How was the trial on early breeding of rice field eel fry in greenhouses carried out?

At present, cage culture of rice field eels is concentrated on collecting fry in July, which gives a high survival rate and low farming risk. But concentrating purchases means few fry sources, high prices, and a low weight-gain multiple. If fry are purchased earlier, the unstable temperature leads to a high mortality rate of the fry. How can this problem be solved? We carried out a trial on early breeding of rice field eel fry in greenhouses, and a brief summary of the trial is as follows:

I. Trial conditions

1. Trial pond: a newly excavated artificial water surface, 250 m long, 4 m wide and 1.5 m deep, with an area of 1000 sq m. The water source is lake water, with water inlet and drainage facilities.

2. Cages: made of knotless polyethylene netting with a mesh of 20 mesh. Cage specifications: 3 m long, l m wide and 1 m high, with 40 trial cages in total. The cages were installed on March 2, alligator weed was placed in the cages on March 3, and a plastic greenhouse was covered on March 4 to let the aquatic plants sprout and grow early.

II. Trial situation

1. Stocking: in late March, the aquatic plants in the cages had filled the whole cages. Fry collection and stocking into the cages began on April 10. The fry were all selected from the same-day fry caught with cages by nearby farmers, purchased on sunny days and handled with water. 30-100 kg of fry were collected each day until May 11, with a total of 1200 kg collected, of which: 10 cages were stocked 3 times, and the remaining cages were stocked 2 times. The fry size was 20-30 g/fish, stocked at 5 kg per sq m. No drug disinfection was used before stocking. Starting from the second day after stocking, a bactericidal red preparation was splashed for two consecutive days to comprehensively disinfect the water body, fry and aquatic plants at a concentration of 0.3 mg/L.

2. Food training: on the afternoon of the day the fry were stocked, fresh minced fish was fed to attract them to eat. Three feeding platforms were made in each cage with alligator weed, and the minced fish was placed on the feeding platforms. Feeding sounds could be heard that same day. Feeding was done only once a day at 5 p.m. The feeding amount started at 1% of the eel body weight and gradually increased to 5%. Minced fish was fed throughout the whole training period.

3. Water exchange: at first the water quality in the greenhouse was relatively stable, and water was changed once every 7 days. In the latter half month, as the temperature rose, water was changed once every 2-3 days. The method was to discharge 30%-50% of the old water on a sunny morning, and fill up with new water at noon.

4. Temperature regulation: the air temperature in the greenhouse fluctuated greatly, while the water temperature did not change much. The day-night temperature difference was about 3°C, and at night the water temperature in the greenhouse was 2-3°C higher than outside. The fry in the greenhouse fed normally on rainy days. After April 20, a shade net was added outside the greenhouse. In May, both sides of the greenhouse were opened during the day for ventilation and covered at night.

5. Disease prevention: a water quality detoxification protective agent was splashed in the cages once every half month, followed by another splash of bactericidal red for sterilization and disinfection. No eel diseases occurred.

III. Trial results

1. The training time of the fry in the cages depended on their feeding situation and the weather. If the food intake remained stable at above 5% for more than 7 consecutive days, there was continuous fine sunny weather, and the water temperature could remain stable above 20°C, the fry could be moved into large open-air cages for culture. We began moving the greenhouse fry out on April 26 and finished on May 18. A total of 1152 kg of fry were moved out of the greenhouse, with a survival rate of 96%. The increase in fry size was not obvious.

2. Greenhouse training costs: cages, greenhouse and agricultural film totaled 1900 yuan; purchased fry 1200 kg x 15 yuan = 18,000 yuan; fresh trash fish 900 kg x 3 yuan = 2700 yuan; drug costs 200 yuan; electricity 100 yuan; total cost 22,900 yuan. Calculated on the basis of 1152 kg of fry leaving the greenhouse, this is 19.87 yuan per kg, much cheaper than the 24-30 yuan per kg for fry collected in July. More importantly, it provides 2-3 months of additional growth period, so the weight-gain multiple and farming benefits are much better.

IV. Summary

1. The main factor affecting the survival rate of greenhouse fry is the quality of the fry. In the trial, we strictly controlled the fry purchasing methods: first, choose fine sunny weather; second, purchase nearby; third, handle with water, ensuring the ratio of fry to water is above 1:2; fourth, keep the time from purchase to stocking within 4 hours, and buy only same-day fry; fifth, strictly sort the purchased fry and resolutely remove abnormal fry with injuries or floating-head symptoms. In this way, the survival rate after stocking is high. With good fry collection, only two or three fish died out of 15 kg of fry in a cage. If the fry are poorly collected, the mortality rate after stocking is also very high.

2. The water temperature in the greenhouse is high, and water quality is prone to deterioration. Neither the cage placement density nor the fry stocking density in the cages should be too high. We believe that the cage area should not exceed 50% of the total water surface, and the fry stocking density should preferably be 3-4 kg per sq m.

3. Water exchange in the greenhouse must be carried out on sunny days, draining water in the morning and adding water at noon when the water temperature outside the greenhouse rises, to prevent an excessive temperature difference.