What is an Otolith and Why Do I Care About It?



Fish otolith

An otolith (inner ear bone) can be found in any animal with a vertebra---mammals, birds, amphibians, fish, etc. Otoliths are sensitive to acceleration and gravity; these organ and bone structures tell the brain how the body is moving in relation to the surrounding environment. We are currently looking at kokanee salmon otoliths to evaluate whether hatchery-reared kokanee are helping us recover the native kokanee population in Lake Sammamish. 

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!

This is what I and other federal, state, and county biologists are doing in the field and in the lab. We are collecting kokanee from Lake Sammamish streams, spawning them, extracting the otolith, collecting biological information on the fish, and then reading the otoliths to see if marking is present. The marking will assist with the research and progression of the Lake Sammamish kokanee supplementation program. This year we saw a larger kokanee run; this is also the first year that the hatchery-reared supplementation fish were old enough to return to spawn. The information we gather from these otoliths will shed new light on the future of kokanee in Lake Sammamish and the human efforts to save this unique population.
    

--Kira Mazzi, Biological Science Technician

 

Photo credit:  WDFW Otolith Thermal Marking Lab