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Tilapia, Life History and Biology

Tilapia

Life History and Biology

Thomas Popma and Michael Masser

World harvest of farmed tilapia has now surpassed 500,000 metric tons, second only to carps as the most widely farmed freshwater fish in the world. "Tilapia" is the generic name of a group of Cichlids endemic to Africa and consists of three aquaculturally important genera:Oreochromis, Sarotherodon, and Tilapia.  Several characteristics distinguish these three genera but possibly the most critical relates to reproductive behavior. All Tilapia species are nest builders, with the fertilized eggs being guarded in the nest by a brood parent. Both Sarotherodon  and Oreochromis  are mouth brooders: eggs are fertilized in the nest but brood parents provide even greater protection to offspring, immediately picking up the eggs in their mouth and maintaining them through incubation and for several days after hatching. In Oreochromis species only females practice mouth-brooding while in Sarotherodon  either male or both male and female are mouthbrooders.

During the last half century several tilapia species have been introduced throughout the tropical and semi-tropical world for fish farming purposes. Today all commercially important tilapia outside of Africa belong to the genus Oreochromis. The first and most widely distributed species was the Java tilapia (O. mossambicus). The Nile tilapia (O. niloticus) eventually proved to be a better aquacultural species, and now more than 90% of all commercially farmed tilapia outside of Africa are Nile tilapia. Other less commonly farmed species include Blue tilapia (O. aureus) and the Zanzibar tilapia (O. urolepis hornorum).  

Considerable revision of the scientific names of tilapia species has occurred within the last 30 years, creating the possibility for misunderstanding by readers of technical literature. Consequently, depending on the date of publication, the scientific name of the Nile tilapia has been given as Tilapia nilotica, Sarotherodon niloticus and now as Oreochromis niloticus.

The "red" tilapia is becoming increasingly popular in recent years because of increased market value due its similarity to the marine red snapper. The original red tilapias were genetic mutants. The first red tilapia, produced in Taiwan in the late 1960's, was a cross between a mutant reddish-orange female Java tilapia and a normal male Nile tilapia and was called the Taiwanese red tilapia. Another red strain of tilapia was developed in Florida in the 1970's by crossing a normal colored female Zanzibar tilapia with a red-gold male Java tilapia.  A third strain of red tilapia was developed in Israel from a mutant pink Nile tilapia crossed with wild-type Blue tilapia. All three original stains have been crossed with other red tilapia of unreported origin or with wild-type Oreochromis  species.  Consequently, most red tilapia found in the Americas are mosaics of uncertain origin. The confused and rapidly changing genetic composition of red tilapia, as well as lack of "head-to-head" growth comparisons between the different lines, make identification of a "best" red strain difficult for interested producers.

The most positive aquacultural characteristics of tilapia are tolerance to poor water quality and the ability to utilize a wide range of natural food organisms. The greatest biological constraints in the development of commercial tilapia farming are an inability to withstand sustained water temperature below 50°-53°F and pond spawning before reaching market size. The remainder of this publication addresses these positive and negative biological characteristics of tilapia.

Natural feeding behavior

Tilapias ingest a wide variety of natural food organisms, including plankton, succulent green leaves, benthic organisms, aquatic invertebrates, larval fish, detritus and decomposing organic matter. In ponds with heavy supplemental feeding, natural food organisms typically account for 30% to 50% of tilapia growth, whereas in full-fed channel catfish ponds only 5-10% of the fish growth is traced to ingestion of natural food organisms.

Tilapias are often considered filter-feeders because they can efficiently harvest planktonic organisms from the water. However, "filter-feeder" is somewhat a misnomer because tilapia do not physically filter the water through gill rakers as efficiently as gizzard shad and silver carp.  Gills of tilapia secrete a mucous that entraps planktonic cells; the plankton-rich bolus is then swollowed. Digestion and assimilation of plant material occurs along the length of a long intestine, usually at least 6 times the total length of the fish. The Java tilapia is less efficient than the Nile or Blue tilapias in ingesting planktonic algae.

Digestion of filamentous and planktonic algae and higher plants is aided by two mechanisms: by physical grinding of plant tissues between two pharyngeal plates of fine teeth and by a stomach pH below 2 which ruptures the cell walls of blue-green algae and bacteria. The commonly cultured tilapias digest 30% to 60% of the protein in algae, with blue-green algae being digested more efficiently than green algae.

When feeding, tilapias do not disturb the pond bottom as aggressively as common carp, but they effectively browse, primarily during daylight hours, on live benthic invertebrates and bacteria-laden detritus. Tilapias also feed on mid-water invertebrates. They are not generally considered piscivorous, but juveniles actively attack larval fish.

In general, tilapias utilize natural food organisms so efficiently that standing crops of fish exceeding 3000 kg/ha can be sustained without supplemental feed in well fertilized ponds.  The nutritional value of the natural food supply is important, even for commercial operations with heavy feeding.

Tolerance to poor water quality

Tilapia are more tolerant than most commonly farmed fish to salinity, high water temperature, low dissolved oxygen and high ammonia concentrations:

Salinity

The most commonly cultured tilapias are freshwater species, but all are tolerant to brackishwater. The Nile tilapia is the least saline tolerant of the commercially important species but grows well at salinities up to 15 ppt. The Blue tilapia grows well in brackishwater up to 20 ppt salinity, and the Java tilapia grows well at salinities near or at full-strength seawater. Due to tolerance to high salinity, the Java tilapia and some mossambicus-derived "red" tilapias are preferred for culture in saltwater.

Some lines of the Java tilapia reportedly spawn in full strength seawater, but generally, its reproductive performance begins to decline at salinities above 10 to 15 ppt. Many red tilapia hybrids with Java tilapia genes have a reproductive performance in saltwater similar to pure Java tilapia.  The Blue and Nile tilapias  can reproduce in salinities up 10 to 15 ppt. However, they have better reproductive performance at salinities not exceeding 5 ppt, with fry numbers declining substantially at 10 ppt salinity.

Water temperature

Inability of tilapia to tolerate low temperatures is a serious constraint for commercial culture in temperate regions. The lethal low temperature for most species is 50° or 52°F for a few days, but the Blue tilapia, the most cold tolerant, tolerates to about 48°F.

Feeding by tilapia generally ceases when water temperature falls below 63°F.  Disease-induced mortality after handling seriously constrains sampling, harvest and transport below 65°F. Reproduction is inhibited at water temperatures below 68°F, is slow in waters of 70°-75°F and is most abundant in waters above 80°F. Subtropical regions with a cool season will see a reduction in the number of fry produced during times when daily water temperature averages below 75°F.  For example, fry recovery from ponds after 16- to 20-day spawning cycles with half-pound Nile tilapia was about 600 fry/female brooder at water temperature of 82°F but only 250 fry/female at water temperature of 75°F

Preferred water temperatures for tilapia growth are approximately 85° to 88°F. If fish are fed to satiation, growth at the preferred temperature is typically three times greater than at 72°F.

Dissolved oxygen concentration

Low dissolved oxygen concentration is usually the first water quality constraint to growth in intensively managed fish ponds. Commonly cultured species of tilapia survive routine dawn dissolved oxygen (DO) concentrations less than 0.3 mg/l, levels considerably below the tolerance limits for most other cultured fish. Nile tilapia were greatest when aerators were used to prevent morning DO concentrations from falling below 0.7-0.8 mg/l but fish yields were not further improved if additional aeration was used to prevent DO concentration from falling below 2.0-2.5 mg/l.

In spite of the ability to survive acute low DO concentrations for several hours, tilapia ponds should be managed to generally maintain DO concentrations above 1 mg/l because metabolism, growth and, possibly, disease resistance are depressed when levels are below this level for prolonged periods.

Ammonia

Massive mortality of tilapia occurs within a couple days when fish are suddenly transferred to water with unionized ammonia concentrations greater than 2 mg/l. However, when acclimated to sub-lethal levels, approximately half the fish will survive three or four days at unionized ammonia concentrations as high as 3 mg/l. Prolonged exposure (several weeks) to unionized ammonia concentration greater than 1 mg/l causes losses, especially among fry and juveniles in water with low DO concentration.  The first mortalities from prolonged exposure may begin at unionized ammonia concentrations as low as 0.2 mg/l.  Unionized ammonia begins to depress appetite of tilapia at concentrations as low as 0.08 mg/l.

Age and size at sexual maturity

In all Oreochromis species the polygamous male excavates a nest in the pond bottom, generally in water shallower than 1 meter. After a short mating ritual, the female spawns in the nest (about 2-4 eggs/g of brood female) and incubates the externally fertilized eggs in her buccal cavity until they hatch. Fry remain in the female's mouth through yolk sac absorption, and often seek refuge in her mouth for several days after they begin to feed.

Sexual maturity in tilapia is a function of age, size, and environmental conditions. The Java tilapia reaches sexual maturity at a smaller size and younger age than the Nile and Blue tilapias.  Tilapia populations in large lakes mature at a later age and larger size than the same species raised in small farm ponds. For example, the Nile tilapia matures at about 10 to 12 months and 350 to 500 g in several East African lakes. This same species under good growth conditions will reach sexual maturity in farm ponds at an age of 5 to 6 months and 150 to 200 g. When growth is slow, sexual maturity in O. niloticus  is delayed a month or two but stunted fish may spawn at a weight as low as 20 g.  Under fast-growing conditions in farm ponds the Java tilapia may reach sexual maturity in as little as three months of age, at which time they seldom exceed 60 to 100 g. In poorly fertilized ponds sexually mature fish may be as small as 15 g.

Fish farming strategies employed to prevent overcrowding and stunting include: 1) cage farming where eggs fall through the mesh to the pond bottom before the female can collect the eggs for brooding; 2) polyculture with a predator fish, such as fingerling largemouth bass at 400/acre; and 3) farming of only males. All-male culture is desireable in ponds, not only as a means to prevent overpopulation and stunting, but also because of the superior growth of males which typically grow twice as fast as females. All-male or monosex fish have been most often obtained by: 1) manual separation of the sexes based on visual examination of the genital papilla of juvenile fish ("hand-sexing"); 2) hybridization between two selected species that produce all-male offspring (for example, Nile or Java female crossed with Blue or Zanzibar male); 3) feeding a male hormone treated feed to recently-hatched fry for 3-4 weeks to produce reproductively functional males ("sex reversal").

Legal restriction on tilapia distribution

Tilapias are native only to Africa. Consequently many states in the United States have put restrictions on the transport and culture of many or all tilapia species. Perspective tilapia producers should check with relevant state agencies to determine legal implications.

(Maybe put more species data for SE states).