Accurate growth models are vital in implementing meaningful management strategies. This is especially true for ectothermic aquatic species, whose growth is tightly coupled to the external environment. Evidentially, predicted global climate change will therefore alter the growth rates of such species. Unfortunately, the necessary information to create accurate growth models can be difficult to collect for elusive and endangered species, such as the critically endangered largetooth sawfish (Pristis pristis). As such, an existing collection of largetooth sawfish vertebrae from the Fitzroy River (WA) was utilised to provide a means of assessing growth by measuring banding caused by differing seasonal growth rates. Banding was then assigned year-specific environmental data. However, the accuracy of these measures relies on the assumption of annual seasonal banding, which is yet to be validated. Therefore, validation was attempted by measuring vertebral oxygen isotope concentrations using a secondary ion mass spectrometer (SIMS); a first for elasmobranch vertebrae. As oxygen concentrations in the Fitzroy River fluctuate significantly between wet and dry seasons, it was hypothesised this pattern would be mirrored in the vertebral bands. Upon validation, inferences regarding differing growth rates between ages, season, and environmental factors, such as flood season magnitude or temperature, become more reliable.