The Otolith Sectioning Process
Otoliths, commonly called ear stones, are removed from the fish's head either by entering through the top of the head or by pulling back the gills. The sagittal otoliths, the largest of three pairs of ear stones, are removed, cleaned, dried, and stored in a vial.
Thin crosswise sections (approximately 300 to 400 micrometers [µm] wide) are cut with a low-speed saw. Generally, four blades separated by aluminum spacers are used to yield three sections. For consistency, the left otolith is processed. The right otolith is archived if the left is successfully processed and aged.
Different species have otoliths of different shapes and sizes. The otoliths shown below are from a red drum, Sciaenops ocellatus. The whole, uncut otolith is gently curved. On the convex surface, which faces the center of the fish's body, is a two-part, tadpole-shaped feature called the sulcus acousticus, to which an auditory nerve attaches. The sulcus generally runs parallel to and closer to the dorsal (back) edge, curving downward toward the ventral edge. The narrow portion of the sulcus is a shallow groove called the cauda. This joins the ostium, a rounded, sometimes oval area whose shape varies among species. The cauda curves toward the posterior (the tail) of the fish, and the ostium points toward the anterior (the head). In this figure, the sulcus that looks like a tadpole facing right is on the left otolith.
To section the otolith, specimens are hot-glued to small pieces of tag paper, which are held to the saw arm by a small binder clip. The specimen is slowly lowered onto the spinning saw blades, which dip into a water bath for lubrication.
The sections are rinsed in water, dried on blotting paper, mounted on a microscope slide, and covered with a liquid mounting media that hardens to form a cover slip.
Otolith formation begins at the earliest stage in fish development. The core, or focus, of the otolith is generally a thickening located behind the ostium and is the point at which the otolith originates. The exact location of the core in an adult specimen is different for every species. The core is marked before cutting to ensure that a perfect section is obtained, because sections that are either anterior or posterior to the core are difficult to age. The figure below shows examples of anterior, core, and posterior sections from a spotted seatrout, Cynoscion nebulosus. A perfect core section shows a sharp V-shape whose point ends at the core. An anterior section has an incomplete V that does not come to a point, whereas a posterior section has a funnel- or tornado-like shape. Notice that the first annulus (arrows) fades in the anterior and posterior sections. The annulus is a continuous, concentric zone that forms once a year during a period of slow growth.