2.5 Million Perfect Little Time Capsules of Ocean Chemistry

Much like counting tree rings, scientists study fish ear stones-known as otoliths-to measure age and growth rates. These small, hard calcium carbonate structures sit in a fluid-filled pocket behind the fish's brain, helping it balance and hear.

Beyond revealing age, otoliths act as natural recording devices. As a fish grows, its ear stones absorb trace elements from its diet and the surrounding water. By analyzing these elements scientists can:

• Determine what fish have eaten
• Trace their origins back to specific estuaries from which they hatched
• Track lifelong migrations
• Reconstruct decades of changing ocean chemistry

An Archive Finds a Safe Home

Across 40 years of research, Alaska Fisheries Science Center personnel, along with the North Pacific Observer Program , gathered otoliths from 83 different species. Their efforts span annual shelf and slope surveys from California to Alaska. They stretch across the Gulf of Alaska, the Aleutian Islands, the Bering Sea, and the Pacific sector of the Arctic Ocean.

Around 2 million pairs of otoliths once sat inside a 1930s wooden airplane hangar that's now part of NOAA's Western Regional Center in Seattle. This created a major fire hazard for millions of ethanol-filled vials. In 2012, a National Science Foundation grant allowed NOAA and the Burke Museum of Natural History and Culture to relocate them. They moved the otoliths to the University of Washington's Fish Collection-North America's largest facility by fish specimen count and curated by the Burke Museum. The relocation safeguarded the collection and cemented a deep partnership.

"Our Fish Collection contains not only otoliths, but millions of egg and larval specimens as well as a great diversity of juvenile and adult fish specimens," said Katherine Maslenikov, Ichthyology Collections Manager at the Burke Museum. "Out of the 13 million specimens in the repository, NOAA has supplied approximately 12 million, and that number continues to grow." The arrangement continuously drives collaboration with the University's School of Aquatic and Fishery Sciences and the partnership has also supported training and career pathways for graduates into biologist roles at the Alaska Fisheries Science Center.

Burke Museum staff transformed the once-haphazard storage system for the otoliths into a meticulously organized repository sorted by year, survey, vessel, and species. While Center scientists borrow samples most frequently, researchers worldwide can pull from the archive to advance research in many fields, including geochemistry, archaeology, and paleontology.

Tiny Samples, Big Insights

Despite ranging in size from a grain of rice to a quarter, otoliths pack a gigantic amount of information. A single specimen provides critical data on fish age, length, sex, and weight relationships for key commercial species.

"Otoliths are a perfect little time capsule," says Derek Chamberlin, a supervisory research fisheries biologist with the center's Age and Growth Program. "Whether it's the chemical signatures they absorb from the water or how their growth increments change, these samples give us incredible insight into the health and productivity of fish populations across time."

Using micro-drills, scientists extract material from an otolith's individual annual rings to measure ocean chemistry year by year. Because some species live remarkably long lives-rougheye rockfish can reach projected ages of 205 years-historical samples unlock deep environmental timelines.

"If you have otoliths collected in 1980 from a rockfish that was 150 years old, you have ocean chemistry dating back to the Industrial Revolution," says Maslenikov. "You have a record of everything we put into the ocean, including radioactivity from mid-century nuclear testing in the 1950s and 60s."

It turns out that nuclear testing is an important benchmark for scientists. Since fish don't exactly carry a driver's license, scientists use this nuclear footprint in Bomb Radiocarbon Age Validation . This technique confirms fish age estimates by tracking the mid-20th-century spike in atmospheric radiocarbon. Because researchers rarely catch wild fish today that lived through that era, the Burke Museum's historical baseline provides the data needed to verify modern aging methods.

The archive also drives research outside marine biology. Teachers bring specimens into classrooms; paleontologists compare fossilized otoliths to modern samples to map evolutionary shifts. Archaeologists recently used otoliths to track Viking-era cod fisheries and reconstruct ancient trading patterns.

A Portal to the Past, A Plan for the Future

Collecting otoliths demands significant labor and financial investment, but maintaining an archive back to the 1960s maximizes the payoff. By pairing historical baselines with modern data, researchers are better able to project how fish populations will respond to shifting ocean temperatures, changing species distribution, and evolving fishing pressure.

And otolith analysis is rapidly evolving. Researchers are building new models that can leverage artificial intelligence to count otolith rings efficiently and accurately. The technique uses Fourier Transform Near-Infrared Spectroscopy to streamline the aging process. Near-infrared light generates spectral signatures from otoliths that machine-learning algorithms correlate directly to age. Scientists spent a decade analyzing tens of thousands of physical samples from the archive to train these AI models. This breakthrough was made possible largely by the museum's extensive otolith collection. Without such an organized resource built over decades, cutting-edge discoveries like this would be far less likely.

It's exciting to imagine what else otoliths can unlock as they help scientists reconstruct the past. These 2.5 million time capsules don't just track fish populations, shape stock management, and reveal ocean history. They also equip scientists to manage tomorrow's seas and drive bold new research.

Interested in learning more or accessing otolith samples for research? Contact Derek Chamberlin or Katherine Maslenikov (@[email protected]) for additional information on the otolith archive.