Determining the Age of Sculpin Using the Otolith Burnt Cross-Section Technique



Prickly sculpin

The prickly sculpin (Cottus asper) is a native and little-studied freshwater fish found throughout western Washington. Over the past few weeks, Roger Tabor, Kira Mazzi, Mike Elam, and I have been trying to unlock the information found in a small ear bone (otolith) hiding inside these fishes' heads. The otolith contains a record of a sculpin's growth, much like rings in a tree--the growth of the fish influences both the size and density of the otolith. Through our analysis of these otoliths, we hope to discover if different populations of prickly sculpin are growing faster and living with or without competition from other fish, at different elevations, and consuming different diets. To do this, we removed otoliths from roughly 200 preserved specimens of prickly sculpin and prepared them using a common aging method called the "otolith burnt cross-section technique."

The first step in preparing the otolith is breaking it as closely through the center as possible. I used my fingers to break them in half, but many people use forceps or a hard surface to crack them open. A good break will allow you to view each annulus (yearly growth ring). Next, and most importantly, is the burning procedure. Using lighters, we burned the otolith until we achieved a uniform dark brown. This turns the annuli dark brown and keeps the remainder of the otolith light, giving you distinction between years. Then we added a dab of mineral oil for clarity and had a perfectly prepared otolith to read.

Sculpin otolith

In the photo above, you can see the distinction between the dark brown annuli and the light summer growth. This otolith is from a mature 13-year-old prickly sculpin. As fish mature, they experience a change in growth rate. You can see this by the small separation of annuli in this adult. In juvenile fish, the translucent growth zones are much larger than those later in life (see center of the image).

We have finished all the aging for this study and will now move on to analyzing our data with the environmental and biological information we collected about the fish and their habitats early this year.
　

--Timothy Grun, Biological Science Technician