Growth rate of the fish, market demand, desired market size, availability of seed and pond condition, are the major considerations for deciding on the time of harvesting. Harvesting the fish stock should be started before the “Law of diminishing returns” starts operating, i.e. the rate of growth of the fish for the invested inputs such as feed and fertilizers start declining. This happens mainly because the growth rate of fish is not linear. Further, the biogenic capacity of the pond, i.e. capacity of water for providing food and oxygen for the fish, cannot be increased after a certain stage according to the need of the increasing fish biomass.
In nursery ponds the fry usually grow to a size of 25–35 mm in about a fortnight with more than 70–80% survival, when they become ready for harvesting. Harvesting should be done by seining the pond water using a close meshed (1.45 mm) drag net. Several netting operations should be done to ensure near total harvesting of the stock. No harvesting should be done on a bright sunny day or in cloudy weather as there might be heavy mortality of tender fry due to high temperature related increased metabolism and the depletion in available dissolved oxygen. The most suitable time for harvesting is the early hours of the morning. Feeding should be stopped a day before harvesting to minimise the conditioning time required for transporting fry over long distances.
Harvesting of fingerlings should be done after three months of rearing when they attain the desired size of 100–150 mm. However, in some cases fingerlings are to be kept for a prolonged period for marketing during the period of scarcity of seed to fetch better price. Harvesting should be done by seining the pond using a drag net of about 8.0 mm mesh. However, complete harvesting of all the species, especially the bottom feeders is usually difficult and hence several netting should be done to ensure near total harvesting. A modified form of net which is described below is very effective in catching all the species of carps even in rearing ponds. Feeding should be stopped a day prior to harvesting.
Usually the carp species attain marketable size within one year and hence the shorter rearing period of less than a year is not recommended unless there is an exceptional threat of flood or outbreak of disease, or for financial reasons. In some exceptional situations when the pond is of a seasonal nature retaining water hardly for 6–7 months, and also in cases where the pond is prone to serious flooding, the rearing period should be synchronised accordingly. Post-flood stocking and pre-flood harvesting should be done in flood prone ponds while in seasonal ponds harvesting should be done before the water level falls below the critical level. Usually these cultivated fish species do not grow well in winter months. Hence in agroclimatic zones having severe winter months the stocking and harvesting phases should be adjusted so as to have complete harvesting before the onset of winter. In regions where seeds of desired cultivated species are available only during post-monsoon period, i.e. October/November, the stocking should normally be done during this period and the crop should be harvested by next October.
It has been experienced that even under the best management, Indian major carps on an average attain hardly 1 kg in size in a year, while Chinese carps reach over 2 kg or so. This kind of differential growth complicates the final harvesting programme and hence in such areas partial harvesting of marketable size fish should be carried out. However, while harvesting, interrelationship of the species cultured should also be seriously considered. Bottom feeders like common carp and mrigal partly subsist on the faecal matter of grass carp and hence an unplanned removal of grass carp may affect the growth of these species. On the other hand, removal of only bottom feeders may create some ecological problems.
Further, the market price of fish is directly related to its size. This factor should also be considered before deciding on the harvesting programme.
Possibility of partial harvesting very much depends on the availability of fingerlings of desired carp species. In such cases the fish already reached the marketable size should be harvested and the stock should be replenished. Usually fish over 500 g should be harvested every 3–4 months with simultaneous stocking with fingerlings. Such partial harvesting programme should be synchronised with peak market demands depending on seasons, festivals, etc.
Harvesting of fish in undrainable ponds should be done by seining the entire pond using desired size of drag net. However, in larger water bodies and especially for partial harvesting, cast nets and gill nets too can be effectively employed. It has been observed that by three subsequent operations of simple net of dragging type, about 90% of the surface and column fishes are caught, whereas the catch of bottom dwellers fall in the range of 20–40% of their entire population. A new gear has been designed by Rout, Lakshmanan and Kanaujia (1979) which can be operated in rearing and stocking ponds with increased efficiency and significant reduction of manpower.
The net is prepared by joining net pieces of 15 × 5 m with 8 mm mesh for fingerlings and 25 mm bar mesh for large sized fish. The free bottom part of the net is provided with a nylon twine (3.0 mm) with sinkers and passed through the bottom series of meshes. The free end of the net is then turned over to the main net and attached at equal intervals to a second line of nylon twine which functions as a false foot rope. This results in the formation of pockets of 20 × 30 cm. Metal sinkers are tied to the first foot rope in each pocket to keep the mouths of the pockets open and also help the net sink in the bottom silt. The net is provided with a strong head rope with polythene floats (Fig. 50).
Weed infestation if any should be removed before harvesting.
All the anti-poaching devices kept in the pond should be removed before netting.
Feeding should be stopped 2–3 days before harvesting.
Harvesting should be done during cool, clear weather and time should be adjusted according to the market hours.
Proper care and prophylactic measures should be taken before releasing back the potential breeders.
Harvested fish should always be kept under proper shade after washing.
