Age validation, growth and mortality rates of the tropical snappers (Pisces: Lutjanidae) Lutjanus adetii (Castelnau, 1873) and L. quinquelineatus (Bloch, 1790) from the central Great Barrier Reef, Australia

Abstract

The age and growth of Lutjanus adetii and L. quinquelineatus from the central Great Barrier Reef were determined from studies of annuli in sectioned otoliths (sagittae). The period of annulus formation was validated by oxytetracycline labelling of externally tagged fishes. For L. adetii, validation was obtained from tagged fishes that were recaptured after a minimum of 12 months at liberty, the first time this has been achieved for a Lutjanus species. A single opaque and translucent zone was formed once a year, with the opaque band (annulus) being formed during the winter months. Otolith weight was strongly correlated with age for both species. There was significant differential growth between the sexes in length-at-age and weight-at-age for both species, with males growing larger than females. The oldest individuals found were a male L. adetii of 24 years of age and a female L. quinquelineatus of 31 years of age. The shape of the growth curves were steep for the first few years and then became asymptotic. The annual instantaneous rate of natural mortality (M) was 0.235 for L. adetii and 0.154 for L. quinquelineatus, representing an annual survivorship of 79% and 86%, respectively. The protracted longevity and low natural mortality rates imply that both L. adetii and L. quinquelineatus are vulnerable to overfishing despite their small size.