Major and medium-size tanks in the country number over 250. It may be possible to improve the fisheries in, and increase production from, these tanks by establishing carps. Such a programme should be designed so that it does not affect the present tilapia production. Considering the infrastructure required and technological support available, the programme will be of long duration. The general approach for such a programme could be as follows:
Four or five large tanks covering a total area of about 10 000 ha at full supply level, located in four or five districts, presently having relatively poor tilapia fishery, are selected for a 10-year project.
Each of these tanks would be stocked every year in September–October with carp fingerlings at a minimum density of 100/ha, as follows:
| No./ha | ||
| Chinese silver carp or big head carp | 30 | |
| Catla catla | 25 | |
| Labeo rohita | 20 | |
| Cyprinus carpio | 20 | |
| Ctenopharyngodon idellus | 5 | |
| Total | 100 |
Chinese silver carp (Hypophthalmichthys molitrix), and big head carp (Aristichthys nobilis) are fast-growing phytoplankton feeders. Since big head carp is already available in the country there is no real need for introducing silver carp. Besides, it has already been seen that big head carp can grow fast in large reservoirs like Udawalawe. Grass carp and common carp are also available in the country.
A ten year project would need 10 million fingerlings. The Mission has calculated their cost to about Cey.Rs. 0.60 each. Thus, the project would spend Cey.Rs. 6 million on carp fingerlings alone. Is that a worthwhile expenditure? To find out, it is necessary to compare the expected fish production, following a stocking programme, with the fish production which is expected if no stocking programme is undertaken. Neither situation is easy to predict.
Present variations in catch rates amongst major tanks, expressed in annual landings per hectare (see Annex 7), reflect not only fishing efforts of varying intensity and differences in limnological characteristics, but also variations in the reliability of the statistical data. Even were no stocking project undertaken, it is obvious that the rate of exploitation in kg/ha and year would increase in most tanks; in some, it might decrease as fishing pressure increases.
In the event that stocking is undertaken, it is not likely that commercial harvesting of the carps can be undertaken until some 10 years after the project starts, that is, when the stock of the introduced carps has established itself and become capable of sustaining a fishery.
In order to appraise the economic merits of a stocking project, the following assumptions have been made:
commercial fishing will be possible from 1990 onwards;
carps will then sell at a price similar to that of tilapia (this assumption reflects the present situation with regard to the common carp offered for sale at some tanks at high altitudes);
the magnitude of future fishing effort will not be influenced by the fact that a stocking programme is carried out.
It was established above (5.1.3) that the Government would have to subsidize the introduction of culture of fish in seasonal tanks to the extent of Cey.Rs. 0.50 per kg. In view of the fact that the commercial harvest of carps is not likely to start until 1990 at the earliest it is suggested that it would be rational if Government de facto spent relatively less of its resources for establishing carps in perennial tanks (reservoirs). The Mission has used the amount of Cey.Rs. 0.25 for all carps landed in the period 1990 to 2010. Carps landed later than 2010 thus are considered of no value today.
The project as described above, would need a total of 10 million fingerlings worth Cey.Rs. 6 million over a period of 10 years for stocking 10 000 ha. In effect this amount is the Government's subsidy to those fishermen who would fish for carps in the major tanks. In order that this amount does not exceed the equivalent of Cey.Rs. 0.25 per kg (Cey.Rs. 6 million:Cey.Rs. 0.25 = 24 million) a total of 24 000 tons of carp would be the smallest quantity necessary during the 1990 to 2010 period.
A total catch of 24 000 tons over 20 years is equal to 1 200 tons a year. As the stocking would be done in 10 000 ha, this means a yield of carps in the order of 120 kg/ha and year.
On the assumption made above there is little reason to believe that even with modified gears the fishermen would be able to land 120 kg of carps per ha and year in addition to the quantity of tilapia that they would have caught anyway, without the stocking programme.
The conclusion is that the stocking is not economically viable. The conclusion should be reviewed, if it becomes obvious, as a result of the culture of carps in seasonal tanks that carp have a significantly better and larger market than tilapia.
