Bedford Institute of Oceanography
Otoliths are commonly referred to as "earstones" or "fish ear bones." They are hard, calcium carbonate structures located directly behind the brain of teleost (bony) fish. Otoliths help with balance, orientation, and sound detection-much like the inner ear of mammals. They are not attached to the skull, but "float" beneath the brain inside the soft, transparent inner ear canals.
There are three pairs of otoliths in each fish: 1 large pair (the sagittae) and 2 small pairs (the lapilli and the asteriscii). In Pacific Salmon, the asteriscus and lapillus are very small, usually only about a millimeter in size. The sagittae are much larger (about 5 mm), and are the most studied of the three. They are the pair most often used in determining age.
Thin sections of an otolith reveal bands of opaque and translucent material, sort of like the rings on a tree trunk. The growth of otoliths is a one-way process: new material (protein and calcium carbonate) is added to the exposed surface of the otolith over time, but existing material cannot be removed. The one-way growth process explains why otoliths can form and retain such delicate structures as daily rings, while bones cannot.
Steven Campana, Bedford
Institute of Oceanography
Otoliths have a very distinct shape, which is characteristic of the species of fish. That is, different fish species have differently shaped otoliths. Indeed, the shape is so distinctive that biologists can use the otoliths recovered from seal and bird stomachs and droppings to determine the type of fish they ate.