International Journal of Aquaculture, 2015, Vol.5, No.3 1
-
13
10
and deterioration of water quality in the experimental
tanks during the course of the experiment. This is in
agreement with that of El-Saidy and Gaber, 2002b.
Siddiqui et al. (1991) reported that effects of water
exchange on water quality in out door concrete tanks
stocked with Nile tilapia fingerlings were with respect
to dissolved concentrations and unionized ammonia
levels deteriorated with batch flow. In tanks with flow
through system, oxygen concentrations above 3.9 mg/L
were maintained, as the metabolites, excreta and left
over more continuously removed and incoming water
was rich in oxygen, however, the ammonia levels for
different water exchange rates were not significantly
different from each other. Ng. et al. (1992) found that
the quality of water supplied to the fish rearing areas,
even without filtration to be acceptable at the present
fish stocking levels. The discontinuous operation of
pumps for 9h a day, which resulted in only 8%
exchange of water through the fish rearing volume,
seemed to be adequate for the existing fish
population.
Besides water quality, the effect of stocking density on
tilapia fingerlings might be dependent upon the
biological characteristics of fish, such as, tolerance to
environmental change, life stage, sex, social
interaction and behavior, so that the density effect on
growth and production might be explainable by their
competition for territories, with similar case found for
African catfish (Haylor 1991). Behavioral studies on
red tilapia indicating that growth inhibiting
antagonistic behavioral patterns was generally
unabated even at the highest stocking density (Suresh
and Lin 1992). The stress on fish caused by the
crowdedness may be the other explanation for the
effect of stocking density. Hogendoorn and Koops
(1983) also found that the highest biomass (Harvest)
was achieved at the highest stocking density for
African catfish cultured in ponds. Culture of Nile
tilapia,
O. niloticus
in cages showed that the highest
stocking density (100 fish /m
3
) achieved the highest
biomass after five and half months (Daungsawasdi et
al. 1986). In our experiment, the highest biomass was
achieved at stocking density of 100 fish /m
3
at either
8L/min. or 12 L/min. water exchange rates.
It has been found that the growth of Nile tilapia was
affected significantly by the stocking density not by
the water exchange rates. Fish reared at low density
grow better than those reared at high density (Table 2),
and the differences were highly significant. Final
mean weight were inversely proportional to stocking
density, which was particularly evident when average
weight of fish reared at the lowest stocking density
was significantly different from weight of fish reared
at the higher densities. Youssif (2002) studied the
effects of stocking density and water exchange rate on
size variation of juvenile
O. niloticus
. He found that
the fish subjected to the lowest stocking and highest
water exchange rate achieved the best final body
weight. Also, he reported that manipulation of water
exchange rates and hence water quality was found to
be an effective approach for minimizing the adverse
effects of high stocking densities of juvenile
O.
niloticus
. Baker and Ayles (1990) resulted that it is a
generally accepted principal that increasing the
number of fish or reducing water turn over (lowering
water quality) will adversely affect fish growth.
Stocking density also affected the growth of
C.
macrocephalus
x
C. gariepinus
hybrids cultured in
concrete ponds at three different densities (Jarimopas
et al. 1999). Fish reared at the highest density had the
lowest final mean weight. These results may be
attributed to the fish at low density consume
maximum amount of food available and growing fast
(Essa and Nour, 1988). Also, Hepher et al. (1989)
reported that slow growth of fish at high density was
probably due to that the individuals disturbing each
other during feeding and normal activity. Holm et al
.
(1990) attributed the decrease in growth rate with
increasing density to the reduced food consumption
and thereby the feed efficiency ratio. The data on feed
conversion ratio given in Table (6) confirm this
finding, whereas the fish reared at low density and at
either 8 L/min. or 12 L/min. water exchange rates
possessed the better feed conversion (the fish used
less feed to produce one unit of gain in body weight)
than those reared at high density and at either 8 L/min.
or 12 L/min. water exchange rates.
While final harvest and production values were
directly related to stocking density, there must be
some density at which mortality is severe for a variety
of causes and growth rate is reduced. When this
occurs, production will be reduced. This critical level
was not reached in our experiment although the
stocking density of 100 fish/m
3
was high. One reason
for the ability of Nile tilapia to maintain high
