Validation of otolith age determination in Australian longfinned river eels, Anguilla reinhardtii

The primary objective of the present study was to validate the annual nature of growth rings in otoliths of 'yellow eel stage' (fully pigmented, premigratory) Australian longfinned eels ( Anguilla reinhardtii ) in New South Wales, Australia, using a combination of laboratory and field experiments. Eels were injected with oxytetracycline (OTC) and tagged with external 'T-bar' tags. Microscopic examination of thin transverse sections of the sagittal otoliths from recaptured eels showed that one opaque annulus was typically formed in the otolith during each year subsequent to OTC marking. The seasonal timing of opaque annulus formation was highly variable, but generally occurred between May and November. Supernumerary (false or incomplete) rings were observed in many of the otoliths. Examination of otolith sections from a sample of very small, untagged yellow eels verifed the age at first annual increment formation. Marked and tagged longfinned eels did not have a significantly higher mortality rate than controls in the laboratory experiment, but tag loss rates may be high. The field study also indicates that Australian longfinned eels generally have a very restricted home range of 300 m or less. This study is believed to provide the first otolith age validation of a tropical anguillid.