A chemical signature recorded on otoliths (ear bones) of Chinook salmon could tell scientists where the fish lived, says a new study published in the journal Science Advances.
Chinook salmon (Oncorhynchus tshawytscha). Image credit: Zureks / CC BY-SA 3.0.
“Each fish has this little recorder, and we can reveal the whole life history of the fish from the perspective of the otolith,” explained Dr Sean Brennan of the University of Washington, who is the lead author on the study.
Otoliths accumulate layers as a fish grows, similar to trees. “Each growth ring is a direct reflection of the environment the fish was swimming in at the time it was formed,” Dr Brennan said.
“This chemical signature comes from isotopes of the trace element strontium, found in bedrock.”
Strontium’s chemical makeup varies geographically. As rushing water weathers the rocks, the element is dissolved and released into the water.
The dissolved strontium ions get picked up by fish, either through the gills or gut lining, then are deposited onto the otolith.
As strontium makes its way from rocks into the otoliths of Chinook salmon (Oncorhynchus tshawytscha) swimming in the rivers, its chemical signature does not change, and so it serves as a robust tag that can tie each fish as being in a specific location in the river at a specific time.
“This particular element and its isotopes are very strongly related to geography. It is a really good marker for where animals have been and whether they move around in their environment,” said co-author Dr Matthew Wooller from the University of Alaska Fairbanks.
“This process relies on a river system that has been mapped extensively for its strontium isotope variation,” Dr Brennan added.
“In general, watersheds that are diverse in the types and ages of rocks will also have a lot of variation in strontium isotope signatures – and thus are good candidates for using this technique.”
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Sean R. Brennan et al. 2015. Strontium isotopes delineate fine-scale natal origins and migration histories of Pacific salmon. Science Advances, vol. 1, no. 4, e1400124; doi: 10.1126/sciadv.1400124
