The Otolith Tells Life Story of Fish

To the untrained eye, the concentric rings of an otolith (fish earbone) that can be seen through a compound microscope are just a mysterious pattern-like the swirls in a thumbprint. To the fisheries research scientist, these rings are the key to a secret language that reveals the fish's entire life story.

From the otolith, the scientist can determine:

• the fish's age. Up to one year of age, a ring is formed for each day of life. After one year, the daily rings continue but become intermittent. However, yearly rings are also formed, and can be read right up until the time of death.

• the date of hatch. By identifying the very first ring at the otolith's core, the research scientist can calculate the fish's birthdate.

• the fish's growth rate. The thicker the daily ring, the greater the growth on that corresponding day.

• migration patterns. By analyzing the chemical composition of the different rings of the otolith, scientists can discover when the fish swam in warm water (such as near shore), and when it swam in cooler water (such as offshore).

• the most stressful times of the fish's life. Check lines indicate major stress. Examples include spawning, and times when the fish may have been caught and then thrown back into the water.

• age, even when the traditional method of counting rings cannot be used. In redfish, for example, the rings can be counted two different ways. One set of scientists believed they lived to age 30, while others claimed age 75. A Department of Fisheries and Oceans scientist developed a new method for aging redfish based on an analysis of the otolith similar to that of Carbon-14 dating. This method is now being used by research laboratories around the world.

As Department of Fisheries and Oceans scientists learn more of the secrets contained within the otolith, fisheries management will continue to benefit worldwide. The more that is learned about the different species of fish, the more effectively the stocks can be managed. And all of this is made possible by tiny fish earbones-less than half-an-inch long.

Published by:
Dept. of Fisheries and Oceans
Communications Branch
P.O. Box 550
Halifax, Nova Scotia
B3J 2S7