Improved otolith preparation, ageing and back-calculation techniques for New Zealand freshwater eels

New Zealand freshwater eels are long-lived and their otoliths are difficult to age using conventional “burn-and-break” techniques. This study developed a number of new and improved techniques which increased the accuracy of age estimates and enabled growth rates to be back-calculated. The “burn-and-break” technique was significantly improved by sawing otoliths through the nucleus before burning. Criteria were developed to distinguish annuli from summer growth bands and supernumerary checks on the basis of colour, width, structure, continuity, and spacing. Annuli and supernumerary checks were most easily identified by examining thin sections stained in an acidic toluidine blue solution. Back-calculation techniques were also improved by using a curvilinear model, originating from a common biological origin, to describe the relationship between fish length and otolith radius. Annulus radii were measured accurately from the flat surface of sawn, burnt, half otoliths and back-calculated lengths were validated by comparison with the lengths of aged wild fish. Using the improved techniques developed in this study it may now be possible to determine factors influencing annual variations in growth and to study inter-relationships between sex and growth rates.