The highest mortality of the production cycle (about 20 percent) occurs during the fry rearing stage. Much of this is due to predation. As the fish grow and become hardier, mortality decreases significantly at each stage so that no more than 2 percent of the fish are expected to die during final growout.
Fry are given a complete diet of powdered feed (40 percent protein) that is fed continuously throughout the day with automatic feeders. The initial feeding rate, which can be as high as 20 percent of body weight per day under ideal conditions (good water quality and temperature: 86°F), is gradually lowered to 15 percent by day 30. During this period, fry grow rapidly and will gain close to 50 percent in body weight every 3 days. Therefore, the daily feed ration is adjusted every 3 days by weighing a small sample of fish in water on a sensitive balance. If feeding vigor diminishes, the feeding rate is cut back immediately and water quality (DO, pH, ammonia, nitrite) is checked.
Feed size can be increased to various grades of crumbles for fingerlings (1 to 50 grams), which also require continuous feeding for fast growth. During the growout stages, the feed is changed to floating pellets to allow visual observation of the feeding response. Recommended protein levels are 32 to 36 percent in fingerling feed and 28 to 32 percent in feed for larger fish. Adjustments in the daily ration can be made less often (e.g., weekly) because relative growth, expressed as a percentage of body weight, gradually decreases to 1 percent per day as tilapia reach 1 pound in weight, although absolute growth in grams/day steadily increases.
The daily ration for adult fish is divided into three to six feedings that are evenly spaced throughout the day. If feed is not consumed rapidly (within 15 minutes), feeding levels are reduced. DO concentrations decline suddenly in response to feeding activity. Although DO levels generally decline during the day in tanks, feeding intervals provide time for DO concentrations to increase somewhat before the next feeding. Continuous feeding of adult fish favors the more aggressive fish, which guard the feeding area, and causes the fish to be less uniform in size. With high quality feeds and proper feeding techniques, the feed conversion ratio (fish weight gain divided by feed weight) should average 1.5 for a l-pound fish.
Total production levels range from 3 to 6 pounds/ft3 of rearing space and 6 to 17 pounds/gallon/minute of flow. Monthly production levels range from 0.4 to 0.6 pounds/ft3. The higher production levels are generally obtained in flow-through systems. Production can always be increased by increasing the inputs, but this may not be economical
Nitrate is relatively non-toxic to fish, but an accumulation of ammonia and nitrite can cause mortality. Tilapia begin to die at ammonia concentrations around 2 mg/liter (expressed as NH3-N) and nitrite levels of 5 mg/liter (as NO2-N). Gravel biofilters, which once were common, are being replaced by plastic- media biofilters because they are lightweight and easy to clean. Biofilters now consist of self-supporting stacks of honeycombed modules, columns or tanks containing loosely packed rings, or a series of discs on an axle that floats at the water surface and rotates, alternately exposing the media to water and air.
Regardless of design, biofilters generally have the same requirements for efficient vitrification: 1) DO of not less than 2 mg/liter or 3 to 5 mg/liter for maximum efficiency; 2) pH 7 to 8; 3) a source of alkalinity for buffer since vitrification produces acid and destroys about 7 mg of alkalinity for every mg of NH3- N oxidized; 4) moderate levels of organic waste (less than 30 mg/liter measured as biochemical oxygen demand), thereby requiring good clarification; 5) water flow velocities that do not dislodge bacteria. Biofilters can be sized by balancing ammonia production rates with ammonia removal rates. Unfortunately, these rates are highly variable. In a growout study on tilapia in tanks, ammonia production averaged 10 grams/100 pounds of fish/day (range:4 to 21). Ammonia production depends on quality of feed, feeding rate, fish size and water temperature, among other factors.
Ammonia removal rates may range from 0.02 to 0.10 grams/ft2 of biofilter surface area/day depending on type of media, biofilter design, and the factors that affect vitrification. The required biofilter surface area can be obtained by dividing total ammonia production for the maximum standing crop by the ammonia removal rate. The filter volume can be determined by dividing the required biofilter surface area by the specific surface area (ft2/ft3) of the media. For example, assume that a biofilter containing l-inch pall rings is being designed to support 1,000 pounds of tilapia. The ammonia production rate is estimated to be 10 grams/100 pounds of fish/day. Therefore, total ammonia production would be 100 grams/day. The ammonia removal rate is estimated to be 0.05 grams/ft2/day. Dividing total ammonia production by the ammonia removal rate gives 2,000 ft2 as the required biofilter surface area. One-inch pall rings have a specific surface area of 66 ft2/ft3. Dividing the required biofilter surface area by the specific surface area gives 30 ft3 as the biofilter volume needed to remove ammonia.
Aeration requirements depend on the rate of water exchange. If water is exchanged rapidly, one to four times per hour, in a tank with moderate fish densities, aeration devices may not be required. The oxygen supply will be renewed by the DO in the incoming water. A flow rate of 6 to 12 gallons/minute is needed to support the oxygen requirement of 100 pounds of tilapia. DO, which should be maintained at 5 mg/liter for good tilapia growth, is the primary limiting factor for intensive tank culture. Flow- through systems should ideally be located next to rivers or streams to take advantage of gravity-fed water supplies, but pumping is practical in many situations. Limited water supplies frequently restrict exchange rates to a few times a day or as little as 10 to 15 percent per day.
In this case, aeration is needed to sustain tilapia at commercial levels. Paddlewheel aerators, agitators and blowers (diffused aeration) are some of the devices used to aerate tanks. Aerators are rated according to their effectiveness (pounds of oxygen transferred into the water per hour) and efficiency (pounds of oxygen transferred/horsepower- hour). Aeration requirements can be estimated by using aerator ratings and oxygen (O2) consumption rates of tilapia, which consume 4.5 grams O2/100 pounds of fish/hour while resting and several times more oxygen while they are feeding and active. For example, a tank with 1,000 pounds of tilapia would consume 45 grams of O2/hour at resting, but maximum oxygen consumption may be at least three times higher (135 grams O2/hour) depending on water temperature, body weight and feeding rate. Aeration efficiency (AE) of diffused-air systems (medium bubble size) ranges from 1,000 to 1,600 grams O2/kilowatthour under standard conditions (68°F and 0 mg/liter DO). However, AE declines to 22 percent of the standard at 5 mg/liter DO and 86°F. Therefore, AE would range from 220 to 352 grams O2/kilowatt-hour under culture conditions. Dividing the maximum oxygen consumption rate (135 grams O2/hour) by the median AE (286 grams O2/hour) gives 0.47-kilowatt (0.63-horsepower) as the size of aerator needed to provide adequate DO levels.
A current trend for intensive tank systems has been the use of pure oxygen for aeration. Oxygen from oxygen generators, compressed oxygen tanks, or liquid oxygen tanks is dissolved completely into the culture water by special techniques to help sustain very high fish densities.
Tanks come in a variety of shapes, but the most common forms are circular and rectangular. Raceways are rectangular tanks that are long and narrow. Variations of circular tanks are silos, which are very deep, and octagonal tanks. Circular tanks are very popular because they tend to be self-cleaning. If the direction of the inlet flow is perpendicular to the radius, a circular flow pattern develops which scours solids off the tank bottom and carries them to a center drain. Rectangular tanks are easy to construct but often have poor flow characteristics. Some of the incoming water may flow directly to the drain, short-circuiting the tank, while other areas of the tank maybecome stagnant, which allows waste to accumulate and lowers oxygen levels. For these reasons, circular tanks provide better conditions than rectangular tanks for tilapia culture.
Circular culture tanks may be as large as 100 feet in diameter, but common sizes range from 12 to 30 feet in diameter and from 4 to 5 feet in depth. Rectangular tanks are variable in dimensions and size, but raceways have specific dimension requirements for proper operation. The length to width to depth ratio should be 30:3:1 for good flow patterns. If the volume of water flow is limited, shorter raceways are better to increase the water exchange rate and prevent tilapia from concentrating near the inlet section where DO levels are higher.
The southernmost parts of Texas and Florida are the only areas where tilapia survive outdoors year-round. Elsewhere, tilapia must be overwintered in heated water. Flow-through systems are only practical for year-round culture in temperate regions if geothermal water is available. In the winter it would be too expensive to heat water and soon discard it. There has been some promising research on the use of heated effluents from power plants to extend the growing season. Indoor recirculating systems are more appropriate for year-round culture because buildings can be insulated to conserve heat and the heated water is saved through recycling.
Indoor recirculating systems have potential for extending the geographical range of tilapia culture throughout the U.S. Systems could be located in urban areas close to market outlets.
Intensive tank culture offers several advantages over pond culture. High fish density in tanks disrupts breeding behavior and allows male and female tilapia to be grown together to marketable size. In ponds, mixedsex populations breed so much that parents and offspring compete for food and become stunted. Tanks allow the fish culturist to easily manage stocks and to exert a relatively high degree of environmental control over parameters (e.g., water temperature, DO, pH, waste) that can be adjusted for maximum production. With tanks, feeding and harvesting operations require much less time and labor compared to ponds.
Small tank volumes make it practical and economical to treat diseases with therapeutic chemicals dissolved in the culture water. Intensive tank culture can produce very high yields on small parcels of land. Tank culture also has some disadvantages. Since tilapia have limited access to natural foods in tanks, they must be fed a complete diet containing vitamins and minerals. The cost of pumping water and aeration increases production costs. The filtration technology of recirculating systems can be fairly complex and expensive and requires constant and close attention. Any tank culture system that relies on continuous aeration or water pumping is at risk of mechanical or electrical failure and major fish mortality.
Backup systems are essential. Confinement of fish in tanks at high densities creates stressful conditions and increases the risk of disease outbreaks. Discharges from flow-through systems may pollute receiving waters with nutrients and organic matter.
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 has 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).
times the total length of the fish). The Mozambique tilapia is less efficient than the Nile or Blue tilapia at harvesting planktonic algae. Two mechanisms help tilapia digest filamentous and planktonic algae and succulent higher plants: 1) physical grinding of plant tissues between two pharyngeal plates of fine teeth; and 2) a stomach pH below 2, which ruptures the cellwalls of algae and bacteria. The commonly cultured tilapias digest 30 to 60 percent of the protein in algae; blue-green algae are digested more efficiently than green algae. When feeding, tilapias do not disturb the pond bottom as aggressively as common carp. However, they effectively browse on live benthic invertebrates and bacteria-laden detritus. Tilapias also feed on midwater invertebrates. They are not generally considered piscivorous, but juveniles do consume larval fish. In general, tilapias use natural food so efficiently that crops of more than 2,700 pounds of fish per acre (3,000 kg/ha) can be sustained in well-fertilized ponds without supplemental feed. The nutritional value of the natural food supply in ponds is important, even for commercial operations that feed fish intensively. In heavily fed ponds with little or no water exhange, natural food organisms may provide one-third or more of total nutrients for growth. In general, tilapia digest animal protein in feeds with an efficiency similar to that of channel catfish, but are more efficient in the digestion of plant protein, especially more fibrous materials. Tilapia requires the same ten essential amino acids as other warmwater fish, and, as far as has been investigated, the requirements for each amino acid are similar to those of other fish. Protein requirements for maximum growth are a function of protein quality and fish size and have been reported as high as 50 percent of the diet for small fingerlings. However, in commercial food fish ponds the crude protein content of feeds is usually 26 to 30 percent, one tenth or less of which is of animal origin. The protein content and proportion of animal protein may be slightly higher in recirculating and flow-through systems. The digestible energy requirements for economically optimum growth are similar to those for catfish and have been estimated at 8.2 to 9.4 kcal DE (digestible energy) per gram of dietary protein. Tilapia may have a dietary requirement for fatty acids of the linoleic (n-6) family. Tilapia appear to have similar vitamin requirements as other warmwater fish species. Vitamin and mineral premixes similar to those added to catfish diets are usually incorporated in commercial tilapia feeds. The feeding behaviour of tilapia allows them to use a mash (unpelleted feeds) more efficiently than do catfish or trout, but most commercial tilapia feeds are pelletized to reduce nutrient loss. In the absence of feeds specifically prepared for tilapia, a commercial catfish feed with a crude protein content of 28 to 32 percent is appropriate in the United States.
method of distinguishing tilapia species because environment, state of sexual maturity, and food source greatly influence color intensity. The ÒredÓ tilapia has become increasingly popular because its similar appearance to the marine red snapper gives it higher market
value. The original red tilapias were genetic mutants. The first red tilapia, produced in Taiwan in the late 1960s, was a cross between a mutant reddish- orange female Mozambique tilapia and a normal male Nile tilapia. It was called the Taiwanese red tilapia. Another red strain of
tilapia was developed in Florida in the 1970s by crossing a normal colored female Zanzibar tilapia with a red-gold Mozambique tilapia. A third strain of red tilapia was developed in Israel from a mutant pink Nile tilapia crossed with wild Blue tilapia. All three original strains have
been crossed with other red tilapia of unreported origin or with wild Oreochromis species. Consequently, most red tilapia in the Americas are mosaics of uncertain origin. The confused
and rapidly changing genetic composition of red tilapia, as well as the lack of “head-to-head” growth comparisons between the different lines, make it difficult for a producer to identify a “best” red strain. Other strains of tilapia selected for color include true breeding gold and
yellow Mozambique lines and a “Rocky Mountain white” tilapia (a true breeding line originating from an aberrant Blue tilapia, subsequently crossed with Nile tilapia). Most
strains selected for color do not grow well enough for food fish culture. Identifying the species of an individual fish is further complicated by natural crossbreeding that has occurred between species. Electrophoresis is often used to determine the species composition of a group of tilapia.
SRAC Publication No. 283
Thomas Popma1 and Michael Masser2
SRAC Publication No. 283
Thomas Popma1 and Michael Masser2
below 50 to 52o F and early sexual maturity that results in spawning before fish reach market
An Open Letter regarding recent reports that low-fat fish like tilapia are unhealthy. (July 16, 2008)
Eating fish, especially oily fish, at least twice per week is recommended for heart disease prevention. Fish is low in total and saturated fats, high in protein and essential trace minerals, and contains long-chain omega-3 fatty acids (EPA and DHA). Oily fish rich in these healthy omega-3s include salmon, trout, albacore tuna, sardines, anchovies, mackerel and herring. Our omega-3 needs can also be met by eating less-oily (lower-fat) fish more often.
Tilapia and catfish are examples of lower-fat fish that have fewer omega-3s than the oily fish listed above, but still provide more of these heart-healthy nutrients than hamburger, steak, chicken, pork or turkey. Actually, a 3 ounce serving of these fish provides over 100 mg of the long chain omega-3 fatty acids EPA and DHA. Considering that this is about the current daily intake of these fatty acids in the US, even these fish should be considered better choices than most other meat alternatives. Since they are also relatively low in total and saturated fats and high in protein, they clearly can be part of a healthy diet.
US Department of Agriculture statistics indicate that farmed tilapia and catfish contain somewhat more omega-6 fatty acids than omega-3. Most health experts (including organizations such as the American Heart Association and the American Dietetic Association) agree that omega-6 fatty acids are, like omega-3s, heart-healthy nutrients which should be a part of everyone's diet. Omega-6 fatty acids are found primarily in vegetable oils (corn, soybean, safflower, etc) but also in salad dressings, nuts, whole-wheat bread, and chicken.
Replacing tilapia or catfish with "bacon, hamburgers or doughnuts" is absolutely not recommended.
William S. Harris, PhD, FAHA
Sr. Scientist and Director
Metabolism and Nutrition Research Center
Sioux Falls, SD