Chemical tags in ear bones reveal Chinooks’ life histories

The otolith, or "ear bone," is located just beneath a salmon's brain. Credit Brennan / UW
The otolith, or “ear bone,” is located just beneath a salmon’s brain.
Credit Brennan / UW

New research on the Nushagak River – one of the largest Chinook salmon runs in the world – used chemical tags in a fish’s ear bones to tell where it was born and raised. Sean Brennan is a post-doc at the University of Washington’s School of Aquatic and Fisheries Sciences. He and his team hope the study research will help managers better understand how their fisheries work.

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When you catch a salmon in the bay, how do you know where it came from? That’s long been a challenge put to fishery managers, who need that information to make decisions about catch and escapement.

A new study, published May 15 in Science Advances, hones in on habitats where chinook salmon are born and raised by tracking chemical tags in the fish’s otolith.

Sean Brennan, then a doctoral student at the University of Alaska Fairbanks’ School of Fisheries and Ocean Sciences, led the research in Bristol Bay’s Nushagak River, home to one of the world’s largest wild chinook salmon runs.

In June of 2011, Brennan spent several days on the docks of Peter Pan Seafoods in Dillingham, dissecting the heads of chinooks that were on their way to be processed. He collected 255 otoliths, or “ear bones,” using tweezers to pull out the thin white discs.

Sean Brennan doing research on the Nushagak River. Credit UAF
Sean Brennan doing research on the Nushagak River.
Credit UAF

Brennan wanted the otoliths because they contain a chemical souvenir of the fish’s travels: the element strontium. And as he puts it, “not all strontium is created equal.” Some of the strontium in earth is heavier, and some is lighter. Those different weights, called strontium isotopes, are found in the bedrock of Bristol Bay. Water flowing over these rocks picks up dissolved strontium, which makes its way into the bodies of fish.

Over a fish’s life, strontium isotopes are deposited onto the tiny ear bone in layers. “The different stretches of rivers the fish are in are essentially tagging the otolith at that particular time in that fish’s life,” explained Brennan. Co-authors Diego Fernandez and Thure Cerling at the University of Utah analyzed these chemical tags, reading the strontium layers like rings on a tree stump.

Using ear bone data from juvenile fish in the upper Nushagak, researchers put together a map of the strontium isotopes in different areas of the watershed. By comparing that map to the strontium in ear bones, Brennan and his team were able to reconstruct each fish’s life history.

One exciting result of the research, Brennan says, is that he can now identify seven distinct zones – seven strontium isotope groups – in the Nushagak watershed. “So when we catch chinook salmon in Nushagak Bay,” he explained, “we now have the ability to determine which of those seven groups produced that particular fish.”

This is a big deal to scientists like Brennan. Other tracing methods, like genetics, paint broader strokes; there’s just not enough genetic variation between chinook populations in Bristol Bay. But the strontium isotope method can tell the precise tributary where a fish hatched in the Nushagak, and how long it stayed there.

Brennan’s results indicate that 70 percent of Nushagak chinook stay in their natal streams until they make a beeline for open ocean. But 20 percent, he said, move earlier, spending an extended period of time in the lower main stem Nushagak before migrating to the ocean. It’s like a small group of teenaged salmon have a hangout spot that scientists didn’t know much about before.

“What’s interesting about that,” Brennan says, “is the common thought is that the lower Nushagak doesn’t produce that many fish.” The new research shows that, in fact, the lower Nushagak is home to a fifth of juvenile chinook for a significant time period before they leave the river.

Researchers found 7 distinct strontium isotope zones in the Nushagak watershed.
Researchers found 7 distinct strontium isotope zones in the Nushagak watershed.

These results also indicate the life histories of Nushagak-born chinook are more varied than previously expected; some juveniles stay in their natal streams longer, while some move out earlier.

It’s this variety of behaviors and life histories that make the Nushagak chinook population so resilient to changes in the environment, says Christian Zimmerman, a USGS ecologist who advised and co-authored the study. “Say it’s a really cold winter – that might benefit fish that leave later,” he explained, “but a warm winter pays off for fish that leave sooner.” This new tracking tool is just another way to understand that resilience.

On a broader scale, Zimmerman says, the strontium isotope method could help fishery managers – in Alaska and beyond – better understand year-to-year changes in productivity. Knowing where a catch comes from, he says, gives you more power in determining how many fish you can sustainably harvest.

“One of the things we hear throughout Western Alaska is that when we see declines [in salmon returns], it’s a bit of a surprise,” Zimmerman said. Scientists hope this tool will take some of that surprise out of the equation, helping predict changes to the environment that may affect salmon runs.

“Our hope is to better understand how freshwater habitats relate to productivity, Zimmerman said. “So you wouldn’t suddenly find a year where commercial or subsistence fishing would have to be regulated, like it has on the Kuskokwim River. You would have some idea that it was happening beforehand.”

That may be a few years off, but Zimmerman says researchers intend to use isotope tracking on the Kuskokwim and Yukon Rivers soon.

For now, Brennan is working with Daniel Schindler at the University of Washington, where they will expand their research to include sockeye salmon on the Nushagak. They plan to collect three years of Chinook data and two years of sockeye by the end of the project.

But it doesn’t end with fish. Brennan says strontium isotopes could help track migratory mammals like caribou or seal as well. “Being able to link highly mobile species to the critical habitats that they use in the critical times of their life is a fundamental piece of information when you’re trying to come up with some sort of conservation strategy,” said Brennan. This tool provides a reliable way to do that.

Note: Other co-authors on the study were Matthew Wooller and Megan McPhee at the University of Alaska Fairbanks.