How to raise newly hatched fry to improve the survival rate?
In plain terms, newly hatched fry refers to the small seedlings that emerge when fish eggs hatch and shed their shells. Generally, those that can be released into ponds and shipped are seedlings 3-4 days after hatching, with a body length of 0.5-1cm, and the whole body is semi-transparent or light gray-black. If the newly hatched fry are kept in a tank for too long after hatching, there will be a great chance of weakening their foraging ability as well as their growth and adaptability, so newly hatched fry should not be kept for too long before entering the pond. How to raise newly hatched fry to achieve a high survival rate? You must understand ten key points.
1. Fry pond design
1. The fry pond requires a solid pond embankment that does not leak, an adequate water source, and convenient filling and drainage. The fish pond should face away from the wind and toward the sun, be rectangular, and run east-west. The fry pond should not be too large; if it is too large, it is prone to forming wind waves, and the fry collide with the pond walls with the wind waves, affecting the survival rate;
2. Fry pond water level: if it is too deep, the water temperature is difficult to raise. If the area is too small, the water temperature changes quickly, and the living conditions of the fry are difficult to control.
2. Thoroughly clear the pond before releasing the fry
1. Before stocking the fry, drain the pond water, remove weeds, and dig out excess mud. Apply 1500 kg of lump quicklime per hectare after slaking it in water and splash it over the whole pond to kill pathogens, parasites and harmful organisms;
2. When filling with water, the water inlet must be filtered with double-layer dense mesh to prevent wild and miscellaneous fish from entering the pond. After the fry pond is filled with water, patrol the pond regularly every morning, and promptly remove frog eggs and aquatic plants when found.
3. Purchase high-quality fry and seed
The quality of the fry directly affects the survival rate of the fry.
4. Preparation before releasing the fry and seed into the pond
1. Release into the pond after "testing the water": 1 day before stocking the fry, take a few fry and place them in a net cage in the fish pond, observe for 1 day, and if the fish move normally, the fry can be released;
2. Release into fertilized water: the standard for releasing into fertilized water is that the zooplankton (rotifers) in the pond are at their peak reproduction period, ensuring that after the fry enter the pond there is sufficient and suitably sized natural food, which can improve the survival rate of fry rearing. Generally, 6 to 7 days after the fish pond is fertilized, rotifer reproduction reaches its peak, so the fertilization time should preferably be 7 to 8 days before the fry are released into the pond. If the water temperature is low, it can be 2 to 3 days earlier, applying 200 kg to 300 kg of organic fertilizer per mu.
3. Release into the pond after satiation: when the fry are released into the pond, feed 2 to 3 egg yolks per 10,000 fry. The method is to put the fry into a slightly larger plastic basin, dilute the egg yolk with water, filter it through 40-mesh polyethylene mesh cloth, evenly sprinkle it in the basin, and then wait 20 minutes before putting them into the pond.
4. Release into the pond after temperature adjustment: when the fry arrive at the pond edge, note that the water temperature of the oxygen bag and the fish pond must not differ by ±3℃. If the temperature difference is too large, place the oxygen bag on the surface of the pond water, and wait until the water temperature inside the bag is close to the pond water temperature before slowly releasing the fry into the pond. If there are wind waves, choose to release the fry 2 m to 4 m from the shore on the windward side.
5. Stocking density must be reasonable
In fry rearing, generally only one species is cultured in one pond, and mixed culture is not suitable. Stock 80,000 to 100,000 fry per mu. If the density is too low, the water body and food cannot be fully utilized; if it is too high, it is easy to cause the fry to gasp at the surface during the rearing process, or inhibit fry growth due to insufficient food.
6. Feed enough food and fertilize in a timely manner
For the first feeding time when the fry (newly hatched fry) are released into the pond, counting from when the fry enter the pond, they can be fed with slurry two hours later. The feeding amount is 4 kg to 5 kg of dry soybeans per mu (667 sq m) of effective water surface per day, and it is evenly and carefully splashed over the whole pond at a dosage of 18 kg of slurry water made from each 1 kg of soybeans. For ponds where fry are released in the morning, feed slurry 2 to 3 times that day; for ponds where fry are released in the afternoon, feed slurry 1 to 2 times that day. After that, starting from the second day after the fry are put into the rearing pond, feed 3 times a day at a daily dosage of 4 kg to 5 kg of dry soybeans per mu (667 sq m) of effective water surface, arranging each feeding time at: 8:00-9:00 a.m., 12:00 noon to 1:00 p.m., and 4:00-5:00 p.m. Splash evenly and carefully over the whole pond, and continue splashing for 3 consecutive days. After 3 days, according to the specific conditions such as whether the pond water is rich or lean, the amount of biomass (especially the amount of rotifers), and the growth of the fish in the pond, adjust the daily feeding amount up or down as appropriate. Whether to adjust the daily feeding amount is mainly determined by looking at the water quality, looking at the water level, and looking at the fish growth.
7. Regulate water quality
In the early stage after the fry enter the pond, the water depth is controlled at 40 cm to 60 cm, which is conducive to raising the water temperature and the growth and reproduction of food organisms. After 7 days, add water once every 2 to 4 days, adding 10 cm to 15 cm each time, to expand the growth space of the fry and keep the pond water "fertile, lively, refreshing and tender". When adding water, note that the water body should flow into the pond evenly and straightly, and the water addition time should not be too long, so as to avoid the fish swimming against the current for a long time, consuming physical strength, affecting growth or causing running horse disease.
8. Grasp dissolved oxygen and respond to changes
The quality of dissolved oxygen content in the water body of the rearing pond is an important condition for whether pond-cultured fish can survive, grow and develop. To achieve high yield and high efficiency in raising high-density fry, in daily feeding and management work, the dissolved oxygen content in the water body of the rearing pond must be placed in a position as important as water itself.
The specific method is: fertilize according to the water, regulate the water in a timely manner, grasp dissolved oxygen and turn on the machine in time. Keep the dissolved oxygen value in the water body of the rearing pond always above 4.5 mg/L.
9. Pond patrol
Patrol the pond once in the morning and once in the evening every day, observe the water quality to determine the amount of bait and fertilizer, observe the activity and growth of the fish, and promptly treat and remove diseases when found. The pond water also must not be too fertile; silver carp and bighead carp will develop swollen bellies, that is, gas bubbles. In such a case, just add water promptly when the color is too strong. When grass carp are released into the pond, the water quality must not be too fertile, as too fertile water affects the survival rate.
10. Do a good job in fish disease prevention and control
In the summer fry stage, the main fish diseases include running horse disease, white head and white mouth disease, and white skin disease, etc. During the rearing period, splash lime water once every 5 to 7 days, using 15 kg of quicklime per mu each time, which can clarify the water quality and prevent fish diseases. When running horse disease occurs, use reed mats to block the fry's frantic swimming route, stop adding water, and appropriately increase the amount of bait and fertilizer. When white head and white mouth disease or white skin disease occurs, splash chlorine dioxide, bleaching powder or strong chlorine essence over the whole pond to disinfect the pond water, use it for 2 consecutive days, and promptly divide the pond to reduce the density in the pond.