How to tell how old a fish is by Alison Haupt
Otoliths are ear stones, made of Calcium Carbonate, located near the brain cavity of a fish. Each fish has three types of otoliths, and two of each type. The largest is called a sagita, the medium a lapillus, and the smallest asteriscus. For the fish, these ear stones function for balance and hearing, much like our ear bones, but scientists use these to determine the age of a fish.

The otoliths grow with the fish as it ages. Usually a fish will have periods of fast growth and periods of slow growth during a year, these growth changes will provide changes in patterns in the otolith causing annual rings to form much like the trunk of a tree has a ring for each year of its life. If the fish experiences strong seasonal changes these annual rings, called annuli, are easier to see. The otolith, usually a sagitta, can be cut and examined under a microscope to count the rings. Some fish are easier to age than others.

Many parts of a fish can be used to tell its age, including the scales, bones, fin rays, vertebrae, and otoliths. These structures all provide indications of age, since they tend to produce annual growth rings, called annuli, like a tree.

Of these, the otolith (which is kind of like an ear bone) tends to be the easiest to interpret. As the fish grows, it undergoes annual changes in its growth rate due to seasonal variations in temperature, food abundance, etc. As the growth rate changes the structure in the deposited ear bone changes, and thus rings in the ear bone correspond to these annual environmental changes. However, it can be very difficult to count these rings accurately, especially since sometimes growth rates change due to factors other than seasonal change in the environment (for example, the fish might have been injured).

This can create false rings. Also the patterns tend to be complex. Therefore, it takes a lot of experience to read these otoliths accurately, and most scientists like to use other forms of dating (such as radioisotope analysis) to bolster their estimates.

Yes, by analyzing the shape and layered growth pattern of an otolith, the age of a fish can be determined. These layers bear a slight resemblance to the growth rings of trees. Using always newer and more powerful analysis tools, scientist have been able to determine not only the age but also many details of the life of different fish species, previously unknown.

During a research trip to Japan in 2000 I heard a presentation that explained how by measuring the ratio of calcium to strontium by electron microprobe analysis in the otoliths of eels the researchers can tell the time spent by the eels in river water and the time spent out in ocean waters.