Validation of annual growth increments in the Otoliths of two species of Damselfish from the Southern Great Barrier Reef

If the otoliths of fish are to be used as an ageing tool, they must fulfil three criteria: they must display an internal structure of increments, this structure must be relatable to a regular time scale, and the otoliths must grow throughout the lives of the fish at a perceptible rate. These criteria were assessed for two species of damselfish: Pomacentrus moluccensis and P. wardi from One Tree Reef on the southern Great Barrier Reef. Transverse sections of sagittae from both species contained an alternating sequence of opaque and translucent zones that formed an interpretable pattern of increments. Treatment of fish with tetracycline showed that the opaque zones were formed on an annual basis, generally between spring and early summer. The assessment of growth of otoliths from fish treated with tetracycline indicated that those as old as 10 years of age were still laying down increments of regular thickness. Consequently, the otoliths from both species successfully fulfilled the three criteria required for the direct determination of age. Furthermore, this study helped to identify two levels of variation in otolith structure, manifested in the clarity and interpretability of the increments; there was an interspecies difference and a significant intraspecies latitudinal difference.