SRAC Publication No. 282

Revision VI PR June 2009

Tank Culture of Dennis P. DeLong,1 Thomas M. Losordo1 and James E. Rakocy2

The popularity of live tilapia in the water. Intensive tank culture can continuous water pumping, aeration marketplace has driven much of produce high yields, year-round, on or oxygenation is at risk of mechani- the development of the tank-based small parcels of land. cal or electrical failure and major fish industry in the United States. In the Tank culture also has disadvantages. mortality without the proper backup southern region, indoor tank culture Because fish have little natural food systems. Automated alarms, oxygen of tilapia allows year-round produc- in tanks, they must be fed a complete storage, backup generators and quick tion and can be a good alternative to diet containing the protein, vitamins response can be critical in saving a pond or cage culture. and minerals necessary for good crop of fish. Confining fish in tanks Intensive tank culture offers several growth. Finishing (often referred to at high densities can create stressful advantages over the use of ponds. The as on-growing) feed for tank culture conditions and increase the risk of high density of fish in tanks disrupts operations generally has more pro- disease outbreaks, especially when breeding behavior and allows male tein than that used for pond culture, water quality deteriorates. and female tilapia to be grown togeth- usually 32 to 40 percent protein. Pond er. If cultured together, females will culture operations, and those tank Suitability of tilapia be half the size of the males (0.75 lbs operations that are heterotrophic or for tank culture vs. 1.5 lbs; 340 grams vs. 680 grams). “biofloc” systems, usually can achieve Tilapia have a number of character- Females will not reach marketable satisfactory growth rates using feed istics that make them attractive for size at the same time as the males un- with lower protein content because tank culture. They can tolerate the less there is a market for the smaller fish receive a portion of their nutrition crowding and handling that is re- fish. In ponds, mixed-sex populations from the pond biota or from the inges- quired in a tank-based facility. Their breed so prolifically that parents and tion of bacteria. Biofloc systems main- heavy slime coat protects them from offspring compete for food, individual tain an active suspension of bacteria abrasion and bacterial infections that fish growth is reduced, and the popu- and in the culture tanks. These would adversely affect many other lation becomes stunted. bacteria and algae control the ammo- fish. Tilapia grow well at high densi- Using tanks allows the fish culturist nia-nitrogen concentration, while the ties in the confinement of tanks when to manage stocks and have a good cells are kept in a constant state of good water quality is maintained, but deal of control over environmental rapid growth and regeneration. they are also amazingly tolerant of parameters (e.g., water temperature, In tank culture, the cost of pumping poor or variable water quality. Tilapia dissolved oxygen concentration, water and aeration or oxygenation can be grown on diets that are high pH, waste) that can be adjusted to increases unit production costs. The in vegetable matter, such as soy pro- promote maximum production. In ad- filtration technology of recirculating tein, which is a more renewable and dition, feeding and harvesting opera- systems can be complex and capital sustainable ingredient than fish meal tions require less time and labor than intensive, and these systems require derived from wild fish catches. in ponds. In small tanks it is practical close attention. The high densities The simplicity of breeding tilapia and economical to treat diseases with required for profitability also create a means that fingerlings can be readily therapeutants applied to the culture vulnerability to disaster if power out- available year-round. This character- ages or equipment failure occur. Any istic is important to indoor facilities 1North Carolina State University tank culture system that relies on that produce fish steadily for the live 2University of the Virgin Islands market and its customers, which can waste products. Culture systems that waters with nutrients and organic then rely upon a constant, predict- discard water after use are called matter. Under the regulatory struc- able supply. Fillet yield of most tila- flow-through systems, while those ture of the U.S. National Pollutant pias is 30 to 35 percent of the whole that filter and recycle water are Discharge Elimination System body weight, so 65 to 70 percent of called recirculating sys- (NPDES), the discharge of effluent the processed fish is discarded if it is tems (RAS), recycling or water re-use water may require a permit, with re- not sold as a live or whole product. systems. Each type has advantages quired periodic testing and oversight, Therefore, the more profitable prod- and disadvantages. based on the following two criteria: uct is the whole/live form. 1) annual production must total at Flow-through tank systems least 100,000 pounds (45,400 kg) of Species selection Flow-through tank systems depend product, and 2) discharge of efflu- on constant or periodic water ex- A number of tilapia species have ent water must occur for more than change to flush out fish waste prod- been cultured in the U.S. With the 30 days annually. These are federal ucts. Exchange rates are determined growth of the tilapia industry and rules, but states are allowed to de- by the available water quality and stocks moving from state to state, termine whether one or both condi- quantity, the fish biomass, and feed- there has been considerable mixing tions must be met before requiring a ing rates. As a rule, the volume of wa- of strains. Among the reasons for permit. ter needed for a facility is the amount selective breeding, genetic selection, Large facilities that exceed the required to replace 100 percent of the and examination of hybrids has been production level of the first criterion tank water every 90 to 120 minutes. the improvement of growth rate, cold should try to use the effluent for crop tolerance and fillet yield, and more Flow-through systems often are not irrigation or treat it on site and not desirable color variations for the suitable for commercial tilapia tank discharge it off site for more than 30 marketplace. The most popular spe- culture. Tilapia are warmwater days of the year. A permit may also cies currently cultured are the Nile fish that grow best when the water be required, at the discretion of the tilapia ( niloticus) and temperature is in the low to mid-80 state regulatory agency involved, the blue tilapia (). °F range (approximately 27 to 29 if any complaint is received. It is However, other species such as the °C). Unless incoming water is from prudent, therefore, for the prospec- tilapia (Oreochromis a geothermal source or is warmed, it tive aquaculturist to know the rules mossambicus) and several hybrids are will be too cool for optimum growth. of the controlling regulatory agency also cultured. While O. niloticus can Warming large volumes of incoming during the early stages of facility survive temperatures as low as about water is generally not economically planning and design. 10 ºC (50 ºF), O. aureus can survive feasible. Operations with a constant to about 7 ºC (45 ºF) but has a slower source of heated water, such as a Recirculating aquaculture growth rate. Although these tempera- geothermal or low-cost waste heat systems (RAS) tures may be the lethal limits, water source, might be economically viable. Recirculating aquaculture systems, temperature in the 10 to 16 ºC (50 to Using surface waters for tank cul- despite earlier failures, have become 60 ºF) range can stress tilapia, reduce ture is not advisable, although there more common and more economi- their feeding behavior, and make may be exceptions. The quantity of cally viable because of advancements them more vulnerable to disease. surface water available may vary that accomplish the required unit processes: solids removal, biologi- ’ tolerance of lower tem- during a drought, and its quality cal nitrification, oxygenation, and perature may make it possible to can vary because of rainfall runoff, dissolved gas management. Recircu- overwinter stock in some areas of agricultural activity or other develop- lating systems must be designed to the U.S., but cold tolerance can be ment activity in the watershed area. accomplish these processes with less viewed as a negative characteristic Groundwater is a better source, but it new water input in an economically by state agencies worried about the is advisable to gather as much history sustainable manner. Sand filters have survival of escaped fish. As with as possible on the water quality of largely been replaced by better types any aquaculture specie, it is prudent a site before developing the culture of solids removal devices such as for the prospective aquaculturist to operation. Water from shallow wells bead filters and screen filters. Granu- consult regulatory agencies early in may contain organic matter and un- lar plastic media (bead) filters have the planning process. acceptable levels of ammonia or hy- drogen sulfide gas. Geothermal water undergone extensive development Types of culture systems sources may have levels of dissolved and improvement and are readily minerals that affect fish health. It available from commercial suppliers. The successful tank culture of any might be possible to treat groundwa- Screen filters that use rotating drums fish specie is highly dependent upon ter before using it, though the opera- or discs are also widely used. These maintaining good water quality. This tor would need to determine whether filters remove solids well, lose very is accomplished by aeration, oxygen- treatment is economically feasible. little energy as water passes through ation, and frequent or continuous them, and discharge very little water Water discharged from flow-through water exchange to renew dissolved during the cleaning process. oxygen (DO) content and remove tank systems may pollute receiving Recirculating systems for tilapia a relatively large, filterable size. The harvest is determined, a maximum culture have a number of advantages. longer these solids remain in the tank, fish density is established, and as- They can be located in areas that do the smaller they become (as they sumptions about mortality rate are not have sufficient water resources disintegrate) and the more waste am- applied to arrive at initial stocking for pond aquaculture. They can be monia they generate, so tanks should numbers. Maximum density dic- located closer to markets and infra- be designed for the rapid and efficient tates the maximum daily feed rate, structure, such as highway connec- removal of waste solids. All manner which is usually 1.0 to 1.5 percent of tions and utilities. Indoor operations of tank shapes have been used for the biomass for market-size fish of protect the fish stock from seasonal culturing tilapia, including square, 1.5 pounds (680 grams) each. This variations in temperature, allowing rectangular, round, oval, octagonal biomass density (lbs/gal or kg/m3) year-round production that satisfies and “D-ended” or “racetrack” configu- is a design parameter that is highly constant market demand. Depending rations. The most desirable tanks are dependent upon the type and size of upon the manager’s adherence to good those that effectively remove solids at filtration technology used. Efficient operating procedures and the quality an affordable cost while using valu- solids filtration makes it possible to of incoming stock, isolating recircu- able floor space efficiently. use higher feed rates and still keep lating tanks from each other promotes Round or octagonal tanks have a cir- the system operating within safe biosecurity by controlling the intro- cular flow pattern that moves settle- limits of dissolved oxygen, dissolved duction and spread of diseases or able solids toward a central drain, ammonia, pH and suspended solids. parasitic organisms. In some states it which is usually screened to prevent Systems that rely solely on aeration, is easier to obtain tilapia culture per- fish escape. In tanks with an internal without supplemental pure oxygen mits for indoor, recirculating systems standpipe, the center drain can be gas, are limited to lower maximum because there is less chance that these fitted with a larger outer pipe (sleeve) biomass densities unless solids non-native fish will escape. with notches at the bottom to remove removal is highly efficient and the There are also disadvantages to recir- water from the bottom of the tank suspended solids concentration is culating aquaculture systems. Prob- where solids are concentrated. Ex- low. The general rule of thumb is ably the greatest problem is the large ternal standpipes are easily adjusted that highly aerated systems in which capital investment required for build- to accommodate increased water fish are fed for good growth have a ing and starting up facilities. The flows during the culture cycle, while maximum biomass density of about need for specialized equipment and maintaining the same water level 0.25 pound per gallon(30 kg/m3), al- thermally efficient buildings signifi- in the tank. Tank drains must have though some operators report achiev- cantly raises the cost of getting into screens, slots or some type of open ing maximum fish densities of 0.5 the business. Economic analyses for area to prevent clogging that might to 0.6 pound per gallon (60 to 72 kg/ the southern region have shown that cause the tank to overflow. Double m3). Vigorous aeration is necessary to the minimum production required drains, which create an effluent flow dissolve enough oxygen in the water, for proper cash flow is about 200,000 stream with greater solids concentra- especially in the warm water used to 250,000 pounds of live weight tila- tion, perform well and are widely for tilapia culture. But vigorous aera- pia annually. The cost of construct- recommended. tion re-suspends and fractures solids ing and starting up a facility with If tilapia fecal material is not quickly that should be quickly removed this capacity can exceed $750,000. If removed from the tank, the mucous from the tank in order to maintain funds must be borrowed, a large debt membrane will trap gases generated good water quality conditions. In to equity ratio may not be economi- by bacterial decay and cause the systems where supplemental oxygen cally viable. fecal string to float. For this reason, is applied and vigorous aeration is More detailed information about some tanks have an additional sur- not used, maximum fish density can recommendations for the design and face drain at the center (standpipe) often exceed 0.50 pound per gallon 3 operation of recirculating aquacul- or on the tank sidewall to remove (60 kg/m ), and some system designs ture systems can be found in SRAC surface water and floating solids. routinely use a maximum of 0.75 to publications 450-459, available at: 1.0 pound per gallon (90 to 120 kg/ Research has shown that round or 3 http://srac.tamu.edu/. m ). For commercial operations, the octagonal tanks with flat bottoms expense of supplemental oxygen can have better solids removal than those Culture tank design often be offset by the significant in- with sloped or cone-shaped bottoms. crease in maximum biomass density. Tanks for culturing tilapia can be of The slope may prevent solids from different sizes and shapes as long as moving to the center drain. Tanks The key to a well-designed system they allow for the effective removal of with flat bottoms also are simpler is the efficient removal of solids, waste solids. Tilapias produce a solid and less costly to build. because this makes it easier to con- waste that is well-suited for removal trol other water quality parameters. from culture tanks. When fed com- System design When waste solids do not remain in the tank and break down into mercial fish feeds, they produce fecal System design should be approached increasingly finer particles, there is strings held together in a mucous in a reverse manner. The final de- less dissolved ammonia and carbon membrane that maintains the feces in sired fish size and total weight of the dioxide and a lower oxygen demand. biomass and total daily feed increase, tank bottom. They produce approxi- With lower carbon dioxide concen- water flow through the tank and mately 0.14 to 0.23 fry per square trations, the pH of the system water filtration system must be increased foot (1.5 to 2.5/m2) per day. Within is more easily maintained for proper to provide more aeration and biofil- 10 to 15 days after stocking brood biofilter performance. tration. When the maximum flow fish, newly hatched fry can be cap- rate or pumping capacity is reached, tured with a dip net and transferred Tank stocking and density the tank is at its maximum carrying to a nursery unit. Fry that avoid The tank culture of tilapia can have capacity and the fish are moved to capture prey on subsequent spawns higher labor and energy costs for a tank or tanks with greater volume and reduce production. After 1 to 2 pumping water and heating water and a filtration system of greater months, the tank must be drained than pond culture methods. As a capacity. to remove all juvenile fish and begin result, the most efficient strategy for This phased approach improves another spawning cycle. Breeding operating tanks is to keep the bio- biosecurity, disease control and feed can be better controlled when net en- mass at or near the maximum system management throughout the growout closures (hapas) are floated in tanks. carrying capacity and maximize feed process. Newly introduced cohorts Male and female brood fish, which input, while minimizing the costs of fry or fingerlings can be isolated have been kept apart, are stocked of labor and energy. The maximum from other populations within the into the hapa to begin breeding. A carrying capacity of a tank should facility to help prevent or control the sex ratio of two females to one male allow for maximum feed conversion introduction and spread of disease. is used to produce large quantities of efficiency throughout the production As a growout history is developed fry. The optimum stocking density cycle. While it is certainly possible with a cohort of fish, that cohort ranges from 0.5 to 1.0 fish per square 2 to grow fish using a “put and take” can be moved into other areas of the foot (5.4 to 10.8/m ). Small brood fish scheme—harvesting larger fish and facility with greater confidence that (0.25 lb, 115 grams) make handling replacing them with smaller fish— they are healthy. The phased grow- easier, although larger brood fish this type of stock management can out strategy also improves population may be used. The brood fish are fed be problematic in a number of ways. data collection. Fish can be sampled high-quality feed at a rate of 2.0 per- The larger fish of a mixed popula- during the transfer process to verify cent of their body weight per day. tion will compete with smaller fish their numbers; determine average The most efficient method when for feed. Different size fish within weight, growth rate and feed conver- hapas are used is to collect eggs at a population may require different sion ratio (FCR); and more accurately 5-day intervals and incubate them size feeds. Sorting and harvesting adjust feed rates for the upcoming in hatching jars. Eggs are collected larger fish can be labor intensive and phase. Fish transfers are opportuni- by passing a 4-inch PVC pipe float stressful on the fish in the tank. And, ties to grade the fish and split the underneath the netting material from it is nearly impossible to distinguish population according to average size. one end of the hapa to the other end genetic runts from other small fish in to concentrate the brood fish in one the mixed cohort population. Genetic Tilapia breeding end. The brood fish are captured runts do not have potential for good and fry production individually with two small scoop growth and should be identified and nets—a large-mesh inner net and a Breeding tilapia is a relatively simple removed as early in the production fine-mesh outer net. The nets are procedure. Regularly producing cycle as possible. held in one hand while the fish is large numbers of high-quality fry, held with the other hand, which is The opposite extreme of tank however, requires greater atten- gloved to prevent injury from the management—stocking fry or fin- tion, good broodstock, high-quality dorsal fin. Using a finger to open gerlings as a batch directly into the feeds, and proper disease controls. the fish’s mouth, the fish is moved final growout tank—is usually not While mating can occur and fry can quickly up and down in the water recommended. A large tank requires be produced from ratios of one or with the nets underneath to wash out a large pump and filtration equip- two females per male, commercial any eggs the fish may be incubating ment, which is an inefficient use of hatcheries usually use four or five fe- in its mouth. Occasionally, a fish will resources for such a small initial males per male. Using a high ratio of expel its eggs as it is being captured. biomass. The alternative is to use females to males is acceptable if the With a double-net scoop net system, a phased approach, initially stock- males are superior, since they will the eggs fall through the large mesh ing fry or fingerlings into smaller, theoretically pass their genes on to net and are retained by the small more manageable nursery tanks many fry. These fry, however, should mesh net. The large mesh net pre- with smaller volumes of water and be used only for growout and not for vents the fish from crushing the eggs. smaller pumps and filtration compo- further selective breeding. nents, and then growing the popula- After each fish is inspected, it can Tilapia are commonly bred in tanks. tion until maximum tank capacity be returned to the other end of the Brood fish are stocked at a rate of is reached. During that phase of hapa. This method produces approxi- 0.06 to 0.14 pound of brood fish per growth, average fish size and over- mately one fry per square foot square foot (0.29 to 0.68 kg/m2) of 2 all tank biomass increase. As fish (10.8/m ) per day. Monosex culture offered. The strategy in choosing and When oxygenation is used, there is changing feed sizes should follow the less need to adjust oxygen input to The culture of nearly all-male popu- goal of always offering the largest maintain optimum dissolved oxy- lations is being conducted with good size feed that the smallest fish of the gen levels. When dissolved oxygen success. All-male populations have population can consume. If feed is concentrations are low, feed conver- better growth rates than mixed-sex too large for the smaller members of sion ratios can be affected; when populations because there are few the population, their feed intake will dissolved oxygen concentrations are slower-growing females, which con- be reduced, their growth rate will de- high (supersaturation), oxygen is be- vert some feed into egg production. cline, and there will be a greater size ing wasted. There are two methods for producing difference within the tank popula- all-male tilapia fingerling batches: 1) With hand feeding, an operator can tion. And by offering the largest size sex reversal of fry using a synthetic observe feeding behavior and reduce feed that the fish can consume, indi- male androgen (17-alpha methyltes- or increase feed depending upon the vidual fish will expend less energy in tosterone) administered in feed for reaction of the fish. Feed can also be the feeding process. 28 days post-hatch; and 2) spawning spread over the tank surface to allow female tilapia with tilapia males that In their natural environments, tila- more fish to feed. Should fish con- have two Y chromosomes. A third pias are feeders. They feed centrate and feed in one area, feed method, using interspecific - by “grazing,” and for longer periods can be applied in a different area to ization of female Oreochromis niloti- than predator fish that pursue and attract those fish that were crowded cus and male Oreochromis aureus, capture prey. Therefore, it is possible out. Trained feed applicators can has been described in the scientific to use an extended feeding regime also become skilled at identifying literature and is sometimes used in for tank-cultured tilapia, feeding a changes in water quality from the countries where chemical sex rever- daily ration over a period of 10 to 12 feeding behavior of the fish. As feed- sal is prohibited. It is not widely used hours or longer. Some researchers ing slows, feed application can be in commercial production in the U.S. have reported that better feed conver- decreased or stopped to reduce the sion will result if tilapia are not fed amount of uneaten feed. The sex reversal of fry can be con- continuously, but are given a rest pe- ducted only by special permit from Tank-cultured tilapia can have very riod, during which their metabolism the federal government. The use of efficient feed conversion ratios (FCR). decreases. The method of feeding breeding males with two Y chromo- The time period for FCR can be used, by hand or by automatic feeder, somes is a patented method; brood- days, weeks, the length of the tank determines whether a feeding sched- stock is available to licensees from production cycle, or a year. FCRs ule can be extended. Each method the patent holder. in the range of 1.4:1 to 1.8:1 are has advantages and disadvantages. common with tilapia and are some Feeding manually over longer peri- Feeds and feeding of the best in agriculture. ods of time increases labor cost but While FCR is one of the most im- One of the characteristics that make allows feeding to be monitored. Using portant benchmarks for measuring tilapias suitable for simple hatchery mechanized feeders makes it easier to the efficiency of an operation, FCR production is that new fry do not schedule feed delivery to individual alone does not give a true measure of need specialized live feeds such as tanks throughout a daily cycle. production. An artificially low FCR Artemia, rotifers or microalgae. They In both flow-through and water can be created by underfeeding, so it can be given commercial dry feeds, reuse systems, extending the feeding is important to consider the growth size 00 or 0, after they have absorbed schedule over a longer period also rate also. their yolk sacs. The fine powder spreads out the impact of feed on wa- form allows some of the feed to float, The cost of feed is a large part of an ter quality parameters. Feed can be encouraging surface feeding. Fry this operating budget so it must be used delivered to the system at more even size may eat as much as 20 percent of wisely. Table 1 contains guidelines intervals to moderate the high and their body weight (biomass) per day. for feed sizes and feeding rates for low variations of these parameters. tank culture of tilapia. At this stage, fry can be given special feed to reverse their sex. This special feed, which must be prescribed by Table 1. Suggested feed size and feeding of tank-cultured tilapia. and produced under the direction of a veterinarian, contains a small concen- Average weight Standard feed size Range of feeding rate (grams) (% biomass/day) tration of male androgen. For up to 28 days post-hatch, this feed is offered to Post-hatch – 0.5 #00, #0, #1 Crumble 20 – 15 the quickly growing and sexually un- 0.5 – 5 #2 Crumble 15 – 10 differentiated fry. Properly adminis- 5 – 18 #3 Crumble 10 – 5 tered, this feed produces populations of more than 90 percent male fish. 18 – 75 #4 Crumble (1 mm) 5 – 3 1 75 – 150 ⁄8 inch (3 mm) 3 – 1.5 As the fry and fingerlings grow, 3 progressively larger feeds should be 150 to market ⁄16 inch (5 mm) 3 – 1.5 To achieve projected weekly weight enough quality to allow daily mea- pH — Tilapia can survive a wide gains, the corresponding total amount surements, so that daily readings can range of pH, from 5 to 10, but are of feed must be fed and consumed be compared to determine the effect said to grow best at pH 6 to 9. In by the fish population during that of increased feed and fish biomass tank systems, dissolved carbon week. In systems that are not prop- on water quality from one day to the dioxide causes pH to decline because erly designed for good solids removal, next. of the formation of carbonic acid biofiltration, dissolved gas stripping, Strict water quality parameters for (H2CO3) in solution. Low pH is not and aeration or oxygenation, poor tilapia culture are difficult to define. as serious a problem in flow-through water quality often occurs before Experience at one site may not reflect systems as in water reuse systems, the required daily ration is fed. This the same results as those reported in which a minimum pH of 6.8 is is especially true near the end of in a scientific publication or from suggested as the lower limit of toler- the growing period when biomass another system at another location. ance for the nitrifying bacteria of is nearing maximum. When this There are variables that influence the the biofilter. Due to the presence of occurs, the manager can 1) reduce effect a particular parameter, such dissolved carbon dioxide, high pH is or suspend feeding or 2) begin water as ammonia concentration, has on generally not a problem in tank sys- exchanges. various fish. Water quality variables tems. (See also Carbon dioxide.) To ensure that targeted growth rates interact in complex and often poorly Ammonia (NH3) — Ammonia exists are achieved for a given week, the understood ways. Variables such as in two forms in the tank environ- manager will estimate the begin- water temperature, pH, hardness, ment, un-ionized NH3 (highly toxic) + ning and ending biomass of the tank general fish health, feeding history, and ionized NH4 (less toxic). Avoid based upon estimated growth rates, and sound and light stressors all concentrations of un-ionized ammo- assume a realistic FCR, and calculate have a role in determining whether nia greater than 1.0 mg/L. Consult the total amount of feed required for the lethal level of a particular param- other sources to understand the rela- that week. For example, if a growth eter has been reached. tionship between pH and the toxicity rate of 5.0 grams per day per fish The following water quality guide- of Total Ammonia Nitrogen (TAN), is expected and the tank contains lines are based on published infor- un-ionized ammonia and ionized 10,000 fish, a total of 350,000 grams mation as well as the authors’ experi- ammonia. (350 kg) of weight should be gained - ence in the tank culture of tilapia. Nitrite (NO2) — Avoid concentra- for the week. Assuming an FCR of For further information, the follow- tions greater than 5 mg/L nitrite- 1.7:1, the manager would need to ing texts are recommended: nitrogen if chloride (Cl–) is low feed 595 kg of feed for the week C. Lim and C.D Webster (eds). 2006. (less than 10 mg/L). Add rock salt (1.7 x 350 kg = 595 kg). The daily to maintain chloride concentration feed rate would have to average at Tilapia—Biology, Culture, and Nutri- tion. New York: The Haworth Press. of 150 to 200 mg/L under normal least 85 kg (595 kg ÷ 7 days/week operating conditions, and increase = 85 kg). If that feed rate cannot be M.B. Timmons and J.M. Ebeling chloride concentration when nitrite reached, the manager should look for (eds). 2007. Recirculating Aquacul- is elevated. The chloride ion allevi- an explanation so the problem can be ture. NRAC Publication No. 01-007. ates nitrite toxicity and can be added remedied. Often poor water quality Ithaca, New York: Cayuga Aqua as sodium chloride (NaCl) or calcium is the cause of the problem. The fish Ventures. chloride (CaCl2). may not be eating aggressively due to Temperature — Optimum growth - Nitrate (NO3) — Nitrate toxicity the stresses of high ammonia levels, for tilapia is achieved at 81 to 84 °F nitrite toxicity, low dissolved oxy- can occur if levels in water reuse (27 to 29 °C), but acceptable growth systems exceed the 300 to 400 mg/L gen, high levels of carbon dioxide, or rates are reported at 77 to 90 °F (25 other water quality problems. nitrate-nitrogen range. Normal water to 32 °C). Temperatures in the ex- exchanges during filter backwashing treme upper range make it more dif- Water quality requirements or solids removal generally control ficult to maintain dissolved oxygen nitrate concentrations. Water ex- Tilapia are some of the hardiest fish concentration. change or a denitrification process being cultured; they can withstand Dissolved oxygen — Operating may be required. water quality conditions and physical levels of between 5.0 and 7.5 mil- handling that would create serious Carbon dioxide (CO2) — Main- ligrams per liter (mg/L) are recom- tain at less than 40 mg/L. Elevated challenges for other species. How- mended. Growth and feed conversion ever, tank culturists need equip- carbon dioxide levels cause lethargic will be affected by chronically low behavior or slow feeding response in ment that analyzes the minimum DO concentrations below 3.5 mg/L. basic water quality parameters of fish. While tilapia can tolerate a wide Survival and recovery are possible range of pH, dissolved carbon diox- dissolved oxygen, temperature, pH, with short-term exposure (less than ammonia, nitrite, alkalinity, chloride ide gas stripping is required in water 10 minutes) to DO concentrations as reuse systems to keep pH above 6.8 concentration, and calcium hardness. low as 0.8 mg/L. The equipment should be of good and promote conditions favorable to nitrifying bacteria in the biofilter. Calcium hardness — Maintain ing tanks during transport to market. comfortable and safe for harvesters. between 50 and 100 mg/L. Dissolved Purging time varies and is influenced A depth of about 50 inches (127 cm) calcium in the water aids in osmoreg- by water quality conditions, the is workable if fish are not so crowded ulation and relieves stress in fish. It type of feed and its ingredients, and that they jump excessively and risk is usually added as calcium chloride the preferences of the marketplace. injury to themselves or workers. (CaCl2), which dissolves readily and Withholding feed for 3 to 5 days Close attention should be paid during also increases chloride (Cl–). before harvesting and marketing is the draining and crowding process Chloride (Cl–) — Maintain between common. During that time, water so that dissolved oxygen levels of 100 and 300 mg/L. See description should be exchanged to improve wa- 4.0 mg/L or greater are maintained. under Nitrite. Also see the Harvest- ter quality and reduce temperature. Stressing fish and causing mortality ing and Marketing section. Lowering the temperature to about at this stage risks the resources that 72 °F (22 °C) slows the activity have been invested in the operation. Alkalinity — This is the measure of and the metabolism of the fish and Minimizing the number of times the the pH buffering capacity of water, increases the dissolved oxygen in the fish are handled between the time and should be maintained at 100 to hauling tanks. Fish grown in recircu- they are captured in the culture 250 mg/L by adding a soluble car- lating systems often develop off- tank and released into the live haul bonate or bicarbonate source. So- flavor, which is thought to be caused tank will significantly improve their dium bicarbonate is commonly used by the fish feed or bacterial content survival when they reach the fish because it is readily available, highly of the culture system. Purging helps dealer’s facility. Scooping the fish in soluble, and safe to handle. Dissolved reduce off-flavor problems and may the same basket in which they are carbon dioxide reduces pH, so higher cause the fish to begin utilizing fat weighed and carried to the live haul alkalinities must be maintained if reserves where the compounds that tank reduces handling and the abra- CO2 stripping is poor. Choosing a cause off-flavor may be concentrated. sion and removal of the fish’s slime water source with higher alkalinity coat. reduces operating expenses because Rock salt or non-iodized salt should less supplemental alkalinity will be be added to the hauling tank to help Harvesters should wear protective needed. alleviate stressful conditions. Food eyewear or face shields, gloves, and grade salt that contains the anti- chest waders or long pants to protect Further water quality guidelines for caking agent yellow prussiate of soda themselves from injury. recirculating aquaculture systems (sodium ferrocyanide) should not All parties involved—the fish pro- can be found in SRAC publication be used, because this cyanide-based 452, Recirculating Aquaculture Tank ducer, the live hauler and the fish compound is toxic to fish. Although dealer—should regard the live fish as Production Systems: Management of it is not currently approved for food Recirculating Systems. a sensitive item, worthy of great care fish, rock salt is classified as Low and proper handling, so that all are Regulatory Priority (LRP) by the Harvesting and marketing rewarded with the reputation of pro- U.S. Food and Drug Administration. ducing and delivering a high-quality In the southern U.S., tank culture Dissolving salt in the transport water product. of tilapia is carried out primarily for brings the salt concentration of the the live fish market. The harvesting hauling water up to the salt con- Conclusion of tilapia requires a certain amount centration of the fish’s blood. Rates Much has been learned about the of preparation and special handling. of about 6.5 pounds of salt per 100 tank culture of tilapia since the early The generally accepted size for the gallons of water are recommended. days of production. Successes and live tilapia market is approximately This equates to a salinity of about 7.8 failures have added to our knowledge 1.5 pounds (680 g), although fish parts per thousand (ppt). With ap- of system design, species selection, weighing as little 0.75 pound (340 g) propriate preparation, live tilapia can stock management and nutrition. and as much as 2.0 pounds (908 g) be transported by truck for 18 hours There are a number of recurring will find acceptance in some areas. or more with little mortality, and can short courses, workshops and confer- As the average weight of the stock be held for live sale for up to 1 week ences where prospective tank aqua- in a tank approaches market size, after harvest. buyers, harvest crews and live haul culturists can learn to plan, design, Proper handling of fish during the transporters should be coordinated. build and operate facilities with harvesting stage enhances survival confidence. Before putting significant Tilapia are usually prepared for sale and ensures that strong fish will be capital at risk, it is to your advantage by purging them, or withholding available to the buyer in the event to use these resources, along with feed for a period before harvest and that fish must be held in a live resources available on the Web and transporting. This allows the fish to facility before sale. Tanks should from your state Cooperative Exten- rid their digestive systems of wastes be drained to a water depth that is sion Service. and improves conditions in the haul-

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software Steering Committee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have not been revised are considered to reflect the current state of knowledge.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 2007- 38500-18470 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.