Using otolith weight–age relationships to predict age based metrics of coral reef fish populations across different temporal scales

We assess the use of otolith weight to predict population age structure of an important harvested coral reef fish at different temporal scales up to 4 years, and explore the implications of age prediction for estimates of key fishery parameters. Fish age and otolith weight relationships were estimated for common coral trout, Plectropomus leopardus (Serranidae: Epinephelinae), at 24 coral reefs located in four different regions spanning 7° of latitude along the Great Barrier Reef of Australia from 1995 to 1999. We explored the robustness of predictions of population age structures from otolith weights where age structure in 1 year was derived from otolith weights using the age–otolith weight relationships derived in another year up to 4 years earlier. The accuracy of predictions of age structure varied depending upon the temporal scale over which the prediction was made. Predictive accuracy was highest for predictions in the same year as the age–otolith weight relationship was derived and worst at the longest scale, when predictions spanned 4 years. Predictions of age based on the age–otolith weight 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. Predicted age structures gave less accurate estimates of total mortality rate than those estimated from directly determined age structures. Fish age–otolith weight relationships generally predicted modal age within ±1 year at all temporal scales. These results have significance for making rapid, initial estimates of key parameters for long-term monitoring of tropical reef fish stocks, especially in circumstances where available resources are insufficient for a comprehensive program of direct age estimation.