Interestingly, fish scales can be used to estimate the age of a fish. As a fish with cycloid or ctenoid scales grows, its scales also grow. The increase in size of the scales causes the development of growth rings known as circuli. Theses rings are similar to the growth rings in tree trunks. When the weather becomes cool, scale growth slows down and the circuli appear more closely together, leaving a dark band. It is possible to estimate how old a fish is by counting the bands, known as annuli.