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International Journal of Aquaculture, 2014, Vol.4, No.16 96
-
101
http://ija.biopublisher.ca
97
Manissery
et al.,
(2001) and Muchlisin
et al
., (2006)
showed that nutrition plays a major role in the
reproductive performance of fish. Early sexual
maturity in tilapias can be achieved by feeding them
commercial feed with all the required nutrients to
support growth. In Tilapia species sexual maturity
advances in stages and gonad maturation has been
categorised into 6 stages where stage (I) consists of
immature or virgin fish, stage (II) are fish beginning
maturation, (III) is a developing phase, (IV)
pre-spawning stage (V) spawning stage and (VI) post
spawning stage (Duponchelle and Legendre,1996;
Nyakuni, 2009). The present study, focused on the
role played by commercial feed on sexual maturity of
Oreochromis niloticus
and
Oreochromis tanganicae
.
1 Materials and Methods
The experiment to determine the effect of commercial
feed (Namfeed) on sexual maturity of
Oreochromis
niloticus
and
Oreochromis tanganicae
(mean wt. 30g)
,
was conducted in six hapas placed in semi-concrete
outdoor ponds for 12 weeks (90 days) at the National
Aquaculture Research and Development Centre
(NARDC) in Mwekera on the Copperbelt Province,
Zambia.
The study was a two by two factorial run in a
Completely Randomized Design (CRD) with three
replicates. Fish were sampled by using a scooping net
at fortnight intervals. Weight and length were
measured using an analytical balance and measuring
board, while maturity was checked by stripping or
massaging the fish on the belly.
Water quality parameters were taken and the figures
recorded using a Horiba U-10 water quality checker
that measures six parameters: pH, Temperature,
Dissolved Oxygen, Electrolytic Conductivity, Turbidity
and Salinity. The water quality checker sensor
measures by being directly submersed in the water as
opposed to collecting samples. pH, water temperature
and dissolved oxygen were measured twice per day, in
the morning (at 08:00 hours) and afternoon (at 14:00
hours) by dipping a probe of the oxygen meter about
20 cm into the water.
One-hundred and eighty (180) juvenile
Oreochromis
niloticus
and
Oreochromis tanganicae
, with an
individual average weight of 30g were used in the
experiment. 30 fingerlings, locally provided by the
NARDC, were randomly distributed among the hapas
measuring 4m×2m×1m. Fish were acclimatized to the
rearing environment for a period of two weeks before
the beginning of the trial.
Throughout the experimental period, fish were fed
with a commercial feed (Namfeed) at the rate of 5% of
their total biomass, twice per day (at 09:00 and at
15:00hrs) for 5 days in a week. Clove (herbal spice)
was used to anesthetise the fish to minimize stress
during sampling. At the beginning and at the end of
the experiment 10% of the fish were individually
weighed and dissected to remove the gonads. The
gonads were weighed using the analytical balance and
results recorded for the calculation of GSI. The
relative gonad weight or gonadosomatic index, an
index of reproductive maturity was calculated by
using the formula: GSI = (Gonad weight (g)/Body
weight of fish (g) × 100 according to Singh and
Dhawan (1996). Gonadosomatic index has been
considered as reliable estimate for gonadal maturity
and spawning of any species. The gonadosomatic
index increased with the maturation of fish and
reaches to its maximum at the peak period of maturity
(Mishra and Saksena, 2012).
Fish were anesthetised using Clove (herbal spice) to
minimize physical injury and stress. Stripping of
sperms and eggs method was used to determine the
stages of sexual development in fish. The fish were
stripped by applying gentle pressure to the abdomen
between the pelvic fin. When the eggs or milt came
out with pressure on the abdomen, the fish was
considered to have reached maturity.
Fish belonging to maturity stage onwards were
considered as mature and used for the purpose of
calculating the size at first maturity.
The water quality parameters included were: water
temperature, dissolved oxygen and pH.
The statistical model used in the experiment was as
follows:
Y
ij
= µ + τ
i
+ β
j
+ (τβ)
ij
+ ε
ij
Where: Y
ij
= observed response variables
µ= overall mean
τ
i
= the effect of feed on
O. niloticus
(i=1,2)