A new guide looks at breeding and hatchery techniques including stocking densities for Nile tilapia.

GLOBAL - A new guide looks at
breeding and hatchery techniques
including stocking densities for Nile
tilapia.
Breeding of tilapia is conducted in
ponds, tanks or hapas. The stocking
ratio for females to males is 1-4:1 with
2 or 3:1 being the most common.
The brood fish stocking rate is variable,
ranging from 0.3-0.7 kg/m2 in small
tanks to 0.2-0.3 kg/m2 in ponds. The
popular hapa-in-pond spawning system
in Southeast Asia uses 100 g brood
fish stocked at 0.7 kg/m2. Spawning
ponds are generally 2000 m2 or
smaller. In Southeast Asia, a common
hapa size is 120 m2.
Brood fish are given high quality feed at
0.5-2 per cent of body weight daily.
Swim-up fry gather at the edge of a
tank or pond and can be collected with
fine-mesh nets. Fry collection can
begin 10 to 15 days after stocking.
Multiple harvests (six times per day at
five day intervals) are conducted up to
a maximum of eight to 10 weeks before
pond drainage and a complete harvest
is necessary.
Tanks must be drained and recycled
every one to two months because
escaped fry are very predaceous on fry
from subsequent spawns. Alternatively
tanks or ponds are harvested
completely after a 2-4 week spawning
period. Production of optimum-sized
(<14 mm) fry ranges from 1.5 to 2.5
fry/m2/day (20 to 60 fry/kg female/
day).
In the South East Asian hapa method,
fish are examined individually every five
days to collect eggs.
This system is much more productive,
but it is labour intensive. Brood fish are
more productive if they are separated
by sex and rested after spawning.
Sex-reversal
Commercial tilapia production generally
requires the use of male monosex
populations. Male tilapia grow
approximately twice as fast as females.
Therefore, mixed-sex populations
develop a large size disparity among
harvested fish, which affects
marketability.
Moreover, the presence of female tilapia
leads to uncontrolled reproduction,
excessive recruitment of fingerlings,
competition for food, and stunting of
the original stock, which may not reach
marketable size. In mixed-sexed
populations, the weight of recruits may
constitute up to 70 per cent of the total
harvest weight. It is therefore necessary
to reverse the sex of female fry.
This is possible because tilapia do
become sexually differentiated for
several days after yolk sac absorption.
If female tilapia receive a male sex
hormone (17 a methyltestosterone, MT)
in their feed, they will develop as
phenotypic males.
Fry collected from breeding facilities
need to be graded through 3.2 mm
mesh material to remove fish that are
>14 mm, which are too old for
successful sex reversal. Swim-up fry
are generally <9 mm.
MT is added to a powdered commercial
feed or powdered fish meal, containing
>40 per cent protein, by dissolving it in
95-100 per cent ethanol, which is
mixed with the feed to create a
concentration of 60 mg MT/kg feed
after the alcohol has evaporated.
The alcohol carrier is usually added at
200 ml/kg feed and mixed thoroughly
until all the feed is moist. The moist
feed is air dried out of direct sunlight,
or stirred in a mixer until dried, and
then stored under dark, dry conditions.
Androgens break down when exposed to
sunlight or high temperatures. Fry are
stocked at 3000 to 4000/m2 in hapas
or tanks with water exchange. Stocking
densities as high as 20,000/m2 have
been used if good water quality can be
maintained.
An initial feeding rate of 20-30 per cent
body weight per day is gradually
decreased to 10-20 per cent by the end
of a three to four week sex-reversal
period. Rations are adjusted daily, and
feed is administered four or more times
per day.
If sex-reversal is conducted in hapas,
the feed must be of a consistency that
allows it to float.
Otherwise a considerable amount of
feed would be lost as it settles through
the bottom of the hapa.
Sex-reversed fry reach an average of
0.2 g after three weeks and 0.4 g after
four weeks. The average efficacy of
sex-reversal ranges from 95 to 100 per
cent depending on the intensity of
management.
Hatchery
After sex-reversal, fingerlings are
generally nursed to an advanced size
before they are stocked into grow-out
facilities. This procedure increases
survival in the grow-out stage and
utilises growing space more efficiently.
Sex-reversed fingerlings are stocked at
approximately 20-25 fish/m2 in small
ponds and cultured for two to three
months to an average size of 30-40 g.
The ponds should be filled immediately
before stocking to prevent the build-up
of predaceous aquatic insects. Final
biomass at harvest should not exceed
6000 kg/ha. In ponds, fingerlings are
given extruded feed (30 per cent
protein) at an initial rate of 8-15 per
cent of biomass per day, which is
gradually decreased to a final rate of
four to nine per cent per day.
A series of small cages (<4 m3) with
increasing mesh size can be used to
rear advanced fingerlings.
Sex-reversed fingerlings can be stocked
at a rate of 3000 fish/m3 and grown for
six weeks until they average 10 g.
Fish of this size can be restocked at
2500 fish/m3 to produce 25-30 g
fingerlings in four weeks. These fish
can be stocked at 1,500 fish/m3 to
produce 50-60 g fingerlings in four
weeks.
A recirculation system stocked at 1 000
fish/m3 will produce 50 g fingerlings in
12 weeks. Fingerlings should be fed
three to four times daily.