Panfili and Tomas: Validation of age estimation and back-calculation of fish length in tilapias 
147 
Figure 7 
Relations between the specific growth rate (otolith length [thin curve] or 
standard length [bold curve] ) and the individual difference of the stan- 
dard length back-calculated at marking ( SL mb ) and the standard length 
observed at marking ( SL m ) for Oreochromis niloticus. 
juvenile Tilapia marine were too thick to allow 
observation of the microstructures without 
previous preparation and decided to work on 
lapilli. Despite the long and time-consuming 
process (about 1 h for preparing each trans- 
verse section of a sagittal, it appears to be the 
best way to observe otolith microincrements 
for both juvenile and adult tilapia. To avoid 
parallax when observing microstructures on 
thick preparations (Campana, 1992; Neilson, 
1992), a thickness of 10 to 40 pm and a fine 
polishing of the sections are necessary. 
Zhang and Runham (1992) observed micro- 
increments of juvenile O. niloticus otoliths un- 
der a maximum magnification 400x with light 
microscopy, whereas Rosa and Re (1985) used 
magnifications ranging from 600x to 1250x 
while working on Tilapia mariae lapilli. Our 
results show that microincrement observation 
and interpretation require a minimum magni- 
fication of lOOOx for adult tilapias. These high 
magnifications with compound microscopes are 
required to observe microincrements in the 
sulcus area where they are found to have such 
a compressed arrangement that no space is left 
for subdaily increments (Zhang and Runham, 
1992). Therefore, to interpret microincrements 
in tilapia otoliths accurately, we strongly rec- 
ommend preparing thin transverse sections 
(10-40 pm), polishing them finely (V3 mm), 
and observing them under high magnifications 
(minimum of lOOOx). 
In interpreting microstructures in tilapia 
otoliths, four types of problems were encoun- 
tered: 1) difficulty in interpreting microstruc- 
tures in the otolith region that correspond to 
first-growth stages; 2) difficulty in having to 
switch the reading axis (starting in the dorso- 
ventral area and finishing along the sulcus ar- 
ea); 3) difficulty in reading some zones; and 4) 
difficulty in identifying microstructures near 
the outer edge of the otolith. Microincrements 
around the hatching check are very faint and 
narrow and require high magnifications to be 
identified. Narrow increments were also re- 
ported on Tilapia mariae lapilli by Rosa and 
Re (1985). The presence of accessory growth 
centers on both sides of the core area in the 
dorsoventral plane of the otolith were also ob- 
served by Karakiri and Hammer (1989) and Zhang and 
Runham ( 1992) on O. niloticus otoliths. These authors esti- 
mated the date of formation of these accessory growth cen- 
ters to be between 21 and 30 days and between 16 and 28 
days after hatching, respectively. Secondary growth cen- 
ters in Ivorian tilapia otoliths were located between the 
13th and the 28th microincrements; therefore our results 
agree with observations made by these authors. It is like- 
ly that the presence of accessory growth centers repre- 
sents a shift in the growth of the otolith, meaning that 
growth along the dorsoventral axis is favored at this stage. 
The use of the dorsoventral axis to read microincrements 
may induce reader error. Ambiguities arise because of the 
numerous subdaily structures deposited during the fast 
growing period (Zhang and Runham, 1992); thus reading 
along the sulcus region is recommended. However, as the 
growth of the otolith along the sulcus is relatively indis- 
tinct during the first 15 to 20 microincrements, proper in- 
formation can only be gathered along the core-ventral ax- 
is. Certain regions in otoliths, especially near the edge, 
have been difficult to read for numerous other species 
(Campana, 1992). Tilapia otoliths also exhibit unreadable 
