- age determination
the age of fish may be determined by counting the annual rings on a scale (by microscopic examination, projection of the scale or its celluloid imprint with a scale projector, or projecting a photographic negative of the scale), or in bony parts such as vertebrae, otoliths, opercular series of bones, pectoral spines; by the known age method (growing fish in ponds or tagging fish in the wild and recapturing them at intervals); by the length frequency method (the different age groups tend to be different lengths apparent when the sizes are grouped in a length frequency graph, from which age may be deduced). Age estimation is often a preferred term because of uncertainties in ageing methods
Dictionary of ichthyology. 2009.