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Fishery Bulletin 99(1 ) 
in an environment as similar as possible to natural condi- 
tions. The second aim was to develop a standard method 
for otolith preparation and examination to be used for both 
species. Both objectives are particularly useful in the case 
of S. melanotheron for which no validation has previously 
been published. We chose to carry out experiments in two 
aquaculture stations in Cote d’Ivoire (Africa), where the 
fish and the otoliths were simultaneously marked with ex- 
ternal tags and tetracycline, respectively, to follow fish and 
otolith growth. Tetracycline labels have been used widely 
to mark calcified structures since the first assays (Weber 
and Rigway, 1967; Meunier, 1974), and we improved this 
universal marker for marking tilapia otoliths. During the 
validation process, special attention was paid to the accu- 
racy of the age estimations (Campana and Jones, 1992). 
Geffen (1992) reported in her review on validation that 
there is a lack of analysis of the variation in increment 
number at a given age and we focused on that particular 
point. Our data on individual fish and otolith tagging were 
useful to test a growth back-calculation model commonly 
used for fishes (Francis, 1990, for review; Campana and 
Jones, 1992; Smedstad and Holm, 1996) and particularly 
to test the problem of uncoupling between somatic and 
otolith growth reported in previous studies (Mosegaard et 
al., 1988; Reznick et ah, 1989; Secor and Dean, 1989). The 
final aim of our study was to obtain an accurate, precise, 
and simple tool for age estimation for future life history 
studies on tilapias. 
Materials and methods 
Rearing experiments 
Fish were reared in two aquaculture stations in Cote 
d’Ivoire (West Africa), a country that experiences a tran- 
sitional equatorial climate with two dry and two rainy 
seasons (Durand and Skubich, 1982; Durand and Guiral, 
1994). Juveniles and adults of Sarotherodon melanotheron 
and Oreochromis niloticus were used in the experiments. 
Juvenile fish were obtained from synchronous layings, 
whereas adults, males and females, were caught in the nat- 
ural environment. Sarotherodon melanotheron was reared 
at the Layo station (Centre de Recherches Oceanologiques) 
located on the Ebrie lagoon. One-hundred and ninety-eight 
adults of S. melanotheron ranging between 90 and 130 
mm FL (fork length) were marked and randomly assigned 
to three 4-m 3 cages (Cl, C2, C3) immersed in the lagoon. 
Another forty four adults between 170 and 210 mm FL 
were marked and kept in a 25-m 3 cage (C4). After hatch- 
ing, juveniles were transferred to a 3-m 3 concrete tank 
supplied with a constant flow of water from the lagoon. 
Fish were fed daily with formulated pellets. Oreochromis 
niloticus was reared at the Bouake station (Institut des 
Savanes). One-hundred and fifty-two adults of O. niloti- 
cus were marked and released in two 400-m 2 ponds (Al 
and A2) that had been previously enriched with organic 
matter (density of 0.2 fish/m 2 ). Juveniles were kept in two 
50-m 2 ponds (J1 and J2) after hatching. No food was sup- 
plied; fish were sustained by natural resources. 
Marking and sampling 
Juveniles were not marked, therefore the “date of mark- 
ing” actually refers to the date of birth. Adults caught in 
the field were marked by injecting tetracycline into the 
peritoneal cavity (50 mg/kg of live weight) and tagged with 
plastic T-bar anchor tags. Fish were measured (standard 
length at marking ( SL m , millimeters), weighed (grams), 
and sex was determined at the date of marking (Table 1). 
All O. niloticus adults and only S. melanotheron adults 
kept in cage C4 were tagged. After marking, both species 
were sampled in cages and ponds at monthly intervals. 
After capture, all individuals were measured (standard 
length at capture ( SL C , millimeters), weighed (grams), 
sexed and their otoliths (sagittae) removed. Randomly 
selected otoliths were prepared for analysis. Table 1 shows 
the dates of marking and sampling, otolith subsamples, 
and the number of days between marking and recapture 
for the two species. 
Otolith preparation 
Only the right otolith was prepared according to the tech- 
nique described by Secor et al. (1992). After testing all pos- 
sible planes of the section (i.e. sagittal, transverse, frontal), 
we chose the transverse section plane. Each otolith was 
then embedded in polyester resin before being sectioned 
transversally (with an Isomet saw) to avoid extra polish- 
ing and taking care to leave material on both sides of the 
core’s plane. The resulting section was attached to a glass 
slide with thermoplastic glue (CrystalBond), ground with 
wet sand paper (grit ranging from 400 to 1200 per paper), 
and polished (polishing cloth with alumina paste ranging 
from 3 to V3 pm) on one side until reaching the primor- 
dium. The block was then turned over, affixed again to 
a slide with the polished face down, and ground and pol- 
ished to remove extra material until the core area was 
reached. The thickness of the resulting sections ranged 
between 10 and 40 pm. Microincrement readability was 
improved by polishing the surfaces with 1 /3 pm alumina 
paste. Otoliths with over-ground surfaces or with damage 
in the reading axis were discarded. 
Otolith interpretation and variables measured 
Terminology used in our study refers to that of Kalish et 
al. (1995). Microincrements were interpreted and counted 
as number of D-zones (reading) along the sulcus, chosen 
as the standard axis for reading (Fig. 1). Otoliths of adults 
were read under an epifluorescent microscope (Leica, 50W 
HBO lamp, 355-420 nm D filter) because the tetracycline 
deposit emits a yellow-green fluorescence under UVB at 
390 nm. Microincrements were counted between the tet- 
racycline mark and the otolith edge on a monitor coupled 
with a video with 1250x magnification. In juvenile oto- 
liths, microincrements were counted from the primordium 
to the outer edge of the otolith under 400x and lOOOx mag- 
nifications on the monitor coupled with video. 
Each otolith was read twice by the same reader, first 
from the primordium to the edge and then back from the 
