Pond Culture of Tilapia (part 2)
Male monosex culture
Males are used for monosex 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 monosex 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 one pound (454 grams) or more. Expected survival for all-male culture is 90 percent or greater. A disadvantage of male monosex culture is that female fingerlings are discarded.
The percentage of females mistakenly included in a population of mostly male tilapia affects the maximum attainable size of the original stock in growout. 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 0.8 pounds (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 monosex 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 2.2 tons/acre. A stocking rate of 8,000/acre is frequently used to achieve yields as high as 4.4 tons/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. To produce a 500-gram fish in temperate regions, overwintered fingerlings should weigh roughly 70 to 100 grams and be started as early as possible in the growing season. A stocking rate of 8,000/acre does require nighttime emergency aeration when the standing crop is high.
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 paddlewheel 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.
Tilapia can be cultured with channel catfish (Ictalurus punctatus) with only a minor reduction in catfish yields. Male tilapia stocked at a rate of 800/acre yield nearly 770 pounds/acre when channel catfish are stocked at 3,000/acre. At this stocking rate, net production of catfish declines by roughly 170 pounds/acre, but for every reduction of 1 pound in catfish production, 4.5 pounds of tilapia are produced. Catfish production does not decline when cultured in combination with tilapia, silver carp (Hypophthalmichthys molitrix) and grass carp (Ctenopharyngodon idellus) at densities of 800, 1,000 and 20/acre, respectively. With no additional feed, total net production can reach nearly 4,120 pounds/acre compared to 2,370 pounds/acre for catfish cultured alone. The incidence of offflavor catfish may be less in catfish/tilapia polyculture than catfish monoculture.
Another promising polyculture system consists of tilapia and prawns (Macrobrachium rosenbergii). In polyculture, survival and growth of tilapia and prawns are independent. Feed is given to meet the requirements of the fish. Prawns, which are unable to compete for the feed, utilize wasted feed and natural foods that result from the breakdown of fish waste. Stocking rates for 1 to 2 gram prawns vary from 4,000 to 36,000/acre, but a rate of 8,000/ acre is often used to obtain a high percentage of market-size prawns (<25 grams) and a yield of about 445 pounds/acre. Tilapia can be stocked in the range of 2,000 to 4,000/acre.
Another type of poylculture involves the use of a predatory fish, such as largemouth bass (Micropterus salmoides), to reduce tilapia recruitrecruitment. Stocking predators with mixedsex 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.
More predators are required to control recruitment when there are larger numbers of mature female tilapia. For tilapia populations ranging from 2,000 to 4,000/acre and containing 50 percent females, the recommended predator/prey ratio is one largemouth bass to 15 tilapia. With 10 percent females, the recommended ratio is one largemouth bass to 65 tilapia.
Use of predators has been effective on an experimental scale, but they have not been used widely in commercial operations because of the difficulty in finding reliable sources of fingerlings. Some of the best predators, such as guapote tigre (Cichlasoma managuense) and peacock bass (Cichla ocellaris), are exotic species and may be illegal to use.
Continue to Pond Culture of Tilapia (part 3)