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 Ore ochromis niloticus (freshwater fish), were marked with tetracycline and rea red in Cote d'Ivoire (West Africa) in intensive (fish fed) and extensive (f ish unfed) conditions. Juvenile and adult otoliths of the two species were examined. They were cut into transverse thin sections (10 to 40 pm), and ot olith microincrements were counted on the sulcus along the ventral axis. Re sults for both species showed that microincrements are laid down daily. The number of days of growth reflects the number of microincrements (regressio n with slope not different from 1 and intercept not different from 0; P>0.0 5). This technique has a tendency to underestimate age (P<0.05): for S. mel anotheron, the mean bias error is 4.4 d for juveniles (48 to 169 d of growt h) 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 adu lts (36 to 65 d of growth). Back-calculation of individual length at markin g is very sensitive to an uncoupling between otolith and fish-specific grow th rates. With the present data, back-calculated lengths overestimated actu al size. When otolith and fish growth were coupled, length was backcalculat ed accurately.