Ž Levels of serum E and T were measured by established radioimmunoassays Duston
2
. and Bromage, 1987 . Serum levels of calcium were measured using an atomic absorp-
Ž .
tion spectrophotometer Perkin-Elmer 2280, UK . Calcium ions are incorporated into the post-translational modification of the vitellogenin molecule prior to its release from the
Ž .
liver Tata, 1978 . As a result, serum calcium levels rise during vitellogenesis; an Ž
increase that is directly correlated to blood vitellogenin concentration Elliot et al., .
1982 . Total serum calcium is therefore considered an accurate indicator of serum vitellogenin concentration. Intra- and inter-assay coefficients of variation were 7.3 and
10.1, 4.6 and 7.4, and 6.2 and 6.9 for calcium analysis, E assay, and T assay,
2
respectively. 2.4. Egg sampling and counting
The mean number of eggs per female was calculated for eggs harvested of different Ž
. stages Little et al, 1993 using the following formula:
Mean number of eggs female
y1
s Total weight of eggsr F = E
Ž .
where F is the total number of females from the same hapa and E is the mean individual weight of eggs.
Mean individual egg weights were based on counts of 200 individual stage 1 eggs from each hapa at each harvest; eggs of stages 2 and 3 were assumed to be of the same
size based on previous AIT studies. 2.5. Statistical analysis
Ž Reproductive performance data egg output, number of egg clutches, growth and
. Ž
. Ž
survival of broodfish were analyzed by a 2 presencerabsence of rohu = 3 female .
Ž .
exchange factorial analysis in which ponds were treated as blocks Table 1 . Percentage Ž
. data were arcsine-transformed before the analysis of variance ANOVA and Student’s
t-test. Correlation analysis was performed for levels of serum T, E , and total calcium
2
Ž .
SPSS for MS Windows . Serum samples obtained from spawned and unspawned fish Ž
. were compared using multifactor ANOVA and Student’s t-test Statgraphics ver. 7.0 .
3. Results
3.1. ReproductiÕe performance Reproductive performance, as indicated by seed output per unit area or as a
percentage of females spawned at each harvest, was enhanced in females regularly Ž
. exchanged for fish conditioned in single sex groups for 7 days P - 0.05 . Mean egg
Ž
y2 y1
. production eggs m
day for females conditioned for 7 days was almost double the
output from hapas in which females were maintained in spawning hapas continuously Ž
. P - 0.05; Fig. 1 . A similar, but less pronounced trend, was found for intensity of
Ž
y1
. female spawning percentage of females spawned harvest
; P - 0.05 . Relative seed
Ž Ž
. Ž
. Fig. 1. Effects of exchange no exchange 0 day , exchanged with conditioned fish every harvest 3.5 days or
Ž ..
Ž . alternate harvests 7 days
of female O. niloticus breeders in large nylon hapas harvested twice weekly on a Ž
y2 y1
. Ž . Ž
y1 y1
. Ž .
egg output eggs m day
, b female reproductive productivity eggs kg total females day
, and c Ž
y1
. spawning frequency females spawned harvest
. Half the hapas were stocked with L. rohita.
output, expressed as the number of seed produced per kilogram total of initial females Ž
. stocked, indicated that frequent exchange 3.5 days reduced productivity of female
Ž .
broodfish significantly P - 0.001 . Exchange strategy did not influence the fecundity of
y1
Ž .
females, or numbers of eggs clutch P 0.05 , mean values of which ranged between
370–415 eggs clutch
y1
. The pond had a significant impact on productivity, with one pond producing nearly
one third more reproductive output than the other. Water quality was consistently higher in the more productive pond, that is, both mean early morning dissolved oxygen and
Ž
y1
chlorophyll levels were higher 1.0 compared to 0.7 mg l and 103.5 compared to
y1
. 61.2 g l
, respectively . The presence of rohu had a negative impact on egg production, reducing the
percentage of females spawning at each seed harvest and resulting in a 12 decline in Ž
. seed output P - 0.05 .
3.2. Growth and surÕiÕal of broodfish The mean total duration spent in the spawning hapa clearly affected fish growth
Ž .
Table 2 and reproductive performance. Females exchanged every 3.5 days, and thus
Table 2 Growth and survival of Nile tilapia broodfish
Notations a, b, and c allow comparison of conditioning periods within the no Rohu or Rohu groups, i.e., to observe the effects of conditioning period for each Rohu and no Rohu group. Values in columns with the same
Ž .
notation are not significantly different P 0.05 .
y1
Ž .
Ž . Ž
. Rohu
Conditioning period day Broodfish survival
Daily weight gain mg fish Female
Male Female
Male
a a
a a
No 985
874 y3734
175
a a
b a
3.5 733
943 3506
3132
a a
ab a
7 853
891 17196
2556 MeanSE
867 902
161112 244
a a
a a
Yes 10514
864 78
y2525
a a
ab a
3.5 8316
955 28783
3914
a a
b a
7 764
748 20819
137152 MeanSE
889 856
16784 5147
spending less time in the spawning hapa overall, gained more weight during the experiment than non-exchanged fish. Females kept in spawning hapas throughout the
Ž .
trial lost weight 0 day . Exchange of females resulted in a significantly poorer
Ž .
P - 0.05 survival to final harvest. Males grew more slowly than females in all
treatments and actually lost weight in four replicates over the 70-day period. A replicate Ž
. in which males gained weight was related to poorer survival of male fish 66 in this
hapa; male survival was more than 80 in all other replicates. The pond had no effect Ž
. Ž
. on broodfish growth
P 0.05 . There were no significant P 0.05
effects of broodfish exchange and the presence of rohu or pond on the survival of broodfish.
Table 3 Ž .
Effects of Nile tilapia female exchange on serum levels of sex steroids and total calcium in spawned S and Ž
. Ž
. Ž
. non-spawned NS female fish removed from spawning SH and conditioning hapas CH after an experimen-
Ž .
tal period of 70 days mean valuesSE Ž
. N.B. mean values with the same superscripts are not significantly P 0.05 different within the same column
and row. Notations a, b, and c, are for comparisons within a vertical column and x, y, and z are for comparisons within a horizontal row.
y1 y1
Ž .
Ž .
Ž .
Broodfish exchange strategy Testosterone ng ml Oestradiol ng ml
Total calcium mgr SH
SH CH
SH SH
CH SH
SH CH
S NS
NS S
NS NS
S NS
NS
a,x a,x
a,x a,x
a,x a,x
Control 7.2
19.5a –
1.7 4.8
– 14.6
13.3 –
0.9 6.7
0.3 1.5 0.7
0.6
b,x b,y
a,y b,x
b,y a,y
b,xy c,y
a,x
Females 18.7
76.5 51.5
3.5 16.6
14.5 23.6
28.8 23.5
exchanged 2.7
15.1 10.7 0.7 2.6
2.3 1.4
2.2 1.3
every 3.5 days
b,x b,y
a,y c,x
b,y a,y
b,x b,x
a,x
Females 21.5
91.4 55.7
8.4 15.3
16.7 21.1
22.7 21.2
exchanged 2.4
14.1 10.6 1.5 1.6
2.7 1.3
1.2 0.6
every 7 days
3.3. Serum testosterone, 17b-oestradiol, and total calcium leÕels In females without conditioning, levels of serum T, E , and total calcium, were
2
Ž .
similar in spawned and non-spawned females Table 3 . In conditioned females,
Ž .
spawned fish had lower levels of both E and T than in unspawned fish P - 0.05 .
2
Levels of both E and T were also higher in fish conditioned for 3.5 and 7 days than
2
Ž .
those that had remained in the spawning hapa continuously P - 0.05 . Levels of both steroids were similar in conditioned fish maintained for both 3.5 and 7 days in the
spawning hapa, and there was no effect of rohu nor of pond. Levels of E and T in
2
Ž .
females removed from conditioning hapas were significantly higher P - 0.001 than
those of spawned females removed from spawning hapas. Serum levels of both hormones in fish removed from conditioning hapas were similar to unspawned fish from
Ž .
conditioned treatments harvested from spawning hapas Table 3 . Calcium levels were similar in spawned and unspawned females, but levels in conditioned females were
Ž .
significantly higher than control females P - 0.001 . In spawned females, there was a Ž
. highly significant P - 0.001 correlation between T and E . Total calcium exhibited a
2
Ž .
significant P - 0.05 correlation with T, but not with E . In non-spawned females, T,
2
Ž .
E and total calcium, were highly correlated P - 0.01 .
2
4. Discussion
