Tank Tilapia Production

7/28/2019 Tank Tilapia Production

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L-2409SRAC Publication No. 282

SouthernRegionalAquacultureCenter

September, 1989**

Tank Culture of Tilapia

James E. Rakocy*

Tank culture of tilapia is a good al-ternative to pond or cage culture ifsufficient water or land is not avail-able and the economics arefavorable. Tilapia grow well at high

densities in the confinement of tankswhen good water quality is main-

tained. This is accomplished by aera-tion and frequent or continuouswater exchange to renew dissolvedoxygen (DO) supplies and remove

wastes. Culture systems that discardwater after use are called flow-through systems while those that fil-

ter and recycle water are referred toas recirculating systems.

Intensive tank culture offers several

advantages over pond culture. Highfish density in tanks disrupts breed-ing behavior and allows male andfemale tilapia to be grown together

to marketable size. In ponds, mixed-

sex populations breed so much thatparents and offspring compete for

food and become stunted. Tanksallow the fish culturist to easilymanage stocks and to exert a rela-tively high degree of environmentalcontrol over parameters (e.g., watertemperature, DO, pH, waste) thatcan be adjusted for maximum

* University of the Virgin Islands

Texas Agricultural Extension Service Zerle L.

production. With tanks, feeding andharvesting operations require muchless time and labor compared to

ponds. Small tank volumes make itpractical and economical to treat dis-

eases with therapeutic chemicals dis-solved in the culture water. Intensivetank culture can produce very highyields on small parcels of land.

Tank culture also has some disad-

vantages. Since tilapia have limitedaccess to natural foods in tanks, theymust be fed a complete diet contain-ing vitamins and minerals. The cost

of pumping water and aeration in-

creases production costs. The filtra-tion technology of recirculatingsystems can be fairly complex and ex-pensive and requires constant and

close attention. Any tank culture sys-tem that relies on continuous aera-tion or water pumping is at risk ofmechanical or electrical failure andmajor fish mortality. Backup systems

are essential. Confinement of fish intanks at high densities creates stress-ful conditions and increases the risk

of disease outbreaks. Dischargesfrom flow-through systems may pol-lute receiving waters with nutrientsand organic matter.

Geographical rangeThe geographical range for culturingtilapia in outdoor tanks is dependent

on water temperature. Thepreferred temperature range for op-

timum tilapia growth is 82 to 86F.Growth diminishes significantly at

temperatures below 68F and deathwill occur below 50F. At tempera-tures below 54F, tilapia lose theirresistance to disease and are subject

to infections by bacteria, fungi andparasites.

In the southern region, tilapia can beheld in tanks for 5 to 12 months ayear depending on location. The

southernmost parts of Texas andFlorida are the only areas where

tilapia survive outdoors year-round.Elsewhere, tilapia must be over-wintered in heated water.

Flow-through systems are only prac-

tical for year-round culture intemperate regions if geothermalwater is available. In the winter it

would be too expensive to heat water

and soon discard it. There has beensome promising research on the use

of heated effluents from powerplants to extend the growing season.

Indoor recirculating systems aremore appropriate for year-round cul-

ture because buildings can be insu-lated to conserve heat and theheated water is saved through recy-

cling. Indoor recirculating systems

Carpenter, Director qThe Texas A&M University System qCollege Station, Texas


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have potential for extending thegeographical range of tilapia culturethroughout the U.S. Systems couldbe located in urban areas close tomarket outlets.

Flow-through systemsThe most durable tank materials are

concrete and fiberglass. Other

suitable but less durable materials in-clude wood coated with fiberglass orepoxy paint, and polyethylene, vinylor neoprene rubber liners inside asupport structure such as coatedsteel, aluminum or wood. Tankmaterial must be non-toxic and non-corrosive. The interior surfaceshould be smooth to prevent damage

to fish by abrasion, to facilitate clean-ing and to reduce resistance to flow.

Both ease and expense of installa-tion are important factors in theselection of construction materials.

Tanks come in a variety of shapes,but the most common forms are cir-

cular and rectangular. Raceways arerectangular tanks that are long andnarrow. Variations of circular tanks

are silos, which are very deep, andoctagonal tanks. Circular tanks arevery popular because they tend to beself-cleaning. If the direction of theinlet flow is perpendicular to the

radius, a circular flow patterndevelops which scours solids off the

tank bottom and carries them to acenter drain. Rectangular tanks areeasy to construct but often have poorflow characteristics. Some of the in-

coming water may flow directly tothe drain, short-circuiting the tank,while other areas of the tank maybe-

come stagnant, which allows wasteto accumulate and lowers oxygenlevels. For these reasons, circular

tanks provide better conditions thanrectangular tanks for tilapia culture.

Circular culture tanks may be aslarge as 100 feet in diameter, but

common sizes range from 12 to 30feet in diameter and from 4 to 5 feet

in depth. Rectangular tanks are vari-

able in dimensions and size, butraceways have specific dimension re-quirements for proper operation.The length to width to depth ratio

should be 30:3:1 for good flow pat-terns. If the volume of water flow is

limited, shorter raceways are better

to increase the water exchange rateand prevent tilapia from concentrat-ing near the inlet section where DOlevels are higher.

Drain design importantDrain design is another important

aspect of tank culture. Center drainsare required in circular tanks for ef-fective removal of solid waste. Waterlevel is controlled by an overflowstandpipe placed directly in the cen-ter drain or in the drain line outsidethe tank. A larger pipe (sleeve) withnotches at the bottom is placed over

the center standpipe to draw wasteoff the tank bottom. The sleeve ishigher than the standpipe but lower

than the tank wall so that water will

flow over the sleeve into thestandpipe if notches becomeclosed. When an external

standpipe is used, the drain linemust be screened to prevent fishfrom escaping. To prevent clogging,the screened area must be expanded

by inserting a cylinder of screen intothe drain so that it projects into thetank.

Aeration requirements depend onthe rate of water exchange. If wateris exchanged rapidly, one to four

times per hour, in a tank withmoderate fish densities, aeration

devices may not be required. Theoxygen supply will be renewed by theDO in the incoming water. A flow

rate of 6 to 12 gallons/minute isneeded to support the oxygen re-

quirement of 100 pounds of tilapia.DO, which should be maintained at

5 mg/liter for good tilapia growth, isthe primary limiting factor for inten-sive tank culture. Flow- through sys-

tems should ideally be located nextto rivers or streams to take ad-vantage of gravity-fed water sup-plies, but pumping is practical in

many situations.

Limited water supplies frequentlyrestrict exchange rates to a few times

a day or as little as 10 to 15 percentper day. In this case, aeration isneeded to sustain tilapia at commer-cial levels. Paddlewheel aerators,agitators and blowers (diffused aera-tion) are some of the devices used to

aerate tanks. Aerators are rated ac-cording to their effectiveness(pounds of oxygen transferred intothe water per hour) and efficiency(pounds of oxygen transferred/horse-power-hour). Aeration requirements

can be estimated by using aeratorratings and oxygen (O2) consump-

tion rates of tilapia, which consume4.5 grams O2/100 pounds offish/hour while resting and severaltimes more oxygen while they arefeeding and active. For example, atank with 1,000 pounds of tilapiawould consume 45 grams of O2/hourat resting, but maximum oxygen con-sumption may be at least three timeshigher (135 grams O2/hour) depend-ing on water temperature, body

weight and feeding rate. Aeration ef-

ficiency (AE) of diffused-air systems(medium bubble size) ranges from1,000 to 1,600 grams O2/kilowatt-hour under standard conditions

(68F and 0 mg/liter DO). However,AE declines to 22 percent of the

standard at 5 mg/liter DO and 86F.Therefore, AE would range from220 to 352 grams O2/kilowatt-hourunder culture conditions. Dividingthe maximum oxygen consumptionrate (135 grams O2/hour) by themedian AE (286 grams O2/hour)gives 0.47-kilowatt (0.63-horse-

power) as the size of aerator neededto provide adequate DO levels.

A current trend for intensive tank

systems has been the use of pure

oxygen for aeration. Oxygen fromoxygen generators, compressed

oxygen tanks, or liquid oxygen tanksis dissolved completely into the cul-

ture water by special techniques tohelp sustain very high fish densities.

Recirculating systemsRecirculating systems generallyrecycle 90 to 99 percent of the cul-

ture water daily. The rearing tank isaerated as in flow- through systemswith low exchange rates. Recirculat-

ing systems require a clarifier (set-

tling tank) to remove solid waste(feces and uneaten feed) and a biofil-

ter to remove toxic waste products(ammonia and nitrite) that areproduced by the fish.

A cylindrical clarifier with a conical


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bottom (60 slope) and center drainfacilitates solids removal, but oftenrectangular tanks are used and the

solids are pumped or siphoned offthe bottom. Baffles are used near theinlet to slow the incoming water flow

and near the outlet to retain floating

sludge. If a few tilapia fingerlings (ofone sex to prevent breeding) areplaced in the clarifier, their move-ment will concentrate sludge in thelowest portion of the tank. They

should not be fed, as they will obtainadequate nutrition from the sludge

and wasted feed. For efficient solidsremoval, clarifiers have a waterretention time of 25 to 30 minutesand a minimal depth of 4 feet.

There are many effective biofilterdesigns, but they all operate on the

same principle of providing a largesurface area for the attachment ofvitrifying bacteria that transform am-monia (NH3), excreted from the gillsof fish, into nitrite (NO2), which inturn is converted to nitrate (NO

3).

Nitrate is relatively non-toxic to fish,but an accumulation of ammoniaand nitrite can cause mortality.Tilapia begin to die at ammonia con-

centrations around 2 mg/liter (ex-pressed as NH3-N) and nitrite levels

of 5 mg/liter (as NO2-N).

Gravel biofilters, which once werecommon, are being replaced by plas-tic-media biofilters because they arelightweight and easy to clean. Biofil-

ters now consist of self-supportingstacks of honeycombed modules,columns or tanks containing looselypacked rings, or a series of discs onan axle that floats at the water sur-face and rotates, alternately expos-ing the media to water and air.Regardless of design, biofiltersgenerally have the same require-ments for efficient vitrification: 1)

DO of not less than 2 mg/liter or 3 to5 mg/liter for maximum efficiency; 2)pH 7 to 8; 3) a source of alkalinityfor buffer since vitrificationproduces acid and destroys about 7mg of alkalinity for every mg of NH 3-N oxidized; 4) moderate levels of or-

ganic waste (less than 30 mg/litermeasured as biochemical oxygendemand), thereby requiring goodclarification; 5) water flow velocities

that do not dislodge bacteria.

Biofilters can be sized by balancingammonia production rates with am-monia removal rates. Unfortunately,these rates are highly variable. In agrowout study on tilapia in tanks, am-

monia production averaged 10grams/100 pounds of fish/day(range:4 to 21). Ammonia produc-tion depends on quality of feed, feed-ing rate, fish size and water

temperature, among other factors.Ammonia removal rates may rangefrom 0.02 to 0.10 grams/ft

2of biofil-

ter surface area/day depending ontype of media, biofilter design, andthe factors that affect vitrification.

The required biofilter surface area

can be obtained by dividing total am-

monia production for the maximumstanding crop by the ammoniaremoval rate. The filter volume canbe determined by dividing the re-quired biofilter surface area by thespecific surface area (ft

2/ft

3) of the

media. For example, assume that abiofilter containing l-inch pall ringsis being designed to support 1,000pounds of tilapia. The ammoniaproduction rate is estimated to be 10

grams/100 pounds of fish/day. There-fore, total ammonia productionwould be 100 grams/day. The am-

monia removal rate is estimated tobe 0.05 grams/ft

2/day. Dividing total

ammonia production by the am-monia removal rate gives 2,000 ft

2as

the required biofilter surface area.

One-inch pall rings have a specificsurface area of 66 ft

2/ft

3. Dividing

the required biofilter surface area bythe specific surface area gives 30 ft

3

as the biofilter volume needed toremove ammonia.

Species selectionThe most appropriate species of

tilapia for tank culture in the U.S.are Tilapia nilotica, T. aurea, Floridared tilapia, Taiwan red tilapia, andhybrids between these species orstrains. The choice of a species forculture depends mainly onavailability, legal status, growth rateand cold tolerance. Many statesprohibit the culture of certainspecies. Unfortunately, T. nilotica,which has the highest growth rate

under tropical conditions, is fre-quently restricted. Florida redtilapia grow nearly as fast as T.nilotica and have an attractive red-dish-orange appearance. T. aureagrow at the slowest rate under tropi-cal conditions, but this species has

the greatest cold tolerance and mayhave the highest growth rate in

temperate regions at temperaturesbelow optimum.

BreedingTanks are commonly used to breedtilapia. Within 10 to 20 days afterstocking brood fish, newly-hatchedfry appear in schools that can be cap-tured with a dip net and transferred

to a nursery unit. Fry that avoid cap-ture prey on subsequent spawns andproduction declines. At that point,

the tank must be drained to removeall juvenile fish and begin anotherspawning cycle.

More controlled breeding can be ob-

tained with net enclosures (hapas).With hapas, all fry can be removed

at regular intervals, which ensuresuniformity in size among the fry,reduces predation, and eliminates

the need for draining the broodtank. Hapas can be fabricated to anyspecification, but a convenient sizefor spawning measures 10 feet by 4

feet by 4 feet deep. This size fits wellinto a 12-foot diameter tank. Hapasare made from nylon netting (Deltastyle) with a l/16-inch mesh.

Male and female brood fish, whichhave been kept apart, are stockedinto the hapa to begin breeding. A

sex ratio of 2 females to 1 male isused to produce large quantities offry. The optimum stocking density

ranges from 0.5 to 1.0 fish/ft2. The

brood fish are fed high quality feedat a rate of 2 percent of their body

weight per day. All of the fry areremoved a few days after they begin

to appear. This is accomplished bypulling a 4-inch PVC float down thelength of the hapa to concentrate thefry and brood fish to one end. Thebrood fish are captured with a large-mesh dip net and placed into a smallcontainer. The fry are captured witha fine-mesh dip net and transferredto a nursery tank. Each brood fish is


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then captured by hand and its mouthis held open under water to remove

any fry, sac fry, or eggs that it maybeincubating. The fry are moved to thenursery tank while the sac fry andeggs are placed in hatchery jars. Thismethod produces roughly 3 fry and 3

eggs (including sac fry) per squarefoot per day.

Production managementStocking density, which is very highfor fry, is decreased at regular inter-vals throughout the production cycleto reduce crowding, to ensure ade-quate water quality, and to use tankspace efficiently (Table 1). It is noteconomical to pump water for a tanksystem that is stocked initially at one

tenth of its capacity, which is thestandard stocking practice forponds. As density becomes too high,fish stocks can be split in half andphysically moved to new tanks or

given more space by adjusting screenpartitions within the rearing tank.Rectangular tanks or raceways, inparticular, are much easier to useand allow the culture of several sizegroups in one tank. However, fry

and small fingerlings are culturedseparately because they require bet-ter water quality. Each time that

stocks are split and moved, they are

graded through a bar grader to cullout about 10 percent of the slowestgrowing fish, which would probably

not reach market size. Culls could besold as baitfish if permitted by statelaw. Recommended grader widths

are 25/64, 32/64, 44/64, and 89/64ths

of an inch for tilapia greater than 5,10,25, and 250 grams, respectively.

The highest mortality of the produc-tion cycle (about 20 percent) occursduring the fry rearing stage. Much ofthis is due to predation. As the fish

grow and become hardier, mortalitydecreases significantly at each stage

so that no more than 2 percent of thefish are expected to die during final

growout.

Fry are given a complete diet of pow-dered feed (40 percent protein) thatis fed continuously throughout the

day with automatic feeders. The ini-tial feeding rate, which can be ashigh as 20 percent of body weight

per day under ideal conditions

(good water quality and tempera-ture: 86F), is gradually lowered to15 percent by day 30. During thisperiod, fry grow rapidly and will gainclose to 50 percent in body weightevery 3 days. Therefore, the dailyfeed ration is adjusted every 3 days

by weighing a small sample of fish inwater on a sensitive balance. If feed-ing vigor diminishes, the feeding rateis cut back immediately and water

quality (DO, pH, ammonia, nitrite)is checked.

Feed size can be increased tovarious grades of crumbles forfingerlings (1 to 50 grams), which

also require continuous feeding forfast growth. During the growoutstages, the feed is changed to float-ing pellets to allow visual observa-

tion of the feeding response.Recommended protein levels are 32

to 36 percent in fingerling feed and28 to 32 percent in feed for largerfish. 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 inweight, although absolute growth ingrams/day steadily increases.

The daily ration for adult fish is

divided into three to six feedings thatare evenly spaced throughout the

day. If feed is not consumed rapidly(within 15 minutes), feeding levelsare reduced. DO concentrationsdecline suddenly in response to feed-ing activity. Although DO levelsgenerally decline during the day in

tanks, feeding intervals provide timefor DO concentrations to increasesomewhat before the next feeding.Continuous feeding of adult fishfavors the more aggressive fish,

which guard the feeding area, andcauses the fish to be less uniform in

size. With high quality feeds and

proper feeding techniques, the feedconversion ratio (fish weight gaindivided by feed weight) should

average 1.5 for a l-pound fish.

Total production levels range from 3to 6 pounds/ft

3of rearing space and

6 to 17 pounds/gallon/minute offlow. Monthly production levelsrange from 0.4 to 0.6 pounds/ft

3. The

higher production levels are general-ly obtained in flow-through systems.Production can always be increased

by increasing the inputs, but this maynot be economical.

Table 1. Recommended stocking and feeding rates for different size

groups of tilapia in tanks and estimated growth rates.

Stocking Rate Weight(grams) Growth Growth Feeding Rate(number/m

3

) Rate Period (%)Initial Final (g/day) (days)

8,000 0.02 0.5-1 - 30 2 0 - 1 5

3,200 0.5-1 5 - 30 15- 10

1,600 5 20 0.5 30 1 0 - 7

1,000 20 50 1.0 30 7- 4

500 50 100 1.5 30 4 -3.5

200 100 250 2.5 50 3.5- 1.5

100 250 450 3.0 70 1.5 - 1.0

This publication was supported in part by a grant from the United States Department of Agriculture, Number 87-CRSR-2-3218, sponsored jointlv bv the

Cooperative State Research Service and the Extension Service.

Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socioeconomic Ievel, race, color, sex, religion, handicap or

national origin.

Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the

United States Department of Agriculture. Zerle L. Carpenter, Director, Texas Agricultural Extension Service, The Texas A&M University System.

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Tank Tilapia Production