Using otolith weight–age relationships to predict age-based metrics of coral reef fish populations at different spatial scales

An accurate estimate of age structure of a fish population is an important requirement of fisheries stock assessment. The conventional method of age determination, based on counts of annuli in sectioned otoliths, can be time consuming and expensive, especially in the tropics. This study assesses the use of otolith weight to predict age structures of an important exploited coral reef fish at different spatial scales (within reef, between reefs within regions and between regions), and the implications of this for estimates of key fishery parameters. Otolith weight–age relationships were estimated for common coral trout, Plectropomus leopardus (Serranidae: Epinephelinae), at 24 coral reefs located in four different regions spanning seven degrees of latitude along the Great Barrier Reef of Australia. Otolith weight increased linearly with age, with r2 values of the relationships varying from 0.55 to 0.81 for individual reefs. The accuracy of predictions of age structure varied depending upon the spatial scale over which the prediction was made. Predictive accuracy was highest at the local scale of individual reefs, and worst at the largest scale of between regions. Predictions of age based on otolith weight–age relationships generally overestimated the minimum age of a population and underestimated the maximum age. Mean predicted age was generally within ±1% difference of the mean observed age, while mean predicted length at modal age (growth index) was largely within ±5% difference of mean observed length at modal age. Predictions were less accurate, however, for estimates of total mortality rate relative to those estimated from direct age estimates. Otolith weight–age relationships generally predicted modal age within ±1 year at all three spatial scales. These results have significance for making rapid, initial estimates of key parameters for stock assessment of tropical reef fish, especially for minor species or in circumstances where available resources are insufficient for a comprehensive program of direct age estimation.