Detailed Explanation of Key Points on Off-Season Breeding Technology for Ayu Sweetfish

I. Facilities and Methods

(I) Facilities

1. Nursery room. Converted from a plastic shed, 500 cubic meters of water body, black shading net to regulate light intensity, controlling light intensity below 5000 lux.

2. Broodstock rearing pond. Cement pond, rectangular (3 m x 2.65 m), pond depth 1 m, also serves as bait pond and nursery pond.

3. Water supply facilities. Groundwater is used, supplied by pumps through main and branch pipelines.

4. Aeration equipment. 2 ordinary air pumps, one air diffuser stone placed every 1-2 sq m in the nursery pond.

5. Heating and power supply. A 0.2-ton small heating furnace is used for heating, with a self-provided small generator.

(II) Methods

1. Off-season broodstock rearing

Technical measures were adopted to advance the ayu sweetfish from breeding in early September in natural water bodies to breeding in late May in spring. The key lies in how to shorten the time for gonadal maturation of the ayu sweetfish. In this experiment, in a plastic greenhouse cement pond, under the premise of ensuring the ayu sweetfish ingests sufficient nutrition, the water temperature was raised and the light duration was increased, so that the gonads of the ayu sweetfish reached stage IV by the end of April, began transitioning to stage V in May, and spawned and fertilized in late May, changing from autumn breeding to spring breeding.

Since October 2003, this research began, using 4 modes for comparative experiments. Mode 1 only raised temperature without increasing light. Mode 2 neither raised temperature nor increased light. Mode 3 did not raise temperature and only increased light. Mode 4 raised temperature and increased light.

2. Off-season rearing of fry

(1) First feeding bait In late 2004, off-season breeding was carried out, using substitute bait as the first feeding bait for nursery rearing, mainly yeast and micro compound bait. During off-season breeding in 2005, rotifers were used as the main first feeding bait. Rotifers were intensively cultured with chlorella, photosynthetic bacteria and spirulina, etc.

(2) Bait and feeding The bait series adopted in 2005 was rotifers - water fleas - synthetic bait.

Rotifers were stocked in each pond respectively at: 10-11 per/mL, 8-9 per/mL, 1-3 per/mL, 5-6 per/mL.

Water fleas were collected from rivers, lakes and marshes, mainly cladocerans, copepods and chironomid larvae, water @#@254@#@, etc. The main nutritional component contents of the synthetic bait were: protein 45%, crude ash 12%, calcium 0.5%, crude fat 10%. It was evenly broadcast, feeding 5-6 times per day, and when body weight reached above 5 g, reduced to 3 times. The initial feeding amount was 7%-8% of body weight; when fry body weight reached about 20 g, the feeding amount was 5%-7% of body weight; after body weight reached above 60 g, the feeding amount was 3%-5% of body weight.

(3) Salinity regulation Seawater crystal and table salt were used to regulate salinity. When fry reached above 1 cm, salinity was gradually increased by 0.3‰-0.5‰ per day; when salinity reached 8‰, it was stabilized for one week. Afterward it was gradually lowered, adjusted to zero at a daily decrease rate of 0.3‰-0.5‰.

(4) Water quality management Water was changed once per day, and the amount of water changed each time depended on water quality conditions, generally 1/3-2/3 of the total pond water volume, and residual bait and debris at the pond bottom were frequently siphoned out.

(5) Water temperature control Water temperature was kept stable, controlled at 20-22°C.

II. Results

1. Broodstock rearing

Under the fourth mode, with both increased light and raised temperature, by mid-May 2005, more than 90% of broodstock matured. On May 23, they were paired for spawning and breeding, with 234 groups induced to spawn, obtaining 2.82 million eggs, of which 1.549 million were fertilized eggs, with a fertilization rate of 55%. 1.1 million fry hatched, with a hatching rate of 54.9%. This showed that under the condition of meeting nutritional needs, increasing light and raising water temperature can promote early maturation of ayu sweetfish broodstock and achieve the purpose of off-season breeding.

2. Fry rearing

Using rotifers as the first feeding bait is the key to success, and the quantity and quality of rotifers are the key factors affecting fry output. When the rotifer quantity was 10-11 per/mL, the fry output rate was the highest, at 81%; at 1-3 per/mL, the fry output rate was the lowest, at 40%.

III. Discussion

1. On off-season rearing of broodstock

Temperature and light are the two major important factors promoting gonadal development of ayu sweetfish, and the two restrict each other and complement each other. When only temperature was increased without increasing light duration, in May the fish body length reached 18-20 cm and body weight reached 90-110 g. Although the fish grew quickly, the fish itself would undergo physiological disorders, leading to disease occurrence, manifested as red and swollen liver and ascites, and rapid death. If neither temperature nor light was increased in winter, under artificial rearing conditions, broodstock could also reach sexual maturity, but the maturation time would be delayed very late, in the north being delayed to August-September, failing to achieve the purpose of off-season hatching. The fourth mode, increasing temperature while also increasing light, had the best effect. Under such conditions, although occasionally a small number of broodstock had some physiological discomfort and showed abdominal fluid accumulation, as long as some vitamins were added to the bait, the symptoms would be alleviated or disappear.

Regarding the effect of "short-day" treatment on broodstock. "Short-day" treatment can promote gonadal maturation, but its effect can only appear when the gonads of the broodstock have already developed well; that is, it is necessary to first increase temperature and increase light, and then conduct "short-day" treatment, in order to achieve obvious results.

As for whether temperature and light can be further increased, raising the water temperature for gonadal development of ayu sweetfish to 22°C, or a bit higher, or increasing the light duration to more than 6 hours/day, this remains to be further explored.

2. On off-season rearing of fry

(1) Selection of first feeding bait The first feeding bait is an important link in ayu sweetfish rearing, and the best first feeding bait for ayu sweetfish is live rotifers. Ayu sweetfish fry hatched in September 2004 used substitute bait, and as a result the survival rate dropped greatly; when the fry reached 1.6 cm, the survival rate was only about 1%, whereas in June 2005, using rotifers as the main first feeding bait, the survival rate was 82%.

(2) Intensive culture of rotifers The nutrition of the rotifers themselves also affects the survival rate of fry rearing. Using chlorella, photosynthetic bacteria and algae powder, etc. to intensively culture rotifers had a very good effect.

(3) Stocking density of rotifers Ayu sweetfish fry are very voracious and have a large food intake. Practice shows that when the rotifer density in the water body is above 6 per/mL, the larvae are all satiated, while when it is below 6-7 per/mL, the fullness of the larval digestive tract is insufficient, directly affecting the survival rate of fry rearing.

3. On the adaptability of ayu sweetfish fry to salinity

Fry bred from ayu sweetfish broodstock collected from natural waters and ayu sweetfish fry bred from indoor-cultured broodstock differ in their adaptability to salinity. For the former, adjusting salinity to 8‰-10‰ gave a relatively high survival rate; for the latter, adjusting salinity to 3‰-5‰ was sufficient. Up to the acceptance inspection of this experiment, for fry bred from indoor-cultured broodstock, salinity was only 2‰-3‰, and no abnormal phenomena occurred.

4. On ayu sweetfish diseases

The most common diseases of ayu sweetfish include parasitic diseases, vibriosis, enteritis, etc. For general diseases, 0.2-0.5 g/cubic meter of crystallized trichlorfon, or 0.3 g/cubic meter of a mixture of copper sulfate and ferrous sulfate, can be splashed over the whole pond, all of which can achieve very good results.

IV. Existing Problems

1. Problems

During this experiment, some broodstock had already reached the end of their "physiological cycle" before reaching gonadal maturity; some healthy fish matured gonads but died without cause before spawning. The mechanism of this phenomenon is still unclear. Whether it means that the "gonadal development cycle" and "life cycle" of ayu sweetfish have a synchronized process, or perhaps temperature or light, one of the factors, caused the life cycle to proceed too quickly and arrive early, resulting in death. If so, further adjustments to light duration and the extent of temperature increase would be needed. This is only a hypothesis and speculation, and the exact mechanism still requires further research and exploration.

2. Conclusion

Temperature and light are two important factors affecting gonadal development of ayu sweetfish in off-season hatching. Through technical measures to raise temperature and increase light duration, it is entirely feasible to promote ayu sweetfish to breed in spring. The best first feeding bait for ayu sweetfish is rotifers, and rotifers must be intensively cultured to ensure their quality and quantity. The stocking density of rotifers must also exceed 6-7 per mL, otherwise it cannot meet the feeding needs of the fry.