different size group prevails among fries they have to be segregated to reduce the uneven growth associated with size hierarchies within fish population. Even, some of the larger fingerlings at the size of 70-80 mm are to be harvested as soon as they attain this range leaving the smaller ones to grow.

Post-stocking management

Supplementary feeding is compulsory in cage culture. The stocked fishes may be fed @ 4% of their total body weight. The ration level should be readjusted every 15 days on the basis of increased biomass, as the fish grows bigger. Mix locally available feed ingredients like rice polish (RP) and mustard oil cake (MOC) in equal quantities. Soak mustard oil cake in water for overnight, mix with rice polish and make balls. These balls are kept in submerged feeding trays hanged from two opposite corners of the cage.

The conventional feed mixture of RP and MOC can be improved by fortifying with vitamin (2%) and mineral (2%) mixture for better growth performance. For better water stability the feed mixture is cooked with 5% wheat flour (moida) for a while to make dough. Now, make small balls, dry under sun and keep in sealed polythene bags until used. Food should be given in a tray. Feed twice a day, usually in the morning and afternoon at fixed times (around 8.00 AM & 4.00 PM).

Production cycle

Some cage can be used for several crops in a year Fingerling production should start with the availability of fry during April. Three crops of advanced fingerlings, after every 2 months of rearing, can be obtained during April to October. A crop of table fish can be obtained by rearing for 5 months from November to March.

Advantages

Cage aquaculture is easier in handling. It has better control on fish, better growth and better per unit area production. Fish has protection from predators and competitors; they use feed efficiently and show low mortality. In this system we can go for high stocking density and it require minimum supervision and less manpower. Due to total harvesting, immediate return on investment is possible.

Precautions

i. Do not feed newly stocked fish for first 2 days as they are stressed from transport.
ii. Reduce or stop feeding until fish respond actively to feed.
iii. Catch few fishes every 15 days to monitor their growth by weighing in a top-pan balance and subsequently adjust the ration.
iv. Place the broader side of the cage wall facing wind direction to facilitate better water exchange.
v. In the event of reduction in water level, drag the cage frame to a place where at least 30-50 cm gap between cage bottom and bottom of water is available.

Choking of cage wall due to bio-fouling caused by the growth of attached algae can prevent water exchange. To avoid this, 4-5 common carp can be stocked in each cage to graze on interior wall of the cage mesh. If choking problem persist, then exchange the cage sequentially every 2 weeks with a new cage for sun-drying the fouled cages and after drying, rub the cage wall with hands, then jerk vigour sly to detach and remove the fouling material.

The exterior of the cage may be scrapped with the help of a coir-made broom. Remove dead fish, if any, from the cage immediately. Repair any holes formed in cage wall immediately. To avoid entry of floating water hyacinth by wind action, bamboo barricade can be given around the whole cage structure.

Dos and Don’ts

Dos
o Clean the webbing at weekly intervals.
o Since it is a feed-based aquaculture system, feed should be specially formulated, fortified and fed to the fish in feeding rays or by using automatic feeders.
o Discarded or injured fish should be removed.
o Periodic sampling should be done to ascertain growth, health etc.

Don’ts
o Stop feeding if algal bloom appears or water is rich in live food organisms.
o Don’t over feed the fish otherwise left over feed may create pollution problems.

Conclusion

o Rearing of fish seed in the cages is not a point for calculation of the economic feasibility of the cage. The actual economic feasibility will be seen when the stocked fingerlings will convert into the fish of an average size of 1.0 kg. Taking into consideration (50% survival) the 50,000 kg fish @ Rs. 30/- /kg will give a return of Rs. 15,00,000/-. So in a span of 5years the actual feasibility will be visible.

In a nutshell it would be an economical venture and will also enrich the reservoir and community for the livelihood security. Fish farmers, fishers, cooperative societies, community depending on the reservoir fishery or other open water bodied for their livelihood will be the beneficiaries of this technology.

o For the purpose of stocking wetlands and reservoirs it is recommended that the advanced fingerlings (10g/100mm) should be produced in cages installed in the respective water bodies. This will minimize the production cost as well as the cost and mortality involved in transportation for carrying seeds from elsewhere is excluded.

o As the fry will be reared in the same water body they will be easily acclimatized when released in those water bodies for culture and thus the mortality will be minimal.

o Fishes native to the region and having good market value can be selected for producing their seeds.

The technologies of fish culture in cage enclosures have been successfully field tested in the beels of Assam. These two technologies re suitable for other open water bodies of the Northeast like the pats of Manipur, reservoirs, lakes and even in riverine pool zones having mild water currents (especially during the dry season).

Large-scale demonstration and adoption of these two technologies is essential for realization of optimal fish production from the beels, reservoirs, natural lakes and rivers/irrigation canals of the Northeast, thereby contributing substantially to making this region self-sufficiency in fish production.

For further details contact: -
Public Relations & Media Management Cell,
CAU, Imphal.
Email: [email protected]

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