Tilapias
(Cichlidae) are natives of Africa. They have
been introduced into a large number of tropical and subtropical countries
around the world since the 1960s (Pillay, 1990). It is also commercially known as Mango
fish or Nilotica.
Tilapias have been called as the “everyman’s fish” (Pullin, 1985). Nile tilapia (Oreochromis
niloticus) is one of the first fish species that was cultured in the world
(Pompa and Masser, 1999). Tilapia was first introduced in Bangladesh in
1954. About 80 species of tilapia have been described out of which 10 species
are reported to be used for culture (Macintosh and Little, 1995). Until the late 1970's the
tilapias, were all classified into a single genus, Tilapia, however most taxonomists now classify them into three
genera, Tilapia, Saratherodon and Oreochromis according to their breeding
behaviour. The existing strain Nile
tilapia (Oreochromis niloticus L.)
was first introduced into this country by the United Nations International
Children Emergency Fund (UNICEF) in 1974, and later by the Bangladesh Fisheries
Research Institute (BFRI) from Thailand
(Gupta et al. 1992). It is a species of high
economic value and is widely introduced outside its natural range; probably
next to Oreochromis mossambicus, it is the most commonly
cultured cichlid.
Another
promising Genetically Improved Farmed Tilapia strain known as GIFT (Eknath et al. 1993) has recently been
introduced in July 1994 from the Philippines. The GIFT strain was
developed by the
International Center for Living
Aquatic Resources Management
(ICLARM) through several generations of
selection from a base population involving
eight different strains of
Nile tilapia, Oreochromis niloticus (Eknath et
al. 1993). It performs 60% better growth and 50% better survival than the
commercially available strains of tilapia (Hussain et al. 2000). GIFT have generally performed as well or better, and
in some countries much better, than existing farmed tilapia. GIFT and
GIFT-derived strains have proven to be good genetic material for continued
selective breeding.
The
GIFT strain has, meanwhile, proved to be very suitable fish for aquaculture in Bangladesh. The
desirable characteristics of this genetically improved strain are as
follows: High yielding, Excellent
breeder, Efficient converter of organic and agricultural wastes in to high
quality protein, Resistant to disease, Very hardy, Tolerant to over crowding
conditions, Able to grow in either fresh or brackish water.
On
the other hand, NEW GIFU tilapia is the 11th strain of Oreochromis niloticus was introduced in Bangladesh from
China,
which is invented by Dr. Li Sifa, professor, Shanghai Fisheries University, China.
The NEW GIFU tilapia strain was introduced in Bangladesh by Bangladesh
Fisheries Resources Institute from China. In China, experiments showed that NEW
GIFU tilapia strain grew more than twice as fast as the local commercial
strain.
Tilapias
have distributed to so many different types of water, to so many different
types of culture systems in the world that they have been even labeled as the
“aquatic chicken” (Maclean, 1984). Tilapias are fast growers, prolific breeders
and can thrive in many unfavourable culture situations. They have good
resistance to poor water quality and disease, tolerance of a wide range of
environmental conditions, ability to convert efficiently the organic and
domestic waste into high quality protein, rapid growth rate and tasty flavour
(Ballarin and Hallar, 1982). Tilapias can tolerate DO concentration of 1mg/l
and can survive by using atmospheric oxygen when dawn DO concentration dropped
to less than 1mg/l (Chervinski, 1982). Tilapia has the ability to survive under
extremely low dissolved oxygen rich surface water layer by reducing activity
(Chervinski, 1982). Because of this,
tilapias are deemed as the source of animal protein in the diet of many people
of the world. Quite a few years have lapsed since tilapias entered into Bangladesh but
not many of our people could know the biology of this group of fish. They
prolifically reproduce in the culture system and very quickly compete for food
and shelter. At this stage, the culture system is seen to be full of fishes of
different sizes. This happens because when the population in the culture system
is very dense, the maturity age and size both get reduced; in other words,
denser is the population, more is the reproduction. At this stage, the fishes
start dying because of starvation on one hand and on the other hand, they start
eating the young ones due to cannibalism that develops in the adults. However,
due to these factors, very soon the population starts diminishing from the
culture system and at a later stage, completely vanishes away. People observing
such a phenomenon call this fish as a miracle fish, a wonder fish.
The
precautious maturity and frequent spawning in tilapias result in over dense,
stunted population in the culture system. Hence, control of reproduction is
necessary so that the stocking population could be maintained at stable
condition. On the other hand, if commercial culture programme of tilapias is
undertaken, larger numbers of broods have to be maintained for the production
of millions of fry, since the fecundity of cultured tilapias is low. Most of
the cultured tilapias spawn in monthly intervals or so. However, the spawning
frequency can be doubled if the eggs are taken out from the female’s mouth and
artificially hatched in incubators. The problem of over population due to
uncontrolled reproduction in the culture system can be overcome by adoption of
monosex culture i.e., to culture of fishes of one sex only. In tilapias, the
males grow faster than the females. Thus, the culture of males in tilapia
profitable. Separation of male fry from the mixed brood is not possible because
identification of sex in tilapia at the early stage is not possible. Thought it
is possible at a latter stage, substantial risk of wrong identification exists.
Efforts thus went on in different ways for the production of monosex male seed
of tilapia for commercial culture. Some inter-specific crosses in tilapias
produce nearly 100% male hybrids. Several such crosses have been used to
produce male hybrid seen in many countries of the world, especially in Israel and Taiwan.
Unfortunately, as such crosses do not always produce 100% male hybrids, the
prospect of the treating the progeny with androgenic steroid hormones to induce
sex change in females. This method appeared to be the main method for
successful large scale production of male fry in tilapia. With this method, the
amount of hormone which is used in the feed does not have any residual effect
to pose a threat to the health of consumers.
The
choice of conversion of sexes (either all males or all females) depends on
growth performance characteristics of individual sexes of fish species. For
instances, in tilapia males grow faster than female masculinization using
androgen hormones (Shelton et al.
1978; Guerrero, 1979 and Guerrero, 1988) and in case of salmonids and
cyprinids, where females grow faster than males, feminization using estrogen
hormone (Shelton, 1987) have become a popular practice.
Tilapias
have great potential in Bangladesh
and they are going to be the prime culture species in near future for
freshwater and brackish water ecosystem. It is expected that about 50% of total
aquaculture production can be met up by tilapia farming. Among the Asian
countries, very soon Bangladesh
will be one of the leading countries in tilapia production.
Tilapia are shaped much like sunfish or crappie but can be easily
recognized by an interrupted lateral line characteristic of the Cichlid family
of fishes. They are laterally compressed and deep-bodied with long dorsal fins.
The forward portion of the dorsal fin is heavily spined. Spines are also found
in the pelvis and anal fins. There are usually wide vertical bars down the
sides of fry, fingerlings, and sometimes adults.
Besides
phytoplankton, tilapia will also eat zooplankton, detritus, aquatic plants,
insects and even small fish fry.
Commercial pellet, waste food and almost any other type of feed given,
with perhaps the exception of meat, is also eagerly devoured. Very little investment is, therefore,
required in their nutrition.
Male tilapias are usually larger than females of the
same age. The male has two body openings situated just forward of the anal
fins, of which one is the anus. The other is the opening of the urethra, at the
end of the genital papilla (an oval-shaped lobe just rearward of the anus),
from which milt (sperm) and urine are discharged.
The female has three body openings, of which one is
the anus. The genital papilla of the female has two openings. They are the
urethra, which is hardly visible to the naked eye and the opening of the
oviduct (a crescent-shaped slit), from which eggs are released.
These features are more visible and identifiable
when the fish have grown to 10–20cm in length and 100–150g in weight. Mature Nile tilapia can also be distinguished by their
coloration under the jaw- reddish in males and greyish in females.
Tilapia are
more tolerant than most commonly farmed freshwater fish to high salinity, high
water temperature, low dissolved oxygen, and high ammonia concentrations.
All tilapias
are tolerant to brackish water. 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 brackish
water up to 20 ppt salinity, and the Mozambique tilapia grows well at
salinities near or at full strength seawater. Therefore, the Mozambique
tilapia is preferred for saltwater culture. Some lines of the Mozambique
tilapia reportedly have spawned in full strength seawater, but its reproductive
performance begins to decline at salinities above 10 to 15 ppt. The Blue and Nile tilapias can reproduce in salinities up to 10 to 15
ppt, but perform better at salinities below 5 ppt. Fry numbers decline
substantially at 10 ppt salinity.
The
intolerance of tilapia to low temperatures is a serious constraint for
commercial culture in temperate regions. The lower lethal temperature for most
species is 50 to 52o F for a few days, but the Blue tilapia
tolerates temperatures to about 48o F.
Tilapia
generally stop feeding when water temperature falls below 63o F.
Reproduction is best at water temperatures higher than 80oF and does
not occur below 68o F. In subtropical regions with a cool season, the number of
fry produced will decrease when daily water temperature averages less than 75oF.
Optimal water temperature for tilapia growth is about 85 to 88oF.
Growth at this optimal temperature is typically three times greater than at 72o
F.
Tilapia
survive routine dawn dissolved oxygen (DO) concentrations of less than 0.3
mg/L, considerably below the tolerance limits for most other cultured fish. In
research studies Nile tilapia grew better when
aerators were used to prevent morning DO concentrations from falling below 0.7
to 0.8 mg/L (compared with un aerated control ponds). Growth was not further
improved if additional aeration kept DO concentrations above 2.0 to 2.5 mg/L.
Although tilapia can survive acute low DO concentrations for several hours,
tilapia ponds should be managed to maintain DO concentrations above 1 mg/L.
Metabolism, growth and, possibly, disease resistance are depressed when DO
falls below this level for prolonged periods.
In general,
tilapia can survive in pH ranging from 5 to 10 but do best in a pH range of 6
to 9.
Massive
mortality of tilapia occurs within a few days when fish are suddenly
transferred to water with unionized ammonia concentrations greater than 2 mg/L.
However, when gradually acclimated to sub-lethal levels, approximately half the
fish will survive 3 or 4 days at unionized ammonia concentrations as high as 3
mg/L. Prolonged exposure (several weeks) to un-ionized 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 concentrations as low as 0.2 mg/L. Un-ionized ammonia begins to
depress food consumption at concentrations as low as 0.08 mg/L.
Nitrite is
toxic to many fish because it makes the hemoglobin less capable of transporting
oxygen; chloride ions reduce the toxicity. Tilapias are more tolerant of
nitrite than many cultured freshwater fish. When dissolved oxygen concentration
was high (6 mg/L) and chloride concentration was low (22 mg/L), the nitrite
concentration at which 50 percent of the fish died in 4 days was 89 mg/L as
nitrite. In general, for freshwater culture the nitrite concentration should be
kept below 27 mg/L as nitrite.
Oreochromis species usually seek
out shallow areas
and group together
for breeding. Males develop
bright colouration and set up territories in which they build their nest: they
hollow out a small area on the pond bottom approximately 10–15cm in diameter.
They display courtship behaviour and lure females to the nesting site. The nest
is used as temporary site for courtship. If the female is receptive to the
courting male, she will be induced to spawn.
In the nest, eggs are laid and fertilized by the male, who
discharges sperm over the eggs. The female collects the fertilized eggs in her
mouth, and mouth-broods the eggs for around 6–10 days. After hatching, the
newly hatched fry continue to shelter in her mouth for another 4–7 days. The
fry begin to swim freely in schools, but may return to the mouth of the mother
when threatened. Females do not feed while they are incubating the eggs or
mouth-brooding the newly hatched fry.
Male
tilapia can mate with several females (polygyny) and females may mate with
several males (polyandry). In ponds, Nile
tilapia becomes sexually mature at three to five months of age (150–200g
weight). As soon as
sexual maturity is attained, most female
tilapia are able
to undergo successive spawning to produce a new brood every 4 to 6 weeks. Temperature plays an important
role here. If water temperature remains
at 22°C and above, tilapia will spawn throughout the year. Temperature in the
range of 25–30°C is considered ideal.
The
number of eggs per clutch increases as the female fish grows. On average each
clutch of eggs will produce 100–500 fry. As female fish get older (more than
one year old) they produce less fry compared with when they were younger. The
best size of female for breeding is 150–300g. On average a 200g breeder would
produce 200–500 fry per month. Fry production will also depend on the condition
and health of the breeders. In ponds or any confined conditions, egg size and
clutch size may vary.
Kohinoor
et al. (2003) studied the breeding
biology and monosex male seed production of GIFT strain of Nile Tilapia (Oreochromis niloticus) in hatchery under
different conditions in Bangladesh.
But comparison of egg production, hatching rate, fry survival rate and
percentage of monosex fry production between GIFT tilapia strain and NEW GIFU
tilapia strain have never been reported. So the present investigation was
undertaken to find out the comparison of egg production, hatching rate, fry
survival rate and percentage of monosex fry production between GIFT tilapia
strain and NEW GIFU tilapia strain in Bangladesh.
From
the above discussion it is evident that mono-sex male tilapia is fast growing
and increased total production. Therefore, the present study was undertaken to
know the performance of mono-sex tilapia production in a hatchery.