Key Technical Points for Fry Rearing in Yellow Catfish Aquaculture

Rearing yellow catfish fry (commonly known as "water flowers") with a body length of 1.5-1.2 cm to fry of 5-6 cm in size (known as "fire-flake fingerlings") takes about half a month. The rearing methods at this stage include pond rearing, flowing-water tank rearing and net-cage rearing. Compared with fingerling rearing, fry-stage rearing demands a relatively higher level of technical expertise. Therefore, great attention must be paid to fry rearing.

(1) Requirements for the conditions of rearing ponds or net cages

1. Structure of flowing-water rearing tanks. Flowing-water tanks are used to rear fry at relatively high density, so the requirements for the rearing tank are also higher. The flowing-water rearing tank should be a small concrete tank with an area of about 10-30 sq m, no more than 50 sq m at most, and a depth of 1 m. A rectangular or circular shape is preferable. The bottom of the tank should be basically flat with a slope of 3°-5°; water inlet and drainage should be convenient, with an inlet screen and an escape-prevention screen installed.

2. Condition requirements for pond rearing. The fry pond should be conducive to fry activity, growth, feeding management and seine netting operations. It should generally meet the following conditions: abundant water source, fresh water quality, and convenient filling and drainage. The pond should be regular in shape, with an area of 0.5-1.0 mu and a water depth maintained at 50-100 cm, shallow in the early stage and deep in the later stage. The pond bottom should be flat with an appropriate amount of silt (about 10 cm), and there should be no weeds on the pond bottom or banks. A rectangular fish-collecting basin (either a concrete tank or an earthen tank) should be set at the outlet to facilitate concentrated harvesting of fry. The pond dikes should be firm and not leak. The surrounding environment should be good, and the fry pond should face the sun with sufficient light. The turbidity of the pond water should not be high, with a pH of 7-8, a dissolved oxygen content above 5 mg/L, and a transparency of 30-40 cm.

3. Structural requirements of net cages for fry rearing. For rearing fry in net cages, the mesh size required differs according to the body length of the fry. For newly hatched larvae, the mesh size of the net cage is 40-50 mesh; for fry of 0.9-1.0 cm, the mesh size is 20-30 mesh; for fry of 3-5 cm, the mesh size is 10-20 mesh. The net cage can be rectangular or square in shape. The specifications of the net cage are determined according to the size of the fry at different stages: for young fry of about 1 cm, the net cage is 3-5 sq m; for fry of more than 2 cm, the net cage is 5-10 sq m; for fry of about 5 cm, a net cage of 10 sq m is appropriate. When setting up the net cage, all four corners, top and bottom, must be firmly fixed to the supporting frame of the net cage.

(2) Preparatory work before stocking fry

This mainly involves clearing and disinfecting the ponds used for rearing fry, as well as fertilizing the water to cultivate natural food.

1. Pond clearing and disinfection. Repairing and clearing the fish pond is an important measure for improving pond environmental conditions and increasing fry survival rate. After fish have been raised in a pond, large amounts of undecomposed organic fertilizer residue, leftover fish feed and feces are deposited on the pond bottom, the bottom silt increases, various harmful microorganisms lurk in large numbers, and the pond may develop leaks and collapsed banks due to wind and wave action, rain erosion and burrowing by various other animals. Thus, the pond needs to be repaired, cleared and disinfected with chemicals.

The method for repairing the fish pond is to drain the pond water in winter or early spring, allowing the pond bottom to freeze and be exposed to the sun, making the soil loose and reducing disease. Then, excess silt is dug out, dikes are repaired, leaks are filled, and the pond bottom is leveled. Pond clearing should be carried out after pond repair is completed and 10-15 days before fry stocking, that is, using various chemicals to kill wild trash fish, pathogenic bacteria, parasites and other harmful organisms. There are many types of chemicals for pond clearing, and farmers should choose and apply them according to their own specific circumstances.

2. Fertilizing the water. Generally, about 1 week after pond clearing and before fry are released into the pond, water is filled to a depth of 40-60 cm. Then organic fertilizer is piled in one corner of the pond to cultivate suitable natural food for the fry, so that the fry can eat sufficient food after entering the pond. This method is especially suitable for rearing yellow catfish before they reach 5 cm. The amount and type of base fertilizer applied should be adapted to local conditions. According to the source of fertilizer, there are mainly two types: first, organic manure, such as livestock, poultry and human feces and urine, generally applied 6-7 days before fry are released into the pond, at about 300-500 kg per mu. However, this should be increased or decreased depending on the original fertility of the pond; for example, new ponds and sandy-bottomed leaking ponds should receive more, while old ponds with thick bottom silt should receive less. Second, green manure from large grasses, generally composted 7-9 days before fry are released into the pond, at 300-400 kg per mu. To accelerate water fertilization, chemical fertilizers can also be applied, generally 5-10 kg of ammonia water per mu, or 4 kg of ammonium sulfate, ammonium nitrate, urea or ammonium chloride, and 3-4 kg of superphosphate.

3. Fry stocking

1) Identification of fry quality. The quality of fry is mainly reflected in whether their constitution is strong. Fry with good constitution show bright body color, plump and uniform body shape, neat and uniform individual size, and active swimming when placed in a white porcelain basin (dish).

2) Stocking density in flowing-water tanks. Newly hatched larvae can be stocked at 5000-8000 per cubic meter of water. When they grow to about 1 cm, they are divided into separate tanks for rearing, with a stocking density of 3000-4000 per cubic meter for 1-3 cm fry. After 3 cm, they are divided again, at 2000-3000 per cubic meter.

3) Stocking density in pond rearing. Because pond rearing conditions are generally poorer than those of flowing-water tanks, the stocking density is also much lower, at about 20-30 fry per sq m. If the density is too high, the pond conditions and natural food in the water cannot keep up with the needs of the fry, so growth rate is slow. Note that other fish fry should not be stocked together during fry rearing.

4) Stocking density in net-cage rearing. The stocking density for rearing yellow catfish fry in net cages is much higher than in pond rearing. However, it must be determined according to the size of the net cage, water quality conditions and food conditions. That is, the stocking density varies with factors such as the quality of the water body; in water bodies with good water quality and high transparency, the stocking density in net cages should be greater than in net cages in water bodies with low transparency. Generally, it is appropriate to stock 3000-4000 newly hatched larvae per cubic meter of water, 2000-3000 for fry of about 1 cm, and 1500-2000 for fry of about 2 cm.

4. Feed and feeding. After yellow catfish fry of 0.9-1 cm body length are released into the pond, no feed is given for the first few days, followed by a small amount of mixed dough feed. This is because yellow catfish mainly feed on zooplankton, chironomid larvae, nauplii and insects during the stage before they reach 5 cm, while also feeding on artificial mixed feed. Generally, powdered compound feed is used, stirred with water into a dough and fed directly into the fish pond, or spread flat on feeding trays laid on the pond bottom. A reference formula for the compound feed is: fresh fish paste 35%, soybean cake powder 5%, third-grade flour 23%, corn flour 15%, binder 3%. At a water temperature of 20-32°C, feed once each in the morning and afternoon every day, with the feeding amount accounting for about 3%-5% of fish body weight. Based on the schooling feeding habit of yellow catfish, feeding should adopt a relatively concentrated method, with the feeding area accounting for about 6%-10% of the pond area.

In feeding, the "four fixeds" feeding principle must be adhered to, namely fixed time, fixed location, fixed quantity and fixed quality. Feeding times and feeding frequency should be set according to the feeding patterns of the fry; fixed-point feeding should be carried out according to the schooling feeding characteristic of yellow catfish; and the quality and quantity of the feed should be guaranteed. The feeding amount for yellow catfish fry is 5%-7% of body weight, with 3-4 feedings per day; the feeding amount should be greater in the morning and evening, and the specific amount for each feeding can be determined according to the feeding situation of the yellow catfish and the water temperature, with the fry being able to finish the feed within 1 hour as the standard. The quality of the artificial feed or fresh feed used for feeding must be guaranteed.

5. Points to note when stocking. The following aspects must be noted when stocking fry:

1) Yellow catfish fry can only be released into the pond when they have reached horizontal swimming and active feeding. If released too early, the fry have weak activity ability and poor feeding ability, find it difficult to adapt to the new pond environment, and are prone to death.

2) The size of fry released into the pond must be uniform; otherwise, the growth rate of the fry will be inconsistent, with large differences in size.

3) Before transport, the fry should undergo seine netting exercise to eliminate fry with weak constitutions, increase transport survival rate and improve fry quality.

4) If yellow catfish fry are transferred directly from the holding pond to the rearing pond, they should first be fed to satiation before being put into the rearing pond; this can improve the constitution of the yellow catfish fry and increase their survival rate.

5) The time for releasing fry into the pond should be chosen when the amount of plankton in the pond is relatively abundant. Achieving this is rather difficult, because the growth and reproduction rate of plankton in the pond and the growth rate of the larvae are hard to keep in sync, and both are greatly affected by external factors such as water temperature.

6) Before releasing fry into the pond, test the water; the temperature difference between the two should not exceed 2°C. If the temperature difference is too large, the fry bag should first be placed in the fish pond for half an hour to adjust the temperature difference.

7) Handle with care when releasing into the pond. When releasing the fry, slowly pour them out (or put them in) along the pond edge, and keep both feet close to the bottom when moving, so as not to trample the fry to death.