Telltale bones

  • Environment
  • Tuesday, 06 Aug 2013

Fish ear bones offer clues to the health of oceans and species.

A tiny white sliver inside the heads of fish could hold evidence of a century’s worth of humans wrecking the environment: atomic bombs, over-fishing, 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 cataloguing two 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 US West Coast.

The otolith collection, dating to the 1960s, had been sitting in an old hangar belonging to the National Oceanic and Atmospheric Administration (NOAA). Last year, Ted Pietsch, a University of Washington professor and curator of fish at the Burke Museum, got a grant to transfer the otoliths to the museum.

The reason for the move? Fire. Although NOAA scientists had been doing active research on the collection, the thousands of flammable polystyrene boxes – piled 7m-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, US$500,000 grant. Instead of sitting haphazardly and uncatalogued in a hangar, the otoliths will be archived in the Burke and searchable on-line for outside scientists.

The university 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 analysing the chemical make-up of each otolith layer has piqued the interest of archaeologists, geochemists and fish biologists alike.

Fishy tales

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 over-fishing on the US 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 centres 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 2cm long. Others are delicate white filigrees pretty enough for jewellery. 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 Centre, 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 analysed 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 seven 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 behaviour.

Jeremy Harris, a graduate student working with the otolith collection at the University of Washington, 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.

Insight on the climate

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.

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 cataloguing the collection is to aid this wide range of research. – The Seattle Times/McClatchy Tribune Information Services

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