International Journal of Aquaculture, 2015, Vol.5, No.22 1
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7
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40 % moisture level. Diet was passed through a
mincer with die into 3-mm diameter spaghetti - like
strands and was dried under sun for 8 h. After drying
the diet was broken up to sizes and sieved into
appropriate pellet sizes. Fish were hand fed initially
with the small size diet (1.2 mm) at 3 % of body
weight (8:00 am and 5:00 pm). The total daily food
was divided to two equal amounts two times daily.
The total biomass of fish in each cage was used to
readjust the food quantity downwards from 3 to 2 %
body weight daily from week twenty until the end of
the study. The reduction was based on changes in
satiation feeding. After the tenth week the food was
changed to the second size (3 mm dia.) until the
termination of the experiment.
Sampling
Sixty percent (by number) of the fish in each cage was
randomly sampled every four weeks by partially
lifting the cage netting and removing a sample of fish
with a dip net. The purpose was to determine fish
growth in length and weight. On each sampling dry
individual fish from each cage were weighed in grams
using a 2-kg portable balance manufactured by
(OHIOUS). The scale was calibrated in 1-g gradations.
The total length in cm of each weighed fish was also
measured. Mean fish weight at each period was
calculated by dividing the total biomass by the
number of fish in each cage. The number of fish in
each cage was also recorded to provide an estimate of
mortality rate. After 196 days the total weight (kg) of
survivors in each cage was recorded.
Water quality parameters in the area of the cages were
measured during the study as following: water
temperature and dissolved oxygen were measured
every day using a YSI Model 58 oxygen meter. Total
ammonia and nitrite were measured twice weekly using
a DREL, 2000 spectrophotometer. Total alkalinity and
chloride were monitored twice weekly using the
titration method, pH was monitored twice weekly using
an electronic pH meter (pH pen; Fisher Scientific,
Cincinnati, OH). During the 28-week rearing trial, the
water-quality parameters averaged (
SD): water
temperature, 26.4
0.8 C: dissolved oxygen, 5.7
0.5
mg/l : total ammonia, 0.20
0.14 mg/l : nitrite, 0.07
0.05 mg/l : total alkalinity, 189
46 mg/l : chlorides,
575
151 mg/l : pH, 8.5
0.16. All parameters were in
the educate levels for rearing Nile tilapia.
Growth performance and feed conversion were
measured in terms of final individual fish weight (g),
total length (cm), survival (%), specific growth rate
(SGR, %/ day), total production, net production, feed
conversion ratio (FCR), protein efficiency ratio (PER),
and food intake. Growth response parameters were
calculated as follows: SGR (% /day) = ({Ln W
t
- Ln
W
i
}/ T ) x 100, where W
t
is the weight of fish at time t,
W
i
is the weight of fish at time 0, and T is the rearing
period in days : FCR = total dry feed fed (g) / total wet
weight gain (g): PER = wet weight gain(g) / amount
of protein fed (g): Food intake = ( % of body weight)
(Richardson, et al., 1985).
Statistical analysis
Data were analyzed by analysis of variance (ANOVA)
using the analysis of variance (ANOVA) procedure
Statistical analysis system (SAS, 1988). Duncan’s
multiple range test was used to compare differences
among individual means (Duncan, 1955). Treatment
effect were considered significant at P
0.05. All
percentage and ratio were transformed to arcsin values
prior to analysis (Zar, 1984).
Results
Growth performance, production and survival rate of
mono sex male Nile tilapia,
O. niloticus
(Average
initial wt. 30.2 g/fish) reared in floating cages for a
period of 196 days at different stocking rates are
presented in Table 2 and Figures 1-3. There were
insignificant differences in initial weights and lengths
among the stocked fish across the four groups. Fish
survival rate, which was 100%, was also not significantly
different among the fish groups. During experimental
period (196 days), stocking density of 25 fish/m
3
showed significantly (
P
<0.05) heavier final body
weight and body weight gain percentage, compared to
the highest stocking density as 100 and 150 fish/m
3
. A
similar trend of significant effect of stocking rate
(
P
<0.05) was detected with final length and length
gain percentage. On the other hand, the higher total
crop and net production of fish with significant
differences (
P
<0.05) were observed with high density
group, 150 fish/m
3
. This study revealed that the specific
growth rate (SGR) and daily gain were significantly
higher (
P
<0.05) with low density group (25 fish/m
3
)
than that of other tested fish groups. Also, the results
showed that, the highest fish density group (150 fish/m
3
)
consumed more feeds and had lower FCR compared
