الفهرس 
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Abstract
This study was carried out at the intensive fish fanning of Maryut
Aquaculture company in Alexandria governorate. Florida red tilapia (T.
mossambicus male x T. hornorum female) were used to define and
quantify growth performance of this hybrid under condition of northern
western coastal :Zone of Egypt. The main objective of this research is to
investigate the combined effect of stocking density and initial weight in
addition to sex, colour segregation and water quality on gorwth
performance.
The experiment of this study lasted for a period extended from 20
of April 1996 to the 5th of October 1996. The fish culture system of
Maryut farm consists of four square concrete ponds of 750 m
3 water volume for· each, three pond were only used (2nd ,3rd and 4th pond). The water supply of the farm was derived from lateral canal of Maryut
drainage. The ponds were aerated by three submersible pumps. Twenty
percent of the water volume of each pond was exchanged each day and all
the water volume was completely changed once a week.Fingerlings kept in green houses were stocked after being graded
according to fish weight to three homogenous groups. Pond 2 was stocked
with 50000 fingerlinges with stocking mean weight of 8.6 gm; pond 3 was
stocked by 40000 fingerlings with stocking mean weight of 18.4 gm and
pond 4 was stocked by 55000 fingerlings with stocking mean wiehgt of
2.69 gm.
The fish were fed three times daily at 8.0, 12.0 and 17.0 hours on
pelleted ration 30% protein related to fish size.Random sample of 50 fish was taken from each pond 15 day post
stocking and, thereafter every 15 days. Colour, sex, body weight (gm),
body length (em) and body depth (em) of each fish sample were recorded.
Water sample were taken every two weeks before exchange of all
water ponds at 11.00 a.m. to determine water parameters (salinity,
ammonia, nitrite, nitrate, phosphorus, orthophosphorus, alkalinity and
hardness). While the water temperature, dissolved oxygen and pH were
measured directly from the water ponds.
Results show that fish of pond 3 were always the heaviest in weight,
then followed, in most, by those of pond 2 and pond 4 in a descending
order. TIns might be attributed to that pond 3 was stocked by fingerlings of
the heaviest mean stocking weight (18.4 gm) at the lowest stocking density
(40000 fingerlings), pond 4 was stocked by fingerlings of the smallest
mean stocking weight (2.69 gm) and the highest stocking density (55000
fingerlings) and pond 2 was stocked with fingerlings of in between those
of pond 3 and pond 4, in mean stocking weight and stocking density (8.6
gm and 5000 fingerlings, respectively). The effect of pond on growth
parameters of body weight was mostly non-significant.
Fish of the normal colour ranked first in this respect, i.e. recorded
the heaviest weight during the experimental period then followed by those
of spoted, red and alpino colours in a descending order. The mean of initial
body weight for normal spoted, red and alpino coloured fish, respectively
was 17.7, 16.6, 15.2 and 5.4gm. The corresponding means affinal body
weight were 91.6,81.5,80.8 and 78.8gm. No significant effect for colour
segregation on body weight parameters of the fish of the study was
detected.Initial mean weight of male and female were 13.3 and 14.1 gm,
respectively. Fish weight at the end of the rearing period reached 85.0 and
72.1 gm for males and female, respectively. Males weighed more than
females almost all the time of the rearing period although the difference
was not signif,icant.
Effect of stocking density, initial body weight, sex and colour on
body length and depth of red tilapia showed similar trends to those on
body weight.
Specific growth rate was the highest (1.8930/0) for fish of pond 4
(i.e. the pond with the lowest mean initial weight and the highest stocking
density) and the lowest (1.0840/0)for fish of pond 3 (Le., the pond with the
highest initial weight and the lowest stocking density). Fish of pond 2 (i.e.
with the inbetween pond 4 and pond 3 for mean initial weight and
stocking density recorded intermediate specific growth rate (1.465%).
Condition factor averaged 2.471 for pond 2, 2.395 for pond 3 and
2.399 for pond 4.
Water temperature of the three pond ranged from 23.1 °C in April to
30.80C in June with an average of26.5°C. The effect of water temperature
on daily gain in weight was not significant. It might be due to that the
range of that temperature was in the comfortable zone for red tilapia.
Dissolved oxygen ranged from 5.3 mg/L in August to 10.6 mgIL in
April with an average of 7 .6 mgIL. The effect of dissolved oxygen on daily
gain in weight was significant.
The water salinity ranged from 5ppt in April to 17 ppt in August.
This range of salinity was found to be suitable for rearing of red tilapia.
Regression of daily weight gain on salinity was not significant.
The range of pH of water of the three pond was between 7.8 and 8.5
with an average of 8.2. This value was recommended by many
investigators to develop the aquatic life in order to increase the natural
food in ponds. The effect of pH on daily weight gain was not significant.
’Water content ranged from 0.03 to 0.30 mg/L for ammonia, from
0.0 to 0.7 mg/L for nitrite, from 0.00 to 0.10 mg/L for nitrate, from 0.00 to
0.06 mg/L for phosphorus and from 0.0 to 0.18 mg/L for orthophosphate.
The effects of water concentration during the rearing period from
ammonia, nitrite, phosphorus and orthophosphate on daily weight gain of
red tilapia were significant (P<O.05, p<O.OI or p<0.001), while those of
nitrate were not significant.
Total alkalinity ranged from 51.30 to 205.2 mg/L. This range is
demonstrated as a satisfactory range for most aquaculture purpose.
Analysis of variance showed that alkalinity had no significant effect on
daily gain in weight during the rearing period Hardness value ranged from 87.2 to 18.65 mg/L. and did not show
any significant effect on daily gain in weight during the experimental
period Production of red tilapia was 1457.05, 1873.6 and 1279.6 kg for
pond 2, pond 3 and pond 4, respectively with a total amount of 4610.25 kg
for the three ponds. These results reveal that fish productivity varied with pond and this is mainly due to differences in initial mean weight and in stocking density.Fish of the 3rd pond (i.e. of the highest initial weight and the lowest
stocking density) had the highest productivity and the highest individual
weight.
Weight gain of red tilapia attained during the rearing period from
April 20th to October 5th was 3296.3 kg. Feed intake amounted to 10004
kg of artificial feeding, feed conversion ratio was 3.035 kg feed/kg gain.
Economic evaluation showed that the fixed costs ( costs of pond
construction aerated pumps and annual maintenance) were 3000 L.E. The
variable costs (seed stock, artificial food, power and labour) were 33806
L.E.. The total costs includingvariable and fixed costs were 36806 L.E.
Net profit an,d net profit percentage 9296.5 L.E. and 25.2% respectively.
Net profit percentage was considered high when compared with other
investments.