Panfili and Tomas: Validation of age estimation and back-calculation of fish length in tilapias 
143 
Table 2 
Results of microincrement readings for adults and juveniles Sarotherodon melanotheron . 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 
34 
6 
28.3 
2.8 
10.4 
25.4-31.3 
64 
6 
56.1 
8.9 
16.6 
46.7-65.5 
93 
3 
88.5 
10.0 
12.2 
63.7-113.3 
129 
7 
119.4 
7.7 
6.7 
112.3-126.2 
163 
4 
150.7 
23.5 
16.5 
113.4-188.1 
185 
3 
176.7 
11.0 
6.7 
149.4-203.9 
Juveniles 
48 
4 
48.6 
2.3 
5.0 
45.0-52.3 
77 
4 
76.9 
2.2 
3.0 
73.3-80.4 
114 
5 
109.9 
2.1 
2.0 
107.3-112.5 
147 
4 
132.9 
17.1 
13.7 
105.6-160.1 
169 
5 
164.5 
6.1 
3.9 
156.9-172.1 
Table 3 
Linear regressions between the number of microincrements iN inc ) and the number of days between marking or birth and capture 
(Dl, D = a + b x N inc , and Student tests for slope (6=1) and intercept (a=0) for Sarotherodon melanotheron. F = result of model 
ANOVA; ns = no significant difference (P>0.05); r 2 = coefficient of determination. 
Intercept Slope 
Group 
n 
F 
r 2 (%) 
a 
t 
a = 0 
b 
t 
b = 1 
Juveniles 
22 
524.7 
96.3 
4.348 
0.88 
ns 
0.923 
-1.91 
ns 
Adults 
29 
658.4 
96.1 
-4.844 
-1.11 
ns 
0.968 
-0.85 
ns 
Juveniles and adults 
51 
1176.3 
96.0 
-1.899 
-0.57 
ns 
0.957 
-1.54 
ns 
otoliths than S. melanotheron even though the quantities 
injected in both species were identical (50 mg per kg of 
live weight). In all cases the tetracycline deposit coincid- 
ed with a check in the otolith structure, confirming that 
the marking is synonymous with stress to the fish. Other 
checks present along the sulcal axis interrupted the mi- 
crostructure deposition pattern without any regularity. 
The growth of the otoliths in the region between the tet- 
racycline deposit and the edge (i.e. between marking and 
capture) was always smaller for S. melanotheron than for 
O. niloticus for comparable sizes regardless of the date of 
capture. 
Validation of microincrement deposition in otoliths 
of Sarotherodon melanotheron 
Microincrement interpretation on adult S. melanotheron 
otoliths is difficult and requires a minimum magnification 
of lOOOx for the microincrements between the tetracycline 
mark and the outer edge. Results of otolith reading for 
adults and juveniles are summarized in Table 2. The mean 
of the two readings was used as the value of the micro- 
increment count of each otolith because the difference 
between the two readings was not significant in the whole 
sample (paired f-test, t=0.02, P>0.05). Results showed a 
tendency to underestimate true age, even though this 
underestimation was less pronounced in juvenile fish than 
in adults (Table 2). The mean underestimation was 8.1 d 
for adults and 4.4 d for juveniles; both means were sig- 
nificantly different from 0 (respectively f=4.19 and t= 2.36, 
P<0.05). The number of increments were plotted against 
the days of growth. An ANOVA conducted with the result- 
ing linear models (Table 3) with adults or juvendes, or 
both, showed a coefficient of determination (r 2 ) signifi- 
cantly different from 0 (PcO.OOl). The slopes were not dif- 
ferent from 1 and the intercepts were not different from 0 
(Table 3). Thus the microincrements counted on the trans- 
verse section of S. melanotheron otoliths can be considered 
structures that are deposited daily. The technique appears 
to accurately estimate the age of S. melanotheron in days. 
The dispersal of residuals in Figure 2 was more im- 
portant in large individuals. Underestimation slightly in- 
