Rings (called circuli) form on each scale just like rings on tree trunks. The circuli are visible in the lighter area of the scale. The darker banded areas are called annuli--they are formed during the winter months when the fish grow more slowly. We can estimate the age of each fish by examining these rings.
The age data we gather is then used to estimate the age and survival rate of the returning salmon.
Here are some videos demonstrating the scale collection/analysis process: