Length-at-age data are an important input to the fully age-structured fishery assessment models used by SharkFAG for stock advice to SharkMAC.
School sharks and gummy sharks are currently aged by counting growth-increment bands on the articular faces of vertebrae stained with alizarin red. This method is only partly validated on captive sharks and requires further work. There is a need to validate the assumption that the bands of alizarin red stain are annual and that they provide reliable estimates of age. Appropriate methods for undertaking this validation were developed as part of the completed 'Southern Shark Age Validation Project' (FRDC 91/037). The recently completed 'Southern Shark Tagging project' (FRDC 93/066) was ideally timed to provide sufficient samples to properly undertake this validation.
The project ‘Age Validation from Tagged School and Gummy Sharks Injected with Oxytetracycline’ (FRDC Project 97/110) follows on from four successfully completed projects. These are the ‘Southern Shark Age Validation’ (FRDC Project 91/037), ‘Southern Shark Nursery’ (FRDC 93/061), ‘Southern Shark Tagging’ (FRDC Project 93/066), and ‘Southern Shark Tag Database’ (FRDC Project 96/162). The present project makes use of vertebrae collected from sharks tagged, injected with oxytetracycline, and released into the wild as part of these projects. These vertebrae were collected for the purpose of further age validation.
The present report provides the results of analysis of samples of oxytetracycline marked vertebrae collected and processed in the laboratory by the end of the year 2000. Most available vertebra samples were collected from sharks at liberty for 1–2 years after being tagged and released; only a small number were collected from sharks at liberty 3–5 years. The longer an animal is at liberty, the more valuable it is for age validation purposes. This is because bias and imprecision in counting growth-increment bands are more significant for sharks at liberty for short periods than for those at liberty for long periods. Although the number of vertebra samples continuing to be collected is low, those that are returned in the future will be particularly valuable because of their long periods at liberty. These will be processed from time to time in the future for updating the analyses.
Keywords: Shark vertebra age validation, alizarin stain, microradiography, fishery monitoring.