2. RESULTS

2.1 TYPES OF CAGES TRIED

2.1.1 Bamboo frame cages

Initially, cages were tried using a bamboo frame and a mesh of galvanized wire. The mesh began rusting after six months, not lasting a complete growing season. Therefore it was clear that these cages were not practicable (see photo 1).

A French plastic mesh “Netlon” was tried with better results but the bamboo deteriorated after one to one and a half years while the mesh remained good. Replacement of the frame was very difficult.

2.1.2 Wooden frame cages

A local wood which is very durable in water called sal (Shorea rohusta) was tried together with “Netlon” plastic mesh. The results were better than those with the bamboo frame cages but the wood was very heavy, making transport difficult.

A cage was tried later with the same materials using thin strips of sal wood. These strips began to break within a year during handling of the cage and because of rough water.

Wood frame cages with bamboo strips on the sides were tried (photo 2) but, considering the lifetime of the bamboo, it is felt that farmers are likely to be discouraged if they were to renew their cages every year or two.

2.1.3 Angle iron frame cages

A welded frame made of 3/4 - 1 in angle iron, which is available in Nepal, was tried together with “Netlon” mesh. Two sizes were tried out, one 4m × 2m × 2.5m and the other 4m × 4m × 2.5m with a central partition dividing it into two compartments of 4m × 2m × 2.5m. This double cage has the advantage that fish can be sorted out so that faster growing ones are kept in one side and the rest on the other. The cages were floated with empty oildrums. The latter were bound with wire to holding brackets welded to the main frame so that about 0.5 m of the cage rested above the water and 2.0 m beneath (Photos 3–6). The smaller cages were bound to a wooden walkway type of frame floated by oildrums (Photo 7).

A still smaller size of cage 2m × 2m × 2.5m (height) floated with two oildrums was also made, particularly for holding fingerlings.

The “Netlon” used was of two mesh sizes, one of 5.0 mm for holding small fngerlings and the other of 12.0 mm for large fingerlings and rearing fish to marketable size.

A rig floated on empty oildrums and fitted with a pulley was designed for transporting the cages or lifting them out of water for cleaning and repairs. It can be operated by one man (Photo 6).

2.1.4 Nylon net cages

(i) Norwegian

A few full units, each consisting of 15 mm mesh size net cage, 5m × 5m × 4m, cover, aluminium frame and six CC3-type polyethylene inflatable bouys were tried as experimental/demonstration units. Subsequently, frames were made locally using galvanized iron pipe and angle iron for the various net cages.

Some spare cages, ordered without their frames and with minor modifications, were floated using bamboo frames. The bamboo frame is cheaper than the galvanized iron/angle iron frame but needs replacement every 1–3 years depending on the quality of the bamboo.

The netting was treated with antifouling chemicals, which were effective under the conditions tried and saved labour on the regular cleaning that would otherwise have been necessary.

(ii) British

Two sizes of net cages of mesh size 19.0 mm, and dimension 4.5m × 4.5m × 4.75m (deep) and 7.5m × 7.5m × 4.5m (deep) were tried. These nets were also treated with antifouling chemicals.

(iii) Japanese

These were originally imported by the Japanese Overseas Co-operation Volunteers. These were of the following dimensions:

3m × 3m × 1.5m (depth) - of 2 mesh sizes 0.7cm and 1.0cm
5m × 5m × 2.5m (depth) - 1.5cm mesh
7m × 7m × 1.5m (depth) - 3.0cm mesh

2.2 COSTS OF VARIOUS CAGES

A number of different cage designs are now available, the choice depending on which lake cage fish culture is to be carried out, and the individual preferences of the farmer.

Prices quoted in Table 1 are based on actual expenditure incurred at the time of purchase of materials and construction of the cages during 1978. They may have to be revised in due course.

Table 1

DESCRIPTION OF VARIOUS CAGES

Both net cages and cages made up of rigid frames and materials such as plastic mesh, galvanized iron mesh or bamboo strips, are satisfactory provided there are no losses due to poaching or damage caused to the mesh. The chances of fish escaping are greater in a damaged net cage than in a cage with rigid plastic mesh because the latter tends to spring back into position. On the other hand, nylon net cages are easier to handle during growth checking and harvesting but greater vigilance may be required to prevent poaching.

In the case of net cages, the use of bamboos as floats is cheaper than using empty oil-drums or inflatable buoys but they need to be replaced every 1 1/2 to 3 years depending on the quality of the bamboos; where angle iron frames and empty oil drums are used, they need to be periodically cleaned and repainted to prolong their lifetime.

2.3 CAGE CULTURE RESULTS

The culture of common carp in cages would involve the use of balanced and economical feeds. In Nepal, fishmeal, blood meal and other protein type of feeds are very expensive; the former has to be imported from India. Trash fish is not available and even oilcakes, such as mustard seed cake, are expensive. Moreover, most poor fishermen and other prospective rural cage fish farmers would find it difficult, at least in the beginning to achieve economic conversion ratios.

Hence, it was decided to encourage the culture of plankton feeders as well as grass carp, since feed costs would be negligible.

The culture of these species gave encouraging results, which are presented below. Returns from the culture of these species is comparatively high because, at the present level of activity, feed costs are low.

Bighead carp (Aristichthys nobilis)

Experiments conducted on rearing bighead carp in Begnas and Rupa Lakes have given encouraging results. In Begnas Lake, bighead carp stocked at about 45.0 g in September 1978 grew to 400–700 g from September to February, i.e., within 5 months. In Rupa Lake, they grew from 41 to 700 g in five months.

Rohu (Labeo rohita)

Rohu were observed to grow in cages from 50–85 to 325–600 g during a period of 7 months. They were also able to go through the winter satisfactorily.

Table 2

GRASS CARP GROWTH DATA FROM PRIVATE CAGES IN THREE LAKES

Note: In Begnas and Rupa Lakes, grass carp were cultured in small numbers, with silver carp as the main species.

Table 3

SILVER CARP GROWTH DATA FROM PRIVATE CAGES IN THREE LAKES

Note: In Phewa Lake, silver carp were stocked in very small numbers, with grass carp as the main species.

Table 4

SILVER CARP GROWTH DATA FROM EXPERIMENTAL CAGES IN THREE LAKES

2.4 LIMNOLOGY AND BIOLOGY OF THE LAKES

2.4.1 Limnology

Table 5

COMPARATIVE LIMNOLOGICAL DATA OF THE THREE LAKES
(after Ferro, 1979b, and Ferro and Swar, 1979)

2.4.2 Biology

The three lakes differ significantly with regard to their biology. A summary of the data obtained as a result of studies carried out by the present authors is given below.

Table 6

SOME COMPARATIVE BIOLOGICAL DATA ON THE THREE LAKES

2.4.3 Gut Contents Analysis

A summary of gut contents observed in silver carp and bighead carp in February 1979. is given in Table 7.

Table 7

COMPARATIVE GUT CONTENT STUDIES OF SILVER CARP AND BIGHEAD CARP

2.4.4 Quantitative studies on plankton (Based on a study carried out in August 1978 in Phewa Lake and in September 1978 for Begnas Lake and Rupa Lake).

The figures given below are based on 5-litre samples taken at various depths with the help of a litre-sample bottle operated with a messenger at the required depth. The volumes given are after centrifuging for one minute with a hand centrifuge.

Table 8

QUANTITATIVE PLANKTON DATA FROM PHEWA LAKE FOR 31 AUGUST 1978

The finer phytoplankton was not retained by the net used in the above measurements.

Table 9

QUANTITATIVE PLANKTON DATA FROM BEGNAS LAKE (8 SEPTEMBER 1978)

Table 10

QUANTITATIVE PLANKTON DATA FROM RUPA LAKE (13 SEPTEMBER 1978)