Lake Phewa near Pokhara has a surface area of 400 ha, maximum depth is 19 m and average depth 7.6 m. The lake has a temperature stratification from the end of February until October. The surface water had a minimum temperature of 15.2°C in January and a maximum of 28°C in June. The temperature stratification causes a chemical stratification to build up. The water below 7 to 9 m is anoxic from April to November. Further the pH of this water is lower than at the surface, while the conductivity is higher.
Surface water is mostly saturated or somewhat oversaturated with dissolved oxygen.
Lake Begnas is situated about 10 km east of Pokhara. It has a surface area of 225 ha, maximum depth of 7.5 m and an average depth of 4.6 m. The lake has an instable temperature stratification. At surface the water temperature varies from 15.1 to 30.3°C over the year. Anoxia occurs below 3.2 m depth at times. At 6 m depth dissolved oxygen is very low (less than 1 mg litre) from March to October. The surface water is mostly somewhat oversaturated with dissolved oxygen.
Lake Rupa is situated just east of Lake Begnas. The area is 117 ha, maximum depth is 4.5 m and average depth is 2.3 m. Temperature stratification occurs at times but is never stable. Lake water temperature varies from 15.0 to 30.2°C over the year. The surface water is mostly oversaturated with oxygen. Below 2 m depth oxygen sometimes is low.
The smaller lakes in the Pokhara valley are less than 10 ha each and shallow (maximum depth 2 to 3 m). They are overgrown with aquatic plants.
The difference between Lakes Phewa, Begnas and Rupa originate initially from differences in depth. Lakes Begnas and Rupa are also more sheltered than Lake Phewa.
The potential of the natural fish catch can be estimated on the basis of the Morpho-Edaphic-Index (i.e. conductivity divided by depth) or on basis of the primary production. Estimates for the Pokhara Valley lakes vary from 17.2 to 51.1 t of fish per year. These estimates are, of necessity, rough.
The potential catch estimates show the limit of production of the natural system. Stocking the lakes with fish will not change the overall capacity of production, as the basic limits are abiotic factors such as light, temperature and available nutrients. Stocking could thus only be useful if there are no indigenous fish to use the environment optimally. Alternatively, it could function as an artificial recruitment to boost an overfished stock. However, the same result could be achieved by proper management of the fishing and then the problem would mainly be to know what is socially more desirable: would it be more acceptable to diminish fishing activity with an inevitable initial drop in catches or should money be invested to release fingerlings, thus involving a yearly recurring expenditure?
Fish culture in cages without feeding uses the same food resources as the natural fishery. Thus, production is bound to the same limitations. An advantage is that the cage is someone's property and he can let the fish grow to the right size. Predation is also diminished in cages. In other words, cage culture could be more effective in harvesting a lake than natural fishing; however only plankton is available as food for the caged fish.
By feeding the fish in cages the cage production can be increased. This introduces organic material into the lake and thus eutrophicates the lake. This is turn would mean increased primary production and higher natural fish production. Natural fishing could benefit indirectly from the feeding of caged fish.
The eutrophication, if too much, could produce harmful effects as from any water pollution with organic materials. The basic danger is fish kills through low dissolved oxygen values. This would be one limit on the expansion of cage culture.
About a dozen fish species are regularly recorded in fish catches in the Pokhara Valley. They vary in size, as indicated by the maximum sizes recorded (Table 1). Sahar and katle grow the largest and probably live the longest.
Most species breed from early spring to autumn (Table 1). Only asla is an exception. This is a river fish, while the other species are mostly fished in the lakes.
The food habits of the different species are quite different, indicating that the food resources of the lakes are probably well used.
Besides the species recorded in Table 1, other species are sometimes found, such as bam (Mastooembalus sp.) and buduna (Labeo sp.), The last species is found mainly in paddy fields and is object of a separate small fishery.
Data were collected mainly from the Podhe caste fishermen from March 1977 to February 1978. They are the traditional full-time fishermen, and as such, a target group for the project. However some of them have now moved to other businesses and only fish part time.
The most important gear is gillnets and castnets. Gillnet mesh sizes correlate positively with the size of the nets. Only a few hooks were found but they are probably under recorded. In general, only part of the gear is licensed.
Only six species (groups) are important quantitatively. In Lake Phewa these are especially rewa and phageta. In Lake Begnas and Rupa, junge and bitta are caught the most. Kade is also recorded from all three lakes in relatively important amounts. Sahar is the most valued species but it is only a minor part of the Podhe fishermen catches. The catches made by the Fishery Section were about 44 percent sahar. However the Fisheries Section uses larger meshed gillnets than the Podhe fishermen.
Total catch per year is known for the Fisheries Section from 1963. There has been a great decline in the catch since 1966. These data can be interpreted as showing overfishing of part of the fish stock (i.e., sahar). No decline is apparent in the catches of the Podhe fishermen.
For years when data are available, the catch of the Podhe fishermen was estimated as 16.1 to 18.6 from the lakes. This is comparable to estimates of the potential as discussed above. Especially as it must be taken into account that groups other than the Podhe also fish in the lakes.
The best interpretation seems to be that population growth and the introduction of gillnets have put pressure on the fish stocks in the lakes. This resulted in a fishing up of the big, long-lived species such as sahar and katle. The shift towards small, short-lived species in the catch was probably not accompanied by a great decline in overall catch. The situation could be stable, or made stable, by a small decrease in fishing pressure. However, restoration of the sahar stock would need a strong interference from government.
The riverine fishery is probably fairly well exploited, since 3.1 t of fish were caught by the Podhe fishermen in rivers over one year. However due to the widespread fishery, it is very difficult to evaluate.
In October 1977 a four day visit was paid to Lake Rara in the far western region of Nepal. It is situated at a height of 2 990 metres and is the largest lake in Nepal (10 km2).
It was found that the maximum depth is 167 m and the average depth is 104 m. The sides are steep, with a very flat and deep central area.
It appears that the temperature cycle is as follows: maximum surface temperature 20–22°C, minimum water temperature 6–7 °C, with a summer stratification and one winter turnover. The hypolimnion loses some oxygen over the stratification period but is never anoxic. Conductivity and pH are quite high, perhaps due to limestone in the catchment basin. The lake is very clear.
The fish population is probably monospecific (Sohizothorax sp.) and could be used more as a protein supply for the local people. At least several tons of fish could be harvested per year.
As the lake is part of a national park, the nearby villages will eventually be removed, so that then fishing would cease anyway. Up to that time, however, fishing with spears and maybe castnets could be allowed without endangering the fish population.
It seems logical to include aquatic life under the protection of a national park. Therefore, introduction of exotic species such as rainbow trout, is not desirable. They would compete with the snowtrout (Schizothorax) for food and predate on small snowtrout, which could decimate the snowtrout. Due to normal energy losses, conversion of snowtrout into rainbow trout would entail a loss of maybe 90 percent of the fish biomass.
On the basis of the Morpho-Edaphio Index, it is possible to obtain estimates of the fish production potential of a reservoir. This technique can be applied even before the reservoir is built, given the expected area, mean depth and conductivity.
Thus, it is possible to estimate that the Kulekhani Reservoir could produce some 2.7 t of fish per year. The projected reservoir on the Karnali River would be the largest lake in Nepal and have a potential of 285 t of fish per year.
Although these figures are only indicative, it is clear that the reservoir in the Karnali River would warrant the close attention of the Fishery Section of the Government while the Kulekhani Reservoir would only have a very limited fishery potential and it would not be useful to spend much time on its fishery development.
