Breeding of trouts

Breeding of trouts

8.1. Introduction

• Trouts belong to the family Salmonidae and Order Salmoniformes
• Among trouts, the rainbow trout, Oncorhynchus mykiss, is the most important species.
• Other species of trouts are : brown trout (Salmo trutta fario), brook trout (Salvelinus fontinalis), lake trout (Salvelinus namaycush), etc. 
  Trouts native to India are : snow trouts (Schizothorax spp.) 

8.2. Habitat and biology of rainbow trout

• The rainbow trout (RBT) is native to North America.
• Since 1874, the RBT has been introduced into several Continents, except Antarctica. It is farmed in about 64 countries.
• It is a hardy fish,
• Easy to spawn
• Fast-growing, reaching 5-10 kg in 3 years, with anadromous (steelhead trout) growing faster (7-10 kg)
• Tolerant to a wide range of environments and handling
• Large-scale fry production is possible as they prefer to feed on zooplankton
• It is anadromous as well as purely river or lake resident (spends entire life in river or lake)
• Can tolerate temperature of 0-27^oC, spawning and growth occur at 9-14^oC.

Identifying characters

• Trouts have fusiform body with 60-66 vertebrae, 3-4 dorsal spines, 10-12 dorsal soft rays, 3-4 anal spines, 8-12 anal soft rays, 19 caudal rays.
• Adipose fin present, usually with black edge.
• Coloration blue to olive green above a pink band along the lateral line and silvery below.

Food and feeding

• Adults feed on aquatic and terrestrial insects, molluscs, crustaceans, fish eggs, minnows and small fishes.
• Juveniles are omnivorous.
• Freshwater shrimps when fed to trouts supply carotenoid pigments which imparts orange color.

 

 

8.3. Brood-stock development

• RBT spawn during spring (January-May), but photoperiodic manipulation can advance or delay maturation and spawning which ensures year-round fry production.
• It does not spawn naturally captivity and in aquaculture systems.
• Eggs are artificially obtained by spawning or stripping females.
• Ripe/fully mature females (3-4 year-old) fish is preferred.
• High quality broodstock is used.
• More number of females is required as fecundity is low.
• Females produce up to 2,000 eggs /kg body weight.
• Eggs are large (3-7 mm)
• Sex ratio 1 : 3 (male and female) are kept separately prior to spawning.
• Since broodstock development is costly, eggs are seldom purchased and hatched.
• Disease-free eggs are purchased. But they should be treated with iodine at 100 mg/l for 10 minutes.
• All-female triploid (sterile) trout produced through gynogenesis and triploidy and all-female progeny produced through 17α - MT treatment and selective breeding are also used for grow-out farming. 

8.4. Stripping and fertilization

• Sex of broodfish is identified based on secondary sexual characteristics which are more pronounced during breeding season.
• Females are slightly larger and have swollen abdomen
• Males are slightly smaller and have round abdomen
• Stripping and fertilization technique is well-developed
• Dry method of fertilization is a common approach
• No hormones are used for spawning in this species.
• Eggs are stripped on to a clean basin and mixed with milt from more than one male to ensure better fertilization and reduce inbreeding.
• Fertilized can be transported after 20 minutes and up to 48 hours after fertilization, but then not until the eyed-stage.
• Exposure of fertilized eggs to direct sunlight should be avoided.

8.5. Hatching

• Eggs incubated undisturbed until the eyed-stage, in hatching troughs, vertical incubators or hatching jars.
• Hatching and rearing troughs are 40-50 cm wide, 20 cm deep and up to 4 m in length.
• Two layers of eggs are placed in wire baskets or screened trays.
• Water flow rate is 3-4 l/minute.
• As the eggs hatch, the fry drop through the mesh to a bottom trough.
• Some times up to 16 trays are stacked one above the other.
• Hatching period is 100 days at 3.9⁰C or 21 days at 14.4⁰C.
• Dead eggs are periodically removed to limit fungal attack.
• Fugal infection is controlled using formalin (37% solution of formaldehyde) for 15 minutes.
• Hatching rate is up to 95%.
• Yolk-sac absorption period lasts for 2-4 weeks.
• After hatching, the trays are removed and trough water depth is maintained at 8-10 cm until swim-up stage
• After yolk-sac absorption, the fry actively actively search for feed. 

   

Developing Trout eggs

8.6. Fry rearing

• Fry are traditionally reared in fiberglass concrete tanks (circular ones preferred).
• Diameter of circular tank is 2 m or size of squarish tank is 2 m x 2 m, with 50-60 cm depth.
• The drain is fixed in the centre of the tank and is protected by a mesh screen.
• Fry are fed with specially prepared starter feeds using automatic feeders.
• Feeding rate is 10% b.w. daily for 2-3 weeks using continuous feeders
• Feed pellets contain fish meal (80%), fish oil and grains.
• Feed contains proteins (50%), fat (12-15%) and vitamins (A, D and E), minerals and pigments (astaxanthin and canthaxanthin). 

Water quality requirements

DO : near saturation

CO₂ : < 2 ppm

Temperature : 12-21⁰C

pH : 6.5-8.5

Alkalinity : 10-400 ppm (as CaCO₃)