Similar to trees, the age of a fish may be estimated by examining growth rings on its ctenoid scales. This is because as a fish grows, so does its scales. The increase in size leaves rings, known as circuli, which are similar to those that appear in tree trunks. In winter months or other periods of cold weather, the growth rate of fish and their scales decelerates. Consequently, the circuli occur at much smaller intervals, which leaves a dark band. By counting the bands, known as annuli, an approximate age of a fish may be obtained.