Variability of stable isotope ratios of glassfish (Ambassis jacksoniensis) from mangrove/saltmarsh environments in southeast Australia and the implications for sample size
Scientists concerned with food chains in aquatic ecosystems are increasingly using stable isotopes to understand energy pathways. To date, the scientific literature shows little attention has been paid to the number of individuals in a sample needed to describe adequately the relationship of one species in a food chain to another. To deal with this short coming, samples of the small estuarine fish Ambassis jacksoniensis were taken from two very different locations in Sydney metropolitan estuaries. One location, Towra Point, is near the entrance to Botany Bay and is little disturbed by urban activity. The other, Powells Creek in Homebush Bay, is at an upstream location in the Parramatta River that until the Sydney Olympics in 2000 was seriously degraded over past decades by various industrial activities. These two locations were chosen to maximise any differences that might arise as local variation in isotope signature. Values of ?13C and ?15N were determined, and while the Powells Creek samples showed depleted carbon and enriched nitrogen compared to Towra Point, of more importance for future studies was the need, on cost-benefit basis, for a minimum of five individuals of A. jacksoniensis to enable spatial and/or temporal comparisons. This sample size cannot be extended to other species without additional investigations. At that point, a generally applicable sample size might be resolved. In the interim, a sample size of ten is recommended for comparing isotope signatures for estuarine fauna.