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How to track the last salmon: Ear bones

Salmon are among the most popular commercial American fish, yet their ecology still isn’t 100% known to scientists. That’s a problem for anyone hoping to manage and collect the fish in a sustainable manner. Now, scientists from the University of Alaska have developed a novel method for learning how, when and where Alaskan salmon spend their developmental years: Chemical signatures on the otolith, a bone in their ears.

“Each fish has this little recorder, and we can reveal the whole life history of the fish from the perspective of the otolith. Each growth ring is a direct reflection of the environment the fish was swimming in at the time it was formed,” said lead author Sean Brennan, who completed the study as a doctoral student at the University of Alaska Fairbanks.

Not unlike the rings found in trees, the otolith bone in salmon collects layers as a fish grows. In order to track the fish, scientists can compare the chemical signature in the otoliths with the chemical makeup of the water in which they find them. The element used to track the fish is strontium, a trace element found in Earth’s bedrock. As running river water erodes surrounding rocks, the strontium gets dissolved and spread throughout the water. Eventually, it makes its way into the salmon and into the layers of their otolith bones.

“This particular element and its isotopes are very strongly related to geography,” said Matthew Wooller, director of the Alaska Stable Isotope Facility at University of Alaska Fairbanks and a co-author of the paper. “It is a really good marker for where animals have been and whether they move around in their environment.”

Crucially, the Bristol Bay region of Alaska is home to one of the last wild commercial salmon fisheries on the planet. Chinook salmon from the Nushagak River are among the most popular varieties, and also happen to be the third largest run of salmon in the world. Thanks to the novel tracking technique, the researchers now know a lot more about how these salmon spend their time, though scientists are still unsure as to why Chinook populations have been on the decline in the past decade.

Each summer, when 200,000 Chinook make their way into the river from the ocean to spawn, scientists now know that the vast majority of them spend most of their first year in the same place they’re born, with about 20% venturing out into the river’s lower main streams before heading back to the ocean. With any luck, the new tracking technique can be applied to adult salmon with the intent of helping fishery and land-use managers improve their conservation efforts.

“This is science responding to a societal issue and need,” said co-author Christian Zimmerman, U.S. Geological Survey ecologist and chief of water and interdisciplinary studies at the USGS Alaska Science Center in Anchorage.