Fishy research

Fish ear bones hold biological code identifying hatchery origins and age

Posted: Tuesday, July 14, 2009

Four Alaska Department Fish and Game workers are standing under a departmental garage in the rain, chopping the tops off fish heads. From the attached bar codes and tags, pink with black polka dots, we know they are sockeye salmon caught near Wrangell.

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Charles Westmoreland / Juneau Empire
Charles Westmoreland / Juneau Empire

A chop reveals a small cavern deep inside the head. A technician pulls out a fingertip-size fish brain, tosses it, and tweezes out the real goods, two tiny ear bones called otoliths. He smears them on his wristband, cleans them up and drops them in a plastic cup.

This summer is just ramping up; he and his colleagues will log 20,000 to 30,000 fish heads by the end.

"It's not a number we like to think about," said Bev Agler, the supervisor of the Mark, Tag and Age Lab in the Mendenhall Valley.

This otolith chopping station yields a key datum for salmon biologists to consider with their stream surveys, fisherman interviews and other work. They need to manage fisheries with immediacy and precision midseason, and they need to know whether the fish being caught are wild or hatchery-raised. Because otoliths from hatchery fish bear unique thermal marks, a seasoned otolith reader can read them for where and when a hatchery fish was raised.

These man-made marks are just one piece of what you can get from an otolith, according to Fish and Game biologist Kristen Munk, who has been looking at fish ears since the 1970s. Now, she's aging long-lived rockfish by looking in their otoliths for a radiocarbon spikes that atomic bomb testing in the 1950s and 1960s left in the world's oceans and marine life.

"There's an awful lot we can get from them," she said.

As for the thermal marks, hatcheries mark billions of fish each year this way. There are other ways to mark fish - fluorescent chemicals or microscopic coded wires implanted in their noses, as in the lab down the hall - but thermal marking is a particularly cheap and elegant method.

The hatchery managers do it by manipulating a baby fish's normal growth patterns. Normally, the little nugget that is an otolith grows inside a fish's head, like any other bone, as calcium carbonate in its lymph stream gradually sticks onto it. Under a microscope, its outward growth looks just like tree rings.

But lower the temperature and the fish's metabolism changes. A dark ring grows. Raise the temperature, and a lighter ring of calcium grows. A pattern of temperature changes over a few days translates directly into a pattern of rings, a history of stress that stays on the otolith throughout the fish's life. It is a pattern that Agler and her associates can read precisely.

In their lab, one wall is lined with cases full of slides with otoliths stuck to them. It is the otolith library for the state.

Not all hatcheries produce perfect bands and hatch marks. There are always a few difficult codes to read. Plus, the bands can be squished together, and natural stresses can confound reading the manmade ones.

"It's an art," said Agler.

It's a skill that takes people a while to learn, and longer to learn to do quickly.

"We don't want someone who takes all day to read 100," Agler said.

It is, Agler said, a matter of visual pattern recognition. Some people never manage it; Agler recalled a friend who bombed, though she was a whiz at nailing sound patterns in a different context.

Pattern recognition is a theme for Agler, who studied finback whales in Maine for 17 years and found ways to identify them from their unique scars and asymmetries.

Now Agler focuses on otoliths and fish scales, which look just like fingerprints and can reliably age a fish. Again, it's about the patterns - and a lot of manual dexterity, sharp eyes, and an ability to avoid blinking while counting fish bands.

Incidentally, the biologists tend to use the fish's left otolith in their work. This leaves the right otolith as a backup. Or, on occasion, a testament to the vast number of fish heads they have processed.

The lab's mascot, Otie, sits inside a glass beaker. It is a dried piranha of an unknown age. The piranha is wearing a pirate hat made from a triangular seedpod with otoliths glued onto it, and a dirty Barbie arm is mysteriously glued to the side of the fish. A plastic sword from a cocktail drink lays below on a thick bed of otoliths that resembles a beach of coconut flakes.

• Contact reporter Kate Golden at 523-2276 or

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