Over the course of many years researchers have sought out to find exactly where Alaska's Chinook salmon are hatched. The process is known, the migratory patterns are mapped, yet for any given fish caught in the wide open ocean, the story of its origins are often shrouded in mystery-but now that has changed. With a simple chemical marker, accumulating in the inner ear bone of the salmon known as an "otolith", researchers now believe that they can trace the origins of any Chinook salmon back to the exact waters from which they came before they emerged in Alaska's Bristol Bay.

Similar to trees whose rings can tell a story of the flora throughout its many years, the "otolith" tracks the life of the salmon in the form of growth rings that progress throughout the course of its open-water adventures. So with that record in mind, researchers from the University of Washington and the University of Alaska revealed in a new study published this week in the journal Science Advances that they can find the origins of each fish by matching trace chemicals to chemical signatures unique to each river in the region. Looking to the element strontium for their answers, researchers with the University of Washington compiled the strontium chemical makeup within the bedrock and the waters of Bristol Bay to determine the unique "chemical signature" unique to each location. And with this chemical map in hand, they believe that the robust tags within the otolith will reveal a sort of growing history of the fish's endeavors.

"Each fish has this little recorder, and we can reveal the whole life history of the fish from the perspective of the otolith" lead researcher of the study, Sean Brennan says. "Each growth ring is a direct reflection of the environment the fish was swimming in at the time it was formed."

"This particular element and its isotopes are very strongly related to geography" coauthor, Matthew Wooller says. "It is a really good marker for where animals have been and whether they move around in their environment."

"Alaska is a mosaic of geologic heterogeneity. As long as you can look at a geologic map and see rocks that are really different, that's a good potential area."

But aside from simply being novel, the process of finding the salmons' spawning grounds will also help researchers determine just which fish are the most fit, and which populations are thriving within the species. Roughly 200,000 Chinook salmon migrate each summer to spawn in Alaska's upper tributaries and river streams, creating a new generation of fish to population the Pacific Ocean and the Bering Sea. But with recent declines in the salmons' populations in the past decade, this new tool can help researchers with the U.S. Geological Survey and the United States Fish and Wildlife Service find out just how specific groups of fish are faring, and just how conservation efforts in some regions may seriously impact the Chinook salmons for generations to come.

"This is science responding to a societal issue and need" coauthor with the U.S. Geological Survey, Christian Zimmerman says. "Using this approach, we will be able to map salmon productivity and determine how freshwater habitats influence the ultimate number of salmon."

"With declines in Chinook salmon in Western Alaska, fishery and land-use managers need better information about freshwater habitats to guide conservation."