Tag: otoliths

by Sue Ann Kendall

Last night, our Chapter Meeting speaker was our own Alan Rudd. He happens to know a lot about fish, thanks to his training and profession of managing fisheries and lakes. He used his knowledge to show us all how little things we think don’t matter can have huge significance for those of us studying the natural world.

I have to say he had some excellent props for his talk. The first one he broke out was a dead fish. Yep. Alan proceeded to start the dissection process on a nice-sized crappie (a delicious member of the sunfish family). All he did was break its neck and extract something very small with his tweezers.

What was it? It was a stone-like thing that grows in the ears of all fish, called an otolith or ear-stone. Every fish has them. It helps them hear. What use could that tiny thing be to research? Alan had wondered that himself, when he first studied fish anatomy.

It turns out that otoliths are pretty cool. They are not bones, but more like stones. You see, bones grow in a process of taking away parts and growing new ones, thanks to components of bone called osteoblasts and osteoclasts. That’s how we get the porosity of our bones.

The ear-stones in fish grow by depositing new layers, like an oyster grows. So, they end up being solid. The stones in different types of fish look different, too, so you can tell where one came from even if the fish isn’t there. It turns out you can learn a lot from them, after all.

Alan shared with us a collection of ear stones from fish he’d harvested lately. They were very light, but quite hard. Then he shared how he came to find out how they help research.

He spent a summer doing research on salmon in the Pacific Ocean, abord a gigantic Japanese fishing boat, which had a whole fleet of other boats along with it. In addition to Alan, there was another American who was researching Dall’s dolphins, who sometimes got caught in the giant nets (over a thousand miles were put out every night–that adds up to a lot of salmon). Thanks to the other researcher freezing dolphins that had drowned and sending them off to research facilities, it was discovered exactly what the dolphins ate in the wild. How? By identifying ear-stones in their stomachs! Previously, it had been thought that the dolphins only ate squid, because all they found in formaldehyde-stored specimens were their beaks and that big bone they have. Freezing gave more info and saved the ear-stones. Progress. Yes.

Alan also shared how research had found that fish deposit a white layer every May (no one knows why) in their ear-stones. This lets you figure out many things about a particular fish. You can count the rings to see how old it was, but you can also inspect how closely together the rings are to determine how fast or slowly the fish grew. For commercial fisheries, you want fast-growing and healthy fish, and you can check on that by ear-stones. Who knew?

Bonus fact: Alan once found a 12-year-old fish in a very acidic lake. Its slow growth had let it live longer. Huh.

Bonus fact 2: Crappie breed every year at the full moon in March. They swim around looking at the moon until the right time (I made that part up).

I assure you this knowledge was much more fun to acquire via Alan’s talk. He is a good speaker, and his theme that knowledge builds slowly on the work of past researchers made a good point. Even the little things we discover doing citizen science can help someone make a big discovery someday.