A tiny white sliver inside the heads of fish could hold evidence of a century’s worth of humans wrecking the environment: atomic bombs, overfishing, even climate change.
Fish ear bones, also known as otoliths, are like tree rings for the ocean. A layer of calcium carbonate laid down each year offers a snapshot of both the fish’s yearly growth and its surrounding ocean conditions.
The University of Washington’s Burke Museum has been transferring and cataloging 2 million pairs of otoliths, representing some 80 species. Scientists hope this collection, gathered over the past half-century, will help them track the health of fish populations and ocean conditions up and down the West Coast.
The otolith collection, dating to the 1960s, had been sitting in an old Sand Point hangar belonging to the National Oceanic and Atmospheric Administration (NOAA). Last year, Ted Pietsch, a UW professor and curator of fishes at the Burke Museum, got a grant to transfer the otoliths to the museum — all of a 10-minute drive away.
The reason for the not-so-big move? Fire. Although NOAA scientists had been doing active research on the collection, the thousands of flammable Styrofoam boxes — piled 20 feet high and filled with ethanol for preservation — were a huge fire hazard.
The threat of fire persuaded the National Science Foundation to fund a two-year, $500,000 grant. Instead of sitting haphazardly and uncataloged in a hangar, the otoliths will be archived in the Burke and searchable online for outside scientists.
The UW recently made the first loan of the otoliths to Oregon State University, where researchers are studying the age when flatfish settle to the ocean floor.
Information unlocked by analyzing the chemical makeup of each otolith layer has piqued the interest of archaeologists, geochemists and fish biologists alike.
Tree rings of the sea
Fishermen, not scientists, were the original beneficiaries of the otolith data, which feed population models that help determine catch limits each year. Fish populations are closely managed by NOAA so as not to repeat disasters such as cod overfishing on the East Coast.
Otoliths reveal age, which when aggregated with the sex, size and locations of capture for many fish, paint a portrait of the population’s health. Thus, scientists can estimate how much fishermen can catch without causing a whole species collapse.
Trained observers go to sea with fishermen to gather catch and bycatch data. NOAA’s fishery research centers rely on scientific survey boats as well as these observers to collect otoliths.
“Each fish has its sweet spot,” says Katherine Maslenikov, fish-collections manager at the Burke, who has dug through fish heads herself as an observer on fishing boats.
Three pairs of otoliths — scientists collect only the largest pair — sit in capsules of liquid behind the fish’s head. Technically, they are stones, not bones, because they contain no live cells.
Otoliths are unique to each species. A pollock’s are wing-shaped and about 2 centimeters long. Others are delicate white filigrees pretty enough for jewelry. The salmon has especially tiny and hard-to-find otoliths.
This huge variation among species is what makes aging otoliths such difficult work.
Determining the age of otoliths is the specialty of Tom Helser’s lab at the Alaska Fisheries Science Center, a part of NOAA. His 12 -person lab ages about 30,000 otoliths a year.
The low-tech method for aging otoliths is called “break and burn.” Heating them in a simple flame darkens the annual bands, so they are easier to count.
A more sophisticated method involves drilling out tiny slivers of the otolith. The wisp of powder — barely enough to be visible to the naked eye — is analyzed for oxygen-18, a slightly heavier form of the usual oxygen-16. Because it varies with temperature, the oxygen-18 level rises and falls with the seasons. Each rise and fall represents one year.
But it’s the change in elements like oxygen-18 over many years that has scientists really excited about otoliths.
“Flight recorders” is how NOAA’s Helser likes to describes otoliths.
Ocean temperature, and thus the oxygen-18 level, varies from shallow coastal waters to deeper ones, so otoliths record migration patterns. A recent study in yellowfin sole found a sharp increase in oxygen-18 after the fish turned 7 years old, meaning it moved into deeper, colder waters. As juveniles, they must have lived near the coast.
“This is an animal that responds extremely closely to temperature as it grows older,” said Helser, noting how climate change could interfere with the fish’s usual behavior.
Jeremy Harris, a graduate student working with the otolith collection at the UW, is also leading a project that could uncover the effects of climate change.
He is comparing the otoliths of walleye pollock from 50 years ago with contemporary ones, with the primary goal of figuring out whether the species has gotten smaller over time. This happened in cod when the largest fish were caught and only smaller ones were left to reproduce.
Combining growth data with temperature could also shed light on climate change.
On a longer time scale, 4,000-year old otoliths in prehistoric trash heaps are a record of ancient surface-water temperatures. This is the basis of a collaboration between Helser and archeologists in Newfoundland.
The chemical signature of the atomic bomb is seen in otoliths, too.
Atomic bomb testing in the 1960s caused a sharp spike in carbon-14 in all living things, from rhino horns to tree rings. That carbon-14 signature is one way to validate the age of older otoliths.
Some of the otoliths in the collection date to the 1960s, and that includes rockfish, which can live more than 100 years.
The ultimate goal in cataloging the collection is to aid this wide range of research.
On a Thursday morning, Helser walked through the cavernous NOAA hangar counting pallets of otoliths. Next to boat parts and old office equipment is another 100 or so pallets yet to be moved to the UW. Each pallet contains up to 200 boxes, each holding 100 pairs of otoliths when full.
The UW is about halfway through cataloging the otoliths. A team of undergraduates is deciphering the scrawled handwriting and putting otoliths in orderly, archival-quality boxes.
“I thought it was going to be overwhelming, just the number of samples,” said senior Kali Williams, as she sorted through her umpteenth box.
“This process really highlights the value of collections,” added Maslenikov. “For a long time, they just kept them in boxes, and all of a sudden there’s this wealth of data so many people can use. What if they’d just pitched them?”