Tilapia: Life History and Biology (part 3)
Tilapia are more tolerant than most commonly farmed freshwater fish to high salinity, high water temperature, low dissolved oxygen, and high ammonia concentrations.
All tilapia are tolerant to brackish water. The Nile tilapia is the least saline tolerant of the commercially important species, but grows well at salinities up to 15 ppt. The Blue tilapia grows well in brackish water up to 20 ppt salinity, and the Mozambique tilapia grows well at salinities near or at full strength seawater. Therefore, the Mozambique tilapia and some mossambicusderived “red” tilapia are preferred for saltwater culture.
Some lines of the Mozambique tilapia reportedly have spawned in full strength seawater, but its reproductive performance begins to decline at salinities above 10 to 15 ppt. The Blue and Nile tilapias can reproduce in salinities up to 10 to 15 ppt, but perform better at salinities below 5 ppt. Fry numbers decline substantially at 10 ppt salinity.
The intolerance of tilapia to low temperatures is a serious constraint for commercial culture in temperate regions. The lower lethal temperature for most species is 50 to 52°F for a few days, but the Blue tilapia tolerates temperatures to about 48°F.
Tilapia generally stop feeding when water temperature falls below 63°F.Disease-induced mortality after handling seriously constrains sampling, harvest and transport below 65°F.Reproduction is best at water temperatures higher than 80°F and does not occur below 68°F. In subtropical regions with a cool season, the number of fry produced will decrease when daily water temperature averages less than 75°F. After 16- to 20-day spawning cycles with 1/2-poundNile tilapia, fry recovery was about600 fry per female brooder at a water temperature of 82°F, but only 250 fry per female at 75°F. Optimal water temperature for tilapia growth is about 85 to 88o F. Growth at this optimal temperature is typically three times greater than at 72°F.
Dissolved oxygen concentration
Tilapia survive routine dawn dissolved oxygen (DO) concentrations of less than 0.3 mg/L, considerably below the tolerance limits for most other cultured fish. In research studies Nile tilapia grew better when aerators were used to prevent morning DO concentrations from falling below 0.7 to 0.8 mg/L (compared with unaerated control ponds). Growth was not further improved if additional aeration kept DO concentrations above 2.0 to 2.5 mg/L. Although tilapia can survive acute
low DO concentrations for several hours, tilapia ponds should be managed to maintain DO concentrations above 1 mg/L. Metabolism, growth and, possibly, disease resistance are depressed when DO falls below this level for prolonged periods.
In general, tilapia can survive in pH ranging from 5 to 10 but do best in a pH range of 6 to 9.
Massive mortality of tilapia occurs within a few days when fish are suddenly transferred to water with unionized ammonia concentrations greater than 2 mg/L. However, when gradually acclimated to sublethal levels, approximately half the fish will survive 3 or 4 days at unionized ammonia concentrations as high as 3 mg/L. Prolonged exposure (several weeks) to un-ionized ammonia concentration greater than 1 mg/L causes losses, especially among fry and juveniles in water with low DO concentration. The first mortalities from prolonged exposure may begin at concentrations as low as 0.2 mg/L. Un-ionized ammonia begins to depress food consumption at concentrations as low as 0.08 mg/L.
Nitrite is toxic to many fish because it makes the hemoglobin less capable of transporting oxygen; chloride ions reduce the toxicity. Tilapia are more tolerant of nitrite than many cultured freshwater fish. When dissolved oxygen concentration was high (6 mg/L) and chloride concentration was low (22 mg/L), the nitrite concentration at which 50 percent of the fish died in 4 days was 89 mg/L as nitrite. In general, for freshwater culture the nitrite concentration should be kept below 27 mg/L as nitrite. As a safeguard against nitrite toxicity in recirculating systems, chloride concentrations are often maintained at 100 to 150 mg/L chloride.
Tilapia are more resistant to viral, bacterial and parasitic diseases than other commonly cultured fish, especially at optimum temperatures for growth. Lymphocystis, columnaris, whirling disease, and hemorrhagic septicemia may cause high mortality, but these problems occur most frequently at water temperatures below 68°F. “Ich,” caused by the protozoan Ichthyopthirius multifiliis, can cause serious losses of fry and juveniles in intensive recirculating systems. External protozoans such as Trichodina and Epistylis also may reach epidemic densities on stressed fry in intensive culture. In recent years the bacterial infection Steptococcus inae has caused heavy losses, primarily in recirculating and intensive flow-through systems.
Growth and yields in aquaculture
Under good growth conditions, 1-gram fish are cultured in nursery ponds to 1 to 2 ounces (20 to 40 grams) in 5 to 8 weeks and then restocked into growout ponds. In monosex growout ponds under good temperature regimes, males generally reach a weight of 1/2 pound (200 + grams) in 3 to 4 months, 1 pound (400 + grams) in 5 to 6 months, and 1.5 pounds (700 grams) in 8 to 9 months. To produce 1-pound (400- to 500-gram) fish, common practice is to stock 6,000 to 8,000 males per acre in static water ponds with aeration or 20,000 to 28,000 males per acre where 20 percent daily water exchange is economically practical. After 6 months of feeding with good quality feeds, such ponds can produce 5,000 to 7,000 pounds per acre and 18,000 to 20,000 pounds per acre, respectively.
If growout cycles are longer than 5 to 6 months (in an attempt to produce a more marketable size fillet) there is a risk that offspring from reproduction of the few females that were unintentionally included in the “all-male” culture will have time to reach sexual maturity and overpopulate the pond. Consequently, a farmer who wishes to produce fish yielding 5-ounce fillets (a 2-pound fish) is often forced to add a second growout phase so females and fingerlings can be eliminated from the growout ponds, or to stock a predaceous fish with the males. Dressout percentage on tilapia is relatively low compared to species such as trout and catfish. Tilapia generally have a dressout of 51 to 53 percent of live weight for whole-dressed fish (head-off) and 32 to 35 percent for fillets (pin bones along the lateral line removed).
For additional information about tilapia culture see SRAC publications 280, 281 and 282.
Tilapias are native only to Africa; many states in the U.S. consider them “exotics” or “non-indigenous” species and have restricted their transport and culture. Prospective tilapia producers should check with state game and fish/natural resources agencies or with Extension fisheries/aquaculture specialists to determine legal implications.
Tilapia are a good fish for warmwater aquaculture. They are easily spawned, use a wide variety of natural foods as well as artificial feeds, tolerate poor water quality, and grow rapidly at warm temperatures. These attributes, along with relatively low input costs, have made tilapia the most widely cultured freshwater fish in tropical and subtropical countries. Consumers like tilapia’s firm flesh and mild flavor, so markets have expanded rapidly in the U.S. during the last 10 years, mostly based on foreign imports. In fact, tilapia sales have recently surpassed rainbow trout sales in the U.S.
Southern Regional Aquaculture Center, Publication No. 283, March 1999
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