Domestication of wild golden mahseer (Tor putitora) and hatchery operation (by Joshi, P.L., T.B. Gurung, S.R. Basnyat and A.P. Nepal)

P. L. Joshi*, T. B. Gurung**, S. R. Basnyat***, A. P. Nepal***
*Regional Agricultural Research Station, Tarahara, **Fisheries Research Station, Pokhara
***Fisheries Research Station, Trishuli

ABSTRACT

Wild breeders of golden mahseer (Tor putitora), both males and females, which since 1989 have been domesticated in a large earthen pond (0.10 ha) regulated with running water, were transferred to an earthen pond (0.08 ha) at the Fisheries Research Centre, Trishuli, from 2 February to 9 April 1993. There the feeding rate was 2 percent of the body weight and feeding frequency was once a day. The fish responded to hand stripping for the first time on 6 April 1993 without any hormonal injection. The operation has continued since then without any problem. Hatchery operation of wild breeders became easier with their increasing age of domestication. Eggs were artificially fertilized; mean fecundity was 3 048 eggs per kg body weight. The diameter of eggs varied from 2.65 to 2.85 mm and the eggs hatched 3-5 days after being incubated depending upon temperature variations from 19 to 26°C. At hatching the total length and weight of alevins averaged 9.9 mm and 9.6 mg respectively. The fertility rate was excellent, over 90 percent, and the hatchability was equally good ranging from 75 to 77 percent. Alevins became free swimming fry 6-7 days after hatching. The success of hatchery operation of golden mahseer is an indication that it is possible to restore the endangered species. It has widened the possibility of reintroduction of hatchery-reared progeny in the wild and culture of the species.

1. INTRODUCTION

Golden mahseer (Tor putitora) are found in most of the south Asian countries including Nepal, India, Bangladesh, Pakistan, Afganistan, Sri Lanka, Myanmar. This popular game fish attains over 50 kg (Thapa, 1994). The population of this fish has been declining because of overfishing, also using destructive fishing methods such as electrofishing and poisoning, and because of the degradation of aquatic environment. India has already identified this fish as endangered (Shrestha, 1988a). Nepal and some other countries are in a stage of enlisting the fish as an endangered species. Strict application of the Aquatic Act and regular restocking of natural water bodies with appropriately sized mahseer can revive their stocks. A joint effort of restocking this migratory fish in the respective water bodies in the region can help to restore their stocks, and all countries should join a programme to revive the fish stocks in the lakes and rivers of their own. Nepal, India and Bangladesh have been attempting to develop large scale seed production technology of mahseer. Information on breeding of golden mahseer is readily available (Tripathi et al., 1977; Pathani and Das, 1979; Masuda and Banstola, 1984; Joshi, 1984; Shrestha 1987, 1988; Shrestha et al., 1990; Sehgal, 1991), but information on domestication of wild broodstock and its hatchery production is scanty (Ogale, this volume). The old practice has been to rear the wild mahseer in captivity. Brood fish grown in captivity can produce the required quantity of seed. Masuda and Banstola (1980) did not foresee the possibility of growing the wild breeders to sexual maturity in captivity. Shrestha (1990) believes that mahseer do not breed in stagnant reservoirs where water circulation is poor. However, the wild breeders grown in earthen ponds, not supplied with running water, attain sexual maturity and exhibit sexual play with the male chasing the female making a loop during the spawning time. The fish spawn twice a year. Its first spawning in April/May is followed by the second one in August/September. Males grown in captivity but it is difficult to sort out females just ready to spawn. This study describes the hatchery operation of wild golden mahseer reared in an earthen pond.

2. MATERIAL AND METHODS

The study was carried out at the Fisheries Research Centre, Trishuli, from 2 February to 9 April 1993. Ten wild breeders including six males and four females stocked in an earthen pond of 0.10 ha were transferred to a smaller pond of 0.08ha on 2 February 1993. Wild breeders collected from the Trishuli River and its tributaries had been domesticated in the earthen pond and exposed to running water (<22°C) all year around since 1989. The females sorted out for breeding belonged to different age groups, all being more than five years old. The experimental pond, which had a mean depth of 1.25m, was stocked with fish at a rate of 500 kg/ha. It was manured with 50 kg dried cow dung mixed with 2 kg triple superphosphate every fifteen days. The fish were given 35 percent protein content pellet feed (Table 1) at a rate of 2 percent of their body weight.

Table 1

Feed ingredients and the composition of pellets fed to golden mahseer in 1993

The feeding frequency was once daily. Temperature, pH (glass electrode), total alkalinity, dissolved oxygen (Winkler method) and Secchi disc transparency were checked once a week. The fish were checked for ripeness; next day they were circling at the water surface. Breeders which extruded eggs or milt upon gentle pressure on the abdomen were selected as the best ones. Mature males and females were rushed to the hatchery and anaesthesized with benzocain. Their total length and body weight were measured to obtain condition factor. Females were caught by their flanks, wiped dry with soft cloth before the eggs were stripped into a pre-weighed clean and dry plastic bowl. The males were gently stroked from upside down to squeeze out milt. A few drops of watery milt were discarded and the rest collected into a clean and dried beaker. Nearly 2 ml of milt was spread over eggs collected from each individual female. The eggs were gently stirred with avian feather to complete fertilisation. A small quantity of spring water was added, eggs gently stirred and washed for about 5 minutes to prevent clumping. The milky water was poured out and fresh water added, and the process was repeated until the eggs were cleansed of milt.

After hardening (about 10 minutes) eggs were poured on screen trays of the size 33cm x 33cm. Screen trays were then tied tightly keeping one on the top of the other and placed in a glass fibre incubation apparatus (215cm x 35cm x 36cm) slanted at 30 degree to the incoming water. Eggs were incubated with spring water and spring water mixed with river water in the incubation apparatus, with water flow at a rate of 2 litres per second. Water temperature was measured four times a day to determine incubation period. The developing eggs were covered with black cloth to protect them against light. Dead eggs dull in color or attacked with fungus were removed 24 hours after the commencement of incubation.

3. RESULTS AND DISCUSSION

Water quality in the breeding pond of golden mahsee at the Fisheries Research Centre, Trishuli, was regularly assessed during the period February-April 1993. Water temperature in the pond remained below 20°C in February and was higher than 20°C in the spring, reaching up to 22.9°C. Carps are reported to feed best at 20-25°C (Woynarovich, 1975). pH values, initially 6.5, gradually increased to 8.8. Reproduction diminishes at pH values below 6.5 (Mount, 1973). Waters with high alkalinity tend to be more strongly buffered than waters with low alkalinity (Boyd, 1979). Total alkalinity measured as CaCO₃ in the pond supplied with Trishuli river water, ranged from 27.4 to 39.5 mg/L. Natural waters, which have total alkalinity of 40 mg/L as CaCO₃, are considered to be hard waters, which are generally more productive than soft waters (Moyle, 1946). Dissolved oxygen concentration was more than 5 mg/L throughout the experimental period. It is undesirable to have dissolved oxygen below 5.0 mg/L in the fish pond (Swingle, 1969). Secchi disc transparency ranged between 47.2 cm and 32.8 cm. The visibilities depended on the concentration of planktonic organisms.

Males and females of golden mahseer did not exhibit any variation in color during spawning time. Mature males had prominent tubercles on opercula and front paired fins. Females did not show any spawning features except a slightly swollen soft belly. Two ripe females responded to hand stripping on 6 April 1993. It was the first time the wild breeders reared in captivity spawned without any hormonal injection. These domesticated fish were never known before to spawn when they were subjected to hand stripping every year during spawning season. It seemed wild breeders of mahseer attained sexual maturity in captivity with increasing time of domestication. The diameter of sinking type eggs ranged from 2.65 to 2.85 mm. Eggs incubated at varied temperatures resulted in excellent fertility (over 90%) and hatchability was equally good ranging from 75 to 77 percent (Table 2).

The rate of fertilization and hatching rate were reported to range from 34-100 percent (Joshi 1982, 1986; Joshi and Malkani, 1986; Sehgal, 1991) and 90-97 percent (Shrestha et al., 1990; Sehgal, 1991), respectively. Eggs fertilized on 6 April 1993 were dark orange in colour. Two more spawners, which did not respond to hand stripping three days before, extruded over-mature eggs just three days later on 9 April 1993. Over-mature eggs were faint yellow in colour. This may mean that the fish has a very short spawning period, may be less than a week. It is advisable to check the readiness of the breeders in the spawning time at least twice a week if not alternate days. Hatching started after 72 hours and was completed in 108 hours based on temperature differences (Table 2). Usually, incubation hours decrease with the increasing water temperature. But there was controversy in the incubation hours reported by different authors (Masuda and Banstola, 1984; Joshi and Malkani, 1986; Shrestha, 1987; Shrestha et al., 1990; Sehgal, 1991; Tripathi, 1977). Alevins became to free swimming fry 6-7 days after hatching. Mortality from alevins to fry was as high as 63%. The survival rate of alevins to fry was reported over 90 percent by Shrestha et al. (1990) at 29-30°C. Higher mortality of sac fry might be associated with lower incubation temperature. Walker (1968) found that a higher mortality and weak larvae resulted from an abnormally low hatchery water temperature.

Table 2

The average time between fertilization and hatching of wild golden mahseer reared in captivity at the Fisheries Research Centre, Trishuli

From 1993 onward the wild breeders reared in captivity have been responding to hand stripping every year either in April/May or in August/September, their spawning seasons. Next year the same breeding pond stocked with four mature males of a mean weight of 3.5 kg and two ripe females averaging 5.2 kg, showed a lot of young fry on 12 May nearly a month after being stocked. Unyil May none of the females extruded eggs in spite of repeated checking for readiness until May. On 19 July 1994 when over 1 500 fingerlings of a mean weight of 5.2 g were seen in the same pond with some circular nesting pits in the marginal region. The diameter of the pits ranged from 15 to 20 cm. This may mean that mahseer spawn naturally in a pond under favorable conditions. This finding of natural breeding in captivity and the success of hatchery operation are an indication that there is hope for restoring this endangered species. The successful breeding operation of mahseer will provide stocking material for open waters as well as for aquaculture.

The number of eggs stripped from individual females differed with size and age (Table 3). The number of eggs increased directly with increase in body weight and age of fish. The condition factor remained highest during the spawning season (Khanna, 1989). But the fecundity per kg body weight did not increase proportionately with increase in condition factor (Table 3).

Mean fecundity remained 3 048 eggs per kg body weight. The fecundity per kg body weight of wild ripe breeders ranged from 3 375 to 8 944 eggs (Das, 1984; Nautiyal and Lal, 1985). A little bit lower fecundity of the captive females might be caused by stress, not being well domesticated.

Table 3

Eggs released by wild golden mahseer reared in captivity, their condition factor and fecundity per kg body weight

4. CONCLUSIONS

Well domesticated wild breeders and young mahseer grown to maturity in captivity can be used for breeding without hormonal injection. The fish also spawn naturally in stagnant ponds and no running water is necessary for their gonadal development. The gonadal development is delayed when the broodstock is exposed to cold water, lower than 17°C for a long time (Martyshev, 1983). It is to be confirmed whether the same fish spawn twice a year or not. The spawning time differs a little bit from country to country depending upon climatic conditions. In Nepal the fish spawn from April/May to August/September. The success of breeding promises that it will be possible to produce stocking material under fish farm conditions both for open waters as well as for aquaculture, and to restore this endangered species in open waters. As the fish needs water temperature above 15°C to breed and can withstand temperature above 30-35°C it should not be called a cold water fish but rather a warm water fish (FAO, 1997)..

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