Production of Mahseer

The Deccan mahseer, Tor khudree is common in the upstream regions of rivers originating from the Western Ghats, and is included in the list of endangered species as per IUCN status. Ranching of hatchery produced seed can be adopted as an ex-situ conservation practice. The mahseer inhabits fast flowing streams and rivers of the hilly areas with richly oxygenated waters and rocky bed. It is the most preferred fish in hilly area, attaining large size, having excellent taste and ornamental beauty. It is a powerful fish which exhibit good fighting skill to wriggle- off the angling hook. Hence, it is considered as an ideal sport fish that provide incomparable recreation to anglers and offers excellent prospects for the growth of eco-tourism.

Mahseer is endowed with good power of locomotion. The fish is omnivorous in the larval stage, carnivorous in the juvenile stage, and herbivorous in the adult stage. It has mouth suitable for rasping encrusted organisms and removing algal slime on rocks and boulders. It has narrow gill opening and reduced gill; hence it needs water with DO above 7 ppm. Sexually mature fish ascend to a height ranging from 800- 1800 m above MSL, travelling a long distance and prefer clean water for breeding. During floods, the stream swells due to monsoon rains and the newly inundated area serves as nursery ground and forms a secured site for the young-ones which mostly prefer the marginal upland area having stones constantly flushed by the flow of water rather than the swift currents of lower regions of river. The factors that influence its spawning include water temperature, velocity, pH, turbidity and rain. Breeding season of mahseer is July to September. It breeds several times in a year and usually lays eggs in sheltered rock pool.

The major reasons for depletion of mahseer stock are degradation of aquatic ecological conditions, indiscriminate fishing of broodstock and anthropogenic interventions in breeding habitats. Destructive fishing methods like use of dynamite, poison, electricity and construction of barrier in natural migratory routes prevents the breeding migration.

Seed Production

Broodstock management

The male and female fish attain maturity in 2^nd year and 3^rd year respectively. The brood fish is maintained in earthen pond or cage and fed with a formulated feed having 40% protein prepared with groundnut oil cake, rice bran and fish meal in the ratio of 3:3:4, which results in faster and healthy gonadal development. The gravid fish is also collected from the wild during its spawning migration by using gill net. The gravid fish generally releases ripe eggs with a slight struggle on entangling with the net. So, wild caught ripe fish is stripped immediately, to avoid loss of eggs.

Selection of brooder

The male and female fish is segregated in the early hours of the day. During spawning season the ripeness of the female fish is ascertained by the softness of abdomen, pink colouration of the vent and the free release of eggs on applying a slight pressure on the belly. In male fish, its readiness is confirmed by the oozing out of milt on exerting gentle pressure near the vent.

Spawning

The brooder is induced for spawning by intramuscular injection of synthetic hormone like Wova-FH as a single dose at 0.8 ml/kg of the body weight for female and at 0.4 ml/kg of the body weight for male. (The brooders collected from natural spawning ground are kept in spawning tank with showering but without hormone administration). If the pair does not spawn within 6-12 hours, stripping is done. The female is stripped as explained for walking catfish. Colour of the egg is pale yellow to bright orange. The stripped eggs are adhesive in nature till they become ‘water hardened’. After the eggs are stripped, the milt extruded from the male fish in the similar way is poured over the eggs. One teaspoon of milt is sufficient to fertilise the eggs produced from 2-3 females. The fecundity is 3500-8900 no./kg bodyweight.

Stripping of female & Stripping of male (Courtesy. DCFR)

Fertilisation takes place instantly while mixing the eggs and milt with a quill feather. The eggs are allowed to remain inside the basin to ensure complete fertilisation. The basin is kept undisturbed for 30-40 minutes with protection against direct sunlight for ‘water hardening’. After that, excessive milt and extraneous materials are removed through repeated washing. The fertilised egg is demersal and attains a size of 3.5-4.0 mm on water hardening. The comparatively larger size of the ova is an adaptation to tide over the scarcity of larval food during monsoon, when the fish spawn.

The rate of fertilisation is ascertained by acetic acid method. A sample of water-hardened egg is kept in 5% glacial acetic acid solution for 24 hours. The viable egg is transparent while the unfertilised one becomes translucent. Proper stripping of the ripe brooder ensures more than 90% fertilisation. After water hardening process, the eggs are quantified by volumetric method. Normally, the egg density is 35-60 no./m1. The fertilised eggs are transferred for incubation and protected from direct sunlight.

Incubation

The fertilised eggs are disinfected and placed for incubation in rectangular FRP hatching trays of 75 cm x 50 cm x 10 cm and its bottom and sides are fitted with wire mesh of 1 mm mesh size. Two such hatching trays are placed in an FRP tank of 200 cm x 60 cm x 30 cm size on a stand of 90 cm height with separate inlet and outlet at opposite ends in such a way that the rate of drained water is equal to that of incoming water. The inlets are so designed to sprinkle the incoming water. The incoming water should be cleaned and filtered, having more than 5 ppm DO. The water level inside the tank is maintained in such a way that there is water up to a height of 4 cm inside the tray. Continuous freshwater flow is maintained in the tank and the ideal water flow rate for incubation is 0.5-1 l/min. Each tray can hold 15000-30000 eggs, which mainly depend on water quality. The eggs are uniformly distributed in the tray to avoid crowding, fungal infection and clogging. The eggs hatch out in 76-96 hours at 19-24⁰C with a hatching rate of 80- 85%. Yolk absorption is completed in 10-12 dph.

Rearing of hatchling

Rectangular FRP tank with 200 x 60 x 60 cm size or circular cement cistern with 2 m diameter and 0.6 m height having separate inlet and outlet facility is used for rearing the hatchlings. The swim-up fry is fed with sieved zooplankton like Moina, Daphnia etc or boiled and macerated egg. Micro-encapsulated feed can also be used. Feeding is done 3-5 times daily. It attains a size of 12 mm in 15 dph and is transferred to cement tank with 4 m x 2 m x 0.75 m size. The stocking density is 1000 no./m2. Continuous water flow at 3 1/min is maintained in the tank. It attains a size of 25-40 mm in 30 dph.

Rearing of fry

The fry is further reared in earthen pond with a stocking density of 500 no./m². The continuous water flow is enhanced to 4-6 l/min. The young one is fed with a formulated feed consisting of groundnut oil cake, fish meal, ragi flour and chicken egg yolk in equal proportion.

Packing & transportation

As the hatching period is more than 76 hours, the fertilised eggs after water hardening is placed as 2-3 layers in between moist cotton kept in a plastic box and transported. After proper conditioning, seeds are transported in oxygen filled bag. Use of 0.05-0.3% salt during transportation decreases its metabolic activity and sensitivity to stress.

Source : Department of Fisheries, Government of Kerala