الفهرس 
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Abstract
This study was conducted during a period of two years (1986
and 1987) in eighteen fresh water ponds in Abbassa Farm belongs
to Aquaculture Development Project, Abu-hamad district, Sharkia
Governorate. Ten rectangle 7.5 Fed. earthen ponds (350x75
meters) were used for the first year (1986), and other eight ones
of the same type were used for the second year (1987). This work
was carried out mainly to investigate the effect of mullet stocking
rate on growth traits (body weight and body length), fish
production, carcass traits and chemical composition of flesh for
tilapia, mullet and carp fish under the polyculture system followed
in earthen ponds. Species differences in all traits
studied were also investigated., In addi tion, specific growth
rate as well as all correlations between weights and lengths of
the fish at different growth stages from fingerlings (April-May
up to harvesting time (November or December) were calculated.
Results obtained could be summarized as follows:-
Growth Traits
Means and coefficients of variability
Means of body weight for tilapia fish in 1986 and 1987,
respectively, were 24.9 and 20.2 gm for fingerling stage at April
and May, 48.6 and 82.2 gm at July, 63.6 and 147 4 gm at August,
180.4 and 200.8 gm at September, 166.4 and 268.7 gm at October
and 267.3 and 288.3 gm at November. The corresponding means of
mullet body weight in 1986 and 1987, respectively, for the same
growth stages in the same order were 36.4 and 12.9 gm, 78.2 and
60.9 gm, 118.0 and 80.8 gm, 141.4 and 146.6 gm, 188.1 and 206.1
gm and 483.7 and 293.8 gm. For carp fish, means of body weight
at April-May, July, August, September, October and November of
1986 were 41.8, 155.6, 157.8, 595.5, 689.1 and 1266.8 gm, respectively.
The corresponding means estimated in 1987 were 21.7,
160.8, 229.0, 323.4, 362.9 and 516.4 gm. The average body weight
at December, the end of the first year of production (1986), for
tilapia, mullet and carp was 283.1, 535.8 and 1433.9 gm, respectively.
Differences between the averages of body weight at different
growth stages were generally in favour of 1987 for tilapia
fish but in favour of 1986 for mullet and carp fish. Coefficients
of variability in body weights of tilapia species in 1986 were
higher than in 1987. The same picture was generally shown by data
of carp species.
Rates of increase in body weight between the two years were
small at first and started in general to be sizable after July
for tilapia fish and after August for either mullet or carp fish.
Tilapia fish gained an average of 258.2 gm from May to
December in 1986 and 268.1 gm from May to November in 1987. The
corresponding average gains in the two years, respectively, were
499.4 and 280.9 gm for mullet fish and 1392.1 and 494.7 gm of
carp fish.
The averages of body length of tilapia fish in 1986 and
1987, respectively, were 7.5 and 8.3 em at April-May, 10.6 and
15.2 em at July, 13.4 and 19.1 em at August, 19.2 and 21.7 em at
September, 18.8 and 23.9 em at October and 24.6 and 24.4 em at
November. The corresponding averages for mullet body length in
1986 and 1987, respectively, at the same stages were 12.5 and 9.8
em, 16.7 and 17.1 em, 21.4 and 18.7 em, 24.7 and 22.7 em, 25.8
and 24.1 em and 32.7 and 26.1 em. For carp fish, the averages of
body length at April-May (fingerlings), July, August, September,
October and November 1986 were 10.7, 18.9, 21.5, 29.4, 32.4 and
42.5 em, respectively. The corresponding averages in 1987 were
8.8, 20.4, 22.9, 25.8, 27.5 and 30.8 em in the same order. The
average of body length at December in 1986 were 23.9, 33.4 and
44.0 em for tilapia, mullet and carp fish, respectively.
Body length of the fish of each of the three species of the
study differed with year of production and the differences were
generally in favour of the second year (1987) for tilapia fish,
and of the first year (1986) for mullet fish. For carp fish, the
means were in favour of the first year at August, September, October
and November. Coefficients of variability for body length
of tilapia and carp fish were higher in 1986 than in 1987 at most
growth stages studied, while the reverse was true for mullet
fish.
The rate of change in body length did not considerably differ
between the corresponding stages of the two years of the
study.
Tilapia gained an average of 16.4 cm in body length from May
to December in 1986 and 16.1 cm from May to November in 1987.
The corresponding gains in body length in the two years, respectively,
were 20.9 and 16.3 cm for mullet and 33.3 and 22.0 cm for
carp.
The increase in body weight and length was at its highest
level for carp fish and at its lowest level for tilapia fish in
the two years of production, indicating that fish of carp species
grew faster than fish of the other two species.
Factors affectinQ Qrowth traits.
Body weight and body length of tilapia fish under polyculture
system (with mullet and carp) used in this study varied with
mullet stocking rate (MSR) in the two years of production (1986
and 1987), the differences in body weight and length due to MSR
were significant (P<0.05 or P<O.Ol or P<O.OOl) in most growth
stages studied. The same findings were also observed when dealing
with either mullet or carp fish.
Differences in body weight and length of tilapia fish due to
mullet stocking rate were generally in favour of the lowest rate
in 1986 and 1987 from September to the end of the year. The same
picture was generally shown for body weight of mullet fish, in
1987 and for body weight and length of carp fish in 1986.
Differences in body weight and length of tilapia fish grown
under polyculture system during either 1986 or 1987 due to the
effect of mullet stocking rate tended to be pronounced after
three or four months from the time of stocking. Similar observations
were detected for mullet and carp fish.
Results of body weight and length when data of the three
species (tilapia, mullet and carp) were analysed together, showed
a general trend indicating the decrease of individual body weight
and length of the fish under the polyculture system with the increase
of mullet stocking rate. Differences in individual fish
weight and length due to differences in mullet stocking rates
were generally significant (P<O.05 or P<O.Ol or P<O.OOl) .
Least squares means of body weight and body length of either
tilapia, mullet or carp fish at different growth stages in each
of the two years of the study varied from pond to another within
the different mullet stocking rates used in each year and the
differences were mostly significant (P<O. 05 or P<O. 01 or.
P<O.OOl). The same results were also obtained when analysing the
pooled data of the three species together in either 1986 or 1987.
Body weight and body length of fish at different growth
stages of the study varied with species, the differences were always
significant (P<O.05 or P<O.Ol P<O.OOl) in 1986 and 1987.
Carp fish was heavier and longer than either tilapia or mullet
fish in each of the two years of the study. At the same time,
tilapia recorded the lightest body weight at most growth stages
in 1986 and intermediate weight between carp and mullet in 1987.
Similarly, tilapia recorded the shortest body length while mullet
was intermediate between carp and tilapia for the same trait at
all growth stages in 1986 and 1987.
Specific Growth Rate
Allover specific growth rate (SGR) of carp in 1986 and 1987
and of mullet in 1987 was higher than that of tilapia in 1986 and
1987 and that of mullet fish in 1986. The SGR of all species exhibited
relatively high values from July till November. However,
the highest SGR of tilapia or carp fish was observed in September
of 1986 and in July of 1987, while the highest SGR for mullet
fish was obtained in November of 1986 and in July of 1987.
Correlations Between Growth Traits
Coefficients of correlation between body weight and body
length of the fish at each stage from July up to the harvest time
in the two years were high and tended to increase with advance of
growth stage.
Correlation coefficients between body length at a particular
stage and body length of the successive stages were generally
higher than those between body length at that particular stage
and body weight of the successive stages of growth.
Correlation coefficients between body weight at a certain
stage and body weight at the next stage increased as the fish
grow older, the same picture could be seen when considering body
length.
Fish Production
Means and coefficients of variability.
Production means per pond and per feddan, respectively, for
tilapia fish at the harvest time were 1183.2 and 190.7 Kg in 1986
and 615.1 and 59.4 Kg in 1987. The corresponding means were
495.0 and 79.8 Kg in 1986 and 416.9 and 67.1 kg in 1987 for mullet
fish while were 1660.7 and 266.7 Kg in 1986 and 1678.9 and
270.1 Kg in 1987 for carp fish. Production of tilapia and mullet
fish in the first year (1986) was higher than that in the second
year (1987), while production of carp fish did not differ considerably
from the first to the second year. Also, the total
production of the different fish species was higher in 1986 than
in 1987 (3338.9 vs 2710.9 Kg per pond and 537.2 vs 432.6 Kg per
feddan). The same picture could be seen when considering the total
production of tilapia, mullet carp, carnivorous and skerta
(3858.3 vs 3089.0 Kg per pond and 621.9 vs 493.7 Kg per feddan).
Coefficients of variability of production per pond or per
feddan were shown to be higher in 1986 than in 1987 for mullet,
carnivorous and skerta fish, lower in 1986 than in 1987 for
tilapia fish and nearly similar in the two years for carp fish.
Factors affecting fish production.
The highest fish total production was recorded by fish of
the lowest mullet stocking rate in the first year (15000
fingerlings/ pond) and by fish of the intermediate mullet stocking
rate in the second year (12000 fingerlings/pond). The same
picture was opserved when dealing with fish production per feddan.
The highest mullet stocking rate was generally associated
with the lowest fish production.
Mean production per pond or per feddan of tilapia and mullet
fish was the highest with the stocking rate of 18000 mullet
fingerlings/pond in each of the two years of the study (1986 and
1987). Carp fish production was observed to be the highest with
the stocking rate of 15000 mullet fingerlings/pond in 1986 and
with that of 8000 mullet fingerlings/ pond in 1987.
The increase of carnivorous fish production was associated
with decrease in either mullet or carp production while tilapia
and skerta increased. On the other hand, the increase in carp
production caused a decrease in mullet and tilapia production in
both of the two years of the study. However, statistical
analysis of the effect of MSR on total production, and the
production of each of fish species studied either per pond or per
feddan in the two years were non-significant.
Carcass Traits
Means of carcass traits of fish in 1986 were mostly higher
than their correspondings in 1987.
Carcass traits studied varied with species of the study, the
differences were mostly significant (P<O.05 or P<O.Ol or P<O.OOl)
in 1986 and 1987. Means of weights of whole fish, flesh, head,
skeleton and viscera, in general, were the highest for carp followed
in decreasing order by mullet and tilapia in 1986. In
1987, this ranking was reversed, the highest weight of these
traits were recorded by tilapia followed in a decreasing order by
mullet and carp.
Means of the flesh percentage and other carcass traits (as
percentages) in the three species in any of the two years of the
study followed the same ranking for the whole fish weight.
Mullet stocking rate affected most carcass traits studied
significantly (P<O.05 or P<O.Ol) but without any consistent trend
in the two years.
Differences between ponds within mullet stocking rate in
carcass traits were mostly non-significant in the two years.
Effects of the interactions between mullet stocking rate and
species were significant (P<O.05 or P<O.Ol or P<O.OOl) on all
carcass traits in 1986 and 1987
1987 only.
except by-product pre cent in
Chemical composition of fish flesh.
Average moisture content of fish flesh was found to be the
highest for tilapia fish followed by carp then mullet fish in a
decreasing order in either 1986 or 1957. Spec es differences in
moisture content were significant (P<O.OOl) in the two years of
the study.
Mullet fish recorded the highest dry matter content then
followed in a decreasing order by carp and tilapia. Differences
in dry matter content due to species effects proved significant
(P<O.OOl) in both of the two years of the study.
Protein and fat contents of fish flesh were the highest for
mullet with significant discrepancies from either tilapia or carp
in 1986. The same picture was shown for fat content only in
1987. Species differences for protein and fat percent in the
fish flesh were mostly significant (P<O.05 or P<O.OOl) in the two
years of the study.
Fish flesh content from ash in tilapia was found to be significantly
(P<O.05) lower than in either mullet or carp fish in
1986 and 1987.
The effect of mullet stocking rate was significant
(P<O.OOl) for fat and ash percentages and non-significant for
moisture, dry matter and protein contents in 1986, for 1987, the
same effect was non-significant on all contents.
ConstitUents of fresh fish flesh of the three species
studied (moisture, dry matter, protein, fat and ash) differ between
ponds within mullet stocking rate, but without significant
differences in most cases
The effect of interaction between species and mullet stocking
rate were not significant (P>O.05) on moisture, dry matter
and protein percentages in 1986 while was significant (P<O.05 or
P<O.OOl) on fat and ash percentages. In 1987, effect of this interaction
was found to be significant (P<O.05 or P<O.OOl) on components
of chemical composition except protein and ash.