The choice of an age determination method for a given species involves deciding on an appropriate aging structure (scales, otoliths, vertebrae, spines, etc.) and processing method (impressions, thin sections, etc.) for that structure. The next step is validation, in which the marks used to age fish are verified to occur once per year and at approximately the same time each year. Common validation techniques include direct methods such as tag/recapture studies, including marking with chemicals such as oxytetracycline, calcein, and others; and indirect techniques such as back-calculation, marginal increment analysis, edge progression analysis, length frequency-year class progression analysis, radiometric/isotope analysis, elemental analysis, and others.
To age a fish, an age reader must identify the annual growth marks (annuli) on the structure chosen. In temperate waters, fish growth is fast during the summer months when water temperatures are warm, and slow during the cold winter months. A year of growth is defined as one summer zone plus one winter zone. These zones are identified on scales as areas of wide (summer) and narrow (winter) circulus spacing. On otoliths, these zones are identified as alternating opaque and translucent bands when viewed microscopically.
In 1988, the Fishery Biology Program developed and published a set of detailed descriptions of the age determination methods we routinely use. These descriptions are being updated and will be reprinted here in their entirety. The descriptions are designed for use primarily by fishery biologists and professional age readers and students but may be of general interest.