Tracing Where Salmon Were Born And Bred

Inspecting a bone from a salmon's ear can reveal where it has lived.

Peter Gwynne, Contributor

(Inside Science) -- An intrepid kayaker has combined with a laboratory team carrying out high-technology chemical analysis in a project that could help locate critical habitats for salmon and other fish threatened by climate change, industrial development, overfishing, and other disturbances.

The project used the chemical makeup of bone-like structures in the ears of 255 Chinook salmon to identify the specific waterways emptying into Bristol Bay, Alaska, in which individual fish were born and bred and migrated.

The chemical information comes from the metal strontium. In salmon bones, strontium exists in two particular forms, or isotopes, characterized by the numbers of particles in each atom's nucleus.

The research team measured the ratios of strontium-86 to strontium-87 in structures in salmon ears called otoliths.

These lozenge-shaped features consist of ultrathin layers laid down each day, like tree rings, as the salmon grow. When each layer forms, it possesses the same ratio of the two strontium isotopes as the water in which it swims at the time.

Interpretation of the chemical signals from the otolith layers indicated where each salmon had hatched and where it had swum after hatching, in seven different sets of streams in the watershed of western Alaska's third largest river, the Nushagak.

The watershed, roughly the size of Massachusetts, is home to one of the world's largest runs of Chinook salmon. About 200,000 Chinook swim upriver from the Pacific Ocean each summer to spawn in the Nushagak's upper tributaries and streams.

But in the past decade, Alaska's Chinook salmon population has fallen dramatically. The new study, headed by Sean Brennan, a postdoctoral researcher at the University of Washington, in Seattle, can help to understand and perhaps reverse the decline.

Previous projects of this type have focused on small numbers of salmon.

"What's really new is the sheer number of samples that have been processed," said Thure Cerling, a geochemist at the University of Utah, in Salt Lake City, who participated in the study. "That allows us to think about population scale rather than individual scale."

The information "could be useful for protecting fish and understanding how many salmon we can take from nature," said geochemist Diego Fernandez, also at the University of Utah.

Fernandez measured the strontium ratios that revealed the salmons' origins.

"The study makes connections between the fish migration and the geology of the spawning ground," said Bernhard Peucker-Ehrenbrink, a senior scientist at the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, who did not participate in the research.

"The level of detail is pretty impressive," he added. "The study makes a great case for how this method can be applied."

The research was reported today in Science Advances, and was the third in a series of studies focused on Chinook salmon's origins.

As part of his Ph.D. project at the University of Alaska Fairbanks, Brennan spent three years surveying the Nushagak watershed for evidence of strontium isotope ratios in its various streams and tributaries.

Since the area has no roads, Brennan recalled, "We would generally fly into the headwaters with inflatable kayaks and spend a week and a half there. Then we would get a person from the local village to meet us and take samples in a skiff."

Next, the team needed to check that strontium isotope ratios in fishes' otoliths accurately reflect those in the waters where the fish swim. They did that by measuring the ratios in slimy sculpin. This fish doesn't travel more than 50 yards from its home waters throughout its lifetime. Investigation showed that the sculpins' ratios were always the same as those in the water around them.

The research reported today built on that finding. It used the strontium ratios to determine the swimming history of Chinook salmon caught in Bristol Bay, on the way back to their home waters in 2011.

To measure the ratios, the team applied a technology known as plasma mass spectrometry.

They cut each otolith lengthwise, and used a laser to zap each layer. The process removed tiny particles that then traveled into a plasma -- a very hot flame -- that separated the strontium isotopes and permitted measurement of their relative quantities.

"This laser ablation allows you to follow the life history of a salmon from the moment it was born to the moment it reached the ocean," Fernandez said.

The study revealed four distinct life histories for the Chinook salmon. While almost three-quarters stayed in the waters where they hatched before swimming downstream to the ocean, the rest had more complex young lives.

About 17 percent of the Chinooks mostly stayed where they hatched, but also made short forays downriver just before they set off for the ocean. Seven percent swam to a different stream from the one in which they hatched and stayed there before setting out to sea. And four percent both left their birthplaces for another stream and took short downriver journeys before leaving for the ocean.

"This is one of the prime salmon fisheries in the world," Cerling said. "So it's critical that we understand all the aspects of the salmon runs to manage their resources in the right way."

The technology has potential application beyond salmon and even fish. "The cool thing about the strontium isotope ratio," Brennan said, "is that it works regardless of species."