144 
Fishery Bulletin 99(1 ) 
Table 4 
Results of microincrement readings for adults and juveniles Oreochromis niloticus. D = number of days between marking (adults) or 
birth (juveniles) and capture; SD = standard deviation; Cl (95%) = confidence interval for mean at 95%; CV = coefficient of variation. 
Number of microincrements 
D (d) 
n 
Mean 
SD 
CV (%) 
Cl (95%) 
Adults 
36 
25 
31.1 
2.5 
8.2 
30.0-32.1 
65 
27 
59.7 
2.6 
4.4 
58.7-60.8 
Juveniles 
31 
12 
31.9 
2.5 
8.2 
30.2-32.1 
33 
9 
30.5 
1.3 
4.5 
29.5-31.5 
60 
9 
59.4 
3.7 
6.4 
56.6-62.3 
62 
13 
60.4 
2.3 
3.9 
59.0-61.8 
50 
O 
40 ■ 
□ 
30 ■ 
o 
20 
O 
;o 
O 
□ 
? 10 . 
O 
8 
o 
8 
o □ 
Q 
© 
□ O 
o 
□ 
0 
o 
D 18 
a . 1 . ° 
□ □ 
o 
□ 1 ° 
B o ° 
□ □ 
30 
D 60 
90 120 
150 180 210 
-10 
o 
-20 
0 (d) 
Figure 2 
Individual differences between the number of days of growth ( D ) and 
the number of microincrements counted on transverse otolith section 
(Nin C ) for adults ( O ) and juveniles O Sarotherodon melanotheron. 
creased with the duration of the experiment 
although the coefficient of variation did not in- 
crease with time (Table 2). Variability in preci- 
sion was nevertheless higher for adults than 
for juveniles. Figure 2 also shows that the dif- 
ference between the number of increments and 
the number of growth days was equal to zero 
for only a few individuals. 
Validation of microincrement deposition 
in otoliths of Oreochromis niloticus 
The distinction of microincrements was much 
greater in O. niloticus than in S. melanotheron 
otoliths resulting in an easier interpretation 
of microincrement deposition in O. niloticus. 
Results of microincrement counts for adults 
and juveniles of O. niloticus are summarized 
in Table 4. As for S. melanotheron , the two 
readings of each otolith were not significantly 
different across the whole sample; thus the 
mean of the two counts was used as the otolith 
microincrement value (paired £-test, £=0.85, 
P>0.05). Microincrement values in otoliths of 
adults after 36 and 65 d underestimated the 
true age of the fish by 4.9 and 5.2 d, respec- 
tively. Both values significantly differed from 
0 (respectively £=9.78 and £=10.44, P<0.05) but were not 
significantly different from each other (P>0.05). Therefore 
the deviation between the true age and the estimated age 
was similar between 36 and 65 d and approximately equal 
to five days. 
In otoliths of juveniles, the mean of the differences be- 
tween the number of days and microincrements counted 
did not differ significantly from 0 in ponds J1 (0.9 d) and 
J2 (0.6 d) (£=1.18 and £=0.45 [P>0.05], respectively). For 
juveniles reared in pond Jl, age was underestimated af- 
ter 33 and 62 d of growth (Table 4), whereas ages of juve- 
niles in pond J2 (31 and 60 d of growth) were accurately 
estimated (Table 4). A one-level nested AN OVA was car- 
ried out to check whether the pond or the age, or both, had 
had an effect in the difference between the true age and 
the estimated age. Results showed a significant effect of 
the pond (F a 39) =7.57, P< 0.05) but not of age (P (2 39) =1.13, 
P>0.05). Furthermore, a multiple rank test showed that 
no significant differences existed between the means at 
31, 60, and 62 d and between the means at 33, 60, and 62 d 
(P>0.05), but that a significant difference existed between 
31 and 33 d (P<0.05). Thus an effect of the pond on otolith 
deposition could not be confirmed. 
The relation between the number of microincrements 
and the number of days before capture was established to 
test the accuracy of the age estimation (Table 5). The re- 
sulting r 2 was significantly different from 0 for adults or ju- 
veniles, or for both (P<0.001). The slopes of the model were 
not different from 1 (Table 5, P>0.05), which shows that 
the deposition rate of microincrements is daily. The num- 
