Production management tilapia in tank

Stocking density, which is very high for fry, is decreased at regular intervals throughout the production cycle to reduce crowding, to ensure adequate water quality, and to use tank space efficiently (Table 1). It is not economical to pump water for a tank system that is stocked initially at one tenth of its capacity, which is the standard stocking practice for ponds. As density becomes too high, fish stocks can be split in half and physically moved to new tanks or given more space by adjusting screen partitions within the rearing tank. Rectangular tanks or raceways, in particular, are much easier to use and allow the culture of several size groups in one tank. However, fry and small fingerlings are cultured separately because they require better water quality. Each time that stocks are split and moved, they are graded through a bar grader to cull out about 10 percent of the slowest growing fish, which would probably not reach market size. Culls could be sold as baitfish if permitted by state law. Recommended grader widths are 25/64, 32/64, 44/64, and 89/64ths of an inch for tilapia greater than 5, 10,25, and 250 grams, respectively.

The highest mortality of the production cycle (about 20 percent) occurs during the fry rearing stage. Much of this is due to predation. As the fish grow and become hardier, mortality decreases significantly at each stage so that no more than 2 percent of the fish are expected to die during final growout.

Fry are given a complete diet of powdered feed (40 percent protein) that is fed continuously throughout the day with automatic feeders. The initial feeding rate, which can be as high as 20 percent of body weight per day under ideal conditions (good water quality and temperature: 86°F), is gradually lowered to 15 percent by day 30. During this period, fry grow rapidly and will gain close to 50 percent in body weight every 3 days. Therefore, the daily feed ration is adjusted every 3 days by weighing a small sample of fish in water on a sensitive balance. If feeding vigor diminishes, the feeding rate is cut back immediately and water quality (DO, pH, ammonia, nitrite) is checked.

Feed size can be increased to various grades of crumbles for fingerlings (1 to 50 grams), which also require continuous feeding for fast growth. During the growout stages, the feed is changed to floating pellets to allow visual observation of the feeding response. Recommended protein levels are 32 to 36 percent in fingerling feed and 28 to 32 percent in feed for larger fish. Adjustments in the daily ration can be made less often (e.g., weekly) because relative growth, expressed as a percentage of body weight, gradually decreases to 1 percent per day as tilapia reach 1 pound in weight, although absolute growth in grams/day steadily increases.

The daily ration for adult fish is divided into three to six feedings that are evenly spaced throughout the day. If feed is not consumed rapidly (within 15 minutes), feeding levels are reduced. DO concentrations decline suddenly in response to feeding activity. Although DO levels generally decline during the day in tanks, feeding intervals provide time for DO concentrations to increase somewhat before the next feeding. Continuous feeding of adult fish favors the more aggressive fish, which guard the feeding area, and causes the fish to be less uniform in size. With high quality feeds and proper feeding techniques, the feed conversion ratio (fish weight gain divided by feed weight) should average 1.5 for a l-pound fish.

Total production levels range from 3 to 6 pounds/ft3 of rearing space and 6 to 17 pounds/gallon/minute of flow. Monthly production levels range from 0.4 to 0.6 pounds/ft3. The higher production levels are generally obtained in flow-through systems. Production can always be increased by increasing the inputs, but this may not be economical