Panfili and Tomas: Validation of age estimation and back-calculation of fish length in tilapias 
141 
Table 1 
Number of otoliths for each sampling date and number of days between marking and sampling (in brackets) for Sarotherodon 
melanotheron and Oreochromis niloticus marked in Layo and Bouake. 1 = adults tagged with anchor tags. 
Date of marking 
Species or birthdate Date of sampling 
Sarotherodon melanotheron 
Adults 
22 Nov 93 
22 Dec 93 
20 Jan 94 
25 Feb 94 
31 Mar 94 
22 Apr 94 
Cage Cl 
19 Nov 93 
1 (34 d) 
1 (64 d) 
1 (93 d) 
1(129 d) 
1 (163 d) 
1 (185 d) 
Cage C2 
19 Nov 93 
1 (34 d) 
1 (64 d) 
1 (93 d) 
1 (129 d) 
1 (163 d) 
1 (185 d) 
Cage C3 
19 Nov 93 
1 (34 d) 
1 (64 d) 
1 (93 d) 
1 (129 d) 
1 (163 d) 
1(185 d) 
Cage C4 ; 
19 Nov 93 
4 (34 d) 
4 (64 d) 
4 (93 d) 
4(129 d) 
4 (163 d) 
4 (185 d) 
Juveniles 
22 Dec 93 
20 Jan /94 
26 Feb 94 
31 Mar 94 
22 Apr 94 
Tank 
4 Nov 93 
5 (48 d) 
5 (77 d) 
5 (114 d) 
5 (147 d) 
5 (169 d) 
Oreochromis niloticus 
Adults 
3 Mar 94 
1 Apr 94 
Pond A F 
26 Jan 94 
15 (36 d) 
15 (65 d) 
Pond A2 ; 
26 Jan 94 
15 (36 d) 
15 (65 d) 
Juveniles 
Pond J 1 
2 Feb 94 
7 Mar 94 
5 Apr 94 
15 (33 d) 
16 (62 d) 
Pond J2 
14 Feb 94 
17 Mar 94 
15 Apr 94 
15 (31 d) 16 (60 d) 
edge to the primordium following the same 
growth axis (Campana, 1992). If no significant 
difference was found between these two read- 
ings for the whole sample after a paired f-test, 
the mean was used to estimate the age (Cam- 
pana and Jones, 1992). To maintain unbiased 
readings, physical data (size, date of capture) 
were not given to the reader. 
Three variables were measured on each oto- 
lith: 1) the otolith diameter at capture ( OD c ) 
which corresponds to the maximum length on 
the anteroposterior axis of the otolith before 
sectioning; 2) the otolith radius at capture (R c ) 
which corresponds to the distance between the 
primordium and the edge along the sulcus ax- 
is; and 3) the otolith radius at marking (R m ) 
which corresponds to the distance between the 
primordium and the tetracycline mark along 
the sulcus axis for the adults marked (Fig. 1). 
Variables were measured with TNPC image 
processing software (Visilog software platform, 
Noesis, France). 
Validation of the time of deposition of 
microincrements 
The term validation refers here to the accuracy of the age 
estimation in tilapias by counting the number of micro- 
increments in the otoliths (N mc ). The number of days 
involved in the validation experiment ( D in days) refers 
to the microincrements deposited between hatching and 
the date of capture for juveniles and between the date of 
tetracycline labeling and the date of capture for adults. 
Pannella’s hypothesis (1971) of a daily deposition rate 
for microincrements was tested for the two development 
stages of both species. 
The accuracy of the method was tested by establishing 
the relation between the number of microincrements 
counted and the number of days involved in the experi- 
ment. A Student test on the linear regression was used 
to establish if the microincrements were deposited daily 
(slope equal to 1) and if the deposition started on the 
