System design should be approached in a reverse manner. The final desired fish size and total weight of the harvest is determined, a maximum fish density is established, and assumptions about mortality rate are applied to arrive at initial stocking numbers. Maximum density dictates the maximum daily feed rate, which is usually 1.0 to 1.5 percent of the biomass for market-size fish of 1.5 pounds (680 grams) each. This biomass density (lbs/gal or kg/m3) is a design parameter that is highly dependent upon the type and size of filtration technology used. Efficient solids filtration makes it possible to use higher feed rates and still keep the system operating within safe limits of dissolved oxygen, dissolved ammonia, pH and suspended solids.
Systems that rely solely on aeration, without supplemental pure oxygen gas, are limited to lower maximum biomass densities unless solids removal is highly efficient and the suspended solids concentration is low. The general rule of thumb is that highly aerated systems in which fish are fed for good growth have a maximum biomass density of about 0.25 pound per gallon(30 kg/m3), although some operators report achieving maximum fish densities of 0.5 to 0.6 pound per gallon (60 to 72 kg/m3). Vigorous aeration is necessary to dissolve enough oxygen in the water, especially in the warm water used for tilapia culture. But vigorous aeration re-suspends and fractures solids that should be quickly removed from the tank in order to maintain good water quality conditions. In systems where supplemental oxygen is applied and vigorous aeration is not used, maximum fish density can often exceed 0.50 pound per gallon (60 kg/m3), and some system designs routinely use a maximum of 0.75 to 1.0 pound per gallon (90 to 120 kg/m3). For commercial operations, the expense of supplemental oxygen can often be offset by the significant increase in maximum biomass density.
The key to a well-designed system is the efficient removal of solids, because this makes it easier to control other water quality parameters. When waste solids do not remain in the tank and break down into increasingly finer particles, there is less dissolved ammonia and carbon dioxide and a lower oxygen demand. With lower carbon dioxide concentrations, the pH of the system water is more easily maintained for proper biofilter performance.
Tank stocking and density
The tank culture of tilapia can have higher labor and energy costs for pumping water and heating water than pond culture methods. As a result, the most efficient strategy for operating tanks is to keep the biomass at or near the maximum system carrying capacity and maximize feed input, while minimizing the costs of labor and energy. The maximum carrying capacity of a tank should allow for maximum feed conversion efficiency throughout the production cycle. While it is certainly possible to grow fish using a “put and take” scheme—harvesting larger fish and replacing them with smaller fish—this type of stock management can be problematic in a number of ways. The larger fish of a mixed population will compete with smaller fish for feed. Different size fish within a population may require different size feeds. Sorting and harvesting larger fish can be labor intensive and stressful on the fish in the tank. And, it is nearly impossible to distinguish genetic runts from other small fish in the mixed cohort population. Genetic runts do not have potential for good growth and should be identified and removed as early in the production cycle as possible.
The opposite extreme of tank management—stocking fry or fingerlings as a batch directly into the final growout tank—is usually not recommended. A large tank requires a large pump and filtration equipment, which is an inefficient use of resources for such a small initial biomass. The alternative is to use a phased approach, initially stocking fry or fingerlings into smaller, more manageable nursery tanks with smaller volumes of water and smaller pumps and filtration components, and then growing the population until maximum tank capacity is reached. During that phase of growth, average fish size and overall tank biomass increase. As fish biomass and total daily feed increase, water flow through the tank and filtration system must be increased to provide more aeration and biofiltration. When the maximum flow rate or pumping capacity is reached, the tank is at its maximum carrying capacity and the fish are moved to a tank or tanks with greater volume and a filtration system of greater capacity.
This phased approach improves biosecurity, disease control and feed management throughout the growout process. Newly introduced cohorts of fry or fingerlings can be isolated from other populations within the facility to help prevent or control the introduction and spread of disease. As a growout history is developed with a cohort of fish, that cohort can be moved into other areas of the facility with greater confidence that they are healthy. The phased grow-out strategy also improves population data collection. Fish can be sampled during the transfer process to verify their numbers; determine average weight, growth rate and feed conversion ratio (FCR); and more accurately adjust feed rates for the upcoming phase. Fish transfers are opportunities to grade the fish and split the population according to average size.
Tilapia breedingand fry production
Breeding tilapia is a relatively simple procedure. Regularly producing large numbers of high-quality fry, however, requires greater attention, good broodstock, high-quality feeds, and proper disease controls. While mating can occur and fry can be produced from ratios of one or two females per male, commercial hatcheries usually use four or five females per male. Using a high ratio of females to males is acceptable if the males are superior, since they will theoretically pass their genes on to many fry. These fry, however, should be used only for growout and not for further selective breeding.
Tilapia are commonly bred in tanks. Brood fish are stocked at a rate of 0.06 to 0.14 pound of brood fish per square foot (0.29 to 0.68 kg/m2) of tank bottom. They produce approximately 0.14 to 0.23 fry per square foot (1.5 to 2.5/m2) per day. Within 10 to 15 days after stocking brood fish, newly hatched fry can be captured with a dip net and transferred to a nursery unit. Fry that avoid capture prey on subsequent spawns and reduce production. After 1 to 2 months, the tank must be drained to remove all juvenile fish and begin another spawning cycle. Breeding can be better controlled when net enclosures (hapas) are floated in tanks. Male and female brood fish, which have been kept apart, are stocked into the hapa to begin breeding. A sex ratio of two females to one male is used to produce large quantities of fry. The optimum stocking density ranges from 0.5 to 1.0 fish per square foot (5.4 to 10.8/m2). Small brood fish (0.25 lb, 115 grams) make handling easier, although larger brood fish may be used. The brood fish are fed high-quality feed at a rate of 2.0 percent of their body weight per day.
The most efficient method when hapas are used is to collect eggs at 5-day intervals and incubate them in hatching jars. Eggs are collected by passing a 4-inch PVC pipe float underneath the netting material from one end of the hapa to the other end to concentrate the brood fish in one end. The brood fish are captured individually with two small scoop nets—a large-mesh inner net and a fine-mesh outer net. The nets are held in one hand while the fish is held with the other hand, which is gloved to prevent injury from the dorsal fin. Using a finger to open the fish’s mouth, the fish is moved quickly up and down in the water with the nets underneath to wash out any eggs the fish may be incubating in its mouth. Occasionally, a fish will expel its eggs as it is being captured. With a double-net scoop net system, the eggs fall through the large mesh net and are retained by the small mesh net. The large mesh net prevents the fish from crushing the eggs. After each fish is inspected, it can be returned to the other end of the hapa. This method produces approximately one fry per square foot(10.8/m2) per day.
The culture of nearly all-male populations is being conducted with good success. All-male populations have better growth rates than mixed-sex populations because there are few slower-growing females, which convert some feed into egg production. There are two methods for producing all-male tilapia fingerling batches: 1) sex reversal of fry using a synthetic male androgen (17-alpha methyltestosterone) administered in feed for 28 days post-hatch; and 2) spawning female tilapia with tilapia males that have two Y chromosomes. A third method, using interspecific hybridization of female Oreochromis nilticus and male Oreochromis aureus, has been described in the scientific literature and is sometime used in countries where chemical sex reversal is prohibited. It is not widely used in commercial production in the U.S.
The sex reversal of fry can be conducted only by special permit from the federal government. The use of breeding males with two Y chromosomes is a patented method; broodstock is available to licensees from the patent holder.
Continue to Tank Culture of Tilapia (part 4)