Dear WW: I just remembered that scales on fish grow at different rates durring
in summer vs. winter, giving something like annular tree rings.
"How We Age Fish

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. "