North Pacific Anadromous Fish Commission

Otolith Marking

Otolith Marking

What are Otoliths?

Otoliths or “fish ear bones” consist of three pairs of small carbonate bodies that are found in the head of teleost (bony) fish. Otoliths are used by fish for balance, orientation and sound detection, thus they function similarly to the inner ear of mammals.

These pairs of otoliths differ in location, function, size, shape, and structure. The three pairs of otoliths are most commonly called the lapilli, asterisci, and sagittae.

In Pacific salmon, the asteriscus and lapillus are usually quite small, only a millimeter in size, but the sagittae are much larger, ~5 mm. Thus, the sagittae are the most studied. They are often referred to as “the otoliths,” although this term more correctly applies to all three structures.

The otolith is a crystal; consequently, it grows by the precipitation of ions on its exposed surfaces. During this process, protein and calcium carbonate are laid down on the surface of the otolith, although the relative amounts vary with time and season.

Thin sections of an otolith reveal a detailed microstructure consisting of bands of opaque and translucent material, sort of like the rings on a tree trunk. Fisheries biologists have discovered that they can extract a variety of information about a fish by looking at changes in these patterns. In some cases, these patterns are a natural record; in other cases they are induced by man.

Because otoliths provide useful information on age, growth rate, life history, recruitment, and taxonomy, they are widely used in fisheries management. Fisheries biologists like to think of otoliths as information storage units; a sort of CD-ROM in which the life and times of the fish are recorded. If we learn the code, we can learn about the fish.