Blog Redux: Fish Ears, “Tree” Rings and a Sectioning Saw

We thought we’d dig through the archives to see what we were up to in previous Novembers. Enjoy this look at “Limno in the Lab” from four years ago!

Aaron Koning uses a sectioning saw to cut a slice of otolith that will be mounted on a slide and polished, enabling him to see it clearly under a microscope.

Originally posted 11/13/12 – After the spring and summer field seasons, it’s time to return to the lab to work up all the specimens collected in the field. For many grad students at the Center for Limnology, this means days, if not weeks, hunched over a circular sectioning saw and buffing wheel.
What are they doing using equipment more appropriate for a jewelry store? Cutting and polishing fish ear stones, of course.
These ear stones, or otoliths, are small disks of calcium carbonate that grow on either side of a fish’s brain. Much like the inner ear in humans, otoliths help fish hear, sense vibrations, and maintain balance and orientation. While certainly an essential little piece of anatomy for the fish, otoliths are nearly as essential to fisheries researchers.
Much like trees, the rings of an otolith can help researchers determine the age and growth rate of a fish.

That’s because the little disks grow as layers of calcium carbonate accumulate on their outer edge, forming rings that, much like tree rings, can tell us a lot about age and growth rate.
That’s invaluable information to researchers like Zach Lawson, a grad student in Steve Carpenter’s lab. Lawson is working on data from the Crystal Mixing experiment, and wants to know if the unnaturally warm waters created by that experiment affected the growth rate of invasive cold water smelt in Crystal Lake. Thinner growth rings would suggest Lawson and his colleagues created less than ideal conditions for the fish, which is the point, as they’re hoping to extripate the voracious invaders from the lake.
But otoliths can provide much more information than just the age of a fish. They can also serve as a sort of travel journal.
Dan Oele holds two otoliths taken from a northern pike as part of his study on Green Bay tributaries.

Dan Oele, a grad student in Pete McIntyre’s lab, is using otoliths taken from Northern Pike near Green Bay to better understand their spawning habits. The water chemistry of a shallow road culvert or wetland where the fish begins its life is different than the chemical make-up of the waters of Green Bay where it lives as an adult. And those different environments leave a trace.
“Otoliths are primarily made up of calcium,” Oele explains. “But there can be mistakes in the uptake mechanism, so elements that have a similar atomic size and structure to calcium – like strontium, barium and magnesium – can show up.” By detecting the presence of such elements in the growth rings of an otolith, while also knowing the proportion of those elements in different aquatic habitats, Oele can then determine roughly where a fish has been. He hopes to use this information to determine if adult pike return, like salmon, to the waters of their birth to spawn, or if any suitable wetland will do. It’s an important question for fisheries managers hoping to promote pike spawning. Can they restore any old tributary, or must they target the “home site” of one of Wisconsin’s favorite sportfish?
After cutting a section of otolith, Koning uses the buffing wheel to polish it. The smoother surface will then be easier to view under the microscope.

Aaron Koning, also a grad student in Pete McIntyre’s lab, is currently working on a project similar to Oele’s, but he’s tracking catfish in the Mekong River. It’s assumed that the catfish migrate, but no one knows how far that range is or where, exactly, they go. The catfish are an important fishery for that part of Southeast Asia, but no one knows enough about their movement patterns and life history to put effective conservation efforts in place. (Aaron’s headed to the Mekong in January, so be on the look out for more on his work.)
While the process of cutting, polishing and mounting otolith sections onto slides will surely grow old as the winter months drag on, our students can at least dream of warmer weather and the next field season and know that, with those tiny disks of calcium carbonate, they’re doing some pretty cool science here at the Center for Limnology!
A fully prepared “slice” of otolith. Ready for its close-up under the scope.

 
 
 

1 thought on “Blog Redux: Fish Ears, “Tree” Rings and a Sectioning Saw”

Leave a Reply

Your email address will not be published. Required fields are marked *