Key Technical Points for Pond Culture of the Orange-spotted Grouper

Orange-spotted grouper (Epinephelus coioides), locally called green grouper or grouper, is currently one of the best varieties for grouper culture. The grouper adapts to a salinity of 15-41, with an optimum salinity of 25-33, and adapts to a water temperature of 6-38°C, with an optimum water temperature of 20-30°C. It can tolerate a low temperature of 6°C, but does not feed below 14°C. In the past, the main culture mode was cage culture, and pond culture was rare. In 2005 we tested pond culture of the orange-spotted grouper and compared the use of artificial compound feed with iced fresh trash fish as feed. The techniques and test results of pond culture of the orange-spotted grouper are summarized as follows.

I. Materials and Methods 1. Fry source and culture pond preparation On March 10, 2005, 2,500 orange-spotted grouper fry with a length of 4.0 cm were purchased and evenly divided into indoor pond A and pond B. On March 20, another 2,000 orange-spotted grouper fry with a body length of 3.8 cm were purchased and evenly divided into indoor pond C and pond D. The indoor ponds were 40 sq m each and 1.0 m deep, with oxygen supplied by a blower through air stones. At the same time, one indoor pond and two outdoor ponds were selected and prepared as grow-out ponds, all with hardened bottoms. Pond No. 1 was an indoor pond of 40 sq m, pond No. 2 was 660 sq m, and pond No. 3 was 1,300 sq m. Ponds No. 2 and No. 3 were both 1.8 m deep and were aerated with paddlewheel aerators. Cement pipes 25 cm in diameter and 50 cm long, or waste tires, were suitably placed around the culture ponds to provide hiding places for the grouper. 2. Indoor pond nursery rearing Because large-size fry are expensive, and in order to improve fry survival rate, take advantage of the higher indoor temperature, and stock fry early to ensure they reach market size (500 g/fish) in the same year, we adopted the method of indoor pond nursery rearing. The indoor ponds were cleaned and dried 20 days before stocking, disinfected with potassium permanganate at 100 g/cubic meter 5 days before stocking, and filled with water after draining the disinfectant water 24 hours later. The nursery rearing density was 30-50 fish/sq m, with a water depth of 90 cm. The nursery rearing period was 50-70 days, and the body weight reached 50-60 g/fish. 3. Outdoor pond grow-out On May 20, 2005, the grouper fry were moved from the indoor nursery ponds to the grow-out ponds. Those from indoor nursery ponds A and B were moved to outdoor pond No. 3, with 2,310 fish, a nursery survival rate of 92.4%, and an average body weight of 56.5 g. Those from indoor ponds C and D were moved to indoor pond No. 1 and outdoor pond No. 2, with 210 fish in pond No. 1 and 1,600 fish in pond No. 2, totaling 1,810 fish, with a nursery survival rate of 90.5% and an average body weight of 50.3 g. The grow-out ponds were cleaned and exposed to the sun 20 days before stocking, and disinfected with bleaching powder at 20 g/cubic meter 5 days before stocking the nursery fry, and filled with water after draining the disinfectant water 24 hours later. The nursery ponds were transferred to outdoor ponds for culture around late May, when the weather was stable, the water temperature was higher, and the growth rate was fast. The culture period was 200-250 days, and the weight reached 400-600 g/fish. 4. Feeding management Ponds No. 1 and No. 3 used artificial compound feed, and pond No. 2 used iced fresh trash fish. Feeding followed the principle of "fixed time and fixed place, small amounts and many times." The fish were fed twice a day, at 8 a.m. and 4 p.m. Feeding grouper requires great patience and care, feeding little by little, because grouper like to rush over to feed as the feed sinks, and do not like to eat it once it lands on the pond bottom. Feeding grouper once takes two to three hours, so a fixed place is very important; otherwise feeding takes even more time. A feeding tray was set below the fixed feeding point to observe the feeding situation of the grouper and adjust the feeding amount. 5. Water quality management (1) Water temperature: During nursery rearing, the water temperature was still relatively low. If the water temperature was below 20°C, it had to be heated, to avoid a decline in the fish's constitution caused by not feeding due to prolonged low temperature, and poor economic returns because the slow growth rate would prevent them from reaching the market before the Spring Festival that year. In July and August, when the temperature was too high, 60% shade nets were added over the places around the pond where the grouper hid, to cool the water appropriately. (2) Dissolved oxygen: Dissolved oxygen was controlled above 5 mg/L. In the indoor nursery ponds, oxygen was mainly supplied by a blower through air pipes and air stones, and in the outdoor ponds by paddlewheel impeller aerators. For aeration, ways should be found to make the water flow in one direction, so that the water flows actively and collects waste. (3) Transparency: The transparency requirement was 50 cm. Especially in the later stage of culture, when there was too much organic matter in the water and algae multiplied rapidly, more attention had to be paid to controlling transparency, so water exchange had to be strengthened. (4) pH: pH was controlled at 7.8-8.3. In the later stage of outdoor pond culture, when the pH was low, especially in rainstorm weather, some lime water had to be sprinkled appropriately, generally 10 g/cubic meter each time. (5) Salinity: The optimum salinity for grouper is 25-33, and a relatively high salinity is required, especially in the early stage. The salinity must not be lower than 25, otherwise the survival rate will be reduced. In the later stage, the salinity can be appropriately lower. (6) Ammonia nitrogen: Total ammonia nitrogen must not exceed 0.6 mg/L. (7) Light: Direct light should be avoided, and shade nets should be added where the light is too strong. 6. Daily management (1) Water exchange: In pond culture of grouper, water must be exchanged frequently. Grouper feed is a high-protein feed and very easily pollutes the water quality, and feeding trash fish pollutes it even more seriously. Generally, water had to be exchanged two or three times a week, with each exchange amounting to 20%-30% of the total. In the high-temperature season, a large whole-pond water exchange was carried out once every 2-4 days. Each large water exchange drained the pond water to a depth of only 50 cm, cleaned the pond bottom, and sometimes disinfected at this low water level as needed. (2) Waste removal: In addition to cleaning the bottom and removing waste during each large water exchange, residual feed on the pond bottom also had to be siphoned off every day. (3) Pond division: After a period of culture, individual differences appear. To avoid mutual predation and improve the survival rate, the fish must be divided by size, especially in the early nursery rearing stage, when regular pond division is even more necessary. (4) Pond patrol: Frequent patrols were needed to observe the fish's activity, feeding, and aeration conditions. II. Test Results When the orange-spotted grouper was harvested, large fish were caught and small ones left. Generally, those caught at about 500 g fetched the best selling price, and the small ones continued to be kept in the pond for culture. The first batch was sold on December 24, 2005, before the Spring Festival, at a price of 60 yuan/kg. Pond No. 1 caught 130 fish weighing 74 kg, pond No. 2 caught 1,010 fish weighing 604 kg, and pond No. 3 caught 1,437 fish weighing 780 kg. The second batch was sold on February 5, 2006, after the Spring Festival, at a price of 54 yuan/kg. Pond No. 1 caught 58 fish weighing 30.9 kg, pond No. 2 caught 398 fish weighing 205.5 kg, and pond No. 3 caught 598 fish weighing 291 kg. The two batches caught a total weight of 1,985.4 kg, with an output value of 115,959.6 yuan. After deducting costs of 97,695 yuan, the profit was 18,264.6 yuan. III. Analysis and Discussion 1. The test results showed that the culture effect of ponds No. 1 and No. 3, which were fed artificial compound feed, had clear advantages in economic and ecological benefits compared with pond No. 2, which was fed only iced fresh trash fish. Pond No. 2, fed iced fresh trash fish, had a slight advantage in growth rate. The cost of culturing grouper with artificial compound feed was about 48 yuan/kg, namely 16 yuan for fry, 16 yuan for feed and drugs, 13 yuan for labor, 5 yuan for electricity, 2 yuan for miscellaneous expenses, and 6 yuan for cost increases caused by factors such as mutual predation. However, the cost of culturing with iced fresh trash fish was about 51 yuan/kg, of which the feed cost increased by 2 yuan. With the rapid development of the grouper culture industry, promoting artificial compound feed for grouper is imperative. 2. Grouper by nature prefers fresh water quality. The difference between pond and cage culture is that cages are in a large-area sea area with flowing water, where the water quality is generally better, while in ponds the organic matter per unit is generally higher. Therefore, water exchange must be strengthened, and at the same time some water quality improvers must be used regularly, such as photosynthetic bacteria (15 g/cubic meter), Bacillus (5 g/cubic meter), zeolite powder (20 g/cubic meter), etc. 3. Practice has shown that grouper have relatively high requirements for feed palatability, such as hardness, color, and taste. They like to eat soft pellets and light-colored, bright feed. If the pellets are too hard, they spit them out. Their adaptability to soft pellet feed is clearly better than to hard pellet feed. There is a relatively long adaptation process from feeding small trash fish to changing to artificial compound feed, and feeding training must be carried out before feeding compound feed. During culture, in order to improve the immunity of grouper, it is very necessary to regularly mix some nutritional additives such as vitamin C and multivitamins into the feed.