Production of Walking Catfish

The walking catfish is an air-breathing fish commonly known as magur, which lives in all types of fresh waterbodies. Clarias magur, Clarias dussumieri and Clarias dayi are the main species of walking catfish found in India. The pectoral fin of this fish is characterized by a rigid spine-like structure which is adapted for terrestrial movement even over dry land and hence, it is named as “walking catfish”. The body is elongated and narrow towards the caudal region. It has a flat and broad head with small eyes, four pairs of sensory barbels, wide mouth with fleshy lips and small teeth patches on both upper and lower jaws. As a defence mechanism, the fish can cause painful stings with its pectoral fin spine. It posess arborescent organ for aerial respiration.

The walking catfish is adapted to live in water with less DO (0.1 ppm) and high CO₂ (70 ppm), which makes it suitable as a candidate fish for intensive farming in derelict waters. It undergoes aestivation for months without feeding in order to overcome water scarcity. It is a voracious feeder preferring small fish and insect. The population of walking catfish is being threatened due to the destruction of its natural habitat coupled with excessive use of pesticides and herbicides. Walking catfish has an excellent nutritional profile due to high protein, low fat and high iron content of flesh. It is beneficial for pregnant and lactating women and convalescing persons.

Seed Production

Broodstock management

The fish having minimum 150 g (20 cm) size is procured from farm or wild three months prior to the breeding season, disinfected with 200 ppm formalin dip for 40 seconds and stocked at a density of 2-4 no./m² in outdoor circular cement cistern of 2-4 m diameter and 1.2 m depth. The tank is covered with nylon net (2 mm mesh size) leaving a clearance of 60 cm from the water surface to prevent the fish from escaping. The broodfish is fed twice daily at 10% of the body weight with a mixed diet consisting of clam meat, shrimps and trash fish. It can be replaced with a formulated feed having 35% protein containing a mixture of fish meal, groundnut oil cake, soya bean meal and rice bran augmented with vitamin-mineral mixture @ 2-3% of the body weight. As the fish is active in the dark, feed is given between dusk and dawn. The bottom of the tank is provided with 5-10 cm layer of washed sand as substrate and pieces of PVC pipes (5-10 cm diameter) as hide-outs. Water hyacinth can be maintained in the pond up to 20-25% of the surface area to simulate the natural condition. The plant also acts as shelter as well as a source of periphyton. Fortnightly the tank is cleaned and 30-50% of water is replaced.

Selection of brooder

By the first year,the fish weighing 200-400 g, attains sexual maturity and spawns in open or confined waters during June to August. Sexual dimorphism is exhibited from sub-adult stages. The mature female has swollen abdomen, reddish vent, round and button-shaped genital papilla and releases eggs on applying slight pressure on the highly distended abdomen. In mature male, the genital papilla is elongated and pointed. The ripe male does not discharge milt even by applying pressure on the abdomen. Maturity stage of female is ascertained by collecting the ovaries using a catheter and examining it under a microscope. If the egg size is 1.2-1.4 mm, the female is selected for spawning. Prior to breeding season, mature fish are collected from pond and kept sex wise in seperate indoor tanks.

Spawning

Broodstock collected from farm responds better to induced breeding compared to wild caught ones. Clarias magur is induced to spawn by intramuscular injection with synthetic hormones like Wova-FH @ 1 ml/kg body weight as a single dose and kept in an FRP tank of l t capacity. The broodfish exhibits pre-spawning activities for 6-8 hours and thereafter spawning activity starts and is continued for another 6 hours at short intervals. The fertilised eggs are golden-yellow in colour, while unfertilised eggs are milky white. The eggs are demersal and slightly adhesive. Hence, egg collectors are used for collecting them. Fecundity ranges between 30-60 no./g body weight. The average rate of fertilisation is 75%.

In the case of C. dussumieri, hormone is injected @ 0.6-0.8 ml/kg for female and @ 0.3-0.4 ml/kg for male. The fish is kept unfed at least for one day prior to hormone injection to avoid faecal contamination during stripping. Before stripping the female, testis is collected from the male fish. After anaesthetising the male fish with 300-400 ppm 2-phenoxy ethanol, a cut is made at the vent of the male with sharp scissors. The testis is removed with forceps and the cut portion is sutured with surgical twine, and the fish is given a bath in a disinfectant solution. The collected testis is ground in a mortar after adding 0.9% saline solution. The macerated tissue becomes inactive in this medium and it can be refrigerated. When freshwater is added, the sperm become re-activated.

After 14-17 hours, the injected female fish is cleaned with a towel and stripped into a clean and dry stainless steel, enamel or plastic tray/ basin. The fish is kept outside the water in an inclined position with the head up and the ventral portion over the tray. Slight pressure is applied gently with the thumb and index finger on the swollen belly, slowly descending towards the lower end of the body down to the vent. The fully gravid female normally releases a stream of ripe eggs, which flow out as a jet under this pressure and are collected in the tray. This process is repeated until all the ripe eggs are released. Care is taken to avoid contamination of eggs with blood and excreta from fish. Fully mature eggs are dark brown or brownish-green in colour. The stripped female fish is given a bath in disinfectant solution and released back to the tank.

The stripped eggs in the tray are sprinkled with sperms within one minute, and it is mixed thoroughly for 2-3 minutes by shaking the tray or using a quill feather or a fine fur brush. Then it is washed in fresh water for two minutes. The fertilised eggs are adhesive, demersal, spherical in shape and golden-yellow in colour. It is disinfected and carefully transferred for incubation. Fecundity is 50-80 no./g body weight. The ideal sex ratio is one male for 2-3 females.

Incubation

If egg collectors are used, it is washed thoroughly in freshwater with attached fertilised eggs and disinfected by dipping in malachite green (20-60 ppm) for 10-30 seconds. If fertilised by stripping, the eggs are washed in freshwater and disinfected. The fertilised eggs are incubated in a simple flow-through system which comprises of a rectangular FRP tank with 200 x 60 x 30 cm size on a stand of 90 cm height with separate inlet and outlet at opposite ends with a continuous feeble water flow arrangement 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. Each tank holds four plastic or FRP hatching trays having the dimensions of 50 x 30 x 10 cm and its bottom and sides are fitted with a synthetic net of 1 mm mesh size. 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. Ideal water flow rate for incubation is 0.5-1 1/min.

Continuous freshwater flow maintained in the tank keeps the egg devoid of contamination. Each tray can hold 6000-18000 eggs (1000- 3000 no./l), which mainly depend on water quality. (Water having 5-9 ppm DO is ideal). The eggs are uniformly distributed in the tray to avoid crowding, fungal infection and clogging.

In the case of C. dussumieri, hatching takes place within 17-20 hours while for C. magur, it is 24-27 hours at 27-30°C. The newly hatched larva measures about 4 mm in length. They usually remain at the bottom of the tray, resting on the side and move very slowly due to the heavy yolk sac. The yolk sac free larva moves in the water and starts feeding by 4 dph. No feed is provided during the first three days as yolk sac serves as food.

Rearing of hatchling

Just before completing yolk sac absorption (usually 3-4 dph), the larvae are transferred from incubation unit to indoor nursery rearing unit. In nursery-rearing unit, the larvae are stocked at a density of 4-6 no./l in FRP tank (lt capacity with 40 cm height) having 15-20 cm depth of water. The hatchling subsists on endogenous nutrition for 3-4 dph. On complete absorption of yolk, hatchlings are fed 4-6 times daily with a heterogeneous mixture of zooplankton including Rotifer, Ciliates and Moina at 0.2 ml/1 for the period up to 12 dph. The live feed is thoroughly sieved to avoid entry of Cyclops, Daphnia and other big zooplankton. Artemia nauplii at a density of 300 no./l or powdered yolk of boiled eggs are also given daily from 7 dph onwards. At the time of feeding, the sprinkling of water and aeration is stopped for 30 minutes.

The larvae absorb DO from the water up to 10-12 dph. Hence, mild aeration is provided continuously without disturbing the water always to maintain DO level above 5 ppm. Water quality is also maintained by providing 50% water exchange twice daily by continuous flow-through system. The waste from the tank bottom is removed by siphoning twice daily. The larvae show high cannibalistic behaviour during 2-5 dph and hence survival during this stage is very much crucial. During the early larval stage, size grading is done frequently and the shooters are removed for better survival.

By the 10-12 dph, the larvae start the vertical movement and come to the water surface for taking oxygen directly from the air, which is the critical period in larval rearing. Subsequently, aeration is stopped, and the height of the water column in the tank is reduced to 10 cm for the next two days.

The metamorphic larva (12 mm size) is adapted for aerial respiration and it is kept in 15 cm water depth with the flow-through system up to 22-24 dph at a density of 1000 no./m². Continuous aeration is not required from this stage onwards. The larva is fed ad-libitum with cut pieces of Tubifex worm, clam meat and egg custard. Subsequently the quantity of Artemia nauplii is reduced. It is gradually weaned on micro- encapsulated shrimp larval feed or a powdered mixed diet consisting of fishmeal, groundnut oil cake and rice bran from 14 dph onwards. About 30 minutes after feeding, the left-over feed is removed. Daily siphoning is done to remove wastes accumulated at the bottom of the tank. Water quality parameters are checked once in every 3 days and are kept at optimum levels. Sampling is done once in 6 days for the estimation of growth and length-weight relation. Daily monitoring is done to correlate the number of larval deaths with those survived for estimating the rate of cannibalism. Larvae are assessed daily for its body colouration, clustering, swimming pattern and aggressive behaviour. Maintenance of water quality, nutritionally balanced feed, thinning of larvae and pathogen and predator-free environment, are essential to prevent heavy mortality in the early stages. It attains a size of 20-25 mm by 24 dph with a survival rate of about 20-40%.

Rearing of fry

The fry can be reared in an earthen pond (200-400 m²) or outdoor cement tank of 40-80 m² size having 1.2 m depth. The cement tank is provided with soil at 15 cm thickness. The nursery pond preparation and manuring for live feed production is similar to that prescribed for the major carps. The water level is maintained at 30 cm initially and gradually enhanced to 60 cm. The pond/tank should have a freeboard of 60 cm to prevent the fingerlings from jumping out.

The tank is inoculated with plankton to ensure natural food organisms. The fry is stocked at a density of 100-200 no./m² and fed daily during dark hours with a formulated feed (40-45% protein) or shrimp larval feed at 10% of the body weight, which can be replaced by egg custard with minced clam meat, prawn meat, tubifex or low valued fish or crumbled formulated catfish feed. In the earthen pond, 20-25% of the water surface is covered with floating aquatic plants such as Pistia and Eichhornia in order to provide shelter and to absorb noxious gases that accumulate in the water. The fish seed grows to lg size by 40-50 dph with 50-75% survival rate.

Water quality requirement

Temperature : 27-30°C        Alkalinity           : 90-160 ppm
DO                : >3 ppm        Nitrite                : < 0.25 ppm
Ammonia      : +0.05 ppm   Carbon Dioxide : 15 ppm
pH                 : 7.0-8.5          Hardness           : 80-150 ppm

Packing and transportation

The method of packing and transportation is similar to that explained for the murrels.

Farming in Pond

Selection of pond

Earthen pond of 0.05-0.2 ha size with a water level of 75-100 cm is desirable. The bottom and sides of the pond have to be made free from holes or crevices. Since the fish is an air-breather, it normally comes up to the water surface for taking atmospheric oxygen. This kind of habit attracts birds for predation. Therefore, the pond should be covered with net. Optimum pH is 6.5-8.5, but this fish can survive at low pH up to 5.

Pond preparation

As a preliminary activity, mahua oil cake is applied @ 2.5 t/ha/m to eradicate predatory as well as weed fishes. No manuring is done, but it is dried between crops and treated with lime. Inner bund of the pond should be firm. The pond should have not less than 75 cm free board and be fenced with stiff net (12 mm mesh size) along the margins of the pond to a height of 60 cm to prevent the ‘walk-out’ of fish.

Stocking

The stocking is done 15 days after the application of mahua oil cake. The seed is stocked @ 5-7 no./m². As the fish exhibit cannibalism in the early stage, uniform-sized seeds (1 g) are selected for stocking, however big sized seed (5 g) shows good survival and growth rate. A dip in 200 ppm formalin for 40 seconds before stocking helps to avoid infection.

Feeding

The fish is fed twice daily with a formulated pelletted feed having 30-45% protein at a rate of 5% of the body weight initially and gradually reduced to 2% in the later stage. This feed can be replaced with a mixture of rice bran, groundnut oil cake and chopped fish in the ratio of 1:1:1.

Care and monitoring

Presence of aquatic vegetation like Eichornia, Lemna,etc in the pond helps to control algal bloom and provide shade and shelter to the fish. It also promotes the growth of insects, which forms the basic food for the fish. But these plants should not occupy more than one-fourth of the pond as it may create hindrance to air breathing activity. There may be occasional accumulation of metabolites, rise in ammonia or algal bloom in the pond. Water exchange is done on acute deterioration of water quality. Periodic sampling is done to monitor the fish health and growth and to schedule the management procedures accordingly.

Harvesting

The fish attains a marketable size of 200-250 g within 10 months.

Other aspects of harvesting are same as that of the murrels.

Source : Department of Fisheries, Government of Kerala