Authors: James E. Rakocy (University of the Virgin Islands) and Andrew S. McGinty (University of Puerto Rico)
In the U.S., the most appropriate species of tilapia for culture are the mouthbrooders: Tilapia nilotica, T. aurea, T. mossambica, T. hornorum, and the substrate spawners: T. rendalli and T. zillii. Various hybrids between mouthbrooding species may also be important; e.g., most of the reddish-orange tilapias are hybrids. In general, mouthbrooders have been used more in food fish production, whereas substrate spawners have been used mostly for weed control.
Potential tilapia culturists in the U.S. should first determine which species, if any, can be legally cultured in their state. Assuming there are no restrictions, selection of a species will depend mostly on growth rate and cold tolerance. Rankings for growth rate in ponds are T. nilotica > T. aurea > T. rendalli > T. mossambica > T. hornorum. Most of the hybrids tested grow as fast as their parent species. Cold tolerance may become an increasingly important criterion for selecting a species in more northerly latitudes. Tilapia aurea is generally recognized as being the most cold tolerant.
Pond culture is the most popular method of growing tilapia. One advantage is that the fish are able to utilize natural foods. Management of tilapia ponds ranges from extensive systems, using only organic or inorganic fertilizers, to intensive systems, using high-protein feed, aeration and water exchange. The major drawback of pond culture is the high level of uncontrolled reproduction that may occur in growout ponds. Tilapia recruitment, the production of fry and fingerlings, may be so great that offspring compete for food with the adults. The original stock becomes stunted, yielding only a small percentage of marketable fish weighing 1 pound (454 grams) or more. In mixed-sex populations, the weight of recruits may constitute up to 70 percent of the total harvest weight. Two major strategies for producing tilapia in ponds, mixed-sex culture and male monosex culture, revolve around controlling spawning and recruitment.
There is no restriction on pond size, but for ease of management and economical operation, shallow (3 to 6 feet), small (1 to 10 acres) ponds with drains are recommended. Draining is necessary to harvest all of the fish. A harvesting sump is needed to concentrate the fish in the final stage of drainage. The pond bottom should be dried to eradicate any fry or fingerlings that may interfere with the next production cycle.
Geographic range for culturing tilapia in ponds is dependent upon temperature. The preferred temperature range for optimum tilapia growth is 82° to 86° F. Growth diminishes significantly at temperatures below 68° F and death will occur below 50° F. At temperatures below 54° F, tilapia lose their resistance to disease and are subject to infections by bacteria, fungi and parasites.
In temperate regions, tilapia must be overwintered in heated water. In the continental United States, the southernmost parts of Texas and Florida are the only areas where tilapia survive outdoors year-round with the exception of geothermally-heated waters, most notably in Idaho. In the southern region, tilapia can be held in ponds for 5 to 12 months a year depending on location.
Mixed-sex populations of fry are cultured together and harvested before or soon after they reach sexual maturity, thereby eliminating or minimizing recruitment and over-crowding. A restricted culture period limits the size of fish that can be harvested.
In mixed-sex culture, tilapia are usually stocked at low rates to reduce competition for food and promote rapid growth. One month-old, l-gram fry are stocked at 2,000 to 6,000 per acre into growout ponds for a 4- to 5-month culture period. Newly-hatched fry should be used because older, stunted fish, such as those held over winter, will reach sexual maturity at a smaller, unmarketable size. Supplemental feeds with 25 to 32 percent protein are generally used. At harvest, average weight is approximately 0.5 pound (220 grams), and total production is near 1,400 pounds/acre for a stocking rate of 4,000/acre. Expected survival is roughly 70 percent.
Species such as Tilapia zilli, T. hornorum, or T. mossambica are not suitable for mixed-sex culture because they reproduce at an age of 2 to 3 months and at an unmarketable size of 30 grams or less. Tilapia suitable for mixed-sex culture are T. aurea, T. nilotica and their hybrids, all of which reproduce at an age of 5 to 6 months.
Two to three crops of fish can be produced annually in the tropics compared to only one crop in temperate regions. In temperate regions, mixed-sex culture is referred to as young-of-the-year culture because fry produced in the spring are grown to marketable size by autumn. Early spawning is needed to maximize the growout period. The growout season is shortened by about 2 months to account for spawning and rearing of l-gram fry for stocking growout ponds.
Male fingerling rearing
With male monosex culture, fry are usually reared to fingerling size in a nursery phase, and then male fingerlings are separated from females for final growout. All-male fingerlings can be obtained by three methods: hybridization, sex-reversal and manual sexing. None of these methods is consistently 100 percent effective, and thus a combination of methods is suggested. Hybridization maybeused to produce a high percentage of male fish. The hybrids may then be manually sexed or subjected to a sex-reversal treatment. All three methods are sometimes used. Hybridization and sex-reversal reduce the number of female fingerlings that must be discarded during manual sexing. This saves time, space and feed. Problems nevertheless still exist with hybridization and sex-reversal. Producing sufficient numbers of hybrid fry maybe
difficult because of spawning incompatibilities between the parent species. Sex-reversal is more technically complicated and requires obtaining recently hatched fry and rearing them in tanks with high quality water. Both hybridization and sexreversal may produce less than 100 percent males. Manual sexing is commonly used by producers. Manual sexing (hand sexing) is the process of separating males from females by visual inspection of the external urogenital pores, often with the aid of dye applied to the papillae. Secondary sex characteristics may also be used to help distinguish sex. Reliability of sexing depends on the skill of the workers, the species to be sorted and its size. Experienced workers can reliably sex 15-gram fingerling T. hornorum and T. mossambica, 30-gram T. nilotica, and 50-gram T. aurea.
In the tropics, fingerlings maybe produced year-round. In temperate regions, fingerlings are produced during summer and stored in overwintering facilities for the next growing season. If manual sexing is used, it is done prior to overwintering. The best fingerling size for overwintering depends on the number of fingerlings that will be needed and the available storage capacity. Fry of 1 gram or less are stocked in nursery ponds and fed high-quality feeds. Ponds stocked at 20,000 fry/acre will produce 100-gram fingerlings in 18 weeks, while 40,000 fry/acre will produce 50-gram fingerlings in 12 weeks, and 72,000 fry/acre will produce 27-gram fingerlings in 9 weeks. Fingerlings that weigh less than 20 grams should not be overwintered because their survival rate will be low.
Overwintering facilities consist of geothermal springs, greenhouses and heated buildings. Fingerlings can be held in cages located in geothermal springs or in small ponds or tanks through which warm spring water is diverted. In greenhouses and heated buildings, recirculating systems are used to hold large quantities of fingerlings. Fingerlings can be overwintered in long, narrow ponds that are covered with clear plastic if the winter is mild.
Continue to Pond Culture of Tilapia (part 2)