Precautions for Nursing Freshwater Piranha (Piaractus brachypomus) Fry: What Key Points Must Farmers Master?

Freshwater piranha (Piaractus brachypomus) is an omnivorous fish; the young fish (fry) mainly feed on large zooplankton, and also consume organic debris and various artificial feeds. The following precautions should be noted during its fry nursing stage:

I. Dedicated pond nursing

Because freshwater piranha is omnivorous, grows fast, has few diseases, likes to live in groups, and is particularly sensitive to certain organophosphorus drugs (such as trichlorfon, malachite green, etc.), from the perspective of fish disease prevention it should be nursed in a dedicated pond. In addition, freshwater piranha, whether fry or adult fish, prefers to “sink to the bottom”; when harvested it “burrows” into the net bottom or pond bucket bottom. If other fish species are mixed in, separating the various fry is difficult, and freshwater piranha, because of its fine, dense scales, is easily injured. To reduce unnecessary losses and benefit the growth and development of the fry, dedicated pond nursing at the fry stage truly has more advantages than disadvantages.

The pond for nursing freshwater piranha fry is generally 1-2 mu in area, with a water depth of 80-100 cm. Requirements: the pond must be disinfected 5-6 days before the fry are stocked (the disinfection method is the same as for nursing other fry; quicklime, tea seed cake, strong chlorine, etc. can be used), and the pond water is stable with no seepage. Because freshwater piranha fry are relatively small at this stage and their tolerance to low oxygen is stronger than that of other fish, the stocking density can be appropriately higher, controlled at 200,000-300,000 healthy fry per mu of water surface.

II. Daily management

Freshwater piranha fry start being fed on the second day after being stocked in the pond. Because the fry are tender and small and mainly eat zooplankton in the pond, a certain high-protein, fine-grained paste feed must also be provided, such as egg yolk or soybean milk. Generally, egg yolk (chicken or duck is fine) is convenient to obtain and gives good results. Method: calculate at 1 egg (2 eggs after 3-4 days) per 10,000 fry per day; first boil the egg, then remove the shell and egg white, and take the remaining cooked egg yolk, mix it with water (2-3 kg) through a fine filter cloth and knead it into a paste, then spray it evenly over the whole pond in the morning and afternoon. In addition, the nursing pond should be appropriately piled with easily decaying plants such as tithonia (large grass) or stinkweed, so that the transparency of the pond water is kept at 20-25 cm, achieving good water quality standards.

After 6-7 days of egg yolk feeding, freshwater piranha fry grow and develop to 4-5 zhao (about 1.2 cm total length). At this point egg yolk feeding can be stopped, and feeding of fine powdery mixed feed with high protein content can begin. At present, most mixed feed is composed of soybean, peanut bran, wheat bran, etc., at proportions of 40%, 30% and 30% respectively. The daily feeding amount goes from 0.5 kg per 10,000 fry at the start to 1.5 kg later, increasing gradually and adjusted flexibly. Feeding method: first weigh out the already crushed mixed feed, then mix it with water into a paste and sprinkle it evenly over the whole pond in the morning and afternoon.

III. Prevention of pests and diseases

Although freshwater piranha suffers fewer diseases than domestic carp, once infected the disease comes on fiercely and the mortality rate is high. For the health of the fry and to reduce losses, preventive work is very necessary. First, during the fry nursing stage (especially before the fry reach 6-7 zhao), guard strictly against severe floating-head in the fry; on the early morning of muggy weather or before a storm arrives, patrol the pond frequently and promptly remove grass debris from the pond. Once abnormal phenomena are found in the pond fish, immediately add fresh water or take other oxygenation measures. Second, freshwater piranha is prone to fish diseases caused by parasites (such as Ichthyophthirius, Dactylogyrus, Trichodina, etc.). The symptoms are generally obvious: at first the fish gather in clusters and do not disperse when driven away; then (after about half a day) the caudal fins of weaker fry are bitten, and they begin to die gradually. If not treated at this point, the mortality rate of infected fish reaches over 80%, and may even wipe out the whole pond.

Once freshwater piranha fry are found clustering in groups, the following treatment methods can be used:

1. Mercurous nitrate at 0.1 g per cubic meter of pond water; weigh out the drug, dissolve it in water and spray it over the whole pond, once a day for two consecutive days;

2. If mercurous nitrate is not convenient, ginger slices and chili water can be boiled for half an hour, then mixed with water and sprinkled over the whole pond; the dosage is half a jin of ginger slices and 1 jin of chili chopped and then decocted per mu of water surface at an average depth of 1 m.

IV. Timely pond division and sales

After 20-25 days of rearing and nursing, freshwater piranha fry have fully grown their scales, their lateral lines are complete, and color has appeared on the body surface. Body length and body weight have increased 4-5 times. If the pond is not divided or the fish are not sold in time, the density will be too high, feed will be insufficient, and the survival rate will be seriously affected.

For normal growth and development of the fry and to improve the survival rate, the pond should be divided or the fish sold in time at this point, but the following points should be noted:

1. Prepare in advance the pond into which the fry will be stocked (including disinfection, preparation, and water filling).

2. Stop feeding one day before pond division or sale, and use a soft, fine-mesh net to pull the fry through once, then immediately return them.

3. If the pond used after division is not far from the original pond, after catching, transport them immediately with a small amount of water and slowly release them into the target pond. If the fry are to be sold or the divided ponds are far apart, first slowly drive the fry from the original pond into a previously prepared pond bucket, hold them in water for 2-3 hours, and then transport them with a small amount of water.

4. Before the divided fry or purchased fry are stocked into the pond, it is best to soak and wash them for about 5 minutes in 6‰ coarse salt water or in a potassium permanganate solution at 1 g per cubic meter of water, and then slowly release the fry into the pond for fingerling rearing management.