Common carp Cyprinus carpio are an important pest species in Australia, yet little is known regarding their age and growth there. We examined otolith sections of common carp to validate their utility for age determination. For the 1999 year-class in Hut Lake near Barmah, we confirmed the absolute age at first annulus formation as age 1 by repeated sampling of a discrete young-of-year cohort. We confirmed the annual periodicity of annulus formation for common carp in a mark-recapture experiment when 19 recaptured adult common carp (from an original stocking of 141 fish marked by injection with oxytetracycline [OTC]) showed visible fluorescent marks on their otoliths. Time at liberty for these fish ranged from 6 to 25 months, and their ages on recapture ranged from 3 to 14 years. Increment counts outside the OTC mark agreed completely with time at liberty. We calculated precision estimates on age determinations as the average percent error (APE) and estimated the coefficient of variation (CV ϭ Ϫ0.15 ϩ 1.41APE) between readers and for each of two readers over time. Precision was assessed by rereading subsamples. The APE was less than 5% and CV was less than 8% in all cases. We conclude that examination of thin otolith sections is a suitable method for the determination of annual age estimates for common carp age 0-14.

Variation in strontium (Sr) and barium (Ba) within otoliths is invaluable to studies of fish diadromy. Typically, otolith Sr:Ca is positively related to salinity, and the ratios of Ba and Sr to calcium (Ca) vary in opposite directions in relation to salinity. In this study of jungle perch, Kuhlia rupestris, otolith Sr:Ca and Ba:Ca, however, showed the same rapid increase as late-larval stages transitioned directly from a marine to freshwater environment. This transition was indicated by a microstructural check mark on otoliths at 35–45 days age. As expected ambient Sr was lower in the fresh than the marine water, however, low Ca levels (0.4mgL–1) of the freshwater resulted in the Sr:Ca being substantially higher than the marine water. Importantly, the otolith Sr:Ba ratio showed the expected pattern of a decrease from the marine to freshwater stage, illustrating that Sr:Ba provided a more reliable inference of diadromous behaviour based on prior expectations of their relationship to salinity, than did Sr:Ca. The results demonstrate that Ca variation in freshwaters can potentially be an important influence on otolith element:Ca ratios and that inferences of marine–freshwater habitat use from otolith Sr:Ca alone can be problematic without an understanding of water chemistry.