- Trout is either grown as a food fish or sport fish are released into natural waters for sport fishermen.
- Trout is popular because it is an attractive, active fighting fish and provides very high quality meat.
- Trouts have been released and cultured in water all over the world.
- It is a cold-water fish. It mainly inhabits rivers, streams, brooks, lakes and ponds.
- In India it is found in Kashmir, Himachal Pradesh, Uttar Pradesh, Nilgiris, Kodai hills and Munnar high ranges.
- Many species of trout are grown. But, the three most common of them are the rainbow trout - Salmo gairdneri or Oncorynchus mykiss, the European brown trout - S. trutta and the brook trout - Salvelinus fontinalis.
- Trouts have a streamlined body, narrow gill openings and reduced gills.
- Trouts are adapted to highly oxygenated waters and freezing point temperatures.
- Trouts have great power of locomotion with clinging and burrowing habits.
- Mouth is modified with rasping lips for food collection from pebbles, rocks, etc.
- The spawning season of S. gairdneri is from September to February; S. trutta is from October to December and S. fontinalis is from October to January.
- Trouts prefer gravelly substratum to safeguard their eggs and the eggs stick to gravel and debris.
- Trout build nests and spawn in streambeds.
- Culturists allow artificial fertilization, because streambed fertilization results less hatching rate than artificial fertilization.
- Manipulation of the photoperiod and water temperature can be used to induce gonadal maturation, so that young fishes are generated throughout the year.
- Trouts are caught at or near maturity as they are swimming upstream and raised to maturity.
- The brood fish are placed in small ponds with flowing water and are often covered with netting to prevent them from jumping.
- The milt of a single male can be used to fertilise two females. More females are stocked with few males.
- Trouts exhibit sexual dimorphism. Males become more brightly coloured and the lower jaw develop a hooked beak during the breeding season.
- Females develop extended bellies and the genital papilla becomes larger and reddish.
- When they are fully matured, milt or eggs comes out with little pressure on the abdominal vent. When the trout is ripe, the female fish are stripped and eggs are collected in a black coloured enamel or plastic container to which the milt of the male is added and mixed thoroughly with a quill feather for fertilisation.
- Water is added after mixing and the water causes the eggs to swell. Water should not be added before the mixing. Life span of the sperm is greatly reduced in the presence of water.
- To ensure a better survival rate, the eggs may be collected in a small quantity of saline solution (10 l fresh water + 90 g common salt + 2 g potassium chloride + 3 g calcium chloride).
- The fertilised eggs develop a green tinge and are known as ‘green’, which are then transferred to hatcheries. Before transferring remove the foreign particles and dead eggs to avoid infection.
Transportation of Trout Eggs
- The fertilised and hardened eggs (hardened for 24 hours) of trout are transported in cardboard cartons of 20 X 30 X 20 cm size.
- The inner side of the card board box is lined with styrofoam lining.
- Two moist sponges or cotton pads are arranged, one at lower side and other at upper side.
- Porous polyethylene bags containing about 4,000 eggs are placed in between the moist sponges and cotton pads.
- A polyethylene bag with 1 Kg broken ice is kept for maintaining low temperature, above the upper pad. These cardboard cartons are transported to various places.
- The trout eggs are incubated by keeping them in concrete troughs with flat and horizontally arranged trays, incubators or jar.
- Hatcheries should be provided with circulating filtered and silt - free freshwater.
- In olden days baskets were used for incubation.
- Vertical flow incubators are the most common. It has many stainless steel, fiber glass, aluminium or wood or PVC or plastic trays arranged one above the other.
- The bottom of the trays are provided with perforated zinc sheets, glass grills or mesh cloth for ensuring the passage of water through the different trays.
- The size may vary from 180 X 30 X 10 cm to 500 X 100 X 50 cm.
- Each tray has an upper egg basket and a lower perforated compartment on which basket rests.
- The eggs are placed in the basket for incubation. The water is introduced to the tray in such a way that it flows up through the basket containing the eggs, then down to the tray below and up through that basket and so on through the incubator.
- This upward flow of water through the eggs allows increased aeration and facilitates removal of metabolites.
- Hatching jars are also used for the incubation of trout eggs.
- It consists a galvanised screen of 0.5 mm mesh with gravel bed at the bottom, just above the inlet.
- This gravel bed is useful as filter to remove the unwanted particles.
- The eggs are placed above the filter for hatching. Water passed through the inlet, up wells through the filter and eggs and drains through the outlet.
- After hatching, the hatchlings are maintained for some time in the jars. The eggs are highly sensitive during the hatching period.
- Newly fertilised eggs can be killed if directly exposed to sunlight. During incubation, water must be moving and have high oxygen content.
- Incubation normally takes place in water with 8-12°c temperature.
- The fry can be held in the trays until they become active and are able to begin to feed. They can be released for stocking in natural waters.
Culture of Trouts
- The fry are reared in small rearing troughs before they have completely absorbed their yolk sac, and introduce to live on artificial feeds.
- Then, they are transferred to nursery ponds for rearing to advanced fry stage. The nursery ponds may be concrete or stone-walled with 2.5 X 1 X 0.75 m to 9 X 1 X 0.75 m size.
- The water flow may be maintained 100 l / min inside the nursery pond.
- The advanced fry are reared to adults in rearing pond and raceways.
- Rearing pond is a natural body of water and a raceway is merely a running water fish pond.
- The size of raceways should range from 20 -100 m with a depth of 1.5 m.
- A series of raceways are constructed either side of the stream or river. Each raceway gets water from stream and water goes out of the raceway through the outlet which is found on the opposite side.
- Zinc plate screens are used at inlets and outlets. The water flow is maintained 50 l / sec into the ponds from river.
- The stocking rate may be limited to produce 5-10 Kg / m.
- High production of 200 Kg / m is also possible in raceways, if management is good.
- Cage culture of trout is also common. In an experiment, fingerlings were stocked at 1.4 Kg / m in cages and fed 3 % of their body weight daily.
- These trouts grew to 27-88 g in two months. The feed given to trouts includes cattle spleen, heart and lung and marine or freshwater trash fish.
- Many commercial trout feeds are available in the market. Trout are fed 3 - 4 times daily.
- There are number of ways of giving feed to trouts.
- The feed is either sprayed on the surface of the water or the feed can be kept in a bag or in a container in the corner of the pond.
- It is used for the demand feeders in which whenever a trout bumps into the trigger the feed is released into water or automatic feeders can also be used.