A. Stock Manipulation Method (Mono-specific, multi-size, medium-water-depth method)
This particular method calls for pond bottom drying and fertilizing with organic and chemical fertilizer then watering with 10 to 15 cm of water and slowly deepening this level to 25 cm at which time food (benthic algae) would be well established. Stock of fish is then introduced in two to four sizes as shown below. Usually the bigger ones are the holdovers from previous crop
About 1000 pieces/ha of sizes (6–8 in) 15–20 cm or 8–12 fish to a kg
About 1200 pieces/ha of sizes (4–5 in) 10–13 cm or 40–50 fish to a kg
About 1800 pieces/ha of sizes (2–3 in) 5–8 cm or 150 fish to a kg,
subsequently the pond water is gradually increased to 35 to 45 cm as the fish grows. The above numbers vary depending on the carrying capacity of the pond as observed by the caretaker. After 30 to 45 days the big ones are harvested either by gill net or by a grading grid at the gates. Another batch of small sizes are again introduced. Every 30 to 50 days the larger fish are harvested until it is decided to stop the culture and a final harvest is made and the remainder smaller fish are stocked in holding ponds for the next culture period. The pond then is drained, dried and the whole cycle is repeated. Sometimes a batch of smaller fish is introduced after the big ones are harvested if the food situation in the pond so warrants.
It should however be noted that all through the culture period balance between available food and stock is closely watched by the caretaker. Should food be short as shown by levelling off of the growth rate, the caretaker has three options:
Harvest the larger fish prematurely
Apply supplemental feeding
Reduce stock by transferring to other areas.
One can run 2 to 3 cultures in a year and on average type fishponds an average harvest of 1 ton/ha/year is not difficult to realize.
This particular method would yield well to combination - Chanos chanos/ Penaeus monodon culture as one can run a continuous culture to a period that P. monodon would be in good marketable size which usually is from 4–6 months.
B. Deepwater or Plankton Method
Advocates of this method usually recommond only heavy chemical fertilizer application particularly of phosphorus (P2O5). Water is kept at 0.8 to 1.2 meter in depth and water kept stagnant and in green color. The quantity of plankton is manipulated by the amount of fertilizer applied. If the Secchi disc shows the water is too turbid or too green, then fertilizer is withheld till it comes within reasonable limits. Water may also be let in or out to help prevent plankton bloom.
Stocking is usually in mono-size and harvesting done from time to time either by gill net or through the gates by slight water manipulation.
Different sizes in stocking can also be used in this culture and harvest by gill net.
Fertilizer application in this case is usually through fertilizer platforms where the fertilizer is placed on a table type structure submerged in water so that the fertilizer slowly melts into the water in several days.
In this type of culture the weight and growth curve are critically watched and when the growth rate starts to level off (meaning the pond carrying capacity is approached) 50 percent of the fish is harvested and the rest of the fish allowed again to grow to reach the pond carrying capacity then 50 percent of the remaining fish is again harvested and the last batch or 1/4 of the original stock is finally harvested and another cycle culture is started.
It has been observed that the taste of the fish in this kind of culture is not too acceptable especially the second and final batches.
Production of over a ton per ha/year can be attained by this method.
The feasibility of this system needs further verification. It has been observed that repeated use of this method did not always produce the desired results. There were instances when in subsequent trials the fish were not gaining weight inspite of the good green color of the water. Then too it appears that certain types of plankton were very sensitive to slight salinity and temperature changes producing either blooms or wholesale mortality which invariably have disastrous effects. Fish kills are not uncommon when the BOD (Biological Oxygen Demand) rises rapidly due to massive drying of plankton.
C. Progression Method
This is a system of milkfish culture where a stock of uniform size fish is transferred from smaller ponds which are usually 1/2 in size to the next bigger ponds. The stock movement is done every 30 – 45 days. A series of as much as five pond sizes may be found in each project starting with the nurseries or fry ponds to the rearing or production ponds (Figure 12 a).
The pond preparation consists of drying, fertilizing (organic and chemical) and letting water into Pond A to about 15 cm. In Pond B water depth can be 20 cm; in Pond C about 30 cm during the culture period in these ponds. Harvest is always done from lot C.
This is particularly good to improve production in ponds where the production is not too high or only marginal. A ton or so production is easy to attain. One can also have a very good control as to the size he wants to harvest by shortening (for small sizes) or extending the culture period (for 500 to 700 grams) of the fish to be harvested.
The fish produced in this system have very good quality. Perhaps this is due to good water quality (not stagnant) in the ponds. The food (benthic blue green algae) grown in these ponds are fresh. It has been also observed that the fish is consistently disease-free. Perhaps this is due to the renewed biological cycle in each pond every 30–45 days. It is noticed that the growth rate of the fish picks up sharply upon introduction to newly prepared pond.
Variations - some operators modify the progression method by transferring the stock only once. In the second pond the water depth is gradually increased to about 0.8 meters as the fish grows. This then changes from growing benthic blue greens to plankton. This system is resorted to during rainy seasons when there are less sunny days. The first crop then is on pure progression method and the second crop is on combination progression on the first part and ending as a plankton or deepwater method on the second part.
Fertilizer application - Recently the more progressive farmers have changed their method of chemical fertilizer application from 2 or 3 batches to 10 or more small applications spaced from 5 to 10 days apart and only during favourable days considering climatic conditions. This small but more frequent fertilizer applications has been adopted out of necessity lately when fertilizer prices went up and experiments were run to get the best reuults at the least expense. The rate of application and its frequency is also influenced by the amount and condition of growth of the algae. The usual proportion is 1:1 nitrogen to phosphorus.
Organic fertilizers are extensively used especially chicken droppings. Dried cattle dung may also be applied. The rates range from 0.5 tons to 3 tons per hectare per year.
N - NURSERY
T - TRANSITION
ABC - REARING
NOTE:
A is 1/2 of B; C is twice B in area.
Stock is all transferred from smaller to next bigger pond every 30 to 45 days.
Newly emptied pond is immediately readied for incoming stock.
FIGURE 12 a
LAYOUT OF CHANOS CULTURE PONDS USING THE PROGRESSION METHODS
