Detailed Explanation of Artificial Breeding Techniques for Rainbow Trout

Rainbow trout is native to California, USA, and is a precious cold-water aquaculture fish species. Its flesh is tender, its flavor is delicious, it is rich in nutrients, it grows rapidly, it is easy to catch, and its artificial breeding is simple. It was introduced to China in 1959, and it is currently farmed in most regions of the country. In rural areas of China, there are many streams, cold springs, and bottom drainage outlets of reservoirs, where rainbow trout can be raised according to local conditions.

Artificial breeding

Rainbow trout reaches sexual maturity at 2 years of age. The breeding season is winter and early spring, with water temperatures between 6°C and 13°C. The spawning period is greatly affected by water temperature and light. The incubation time of rainbow trout fertilized eggs is relatively long; at 10°C, it takes about 30 days for the fry to hatch. Its artificial breeding techniques include four stages: broodstock cultivation, egg collection and fertilization, incubation, and fry rearing.

1. Broodstock cultivation

In order to obtain offspring with robust constitutions and rapid growth, in addition to avoiding inbreeding, generally first-time sexually mature broodstock are not used, because the eggs they produce are small and the vitality of the offspring is also relatively weak. Therefore, broodstock for breeding should be selected from 2-year-old fish and cultivated; body weight should be above 1 kg, and the female-to-male ratio should preferably be 3-5:1; the stocking density should be 1-2 fish per sq m, with males and females raised together. About 1 month before breeding, they should be raised in separate ponds to facilitate operations. The year-round water temperature for raising broodstock should not be higher than 17°C; if it exceeds 17°C, they cannot mature. At the same time, egg quality is also closely related to the nutritional composition of the feed. During rearing, vitamins should be appropriately increased, and fatty feeds should not be given in excess. In broodstock feed, crude protein should be higher than 40%, crude fat should be lower than 6%, and carbohydrates should be lower than 12%. August to September is the recovery point for broodstock gonad development, so feeding should be intensified and nutritional supply strengthened, with the feeding amount about 3%-5% of body weight; two months before spawning, feeding should be correspondingly reduced, and feeding should be stopped 2-3 days before spawning. Where conditions permit, before and after spawning, broodstock can be fed entirely on fresh small trash fish and other animal feeds.

2. Artificial egg collection and insemination

After the female fish matures, the front abdomen is large and soft, the genital pore is red, swollen, and protruding, and when the abdomen is touched by hand there is a feeling of separated egg grains; after lightly pressing the rear abdomen, egg grains flow out from the genital pore. In mature male fish, milky white semen flows out after pressing the rear abdomen. At this time, eggs should be collected and inseminated promptly; otherwise, overripening is likely to occur and fertilization capacity will be lost. During the breeding season, the maturity of male fish should be observed once every two days.

Egg collection and insemination should be carried out indoors, avoiding strong light exposure. During artificial insemination, first gently squeeze the eggs into a clean basin, and wash the eggs with an isotonic solution (55.05 g sodium chloride, 1.97 g potassium chloride, 3.14 g calcium chloride, made up to 7.5-8.0 L of solution with distilled water, and adjust the pH to 7 with sodium bicarbonate). Filter out the isotonic solution and quickly squeeze in the semen, stir with a goose feather so that eggs and sperm are evenly mixed, then add a small amount of clean water, stir slightly again, let stand for 1 minute, then wash the eggs with clean water 2-3 times, add clean water and let stand for 2-3 hours, and when the eggs have absorbed water, swollen fully, and become elastic, transfer them into an incubator for incubation.

3. Incubation

During incubation, flowing-water incubation is carried out in incubation troughs. The water used for incubation must be filtered, and the water quality must be fresh with sufficient dissolved oxygen. For every 10,000 eggs, 2-4 L of water per minute is required. The incubation temperature is 7°C-13°C. During the incubation process, light must be strictly avoided, and dead eggs should be removed promptly (use 10% saline to separate dead eggs; dead eggs float because of their low specific gravity). To prevent water mold infection, 3-5 g of malachite green can be added per cubic meter of water for disinfection.

Fry and fingerling rearing

Fry and fingerling rearing refers to raising newly hatched larvae to 2.5 cm-3 cm in length. Newly hatched fry are very delicate, use the yolk sac for nutrition, are afraid of strong light, and the water volume must be well controlled; for every 100,000 fry, the water injection volume is 30-50 L per minute. At a water temperature of 10°C, after 23 days, when 2/3 of the yolk sac has been absorbed, the fry are transferred to rearing troughs or cement ponds for rearing. The stocking density is 10,000 fish per sq m, and every 10,000 fish require 20 L of water per minute. When 60%-70% of the larvae have floated up, live feed should be given (large water fleas, aquatic earthworms); cooked egg yolk and minced fish meat can also be given; the feed can be sifted through a fine sieve with 0.4 mm mesh and then fed, 6-8 times per day, with a daily feeding amount of 10%-15% of fish body weight; at a water temperature of 10°C-15°C, after 20 days of rearing, the body length can reach 2.5 cm-3 cm, and they can then be transferred to outdoor fingerling ponds for rearing.

Before stocking, fingerling ponds should be disinfected and cleared with quicklime, and gates with 2 mm-3 mm mesh should be installed. Before fingerlings are put into the ponds, they should be graded and raised separately by size to prevent large fish from competing for food and affecting the growth of small fish. Stocking density depends on dissolved oxygen in the water, water injection volume, and fingerling size. At a body weight of 3 g-5 g, 200 fish per sq m, with a flow rate of 10 L per second. In the first 20 days of fry and fingerling rearing, feeding is mainly with minced small trash fish, with a daily feeding amount of 6%-10%, 4 times per day; after 20 days, switch to compound feed, with a daily feeding amount of 2%-4%, 4 times per day. In order to meet the nutritional needs of fry growth and development, in addition to feeding artificial compound feed, some live feed such as water fleas and tadpoles should also be given. During the rearing process, attention should be paid to controlling the water injection volume well, frequently brushing the gates to prevent gate blockage and fish death from lack of oxygen, and frequently removing bottom sludge and residual feed to maintain a good environment, which is conducive to fingerling growth.