Main-culture techniques for freshwater pomfret (piaractus brachypomus) in shallow ponds
Among existing ponds, shallow ponds still account for a very large proportion. Such ponds are shallow, lack matched drainage and irrigation systems, and some are formed by seasonal flooding; when traditional fish species are farmed in them, fish yields are low and returns are very poor. To explore ways to increase production, income and efficiency in such ponds, for two consecutive years from 1993 to 1994, we conducted a "trial of main-culture of freshwater pomfret in shallow ponds" in three ponds with a total area of 9.7 mu and a maximum water depth of 1 m. Without aeration facilities, satisfactory results were achieved: in 1994 the average yield per mu was 227 kg, the largest individual pomfret weighed 0.9 kg, the smallest 0.45 kg, and the average individual 0.7 kg; the output value per mu was 2750 yuan and the profit per mu was 1525 yuan. Over the past two years this technique has been extended throughout the county, achieving good economic and social benefits. Its main technical measures are described below.
I. Pond conditions
Ponds ranging from several dozen square meters to several dozen mu in area and with a water depth above 0.8 m can all be used for farming. Seasonally flooded ponds can be used as long as they can hold water from early June to the end of September. Since freshwater pomfret is easy to catch, the condition of the pond bottom need not be considered.
10 to 15 days before stocking the fingerlings, use 100-150 kg of quicklime per mu to drain and clear the pond, or 300-350 kg of quicklime per mu to clear the pond with the water retained, so as to thoroughly kill pathogens, parasites and wild miscellaneous fish in the pond water.
II. Fingerling stocking
1. Stocking time. Since freshwater pomfret is not tolerant of low temperatures, stocking can only be carried out when the water temperature is stably above 18°C. North of the Yangtze River, stocking from late May to early June is generally more suitable.
2. Stocking size. In northern regions, freshwater pomfret requires a heat source to overwinter as broodstock, so fingerlings are rather expensive. To reduce production costs, there is no need to stock larger fingerlings. Based on several years of farming practice, we consider it more suitable to stock pomfret fingerlings of 30-35 g each. As for the silver carp stocked together with them, the size must be above 50 g each.
3. Stocking density and mixing ratio. Because the pond is shallow, too many species should not be mixed in; generally only silver carp need be stocked together, which both fully uses the food organisms in the pond and reduces the fertility of the pond water. Stock 400 fish per mu, of which 350 are pomfret and 50 are silver carp. In ponds with aeration facilities, the stocking rate can be increased to 800-1000 fish per mu.
III. Feeding management
1. Feeding. Ponds mainly culturing freshwater pomfret should be based on feeding compound pelleted feed.
(1) Feed: The crude protein content in feed for grow-out of freshwater pomfret is generally 26%. Its feed formula is: fish meal 15%, soybean cake 20%, cottonseed cake 10%, wheat bran 53%, and mixed salt based mainly on monocalcium phosphate 2%. In addition, add 5% leftover flour as a binder. Extrude into shape with a feed machine. For feeding fish below 100 g, the pelleted feed particle size is 3 mm; for feeding those above 100 g, the pelleted feed particle size is 4-5 mm.
(2) Feeding method and daily feeding amount: Feeding follows the scientific "four fixed" principles. On the second day after the fingerlings enter the pond, begin feeding to tame them. At first, broadcast the feed over the whole pond; afterwards gradually narrow the feeding range each day, and after about a week gather the fish in one place, which serves as the feeding site. Ponds with a large area may have two feeding sites. During taming, before each feeding, strike the feed bucket or make other sounds so that the fish form a conditioned reflex; later, when they hear the sound at feeding time, they will swim in schools to this place to seek food. Feeding adopts the hand-broadcasting method, using a "slow, fast, slow" pattern: at the beginning, when few fish come, broadcast the feed more slowly and less; when most of the fish have been attracted, broadcast faster and more; when most fish are full, broadcast slowly and less again. Each feeding lasts about 20-30 minutes. In June, feed three times a day, with the daily feeding amount at 7% of the total weight of pond fish; in July and August, feed four times a day, with the daily feeding amount at 7%-8% of the total weight of pond fish; in early and mid-September, feed three times a day, with the daily feeding amount at 5% of the total weight of pond fish; in late September, feed twice a day, with the daily feeding amount at 3% of the total weight of pond fish; in early October, feed once a day, with the daily feeding amount at 2% of the total weight of pond fish. Generally, measure the growth of the pond fish every 20 days and correctly estimate the weight of pond fish so as to adjust the daily feeding amount. The specific amount to feed each time should also be determined according to the feeding response of the pond fish; under normal conditions, feeding can be stopped when about 80% of the fish have eaten their fill and swum away.
2. Regulating water quality: Regulating pond water quality includes two parts: increasing oxygen and filling and draining water.
In the middle and later stages of farming, as the fish grow and the pond holds many fish, the pond fish are prone to gulping for air at the surface at night. Where conditions permit, install aerators and turn them on in good time to increase oxygen, generally following "three turn-ons" and "two no turn-ons". That is, on clear days turn the machine on at noon for 1 hour; on cloudy days turn it on from 2-4 a.m. the next morning until the gulping for air is relieved; during continuous overcast and rainy weather with a serious risk of gulping for air, turn it on before the fish gulp for air and keep it on until they no longer do so. Under normal conditions, do not turn it on in the evening; on overcast and rainy days do not turn it on during the daytime, but arrange the limited running time at night. In ponds without aeration facilities, when serious gulping for air occurs, a sprinkler irrigation machine can be started to pump water from the pond itself and spray it to provide relief.
Because pelleted feed is used, the pond water is very fertile. To promote rapid fish growth, ponds with a water source should drain and fill water in good time: once every 20-30 days in the early stage of farming, and once every 15 days in the later stage, draining and filling 40-50 cm each time.
3. Preventing and treating fish diseases.
In the initial stage after stocking the fingerlings, because the fingerlings are easily injured during seining and transport, and because the water temperature is low, freshwater pomfret is susceptible to white skin disease, causing death, so preventive work should be emphasized. First, before the fingerlings enter the pond, soak them in a 2 ppm furazolidone solution for 20-30 minutes. Second, after the fingerlings are put into the pond, splash the whole pond with a 0.2 ppm furazolidone solution for 3 consecutive days. If white skin disease occurs, it can be treated by the latter method.
IV. Points for attention
1. Freshwater pomfret has small, easily shed scales. During seining, transport, weighing and other processes involving the fingerlings, operate carefully and meticulously to avoid injuring the fish.
2. During the farming period, strictly prevent freshwater pomfret from gulping for air at the surface. Over several years we found that although freshwater pomfret has a strong tolerance for low oxygen, if serious gulping for air occurs once at night, the pond fish will have a poor appetite during the day; if even ordinary gulping for air occurs at night for several consecutive days, most of the fish will not eat for 2-3 days; if gulping for air occurs frequently at night, then all the pond fish will not feed. Poor appetite in the pond fish directly affects growth. Therefore, try to avoid freshwater pomfret gulping for air. An effective method is to install an aerator in the pond
3. Freshwater pomfret is especially sensitive to organophosphorus pesticides. Therefore, when preventing and treating fish diseases, choose drugs with caution; do not wash instruments and containers containing organophosphorus liquid in the farming waters, so as to avoid poisoning and death of the pond fish.