Notice of Publication

AQUACULTURE COLLABORATIVE RESEARCH SUPPORT PROGRAM

RESEARCH REPORTS
Sustainable Aquaculture for a Secure Future

Title: Atlas of Tilapia Histology

Author(s): Carol M. Morrison

Kevin Fitzsimmons
University of Arizona
2601 E. Airport Drive
Tucson, Arizona 85706, USA

James R. Wright Jr.

Date: 6 November 2006 Publication Number: CRSP Research Report 06-215

Abstract: Tilapia is the common name applied to several closely related species of fish that have become
the second most important group of farmed fish, after the carps. Global production of the tila-
pias is somewhat greater in volume than that of the salmonids, although probably lesser sales
value, due to the higher prices for salmon products. However, the benefit of tilapia farming
to household income may be greater as the vast majority of tilapia are reared by small farmers
in relatively poor tropical countries for domestic consumption, local trade, and for interna-
tional exports. Tilapia have become a major commodity in international trade. Production
in China exceeded 900,000 metric tons in 2005 and in the United States, tilapia was the sixth
most popular seafood item. Production and consumption continues to rise at an annual rate
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of more than 10%.

Tilapia species are native to Africa and the Middle East, and refer to several genera and species
of fish that were formally classified in the genus Tilapia, in the family Cichlidae. In the reclas-
sification scheme developed by Trewavas (1983) the several hundred species of Tilapia were split
into the genera; Oreochromis, Sarotherodon and some remained as Tilapia. The Oreochromis
are maternal mouthbrooders, the Sarotherodon are paternal mouthbrooders and the Tilapia
are substrate spawners. The species that are most commonly reared in aquaculture are in the
genus Oreochromis. These include the Nile tilapia, Oreochromis niloticus, the blue tilapia, O.
aureus, the Mozambique tilapia, O. mossambicus, and O. urolepis hornorum, sometimes called
the Wami River tilapia. These species all readily hybridize in captivity. There are now many
strains of the parent species along with many hybrid strains available to growers.

There are also several species in the genus Tilapia and the genus Sarotherodon that are of inter-
est to aquaculture. Tilapia, like the other cichlids, are also of special interest to hobbyists and
ecologists. Tilapia in Africa have been intensively studies for the species clusters have evolved
in the Rift Lakes of East Africa. Some lakes contain over one hundred species in a single genus.
Some of the tilapias native ranges extend up into Israel and Syria. One of the most common
names for the fish is St. Peter’s fish. This comes from the fact that two species of tilapia are
native to lakes in Israel and are reputedly the fish that were caught by the Apostles and that
Jesus used to feed the multitudes as recounted in the Bible.

From the 1930’s to 1960’s various tilapia populations were widely distributed around the world
by missionaries, national governments, and international development agencies in efforts to
improve the nutritional welfare of poor farmers in developing countries. In the 1960’s and
1970’s the fish were further distributed into additional watersheds in many countries for use
as bio-control agents to reduce mosquito and aquatic weed populations in irrigation systems.
By the late 1980’s when commercial interests began to consider tilapia for aquaculture, one
or more species had already become established in virtually every tropical country and may
sub-tropical regions.

Domestication of the tilapias started in the 1950's and 1960's with groups working in several
countries. In the 1980's and 1990's several sophisticated breeding programs were begun which
vastly improved the growth rates, average size and profitability of commercial rearing of tilapia.
These improved strains have now also been widely distributed. Tilapia have been important to
aquaculture because of the ease with which they can be bred in captivity and the wide variety
of water conditions in which fish can grow. The tilapia species grown in aquaculture evolved
in ephemeral waters in Africa and the Middle East that are subject to wide swings of environ-
mental conditions as the water evaporates away in the dry season and then greatly increases
in volume and water quality in the wet season. As water evaporates, fish populations density
increases, along with salinity and ammonia concentration, dissolved oxygen decreases, while
pH and temperature go through diurnal swings. When the rains come and water quality and
volume improves, the fish spawn. Thus, various strains can be grown in water varying in
salinity from fresh water to full strength seawater (35 ppt). They will grow in water ranging
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from acidic (pH of 5) to alkaline (pH of 9). Tilapia can survive low dissolved oxygen (<2 mg/l)
and high ammonia levels (50 mg/l) for longer periods than most other fish. Consequently, they
can be grown in densities greater than virtually any other kind of fish. These characteristics
make them ideal for aquaculture.

Another characteristic that facilitates selective breeding and domestication is their reproduc-
tive behavior. The tilapias used in aquaculture are maternal mouthbrooders. A female lays
her eggs in a simple nest prepared by the male, the male fertilizes the eggs and then the female
picks the eggs up and incubates them in her mouth. Even after eggs hatch, fry will remain in
the mother’s mouth. Once the fry are free-swimming they will return to her mouth for protec-
tion. Females can produce several hundred to several thousand young per spawn. The high
levels of parental care allows breeders to quickly raise thousands of young for directed selection
or for stocking into production units. Another advantage is that the adults become sexually
mature in less than six months, when they are still a fraction of their potential size. This is an
additional advantage for selective breeding, allowing several generations to be produced in
the time it takes other fish to reach maturity. The drawback to this high potential for reproduc-
tion is that tilapia introduced to new (exotic) locations can quickly spread and impact native
fish populations. Likewise in production ponds without predators, tilapia can over-populate
ending up with large number of small, stunted fish. This can present a serious problem for
aquaculturalists who are attempting to rear a large size fish for market.

Eggs of tilapia are relatively large and fry are hardy and omnivorous. Fry readily feed on a
variety of foods including periphyton and phytoplankton, zooplankton and powdered feed.
This allows the culturist to further manipulate spawning by removing the young from the fe-
male and raising them independent of the mother. Removal of fry will encourage the female to
begin eating again; she eats little while brooding, and will be ready to spawn again in several
weeks. Sex of the fry can be manipulated in several ways. Undifferentiated sexual organs of
juvenile tilapia can be induced to produce phenotypic all male or all female populations. Males
grow more rapidly and crops of primarily males will avoid problems associated with unwanted
spawning. There are several methods and reasons for this “sex-reversal”. Untreated gonads
are depicted in this volume for references purposes.

Another reason that tilapia are prized as aquaculture species is because they are herbivorous
or omnivorous, depending on the species. In nature, tilapia receive all of their nutrition from
algae, higher plants, detrital matter and/or small invertebrates. This makes it easy to grow
the fish in ponds with minimal inputs of feed or fertilizer in extensive aquaculture. If semi-
intensive systems are used to generate greater production from a facility, fertilizers can be used
to produce algae and zooplankton. In intensive production, feeds containing primarily plant
proteins can be fed. These inputs are considerably less expensive than the costly feeds con-
taining high percentages of fish meal or other animal proteins that must be fed to carnivorous
fish. Consuming herbivorous fish is a more ecologically efficient transfer of energy and protein
to human consumers than using carnivorous fish that require fish or their animal proteins in
their diets.
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These are just a few of the reasons that tilapia have become one of the most important domes-
ticated fish around the world. The authors felt that it was important to provide a reference
work that demonstrated the normal anatomy and histology. Trewavas (1983) provides excellent
descriptions and line drawings of external anatomies of the tilapia species, including teeth, fin
configurations, and gill rakers. However, internal anatomy is not included in that reference.
Oreochromis niloticus was selected to depict the normal healthy condition as it accounts for more
than 70% of global fish production and even higher when its hybrids are included. Although
the fish is very hardy, the increased densities, polyculture and unusual environments in which
the fish is reared, are exposing them to pathogens and stress conditions that lead to disease
conditions. It is our hope that this reference will allow farmers and fish health professionals
to make quicker and more accurate diagnoses. It should also be of interest to producers and
breeders who need references to compare “normal” fish with newly bred strains and hybrids
or fish reared in unusual environmental conditions.

This abstract was excerpted from the original paper which was in, Atlas of Tilapia Histology.
The World Aquaculture Society, Baton Rouge, USA. 96 pp.

CRSP RESEARCH REPORTS are published as occasional papers by the Program Management Office,
Aquaculture Collaborative Research Support Program, Oregon State University, 418 Snell Hall, Cor-
vallis, Oregon 97331-1643 USA. The Aquaculture CRSP is supported by the US Agency for Interna-
tional Development under CRSP Grant No.: LAG-G-00-96-90015-00 and by collaborating institutions.
http://pdacrsp.oregonstate.edu