139 
Abstract— Two tilapia species, Saro- 
therodon melanotheron (brackish water 
fish) and Oreochromis niloticus (fresh- 
water fish), were marked with tetracy- 
cline and reared in Cote d’Ivoire (West 
Africa) in intensive (fish fed) and exten- 
sive (fish unfed) conditions. Juvenile and 
adult otoliths of the two species were 
examined. They were cut into trans- 
verse thin sections (10 to 40 pm), and 
otolith microincrements were counted 
on the sulcus along the ventral axis. 
Results for both species showed that 
microincrements are laid down daily. 
The number of days of growth reflects 
the number of microincrements (regres- 
sion with slope not different from 1 and 
intercept not different from 0; P>0.05). 
This technique has a tendency to under- 
estimate age (P<0.05): for S. melano- 
theron . , the mean bias error is 4.4 d for 
juveniles (48 to 169 d of growth) and 
8.1 d for adults (34 to 185 d of growth); 
for O. niloticus , the mean bias error is 
0.9 d for juveniles (31 to 62 d of growth) 
and 5.1 d for adults (36 to 65 d of 
growth). Back-calculation of individual 
length at marking is very sensitive to 
an uncoupling between otolith and fish- 
specific growth rates. With the present 
data, back-calculated lengths overesti- 
mated actual size. When otolith and fish 
growth were coupled, length was back- 
calculated accurately. 
Manuscript accepted 9 August 2000. 
Fish. Bull. 99:139-150 (2001). 
Validation of age estimation and back-calculation 
of fish length based on otolith microstructures 
in tilapias (Pisces, Cichlidae) 
Jacques Panfili 
Institut de Recherche pour le Developpement, LASAA 
B P 70 
29280 Plouzane, France 
E-mail address: panfili@ird fr 
Javier Tomas 
Port Erin Marine Laboratory 
Port Erin 
Isle of Man IM9 6JA, United Kingdom 
“Tilapia” is the common name for three 
genera of endemic cichlids from Africa, 
Oreochromis, Sarotherodon, and Tila- 
pia. Species belonging to these three 
genera are widely distributed in tropi- 
cal areas and have colonized all kinds of 
continental waters as natural or intro- 
duced species. Their adaptative capabil- 
ities have also been used in developing 
an aquaculture for these species world- 
wide (Lowe-McConnell, 1982; Wootton, 
1984; Pullin et al., 1988). Both the repro- 
duction and growth strategies in tila- 
pias, however, differ among populations 
depending on environmental conditions 
(Kolding, 1993). Therefore, correct esti- 
mates of age is of great importance in 
assessing different growth strategies, 
as well as in characterizing different 
populations, by means of parameters 
such as size at first maturity. Despite 
the need for accurate age estimates, 
age at first maturity has not been 
the subject of many studies in tilapias 
and the question remains whether the 
observed differences in size at matu- 
rity are due to growth or to age dif- 
ferences (Eyeson, 1983; Legendre and 
Ecoutin, 1989; Duponchelle and Panfili, 
1998). Moreover, tilapias can be sexu- 
ally active a few months after hatching 
as shown in studies on tilapia reared in 
aquaculture (Eyeson, 1983). 
Since Pannella’s (1971) work, otolith 
microincrements have been widely used 
to estimate the age of fish in days, 
useful for studies on larvae and juve- 
nile fish (Jones, 1992). Several authors 
have studied microincrements in tila- 
pia otoliths, but their results have not 
been applied to field research. One pos- 
sible reason is that there is not a stan- 
dard and simple method available for 
choosing and preparing the otolith. Pre- 
vious works have used different oto- 
liths and preparation techniques: sagit- 
ta cut transversally and acid etched for 
Tilapia guineensis (Fagade, 1980),sagit- 
ta cut transversally and observed with 
scanning electron microscope for Oreo- 
chromis niloticus (Tanaka et al., 1981), 
lapillus observed whole in photonic mi- 
croscopy for Tilapia marine (Rosa and 
Re, 1985), sagitta cut sagittally and ob- 
served in scanning electron microscopy 
for Oreochromis aureus (Karakiri and 
Hammer, 1989), and sagitta cut trans- 
versally, etched and stained for Oreo- 
chromis niloticus (Zhang and Runham, 
1992). Each of these authors, except Fag- 
ade ( 1980), has validated the daily depo- 
sition of microincrements. Nevertheless, 
all fish used for validation were juve- 
niles and were reared under controlled 
conditions in aquaria, far from their nat- 
ural environment. To date, no attempt 
has been made to estimate growth for 
tilapia in natural waters nor age at ma- 
turity by using microincrements. 
The first aim of our study was to as- 
sess whether examination of microin- 
crements in otoliths yields valuable esti- 
mates of age (in days) for both juve- 
niles and adults of Sarotherodon mel- 
anotheron (brackishwater species) and 
Oreochromis niloticus (freshwater spe- 
cies). To obtain results that could be ap- 
plied to field studies, fish were reared 
