Just like how we estimate the age of a tree, we cut the tree horizontally and count the rings on the trunk. One ring equivalent to one year. Hence, as the tree grows older, it becomes fatter.
As cycloid and ctenoid scales increase in size, growth rings called circuli become visible on the fish scale. These rings look a little like the growth rings in the trunk of a tree. During the cooler months of the year the scale grows more slowly and the circuli are closer together leaving a band called an annulus. By counting the annuli it is possible estimate the age of the fish. This technique is extensively used by fisheries biologists.
