Tilapia in Farm Ponds (Part 2)
Male Mono-sex Culture
Male tilapia fish are used for mono-sex culture because male tilapia grow faster than females. Females use considerable energy in egg production and do not eat when they are incubating eggs. Male mono-sex culture permits the use of longer culture periods, higher stocking rates and fingerlings of any age. High stocking densities reduce individual growth rates, but yields per unit area are greater. If the growing season can be extended, it should be possible to produce fish weighing 454 grams or more. Expected survival for all-male culture is 90 percent or greater.
The percentage of females mistakenly included in a population of mostly male tilapia affects the maximum attainable size of the original stock in grow-out. For example, manually sexed T. nilotica fingerlings (90 percent males) stocked at 3,848/acre will cease growing after 5 months when they average about 365 grams because of competition from recruits. If larger fish are desired, females should comprise 4 percent or less of the original stock and predator fish should be included.
The stocking rate for male mono-sex culture varies from 4,000 to 20,000/ acre or more. At proper feeding rates, densities around 4,000/acre allow the fish to grow rapidly without the need for supplemental aeration. About 6 months are required to produce 500-gram fish from 50-gram fingerlings, with a growth rate of 2.5 grams/day. Total production approaches 1,996 Kg/acre. A stocking rate of 8,000/acre is frequently used to achieve yields as high as 3,992 Kg/acre. At this stocking rate the daily weight gain will range from 1.5 to 2.0 grams. Culture periods of 200 days or more are needed to produce large fish that weigh close to 500 grams.
Stocking rates of 12,000 to 20,000/acre have been used in 1.2 to 2.5-acre ponds, but this requires the continuous use of two to four, one-horse power paddle-wheel aerators per pond. Yields for a single crop range from 6 to 10 tons/acre.
With optimal temperatures, feeding rates depend on fish size and density. Optimal daily feeding rates for fish of 30,50, 100, 175 and 450 grams are 3.5, 3.0, 2.5, 2.0 and 1.5 percent of body weight, respectively. If densities are high, sub-optimal feeding rates may have to be used to maintain suitable water quality, thereby increasing culture duration.
Tilapia are frequently cultured with other species to take advantage of many natural foods available in ponds and to produce a secondary crop, or to control tilapia recruitment. Polyculture uses a combination of species that have different feeding niches to increase overall production without a corresponding increase in the quantity of supplemental feed. Polyculture can improve water quality by creating a better balance among the microbial communities of the pond, resulting in enhanced production. The disadvantage of polyculture is the special equipment (sorting devices, conveyors, etc.) and extra labor needed to sort the different species at harvest. The role of natural pond foods is less important in the intensive culture of all male populations and may not justify the expense of sorting the various species at harvest.
Another type of poylculture involves the use of a predatory fish, such as African Catfish, to reduce tilapia recruit recruitment. Stocking predators with mixed-sex tilapia populations controls recruitment and allows the original stock to attain a larger market size. Predators must be stocked at a small size to prevent them from eating the original stock. Predators may be stocked when tilapia begin breeding.
The number of predators required to control tilapia recruitment in culture ponds depends primarily on the maximum attainable size of the predator species, the ability of the predator to reproduce, and the number of mature female tilapia. In general, as predators grow they eat larger sized tilapia recruits. Eventually this may result in an increasing biomass of small tilapia that are not consumed. However, this problem should not develop in ponds that are completely harvested one or more times a year.