Validation of age estimation and back-calculation of fish length based on otolith microstructures in tilapias (Pisces, Cichlidae)

Two tilapia species, Sarotherodon melanotheron (brackishwater fish) and Oreochromis niloticus (freshwater fish), were marked with tetracycline and reared in Cote d'Ivoire (West Africa) in intensive (fish fed) and extensive (fish unfed) conditions. Juvenile and adult otoliths of the two species were examined. They were cut into transverse 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 (regression with slope not different from 1 and intercept not different from 0; P>0.05). This technique has a tendency to underestimate age (P<0.05): for S. melanotheron, 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 0. 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 overestimated actual size. When otolith and fish growth were coupled, length was backcalculated accurately.