Chemical "signatures" in the bones of Chinook salmon from the Bristol Bay area in Alaska could help wildlife resource managers identify exactly where the fish were born and where they have spent their lives, researchers say.
The tiny bones called otoliths in the fishes' inner ear canal grow as the fish grow, adding layers each year similar to how a tree forms annual rings.
By analyzing chemical signatures in particular layers of the bones and comparing them to the chemical signatures of the water the fish swim in, scientists can locate exactly what body of water they were born in and where they have journeyed throughout their lives, the researchers report in the journal Science Advances.
"Each fish has this little recorder, and we can reveal the whole life history of the fish from the perspective of the otolith," said study lead author Sean Brennan, who did the research while a doctoral student at the University of Alaska Fairbanks.
"Each growth ring is a direct reflection of the environment the fish was swimming in at the time it was formed," said Brennan, now a postdoctoral researcher in the University of Washington's School of Aquatic and Fishery Sciences.
The researchers analyzed the tiny ear bones of salmon for the element strontium, which is dissolved out of river rocks by moving water.
Different amounts of different isotopes are found in different regions, so the distinct isotope ratios in the layers of the fish bones can show where they were at any period of their lives, the researchers say.
"This particular element and its isotopes are very strongly related to geography," said study co-author Matthew Wooller from the University of Alaska. "It is a really good marker for where animals have been and whether they move around in their environment."
That makes the bone arrangement of the salmon ears a ready-made research instrument, other experts say.
"It's an amazing structure," said marine biologist Anna Sturrock of the University of California, Berkeley, who wasn't involved in the study. "It's this little data-logger in the fish's head."
To test their hypothesis, the researchers traveled the length of the Nushagak river in Alaska collecting water samples, from which they identified distinctive strontium profiles for seven regions of the river's streams and tributaries.
With that data, Brennan and his colleagues were able to pinpoint the birthplaces and subsequent travels of more than 400 salmon caught in the region, by analyzing the strontium found in their otoliths.
Such data, and further such studies, could help resource managers better understand how individual populations of fish are responding to habitat loss or changes in their environment, the researchers say.
