Otoliths are accretions of calcium carbonate that develop in the ear canals of the heads of bony fishes. They acquire yearly growth rings, or annuli, much like trees do: concentric rings around year 1 at the center. The age of the fish at the time of collection can be determined by counting the annuli. A general idea of the number of annuli can sometimes be obtained by looking at the surface of the otolith, but a more accurate count can be obtained by using the break-and-burn technique. The process is as follows:
Remove both otoliths from the vial of glycerol-thymol and put them into a dish of water. For flatfish, examine them to determine which one is from the “blind” side of the fish (the underside). On that one, the nucleus will be more centered. Return the other otolith to the vial.
Under the dissecting scope, try to count the annuli. Mark year 1 with a pencil. Using a scalpel, cut across the otolith dorsal-ventrally through year 1.
Using tweezers, hold the cut edge of one of the halves over an alcohol burner until it browns a little. It will smoke and become very hot. Be careful not to overburn it—this may cause it to crack or break.
Allow it to cool, then set the uncut edge in clay so that the cut edge can be examined under the scope. Brushing the cut edge with oil may enhance the readability of the annuli. The otolith can be toasted more if checks obscure the rings. Checks are marks and ripples that are concentric with the annuli, but do not describe a full circle around the center. If the initial half cannot be made clear enough, the other half can be tried, followed, if necessary, by breaking and burning the remaining otolith.
Return to Data Collection Return to Main Page
© 2004 Teresa Jewell
Please ask if you wish to reproduce the text or images on this site.