4. DISCUSSION

4.1 According to qualitative and quantitative plankton studies Ceratium is the principal component in Rupa Lake, while other phytoplankton are insignificant. Likewise, in Begnas Lake, Microcystis is the predominant phytoplankton species, while other phytoplankton are insignificant. In Phewa Lake, phytoplankton nets did not collect much phytoplankton, although the water has a greenish colour, suggesting that the phytoplankton were extremely minute and could not be effectively utilized by silver carp. The faster rate of growth observed in Rupa Lake as compared with Begnas Lake suggests that Ceratium is better than Microcystis as feed for silver carp. Stomach content studies showed that these two species were consumed by the silver carp in the respective lakes. It is also interesting to note that the surface oxygen values varied from 7.5–15.0 ml/l in Rupa Lake whereas the variation was from 5.5–11.0 ml/l in Begnas Lake.

Table 3 indicates that silver carp growth within Phewa Lake can vary from location to location. Results on the growth of silver carp in cages located 100 m opposite the Fisheries Development Centre were not encouraging. The area near Khapaudi, however, gave satisfactory results, even comparable with results of Begnas Lake, although stocking densities were kept low for experimental purposes. Also between Khapaudi and Sedi the former gave better results.

Quantitative plankton studies (Tables 8, 9 and 10) in all three lakes indicate wide differences in plankton densities in different parts of the same lake. This will, of course, have important implications with regard to site selection for cagefish culture within any of the lakes. Further, studies have to be carried out with regard to seasonal variations in any particular location.

4.2 Similar observations were made with regard to zooplankton. In Phewa Lake larger quantities of copepods and nauplii were observed in Khapaudi than in Sedi and the location 100 m opposite the Fisheries Development Centre. Short horizontal tows with plankton nets showed that the highest concentration of cladocerans and copepods was in the 20 m deep trench. This would no doubt be of interest with regard to cagefish culture using zooplankton feeders such as bighead carp (Aristichthys nobilis) and Catla catla. A bloom of cladocerans was observed in most parts of the lake, but particularly closer to the Fisheries Development Centre, in February 1978 and February 1979. Perhaps this has some relationship to the increased fertility of the water as a result of the large number of ducks raised by the project in recent years. Some variations in the abundance of zooplankton were also noticed in the other two lakes; the larger cladocerans were more abundant closer to the submerged aquatic plants and outlets, particularly in Rupa Lake.

4.3 Production figures per cubic meter quoted from other countries for other species are frequently higher than those given from the work reported here. In Viet Nam, Pantulu (1979) reports production figures from 42.5 kg/m³ for Barbus altus and Lepto barbus (over 11 months) to 122.2 kg/m³ for common carp (Cyprinus carpio) over similar periods. Suzuki (1979) reports harvest 20–50 kg/m³ for common carp in Japan. Kuronuma (1968) reports production of 2.96 kg/m³ to 22.55 kg/m³ for common carp in Japan. The lowest production is accomplished in two months, while the highest production took three months. Coche (1979) gives a variety of production figures, some of which fall within the range reported here. Highest figures are reported for common carp (164 kg/m³ in 6.5 months in the Netherlands) and channel catfish (91–175 kg/m³ in 7–9 months in Oklahoma, USA, Collin, 1972). Such figures may be due partly to intensive feeding with balanced diets. Stocking rates per cubic metre tend to be higher in most instances. Usual stocking rates, with hundreds per cubic metre, are reported by Cyprinus carpio, Salmo spp., Ictalurus punctatus and Tilapia spp. (Coche, 1976). Stocking rates of lower than 50/m³ are uncommon. Japanese culture of yellowtail (Seriola quinqueradiata) is as low as 10/m³, (Ling, 1973) and this is the lowest reported.

Table 13

STATUS OF ALL PRIVATE CAGES AS OF 31 DECEMBER 1978

¹ Cost of cage is 50% subsidized as an introductory measure.
² Initial stocking only; a few cages have been partially restocked to replace most fish.
³ Principal only.
⁴ As of growth checks, early December, last four values as of stocking time in mid-November.

The cost behind most of these production figures involve the use of expensive commercial feeds, which the average fishermen in the Pokhara Valley could not afford and which are not locally available at present. Therefore, it was decided to encourage the culture of planktivorous and herbivorous fish, which would require minimal recurrent expenses on feeds.

Only one reference to Chinese carp cage culture was readily available (FAO, 1977). Silver carp fry have been cultured in heated effluent in Poland but harvesting figures were not provided.

The figures reported for cagefish culture of Chinese carp in Nepal may seem relatively low (Table 2, 3, 4 and 11), due largely to an inadequate fingerling supply and the consequent use of smaller stocking numbers per cubic metre. Nevertheless production of grass carp in private cages in Phewa yielded an average of 3.6 kg/m³ after six months and an annual production of 6.7 kg/m³ is projected. Silver carp production for Begnas ranged from 2.7–5.6 kg/m³ after a year of cultivation, while in Rupa production from the same species reached 6–9 kg/m³ after seven months. In the case of silver carp, no feed was introduced to the cages; this production is attributable to the abundance of plankton in Begnas and Rupa Lakes.

In Table 13, total weight increase per cage for the first six cages (totals of columns 7 and 12 averaged) compared to total weight stocked per cage (total of column 6 averaged) indicates an overall weight increase of almost six times within six months.

Considering only sales returns (col. 8) and estimated value of remaining fish (col. 13) for the six cages in Phewa Lake, complete repayment of loans appears possible by the second or third summer, if virtually all sales proceeds go to the bank. If about 50 percent of the sales proceeds are used for subsistance, payment should be easily completed by the third to fifth summer. This is at the 50 percent subsidized rate. If the cage is not subsidized, they should be able to settle the loans by the fifth to sixth summer.

In Begnas and Rupa Lakes, particularly the latter, the settlement period should be considerably less owing to the better growth rates observed.

The cages hitherto used in Phewa Lake were of angle iron frames with “Netlon” mesh and floated with empty oildrums. Their estimated lifetime is about 10 years, subject to proper and regular maintenance. The species cultured in them were principally grass carp, with small number of rohu, common carp and silver carp.

In Begnas and Rupa Lakes nylon net cages were used. The species cultured were principally silver carp, with small numbers of grass carp, bighead carp and common carp.

The culture of grass carp involves feeding with grass or other fodder, while silver and bighead carp are not fed, owing to the present availability of adequate plankton. Addition of cow dung helped to increase production, possibly through its direct consumption along with the insect larvae it often contains, or through local fertilization effects.

The cages in all three lakes were regularly cleaned in order to ensure better circulation of water.