North Carolina Aquariums

How is the age of a Fish Determined?

13 November 1997

Two methods are used to determine the age of a fish -- growth "rings" on scales, and/or ringlike structures found on otoliths (small bones of the inner ear). The rings correspond to seasonal changes, similar to the rings of a tree, and can be counted to calculate the fish's age.

Examined under a microscope, a fish scale reveals a series of fine ridges, called "circuli," in a circular pattern. A series of widely separated, light rings form in the summer, when faster growth takes place.

Longevity studies indicate that the life span of groupers and sturgeons can reach 40-50 years. This grouper, native to North Carolina, can live 20+ years and can reach weights of more than 50 pounds.

During the winter, slower growth is indicated by narrow separations between the rings, resulting in a dark band. Each pair of rings indicates one year.

Because scales and scale rings are sometimes influenced by other factors, such as pollution and contact with harmful materials, researchers often examine otoliths, whose ringlike structures also indicate years of life. Like scales, otoliths exhibit a series of circular rings or bands around a center point. White bands are formed during the spring and summer months, while darker bands are formed during the winter. The fish's age can be approximated by counting the light and dark bands as one year.

Although scientists can obtain an accurate estimate of a fish's age by studying these markings, little information exists on fish longevity. Research reveals that the life span of a fish can range from a few weeks or months (small reef fishes) to 40-50 years (groupers, sturgeons). In fact, scientists believe that some species of groupers may live to be 80-100 years old. Most species, however, probably live no longer than 10-20 years, with larger species generally having a longer life span than smaller species.