The chemical map of otoliths

Arctic Toolik frozen rivers
Frozen rivers of Alaska’s North Slope; when thawed, they provide Arctic grayling with vital habitat for feeding and spawning / Image Cameron MacKenzie

It’s about the size of a diamond and comes from the inner ear of a fish. This tiny construction holds a treasure trove of information, a calcium carbonate microchip made of bone and accessed by a laser. Let’s take a look at the science of otoliths.

Highly magnified view of an otolith, an Arctic grayling inner ear bone / Image Heidi Golden
Highly magnified view of an otolith, an Arctic grayling inner ear bone / Image Heidi Golden

An otolith is a fish ear bone (from oto– ear and lithos– stone). From every population of fish Heidi Golden finds, she preserves a number of otoliths to take back to the lab. These bones are roadmaps of Arctic grayling lifetimes.

Grayling samples

Heidi Golden, Ph.D. candidate at the University of Connecticut (Storrs Campus), is immersed in Ecology and Evolutionary Biology studies. As part of her work researching Arctic grayling, she’s taking samples from fish to map out their genetic distribution across Alaska’s North Slope.

Grayling used to be more widely spread through North America where extremely cold and clean water was found: from Alaska and Canada down to the upper Midwest. Now, they’re absent from the Midwest and struggling in Montana and parts of Canada.

Golden is focused on Arctic grayling which reside in Alaska’s Kuparuk River. The Arctic grayling population which lives in the upper river relies on extensive migrations. They spawn in gravelly riverbeds, travel the river to feed and fatten up, then overwinter in remote Green Cabin Lake. Other populations live further downstream, closer to the Arctic coast. In order to map out genetic differences, Golden visits multiple remote locations to sample fish.

Rings of data

Dendrochronology uses tree-rings, or growth rings, to discover information about the past. Growth rings allow scientists to date when a tree grew and determine information related to environmental conditions. Did the tree grow well and flourish? When did droughts occur?

Otoliths are similar to tree-rings; as a fish ages, its otoliths grow daily rings of calcium carbonate. The chemical composition of the otholith rings matches that of the fish’s environment. As the fish grows and travels from place to place, its ear bones record a chemical signature that can tell scientists like Golden where the fish has been.

“I’ve taken 10 fish from each of the populations that I’ve gone to and I’m going to be analyzing those earbones by aging the fish and looking at the chemical composition. When you look at an otolith, the center of the otolith comprises the birth of the fish and the edge of the otolith constitutes the death of the fish. For each ring you can go in with a laser and get the chemical compostion of that ring, then match up the otolith chemical signal with the chemical composition of different places along the river.” ~ Heidi Golden

Chemical map otoliths fish ear bones - Arctic grayling otolith ear bone extraction
Extracting an Arctic grayling otolith / Image Heidi Golden

Glaciers and chemistry

Golden explains why different locations of the river she works alongside have chemical signatures.

“The chemistry is dictated by the underlying glacial geology of the area. And the North Slope has a strong signal in its water chemistry because it was glaciated three successive times.” ~ Heidi Golden

Glacial periods describe historical periods thousands of years long when glaciers or ice sheets were growing and advancing across the land. Their warmer counterparts are interglacial periods when ice was receding.

Arctic grayling juvenile fish
Juvenile Arctic grayling, magnified / Image Heidi Golden

The part of Alaska where the Kuparuk River flows was covered by glaciers three times in (relatively) recent history. The first wave of glaciers covered an extensive amount of ground, the second wave a middling amount, and the third a smaller amount.

“Those are three different levels of exposure that the rocks have had. So over time minerals leach out and the isotopes change. As the rocks are exposed you get these differences: it’s a gradient in chemistry from the headwaters down toward the Arctic Ocean. And that gradient allows you to use the micro-chemistry signal as a natural tag. If there is no gradient in your water chemistry, you really can’t use otoliths this way. We’re really fortunate that we have this great signal in our system.” ~ Heidi Golden

Telling patterns

Golden has to trek through Alaska collecting water samples from different locations and analyze those samples for their chemical signatures. After collecting otholiths she must carefully sand down the bone to reveal an ideal surface, use a laser to ablade the bone while simultaneously using a mass spectrometer to analyze the chemical signatures held by the different layers of the bone. Finally, she can chart how those chemical signatures match up with different locations in the river.

“By looking at the pattern of the chemicals in the otoliths as the fish travels through the river throughout its lifetime, you can see peaks and valleys in the chemistry data as the fish traveled from the overwintering site which has one chemical signal to a spawning location which has a different chemical signal to a feeding location which has yet another chemical signal. Not only that but you can track it over the lifetime of the fish.” ~ Heidi Golden

Arctic grayling are very important for the stability and biodiversity of their ecosystems. Studying the otolith microchemistry will help Golden paint a picture of Arctic grayling populations, and determine what specific locations and habitats preserve their species’ health and genetic diversity.

“They are going to be very telling, not only looking at life movement patterns – when and where the fish migrate within their life span – but also looking at site fidelity: How often individual fish return to the same site? Because rivers freeze solid in winter, locations or larval overwintering habitat might be of particular importance. By looking at the core of the otoliths we might be able to locate critical spawning areas and discover where larval fish spend their first year of life. I think those habitats are critical for sustaining a population of grayling on the North Slope.” ~ Heidi Golden

Laura Nielsen 2014

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

  • Sourced from interview with Heidi Golden, Ph.D. candidate at the University of Connecticut, July 2014
  • (Chemical map otoliths fish ear bones)