Site selection and placement of cages
Large bodies of water tend to be better suited for cage culture than small ponds because the water quality is generally more stable and affected less by fish waste. Exceptions are eutrophic waters rich in nutrients and organic matter. Small (1 to 5 acre) ponds can be used for cage culture, but provisions for water exchange or emergency aeration may be required.
Cages should be placed where water currents are greatest, usually to the windward side. Calm, stagnant areas should be avoided. However, areas with rough water and strong currents also present problems.
Cages may be moored individually or linked in groups to piers, rafts, or lines of heavy rope suspended across the water surface. At least 15 feet should separate each cage to optimize water quality. The cage floor should be a minimum of 3 feet above the bottom substrate, where waste accumulates and oxygen levels may be depressed. However, greater depths promote rapid growth and reduce the possibility of parasitism and disease. See SRAC publications Nos. 160-166 for more information on cage culture.
Geographic range for tilapia culture is temperature dependent. Preferred water temperature range for optimum growth is 82° to 86° F. Growth diminishes significantly at temperatures below 68°F and death will occur below 50° F. Only the southernmost states have suitable temperatures to produce tilapia in cages. In the southern region tilapia can be held in cages from 5 to 12 months per year depending on location.
Cages may be used for fingerling production. One-gram fry may be reared in l/4-inch mesh cages at up to 3,000 fish per cubic meter for 7 to 8 weeks until they average about 10 grams each. Ten-gram fish can be restocked into l/2-inch mesh cages. Cages stocked with 10-gram fish at 2,500 per cubic meter will produce 25- to 30-gram fingerlings in 5 to 6 weeks. After grading, 25- to 30-gram fish can be restocked at 1,500 fish per cubic meter to produce 50- to 60- gram fingerlings in 5 weeks, or at 1,000 fish per cubic meter to produce 100-gram fingerlings in 9 to 10 weeks. Fish should be graded by size every 4 to 6 weeks. Stunted fish and females should be culled.
The optimum fingerling size for stocking in final growout cages is determined by the length of the growing season and the desired market size. The shorter the growing season, the larger the fingerlings must be at stocking. The use of male populations which grow at twice the rate of female populations will result in larger fish, greater production and a reduction in the growout period. In temperate regions, overwintered, l-year-old fingerlings of 60 to 100 grams (4 to 7 fish/pound) are generally used to produce fish of 1 pound or greater in cages. If l/2-pound fish are acceptable for market, then it maybe possible to rear smaller, 20- to 30-gram fingerlings (15 to 23 fish/pound) which were produced during the spring of the same year.
Recommended stocking rate of tilapia fingerlings depends on cage volume, desired harvest size and production level, and the length of the culture period. Expected harvest weights of male tilapia are given in Table 1.
High stocking rates can be used in small cages of 1 to 4 cubic meters. Optimum stocking rates per cubic meter range from 600 to 800 fish to produce fish averaging 1/2 pound; 300 to 400 to produce fish averaging 1 pound; and 200 to 250 to produce fish averaging 1.5 pounds.
Water exchange is less frequent in large cages, and therefore the stocking rate must be reduced accordingly. In 100-cubic meter cages, the optimum stocking rate is approximately 50 fish per cubic meter to produce l-pound fish.
In temperate regions, complete or batch harvests are required. Cages for final growout should be stocked when water temperature rises above 70° F and harvested when the temperature falls below 70° F.
In tropical or sub-tropical regions with a year-round growing season, a staggered production system could be used to facilitate marketing by ensuring regular harvests, e.g., weekly, biweekly, or monthly. The exact strategy will depend on the number of cages available and the total production potential of the body of water.
Example: if 10 cages are available for placement in a pond with sufficient production potential and growout takes 20 weeks, then a cage could be stocked every 2 weeks. Beginning on week 20, the first cage would be harvested and restocked, followed by another cage every 2 weeks. A staggered system requires a regular supply of fingerlings.
Total production in cages increases as the stocking rate is increased. However, there is a density at which tilapia become too crowded and water quality within the cage deteriorates to a point that causes a declinein growth rates. In small cages of 1 to 4 cubic meters, a reduction in growth usually begins at production levels around 250 pounds per cubic meter. In 100-cubic meter cages, production should be limited to 50 pounds per cubic meter. Tilapia continue to grow above these levels at gradually decreasing rates, but they convert feed poorly, and the risk of loss due to oxygen depletion or disease is greater. For maximum turnover of marketable fish, it is best to limit production to levels that do not depress growth.
The total number of cages that can be deployed in a pond, and therefore total fish production, is primarily a function of maximum allowable feeding rate for all cages in that body of water. The total feed input is related to number and size of fish in the cages and is limited by surface area of the pond.
If emergency aeration is not available and if all cages in a pond are stocked at once (batch culture), then a maximum daily feeding rate of 30 to 45 pounds per pond acre should be safe for a limited period near the end of the production cycle. At this rate it is possible to produce a total of about 2,000 to 3,000 pounds of caged fish per pond acre every 20 weeks. If a staggered stocking and harvesting system is used for continuous year-round production, then the maximum daily feeding rate should not exceed 20 to 30 pounds per acre because this feeding rate will be applied continuously.
As total feed input is increased water quality eventually starts to deteriorate until it becomes unsuitable for fish in cages. Although tilapia survival is usually better than 95 percent, caged tilapia are more susceptible than non-caged tilapia to stress from poor water quality, particularly low dissolved oxygen (DO) concentrations. DO should be monitored regularly at late afternoon and early morning especially when attempting to maximize total production and emergency aeration equipment should be available.
Recent research has shown that Florida red tilapia can be cage cultured in full strength sea water. Fingerlings must be acclimated from freshwater to sea water over several days and then stocked in cages. Tilapia in sea water are more susceptible to handling stress, and additional care is needed to control parasites and diseases. Additionally, bio-fouling of cage mesh and damage from corrosion and wave action are concerns in sea water. Otherwise, culture techniques are similar to those described here for freshwater.
Continue to Cage Culture of Tilapia (part 3)