Key technical points for the cultivation of summer fry and large-size fingerlings of the yellowtail fish
In some places, the yellowtail fish is called huangbandiao, huangguzi, banhuangyu, shaguzi, huangtiao, huangchuan, huangweiyu, and so on.
1. Pond conditions: area 3-5 mu, water depth 1.2-1.8 m, mud 15-20 cm, water source and water quality up to standard. The pond is rectangular, running east-west, with a length-to-width ratio of 5:4 to 3:2, and convenient water intake and drainage.
2. Pond clearing and disinfection: use quicklime or bleaching powder to clear and disinfect the pond, using the dry method, leaving 10-15 cm of water in the pond. 7-10 days before stocking, use 150-200 jin of quicklime or 25-30 jin of bleaching powder per 1 mu to clear and disinfect the pond.
3. Time for releasing the newly hatched fry into the pond: conventional fry and fingerling cultivation mostly involves releasing into “fertilized water”, but the method for cultivating yellowtail fish fry and fingerlings is slightly different. Because yellowtail fish fry are smaller than domestic fish fry of the same age, and when they first begin feeding they can only eat tiny organisms such as rotifers and planktonic algae, the base fertilizer must not be applied too early or too heavily before the fry are released into the pond, to prevent too many large algae from being unfavorable for the fry to feed on. The water color is better when it is a little “tender”.
4. Fry stocking: before releasing the fry into the pond, first put them into a fry box and feed cooked egg yolk, with 1 cooked egg yolk fed per 100,000 fry. Feeding cooked egg yolk should be carried out according to the following steps:
1) Boil the egg yolk a little longer, wrap the egg yolk in 60-mesh nylon sieve cloth, and wash and filter it by agitating the water in a basin of water; do not squeeze it with your hands, so that fine egg yolk particles pass out through the mesh into the water;
2) Use your fingers to evenly sprinkle the egg yolk water into the fry box. After half an hour, egg yolk particles can be seen in the intestines of the bream larvae (commonly known as flower intestine);
3) After keeping them in the fry box for 3-4 hours, release the larvae into the pond.
5. Stocking density: the general stocking density is 100,000-150,000 fry per mu. If the fry are seined and divided among ponds when they grow to a total length of 2-2.5 cm, the stocking density can be increased, reaching 150,000-200,000 fry per mu.
6. Fry feeding: after the fry are released into the pond, start feeding soybean milk on the 1st day, feeding 2-3 jin per day of soybean milk made from soybeans per 100,000 fry, with water:beans = 30:1, fed three times a day, at 9 a.m., 1 p.m., and 4 p.m. When grinding the soybean milk, the soybeans must be soaked until swollen, and after grinding the okara must be removed. When feeding, sprinkle the milk along the pond edge in two circles, and make the sprinkled milk resemble raindrops and be evenly distributed.
As the fry grow larger and their food intake increases, supplementary feed must be added and the feeding amount appropriately increased. You can continue to apply fermented manure to cultivate the water color and splash an appropriate amount of soybean milk. After half a month, when the fry grow to 3 cm, begin feeding refined feed such as rice bran and wheat bran, with a daily feeding amount of 15-20% of the fish body weight.
Within 15 days after the fry are released into the pond, feeding soybean milk is the main method; after 15 days, refined feed such as fine flour, rice bran, and wheat bran can be fed. After 1 month of cultivation, the size can reach more than 3.3 cm.
7. Precautions:
a. Newly hatched yellowtail fish fry and fingerlings are easily harmed by large water fleas. Therefore, fertilizer must not be applied too early before the newly hatched fry are released into the pond, in order to reduce the reproduction of water fleas. Keep the water color tender, with many small planktonic organisms and suitable palatable food, which is beneficial to fry growth. If many large water fleas are found before the fry are released into the pond, splash 0.05 g of 90% trichlorfon per cubic meter of water over the whole pond to kill them.
b. During the fry cultivation process, attention must be paid to regulating water quality. Because fertilizer and feed are often applied, manure residue and leftover feed easily spoil the water quality, so change 1/3 of the water every 2-3 days.
8. Timely separation and rearing: after 20-30 days of cultivation, the fry can reach a body length of more than 3 cm. At this time the fish scales have fully grown, so they should be timely seined for exercise and divided for thinner rearing. However, during seining operations, special care must be taken to prevent injuring the fish. For fingerlings with a body length of 3-4 cm, 50,000-60,000 per mu can be stocked; when the body length is more than 5-6 cm, 30,000-40,000 per mu can be stocked. As the fish grow, the stocking density should be gradually reduced to facilitate rapid growth of the fingerlings and increase the survival rate.
9. Exercise and pond division: after 20-25 days of rearing, when the fry grow to about 3 cm, they should be separated and reared. Before summer fry fingerlings leave the pond, they must undergo 2-3 seining exercises. This enables the fingerlings, during the crowding process, to increase their ability to adapt to low oxygen. It makes the fish body “hardened” and sturdy, able to withstand leaving the pond for separate rearing, and during transport avoids producing large amounts of mucus and feces that pollute the water quality, thereby increasing the transport survival rate. Seining exercise can also remove harmful organisms and estimate the number of summer fry.
1) First seining: gather the summer fry in the net, check their physical condition, and then immediately release them back into the pond. Because the fish body is delicate, operations must be careful.
2) Second seining: after 1 day, carry out the second seining exercise. After gathering the fish, let them swim against the water and voluntarily enter the net box. After two hours, release them back into the pond. If they are to be kept for self-rearing, then after 2 seining sessions and crowding in the box, separate rearing can be carried out.
3) Third seining: after the second seining, wait another 1 day, then carry out the third seining. Put the fingerlings into a net box in a pond with clearer water and “hang-rear” them for 1 night. The next morning they can be loaded for long-distance transport.
For exercising fry, be sure to choose clear, fine weather and carry it out in the morning when the fish are not gulping air at the surface. The operation of seining and driving the fish must be slow, so as not to injure the fry. When closing the net, do not let the fish stick to the net. After the fish enter the box, immediately wash the box once to wash away excretions and dirt clogging the mesh, so that the water inside the box flows freely with the outside and fry deaths caused by lack of oxygen inside the box are prevented.
10. Disease prevention and control: the main diseases during the fry and fingerling cultivation stage of yellowtail fish are parasites, such as trichodina, fish lice, and anchor worms.
Control method: regularly splash crystalline trichlorfon over the whole pond so that the pond water concentration reaches 0.3 g per cubic meter, then splash once more the next day, and it can be controlled.
11. Cultivating large-size fingerlings from yellowtail fish summer fry: yellowtail fish summer fry can be cultivated into large-size fingerlings by two methods: pond monoculture and pond polyculture.
Both methods stock summer fry of about 3 cm. At this stage their habits are close to those of adult fish, mainly feeding on humus, organic debris, and some benthic organisms (such as benthic diatoms, chironomid larvae, etc.). In ponds, they also eat refined feed such as soybean milk, soybean cake, and compound feed. If feeding full-price compound feed, careful domestication is required.
1) Stocking density: stock 15,000-20,000 summer fry fingerlings of more than 3 cm per 1 mu. The feed is mainly soybean cake and soybean milk, fed 3 times a day. After the fry are released into the pond, continue to cultivate water quality (apply topdressing). On the other hand, feed refined feed such as soybean cake slurry, wheat bran, or compound feed.
After about 80-90 days of cultivation, they can reach a size of more than 10-12 cm, and at this time they can enter the grow-out stage.
2) Fingerling feeding: be sure to adopt the “four fixed” method. Use wheat bran combined with a small portion of full-price compound feed. That is: in the daily feed, wheat bran accounts for 60% and compound feed accounts for 40%. The daily feeding amount should be appropriately adjusted according to weather, air temperature, and the feeding situation of the fish. Yellowtail fish do not compete conspicuously for food, and the feeding amount should be based on finishing the food within 1.5 hours after feeding.
12. Daily rearing management: during the period of intensified fry and fingerling cultivation, patrol the pond frequently to observe the activity of the pond fish and changes in water quality, and take timely measures when problems are found to prevent accidents.
a. During the rearing period, the pond water should always remain fertilized water, dissolved oxygen should remain above 4 mg/L, the water color should be yellow-brown or tea-brown, and the transparency should be 20-30 cm. Because summer water temperature is high, the oxygen consumption of various organisms in the water increases, and yellowtail fish fry and fingerlings often die from pond turnover due to deteriorated water quality and lack of oxygen, so water quality regulation is very important.
b. Every 10-15 days, according to the pond water quality situation, add an appropriate amount of fresh water to the pond. When adding water, filter wild miscellaneous fish (carnivorous fish) with mesh at the water inlet. If the fish are seriously gulping air at the surface before dawn, emergency fresh water should be added.
c. When applying topdressing, it is better to apply urea to the pond. It can make plankton flourish, both fertilizing the water, and phytoplankton can absorb the ammonia metabolites of yellowtail fish fingerlings, thereby increasing dissolved oxygen.
d. Turn on the aerator to increase oxygen, generally turning it on at noon on clear days, at night on cloudy days, and before the fish gulp air at the surface.
13. Transport of fry and fingerlings: in the process of transporting fry and fingerlings, no matter what method is used, operations must be meticulous, and attention should be paid to the following aspects:
1) The fry are robust, the swim bladder is inflated (long waist spot), the mouthparts are open, and they swim actively, so they can be loaded for transport. The loading amount for oxygen-filled nylon bag transport is about 80% of that of a transport box.
2) The water quality is fresh and non-toxic, with high dissolved oxygen. Water from rivers, lakes, reservoirs, and ponds can all be used; choose colorless, transparent water with low mud content. Tap water must be dechlorinated and left to stand for 24 hours before use. In summer, use cool water.
3) Before fingerlings are transported, the fish body must undergo seining exercise to expel feces. With a sturdy body and reduced mucus secretion, the survival rate can be increased.
4) Pay attention to temperature balance; the temperature difference must not be ±2℃. After arriving at the destination, first put them into a box to revive the fish, and one hour later feed the fry cooked egg yolk. Feed one egg yolk per 100,000 fry. After seeing the fry flower intestine (swallowing egg yolk into the intestine), wait another hour before releasing the fish into the pond. For fry in nylon bags and transport boxes, before putting them into the box, first place them in the water so that the inside and outside water temperatures are consistent. Then open the nylon bag or transport box, pour in pond water, put the bag into the box, and let the fry (fingerlings) swim out on their own.