146 
Fishery Bulletin 99(1) 
(mean=8.5 mm, standard deviation=8.7 mm) was signifi- 
cantly different from 0 (t=6.94, P< 0.05). As a result, back- 
calculation of fish length overestimated the length of fish 
that had grown for one or two months. 
The relation between the specific growth rate of the fish 
(G sl ) and the difference between back-calculated and ob- 
served length at marking was positive because overesti- 
mation increases with the specific growth rate, regardless 
of the duration of the experiment (Fig. 6). The 
actual under- or overestimation of back-calcu- 
lated length seems to depend on the coupling 
between the specific growth rates of the otolith 
and the fish (Fig. 7). An uncoupling between so- 
matic and otolith growth rates would explain 
the deviation of the back-calculated length from 
the measured value (Fig. 7). That is to say, if 
the specific growth rates of the fish and the oto- 
lith are identical, the back-calculation gives a 
very good approximation of the length at mark- 
ing (i.e. the difference is almost equal to zero), 
whereas if the otolith growth rate is higher than 
the fish growth rate, then the back-calculation 
underestimates the length at marking. Inverse- 
ly if the somatic growth rate is higher than 
the otolith growth rate, the back-calculation will 
overestimate the length at marking. Moreover, 
when the uncoupling between otolith and so- 
matic growth rates rises, the overestimation of 
the back-calculation increases (Fig. 7). 
Discussion 
Otolith preparation and interpretation 
of microincrements in tilapias 
Transversal sections (in contrast to sagit- 
tal or frontal sections) of otoliths are the 
clearest and most reliable way to inter- 
pret microincrements in tilapia otoliths. 
Although some authors have worked on 
sagittal sections (Fagade, 1980; Karakiri 
and Hammer, 1989) the concavoconvex 
shape of the otoliths in adults makes it 
very difficult to obtain a plane that would 
include both the core area and the otolith 
edge. Zhang and Runham (1992), who 
prepared transverse sections of adult O. 
niloticus otoliths with their histological 
technique (Zhang et al., 1991), obtained 
sections with clear microincrements. Rosa 
and Re (1985) reported that sagittae in 
Table 6 
Relations between SL, (standard length at capture) and R c (otolith radius at capture) or OD r (otolith diameter at capture) for 
Oreochromis niloticus. F is calculated from the ANOVA for testing the model, r 2 = coefficient of determination. 
Model 
n 
Regression 
F 
r 2 (%) 
Linear 
108 
SL C = 17.13 + 130.31 x R c 
612.5 
85.1 
Multiplicative 
108 
Log(SL c ) = 5.02 + 0.899 x Log(R c ) 
1117.3 
91.3 
Linear 
119 
SL c = -0.609 + 20.33 x OD ( 
4163.8 
97.2 
