Otolith
A small stone of calcium carbonate in a fish's inner ear, used for balance and hearing, and by scientists to age fish.
Also: ear stone
Bony fish have three pairs of otoliths in their inner ears. These dense little stones rest on sensory hair cells and tell the fish which way is down and how it is accelerating. Otoliths grow through life, laying down a new layer each day and a visible band each year, so scientists cut them in thin slices to count a fish's age, sometimes to the day in young fish. The chemistry of each layer also records the water the fish lived in, which shows when a salmon or hilsa moved from sea to river.