Most of us know about tree rings—a tree forms a new one every year in its trunk. When counted, they tell us the tree’s age. But trees are not the only living things to produce annual rings. All fish produce small calcium carbonate structures just below their brains called “otoliths.” Fish add calcium carbonate and protein to these structures throughout their life. In a similar process to trees, rings occur on an otolith when a fish’s growth slows down during colder months and the deposit appears white, compared to darker/translucent deposits as their growth speeds up in warmer months. This results in yearly rings that coincide with the fish’s age.
Biologists can extract a fish’s otoliths and, using a microscope, count the rings to get an estimate of the fish’s age. Aging fish is useful for understanding age structure and population trends in a fishery. Otoliths vary in size and shape from one species to another, and can be used to identify the species it came from. This helps biologists trying to understand marine food webs—they can use otoliths found in the stomachs of seals, sharks, and other fish to identify what fish they eat.