Chapter 5 POND CULTURE OF FOOD FISH (Contd.)

Management of Rearing Food Fish

Feeding

In high-density, polycultured ponds, each fish gets only a small amount of natural food. Therefore, supplemental feeds and manures are essential to ensure the normal growth and high yield of fish. Feeding is the most important aspect of intensive fish culture and responsible for most of the management work around the grow-out ponds. The expenditure on feeds normally accounts for more than 50 per cent of the total production cost. Therefore, the selection and processing of fish feeds and feeding technique will significantly affect the output, production cost, and economic efficiency of fish farming.

Table 5.17. Stocking and harvesting in pond A.

Note: The net food fish yield in this pond is only 75–100 kg/mu in early May. The pond can also be used to spawn carp, crucian carp and Wuchang fish or as a fodder plantation.

Table 5.18. Stocking and harvesting in ponds B and C.

Table 5.19. Feeding and manuring (kg/mu) in pond A.

Note: Before May the net yield of food fish is 87.35 kg/mu. After the second 10 days in May, the yield of the first harvest of fingerlings is 72.13 kg/mu, the second harvest yield is 404.7 kg/mu. The total yield is 564.18 kg/mu.

Table 5.20. Feeding and manuring (kg/mu) in pond B and C.

Note: The net yield of food fish in the first harvest is 215 kg/mu; second harvest yield is 397 kg/mu. The total net fish yield is 612 kg/mu.

Allocation of fish feeds

In fish farming, it is necessary to work out a feeding plan. The total food demand for 1 year should be determined based on the target fish production, the expected body weight increment of each species, and the food-conversion rate and monthly feed demand according to the water temperature and monthly fish growth. Feed and manure allocations also depend on their local availability; this should be a major determining factor in setting production goals. The monthly feed and manure requirements in Jiangsu province are shown in Table 5.21.

Table 5.21. Feed and manure demand in grow-out ponds in Jiangsu province.

^a Number of applications with the per cent of the total in paren theses.

Feeding techniques

The feeding habits of the various species must be known to ensure feeds are supplied properly. On the one hand, we much achieve high yields and, on the other hand, we should economize on feeds. Therefore, feeds should be supplied according to the season, the weather, water quality, and food intake (“four observations”).

Seasons — More feed should be provided from June to October when the air temperature is comparatively high. Fish have a good appetite and grow rapidly when the air temperature reaches 28°C. June to October is also the growing season for aquatic weeds, molluscs, and terrestrial grasses. The fish feed supplied over this period accounts for about 80 per cent of the total yearly allocation. A small amount of food should be supplied shortly after initial stocking in early spring, when air temperature is low and fish have a poor appetite to stimulate the appetite of the fish. Feeding is reduced in late autumn when the air temperature drops below 10°C. The appetite of the fish and their ability to search for food gradually decline from late October to the end of the year, when the fish are harvested.

Weather — More feed should be provided when the weather is good and the DOC is high. Feed application should be reduced or discontinued when the weather is stifling. A good time for feeding is after a foggy period because of the low air pressure.

Water quality — If the pond water is fertile and fish have a good appetite, more feed can be applied. If the pond water is sheer, the quantity of feed should be increased; however, feed application should be decreased if the pond water is overfertile.

Food intake status — Normally, all the feed should be consumed in 7–8 h. If all the feed is not consumed in this time, the fish have an abnormal ingestion status and the quantity of feed should be reduced.

Fixed quality, fixed quantity, fixed time, and fixed position of feeding (“four fix”) are important in a proper feeding schedule.

Fixed quality — Feeds should be fresh and palatable with a high nutritive value. Spoiled foods should be discarded to prevent disease.

Fixed quantity — Fish should be provided with a fixed amount of food every day. Uneven feeding will cause poor digestion, bad absorption, and slow growth. Daily feeding rates are mainly dependent on the ingestion status of the fish.

Fixed time — Feeding is usually done before 1000 hrs every day. To raise the food-utilization rate, fish should be fed when the DOC is high.

Fixed position — Feeds such as molluscs, pellets, and fine feeds should be supplied at a fixed position (e.g., on a fixed feeding platform). This allows easier examination of the fish. Floating feeds may be supplied in a floating framework.

Manuring

Like fry and fingerling ponds, grow-out ponds need fertilization. A heavy supply of base manure should be added to newly dug ponds because these ponds initially have little or no silt and this would make it difficult to get the water fertile. Therefore, organic manures (animal manure, green manure, or stable manure) are used at a rate of 400–500 kg/mu as the base manure. After the decomposition and mineralization of the base manure through exposure to the sunlight for 3 or 4 days, the pond can be filled with fresh water.

Fingerlings can be added after another 7 or 8 days. Additional manure should be applied evenly, frequently, and in small amounts to ensure the continued propagation of natural food organisms.

Daily Routine of Pond Management

High and stable fish yields can only be ensured through the daily routine of pond management, which should be performed carefully, diligently, and unremittingly throughout the rearing period.

Surfacing

The pond should be examined two or three times daily for fish surfacing at dawn, food intake in the afternoon, ingestion status of the fish at sunset, and fish gasping for air. From late spring to mid summer, when the weather changes suddenly, the pond should be examined around midnight for fish surfacing. Surfacing is common due to high water temperatures, intense decomposition of organic materials, the cessation of photosynthesis by phytoplankton at night, and the oxygen consumption of other aquatic life.

Forecasting surfacing — When the weather is hot and the water temperature is high or the water is fertile, surfacing will occur at dawn or around midnight. Fish are not diseased, however, they do lose their appetite; this indicates that the pond water lacks dissolved oxygen. When the pond water is overfertile or when there is a sudden deterioration of water quality caused by the weather, the decomposition of large amounts of plankton not only consumes dissolved oxygen but also produces toxic substances such as hydrogen sulphide (H₂S) and ammonia, which will cause serious surfacing.

Diagnosis of degree — The degree of surfacing is based on the time of surfacing, the location of surfacing, the species sequence of surfacing, and the fish response during surfacing. Surfacing before dawn is not serious. As the sun rises, photosynthesis of phytoplankton accelerates and the DOC of the pond water increases, alleviating the hypoxic condition. Surfacing in the evening or around midnight is more serious. Because the hypoxic condition is enhanced by the respiration of plants throughout the night, the surfacing of fish will become more and more serious. Surfacing at the centre of a pond is less serious than surfacing all over the pond.

Fish swim deeper in the water when scared. However, if fish show no response and appear to be in a coma when frightened, surfacing is serious and the dissolved oxygen in the pond water is depleted. Different species surface at different times because of their different oxygen-consumption rates and asphyxiation points. The order of seriousness, from slight to severe, is as follows: wuchang fish (slight); silver carp and bighead (moderate); grass carp and black carp (serious); common carp and crucian carp (severe). When common carp and crucian carp surface, fish mass mortality will result.

Remedies for surfacing — When symptoms of hypoxia appear, measures should be taken to increase the DOC of the pond water (addition of fresh water). When pumps are used, the outlet should be placed horizontally to ensure that the fresh water enters at one level. All the surfacing fish will be attracted to the fresh water area where the DOC is higher.

Water Quality

Water quality is closely related to fish growth and yield. During production, water colour is the indicator of water quality, and there are various means of controlling water quality: feeding and manuring, use of aerators, adding fresh water, and turning the pond silt.

There are four types of water: “fertile”, “sterile”, water bloom, and deteriorated. Experience has shown that pond water should be kept “fertile” including “flexible” and “crisp”. “Fertile” water is biologically active and rich in plankton. “Flexible” water frequently changes colour. There are diurnal and monthly variations. A common saying among fish farmers is “brown in the morning and green at night.” This diurnal variation is due to a change in the dominant species of phytoplankton. “Crisp” water is fertile but not turbid, moderately transparent and has a high DOC.

“Fertile” water has a heavy water bloom of a desirable species of phytoplankton with a biomass from 20 to 100 mg/L. “Flexible” means that there is diurnal vertical movement of Gonyostomum and other flagellates. “Tender” or “bright” means that algae are experiencing exponential growth' cell multiplication and feeding are accelerated during this period (He Zhihui 1985).

Aeration

Impeller aerators (Fig. 5.3) are commonly used in China. They have three functions: oxygenation, water stirring, and gas exposure.

Fig. 5.3. Impeller aerator. “Zhengyangji” in Chinese means “oxygen enhancing machine.”

The aerator need not be turned on for the whole day; it should be only used for a short time at one or two critical moments in the day. The time and period of operation of the aerator should be set according to the diurnal fluctuation of water quality to maximize its utilization and effect and minimize its energy consumption. There are three main reasons to use aerators in stagnant fish ponds: to break the metalimnion and avoid the loss of oxygen to the atmosphere, to prevent the depletion of oxygen at the bottom of the pond, and to directly increase the DOC.

On days with good weather, aerators are operated around noon to break the metalimnion and maximize oxygen utilization. The operation time should be short. In a pond of 3–5 mu, a 3 kw impeller aerator should be activated for 30 min. In a pond 7–10 mu, 1 h of aeration is required.

On a cloudy day, the aerators should be used early next morning. This is because photosynthesis is rather weak on a cloudy day and the DOC is low. After one night, the DOC will decline to a minimum. Aerators should be activated from 0300–0500 hrs to sunrise.

Aerators are often operated at midnight on rainy days before surfacing occurs. However, aerators are not used at dusk during the day when the weather is rainy because photosynthesis is weak and the DOC is low in the upper water layer, and therefore, the aerator would have little or no effect.

When an aerator is used rationally, fish surfacing and asphyxiation can be controlled. In addition, the recycling of material in the ponds can be accelerated, the distribution of dissolved oxygen can be improved, and the metabolic intensity of the fish can be increased, thereby decreasing the food-conversion factor, purifying the pond water, stabilizing water quality, and preventing disease.