VARIATION IN THE RELATIONSHIP BETWEEN OTOLITH WEIGHT AND AGE - IMPLICATIONS FOR THE ESTIMATION OF AGE OF 2 TROPICAL DAMSELFISH (POMACENTRUS-MOLUCCENSIS AND P-WARDI)

The weight of otoliths may provide an objective and economic method of ageing fish. To investigate this potential, populations of two specie s 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 bot h species. Two factors appeared to have the potential to undermine the use of otolith weight to estimate the age of individuals. First, over lap in the range of otolith weight among fish of different ages caused normally distributed errors in the subsequent estimation of age by ot olith weight. Second, variation among reefs in the relationship betwee n otolith weight and age biased subsequent age determinations. Both th ese types of error also affected the estimation of the age structure o f the populations. Random errors in age determination resulted in the smoothing of the age structure whilst bias either shifted or compresse d the age structure. While smoothing of an age structure will not comp romise the results of several types of analysis, bias will be detrimen tal to most. Consequently, randomization methods were used to estimate the power of tests to detect variation in the relationship between ot olith weight and age. Our results show that by frequently recalibratin g the relationship between otolith weight and age, it can be used as a n objective and economic method of age determination with the potentia l to be as reliable as counting annuli in sectioned otoliths.