Fisheries scientists have figured out that fish otoliths grow much like the rings of a tree trunk. In the summer, when growth is high, the fish lays down a lot of otolith material. In contrast, during the winter when growth is minimal, the fish lays down very little growth material, resulting in the “ring” pattern. Much like a tree, if you count the “rings” you can determine the age of the fish.
Now, bear with me on this one because what I am about to tell you is kind of hard to believe but it's true . . . .
In the hatchery, the temperature of the water the fish are reared in can be controlled. By controlling the water temperature over a short time period (usually a few weeks), a barcode-type pattern is created in the rings of the otolith. This technique is called marking and is permanent. After the fish’s death, the otolith can be extracted, shaved down, put under a microscope and read. By collecting the fish and reading the otoliths, we can tell how old the fish is, what stream its parents came from, whether the fish strayed from its natal stream. That is really, really cool! And all of this from an inner ear bone about 1-4 millimeters in length!
--Kira Mazzi, Biological Science Technician
Photo credit: WDFW Otolith Thermal Marking Lab