Variation in the relationship between otolith weight and age: implications for the estimation of age of two tropical damselfish (Pomacentrus moluccensis and P. wardi)

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Abstract
The weight of otoliths may provide an objective and economic method of ageing fish. To investigate this potential, populations of two species of fish (Pomacentrus moluccensis and P. wardi) were sampled from the lagoons of seven coral reefs within the Great Barrier Reef, Australia. The weight of otoliths increased with age throughout the life of both species. Two factors appeared to have the potential to undermine the use of otolith weight to estimate the age of individuals. First, overlap in the range of otolith weight among fish of different ages caused normally distributed errors in the subsequent estimation of age by otolith weight. Second, variation among reefs in the relationship between otolith weight and age biased subsequent age determinations. Both these types of error also affected the estimation of the age structure of the populations. Random errors in age determination resulted in the smoothing of the age structure whilst bias either shifted or compressed the age structure. While smoothing of an age structure will not compromise the results of several types of analysis, bias will be detrimental to most. Consequently, randomization methods were used to estimate the power of tests to detect variation in the relationship between otolith weight and age. Our results show that by frequently recalibrating the relationship between otolith weight and age, it can be used as an objective and economic method of age determination with the potential to be as reliable as counting annuli in sectioned otoliths.

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