The most prevalent method of aging bony fish is known as Otolith Analysis. This procedure entails the extraction and microscope analysis of the fish’s otoliths – small calcium carbonate structures that are located slightly posterior to the fish’s eyes.
These structures, which are used as gravity, balance, and movement indicators, grow continuously throughout a fish’s life and exhibit a unique growth pattern. This growth pattern is thought to be a result of seasonal temperature changes – during the winter, the otoliths grow slowly, accreting lightly-colored calcium carbonate; during the summer, the otoliths grow quickly, accreting darker calcium carbonate. The contrast between lighter calcium carbonate and darker calcium carbonate forms rings known as annuli. Since each annuli represents one year, scientists may determine the age of the fish by counting them.