A.K. Rai, B.R. Pradhan*, S.R. Basnet**
Fisheries Research Division, Godawari, P.O. Box-13442, Kathmandu, Nepal.
*Nepal Agricultural Research Council, Singha Darbar Plaza, Kathmandu.
**Fisheries Research Centre, Trishuli, Nuwakot.
ABSTRACT
Snow trout, a cold water riverine and short migratory fish is locally known as asla. It belongs to the family Cyprinidae and sub-family Schizothoracinae which are widely distributed in the Himalayan and sub-Himalayan region and much of the rest of Asia. Altogether 28 species of snow trout are reported of which 6 species of genus Schizothorax and 3 species of genus Schizothoraichthys are recorded in Nepal. Schizothoraichthys progastus (altitude distribution 300-1 820 m), locally called Chuche asla, and Schizothorax plagiostomus (altitude 345-3 323 m), called Buche asla, are high value sport fish and are common in Nepal. Asla is a phytophagous fish and has developed a special mouth to scrape the algae attached on stones. Asla spawn twice a year during September/October and March/April, but September/October is the best season for spawning. Clear water, stony bottom of creeks composed of fine pebbles and gravel, and water flow of 2.8-4 m/sec, pH 7.5 and dissolved oxygen concentrations of 10-15 mg/L form good spawning conditions in the natural environment. Asla has been bred artificially in the Fisheries Research Centre Trishuli since 1971 but the hatching rate is low and mortality of alevins is still high (>50%). Fully mature females release orange and sticky eggs when gently pressed on the abdomen but it requires a very specific time for spawning to get good quality of eggs.
Asla brood fish maintained in concrete raceway ponds and fed 35% protein content diet at 1-2% of body weight produce about 12 000 eggs/kg. Hatching takes place within about 11 days at water temperature 15-20°C and Buche asla is easier to breed than Chuche Asla. The development from one- day hatchling to free swimming stage varies inversely with temperature, ranging from 5 days at 24°C to 24 days at 12°C, and the survival rate is higher (>90%) at 20-24°C, lower (65%) at 17°C and <50% at 11-14°C. Hatching rate is higher (50%) during October/November than during February/March (<25%). Asla grow up to 20-25 g in two years and reach 100 g in the third year and increase considerably in the following years. Asla has specialized feeding habit due to its mouth structure, which cannot take larger size of food. No technology has been developed yet for commercial culture, particularly due to not being able to develop appropriate feed for asla. Further studies are needed, with emphasis on suitable feed development required for larval rearing, which seems to a very important as well as critical stage.
Nepal, a Himalayan kingdom, covers about 147 000 km2 with the altitude varying from 60 to 8 850 m above sea level. Geographically it is divided into Himalayan region, mountain region, mid-hill region and Terai - the flat area region. The climatic conditions vary with the altitude. There are also seasonal changes that can be defined as autumn, winter, summer and monsoon seasons. The water originates in the Himalayas in the north and flows to the lowlands in the southern part, with water temperature increasing accordingly. The air temperature varies from below 0°C in the Himalayan region to about 44°C in the southern Terai region of the country. Therefore temperature of water bodies differs from place to place and depends primarily on altitude and whether the water source is snow-fed or not. Nepal is rich in water resources, with more than 6 000 rivers and rivulets cover an estimated area of 395 000 ha, lakes 5 000 ha, and small reservoirs 1 500 ha (FDD, 1998). Koshi, Gandaki and Karnali are the three major river systems, which are fed by hundreds of small rivers originating from the snow-capped Himalayan mountains and flowing south.
The great variation in climatic conditions between the north and the south of the country is due to the great differences in altitude.The rivers accordingly exhibit a wide variation of aquatic habitats. The difference in ecological conditions ranging from oligotrophic glacial cold waters in mountainous and hilly regions to slow flowing or stagnant eutrophic warm waters in the lowland Terai is also reflected in the diversity of fish species. Fish species have different form adaptations, different feeding habits and different special organs to be able to attach themselves to rocks in the fast current. Shrestha (1995) has reported a total of 186 fish species in Nepal, of which she identified 18 species as cold water fish. An updated list (Shrestha, J., this volume) mentions 163 freshwater fish species for Nepal.
2.1 Description
Asla has a conical head with slender, elongated and strong body to resist the strong water current of the hill streams and rivers. The body is covered with minute silvery scales and the abdomen with lighter brown scales. Snout bears nuptial tubercles and the size and number are well developed in males. They are grayish black on the dorsal side and silvery on belly and sides. A distinct suctorial disc in addition to 4 barbels is present on the chin for attachment to stones. Maximum weight and size are 1.5 kg (48 cm) reported by Rajbanshi (1971), but up to about 5.0 kg in the Seti River and about 2.5 kg in the Trishuli River and Sunkoshi River according to the local fishermen.
Schizothorax plagiostomus, locally called Buche asla (Fig.1a), and Schizothoraichthys progastus, called Chuche asla (Fig. 1b) are present at altitudes 300-1820 m and 345-3323 m (Shrestha 1995), respectively. Misra (1959) has described Schizothorax and Schizothoraichthys based on the shape of snout, with Schizothorax having a blunt snout and suctorial lip whereas Schizothoraichthys has a pointed snout and no suctorial lip (Fig. 1c). Schizothorax plagiostomus and Schizothoraichthys progastus are common and dominant fish in most of the rivers and lakes of Nepal and are good sport fish as well as delicious to eat. Schizothorax plagiostomus is called golden snow trout by virtue of its silvery golden colour. These two species have been studied at the Fisheries Research Centre, Trishuli since 1971. Culturing of these species is still not fully developed due to the problem of developing appropriate feed. Snow trout is a short-distance migratory fish which enters tributaries for breeding. The presence of dam blocks its migrations. It migrates downstream during winter and upstream during early June when water becomes turbid. The migration pattern varies from species to species and also depends on the volume of water in rivers and on water temperature.
2.2 Distribution
Genus Schizothorax, a cold water river fish locally known as asla and more generally as snow trout, (Cyprinidae, Schizothoracinae), is widely distributed in the Himalayan and sub-Himalayan region of the Indian-Chinese sub-continent. In the Tran-Himalaya region snow trout is found in India in cold waters from Jammu and Kashmir (Sunder and Bhagat, 1979) to Nainital where Menon (1971) reported a new species Schizothorax kumaonensis. Schizothorax plagiostomus (McClelland) forms significant indigenous fishery in upland waters of Jammu and Kashmir and represents about 10-20% in total commercial landings (Raina et al., 1985). Jhingran (1982) reported that this species is distributed from Asam and eastern Himalayas through Bhutan and Sikkim at an altitude of 1 180-3 000 m. In Nepal this fish species has been reported from rivers and lakes/reservoirs at an altitude of 300-3 323 m (Shrestha, 1981). It needs a high dissolved oxygen concentration as also required by the rainbow trout. Some of the high altitude lakes in Nepal are inhabited only by Schizothorax spp (Ferro, 1977; Pradhan, 1982).
Fig. 1 - (a) Buche asla Schizothorax plagiostomus; (b) Chuche asla Schizothoraichthys progastus;
(a) Buche asla
(b) Chuche asla
(c) Buche & Chuche asla
Shaw and Shebbeare (1937) also reported Schizothorax plagiostomus from Nepal. Schizothorax spp. generally prefer rapids and pools of snow-fed torrential streams with temperature ranging from 8°C to 22°C (Sharma, 1989) and can tolerate the high range of water temperature of 0-32°C (FRC, personnel communication). It is also present in lakes without a connection to flowing rivers or streams (Pradhan, 1982). Sharma (1989) has reported 28 species of snow trout in Himalayan and sub-Himalayan regions including Tibet, China and Pakistan. In Nepal 9 species have been recorded of which 6 species are of the genus Schizothorax and 3 species of the genus Schizothoraichthys (Shrestha, 1995). Schizothorax macrophthalmichthys, S. nepalensis and S. raraensis recorded by Terashima (1984) are endemic to Rara Lake (2 900 m altitude) situated in north-western Nepal. However, the populations are decreasing rapidly due to a heavy fishing pressure, natural calamities, as well as human disturbances such as construction of dams and roads.
2.3 Feeding habits
Asla is a phytophagous fish, with its mouth adapted to scraping attached algae from the surfaces of stones. The feeding habits of asla have been reported by many authors (Shrestha, 1979; Masuda and Karki, 1980; Terashima, 1984; Sharma, 1989). It feeds on attached algae including Spirogyra, Ulothrix, Oedogonium, as well as on the benthic insect larvae of mayflies, caddis flies, ephemeropterans, etc. Fry feed on larvae of chironomids and caddis flies, but also on microscopic algae. S. plagiostomus is herbivorous and feeds on aquatic plants and algae attached to stones and rocks (Shrestha, 1979). Asla food was found to contain predominantly green and blue-green algae, folllowed by detritus and aquatic insects. Diatoms attached to rocks, stones and boulders as aufwuchs form the primary source of food in torrential streams whereas aquatic macrophytes, decayed organic matter and green algae are minor diets.
2.4 Spawning
Asla spawns when two years old, depending on food supply. Sexual dimorphism is developed in S. plagiostomus (Rajbanshi, 1971) and in S. macrophthalmichthys, S. nepalensis and S. raraensis (Terashima, 1984) in anal fin, presence of nuptial organs and size of the basal sheath scale. Mature asla has a change in colour during the breeding time. Mature males develop tubercles on either side of the snout, faint yellow colour of the body, and reddish colour of fins. Females spawn in natural as well as in artificial environments. Asla can spawn naturally or by stripping the wild/cultured mature female during the spawning season. It spawns in September/October and March/April.
2.4.1 Natural spawning
Sexually mature asla spawn naturally when they reach 18-24 cm length. They attach eggs to the substrate. Fry stay on sand and gravel bottom. Asla spawn naturally in clear water on gravelly/stony grounds or on fine pebbles at 1-3 m depth (Shrestha and Khanna, 1976). Water current of 2.8-4 m/sec, pH 7.5, dissolved oxygen concentrations of 10-15 mg/L, and gravel size of 50-80 mm are the optimal conditions for spawning (Shrestha and Khanna, 1976).
2.4.2 Artificial spawning
Fully mature female fish both wild and raised in captivity release eggs when gently pressed on the abdomen but have a very specific time for spawning, which should be checked frequently to know the right ovulation time to get good quality eggs during stripping. Since 1971 the Fisheries Research Centre, Trishuli, has been breeding asla. It is still experiencing low hatching rates as well as high mortality of alevins (>50%), which might be due to not being able to develop proper feed for alevins as well as for brood fish. Spawning is better during September/October than in February/March, which means temperature and spawning are closely related. The optimal water temperature is 15-16°C (FRC, personnel communication). Mature females need to be checked at 2/3 day intervals during the spawning season otherwise over-ripe or under-ripe eggs are squeezed out, which affects the hatching and survival rates. Males produce sperm throughout the year. Proper broodstock management with good nutritive feed helps to develop good quality eggs and October/November is probably the best season for spawning of Schizothorax plagiostomus and Schizothoraichthys progastus.
Asla broodstock are maintained in concrete raceway ponds (23.3 m2) in the Fisheries Research Centre, Trishuli. They are fed 35% protein content pellet diet at 1-2% body weight daily, depending on temperature. Broodstock used for spawning are 3-6 years old, and males and females are kept separately 2 months prior to spawning. Females are checked weekly or twice a week during the spawning season. Eggs are fertilized with milt collected from male by stirring them gently with avian feather. Fresh water is added and eggs are washed 3/4 times till they are clean, swollen and not clinging. After about 5 minutes, fertilized eggs are poured in incubation trays made of screen which is placed in a fiberglass incubation apparatus (215 x 35 x 36 cm) (Fig. 2). The incubator is shaded from direct light, inclined at 30 degrees, and has a constant water flow of 20 litres per minute. Unfertile or damaged eggs become dull and may be attacked by fungus. They must be separated from the good eggs so as not to contaminate them. The mean fecundity of asla ranges from 5 000 to 12 000 eggs/kg body weight (FRC, Trishuli, 1994). Shrestha (1978) has reported about 25 000-40 000 eggs per fish. The eggs are sticky, orange in colour, toxic and cause diarrhea and dizziness when eaten. They lose the toxicity after being kept in water for some time. Two females of 242 g and 261 g, respectively, with the egg size of 2.54 mm ±0.2 - 13.3 mg ±3.0, released 11 713 and 11 697 of eggs per one kg of body weight.
Fig. 2 - Incubation trays made of screen and placed in fiberglass incubation apparatus
Incubation water temperature of 15-20°C might be the optimum temperature as it takes about 11 days for hatching (FRC, 1991). Shrestha (1978, 1979) has also reported that hatching takes place within 11-18 days at water temperature of 18-21°C. Buche asla is easier to breed than Chuche asla but egg fertilization from artificial breeding is low. This could be due to the stripped eggs being mixed with blood, faeces and scales during stripping. Eggs have to be cleaned with salt water or other chemicals before fertilizing them with milt. Asla fry were similar to Ayu fry in Japan (Wada, personnal communication). Ayu fry was fed every 2 hours after hatching and then twice a day after one month. The development from one day hatchling to free swimming stage varied inversely with temperature ranging from 5 days at 24°C to 24 days at 12°C and the survival rate was also higher (>90%) at 20-24°C and lower (65%) at 17°C and <50% at 11-14°C (FRC 1994). The hatching rate was higher (>50%) during October/November than during February/March (<25%) (Fig. 3) and October/November might be the best time for spawning. The size of hatchlings varied from 3.9-9.6 mg and survival was about 47%.
2.4.3 Growth
Chuche asla fed 35% protein content diet at 2-5% body weight grew from 0.7 to 7.2 g in 359 days but lost weight in December/January when temperature was 10-12°C. Buche asla grew better than Chuche from 1.8 g to 17 g in 295 days and females grew faster and larger than males (FRC, 1991). S. plagiostomus grow up to 226 g and S. progastus grew up to 363 g in the fourth year (Masuda, 1979). The growth of asla is very slow reaching 20-25 g in two years and 100 g in the third year (FRC, personnal communication). Dhar (1967) reported the fish to reach 175 g in the second year and 330 g in the fifth year. Alevins at 20-24°C reached the free swimming stage within 5-6 days and swam at the surface in 8-10 days.
Fig. 3 - Hatching rate is higher (>50%) during October/November and lower (<25%) during February/March.
2.5 Potential
Asla is a very good sport fish, tasty and liked very much by the people. It also fetches high price compared to other fish species. Asla is considered as an economically important high value fish of the Himalayan and sub-Himalayan regions. To bring it under culture needs proper feed and this is a major task for research. Without it asla cannot be yet produced for markets under aquaculture conditions. Asla is a sport fish and could support sport fishery in the mid-hill region. From this the local people could benefit as it would encourage tourism. The required habitat for asla is present in the hills and mid-hill regions. Once the feed problem is solved production of stocking material for regular releases into streams and rivers can be made. This would also make possible full-cycle fish production under aquaculture conditions. Overcoming difficulties of mass production of asla stocking material will require regional cooperation in which scientists of the countries of the Trans-Himalayan region closely collaborate.
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