By measuring strontium isotope ratios in different layers of otoliths from salmon caught at sea, researchers from the universities of Utah, Washington and Alaska Fairbanks and the U.S. Geological Survey were able to determine not just the watershed, but a set of streams where the salmon hatched and grew before migrating downstream to the ocean.
The new fish-tracking method may help pinpoint critical habitats for fish threatened by climate change, industrial development and overfishing.
University of Utah researchers study a cross-section of an otolith, also known as a fish ear stone or fish ear bone of chinook salmon from Alaska’s Nushagak River to help determine where the fish originated and where they spent their lives in the ocean.
“Using this method, we can trace where the salmon were born and where they moved while they were growing in the rivers and streams,” said University of Utah geochemist Diego Fernandez, co-author of a study published May 15 in the journal Science Advances. “This could be useful for protecting fish and understanding how many salmon we can take from nature.”
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The U.S. Geological Survey and researchers from universities of Washington and Alaska Fairbanks also participated in the project, which involved gathering 255 chinook — also known as king — salmon caught in Bristol Bay. The fish were captured as bycatch during the sockeye salmon season by commercial fisherman for Peter Pan Seafoods in Dillingham, Alaska.
The fishes’ otoliths, or “ears,” were sent to the University of Utah where they were cut and lasered so researchers could track changing strontium isotope ratios from the time the fish emerged as fry.
Otoliths are called bones, but are more like stones found in fish ear canals. The stones are calcium carbonate and help with balance and a sense of sound. Otoliths grow new layers daily as juvenile fish incorporate strontium from eroded rocks in the waters where they swim.
An earlier study involved sampling Nushagak tributary fish that do not leave the watershed to determine strontium ratios in rocks from the streams.
Thure Cerling, a professor of geology, geophysics, and biology at the University of Utah, said the ratios vary because rocks closer to Bristol bay and the Aleutian Islands are volcanics, and rocks father north are older sedimentary rocks.
The study revealed where each fish hatched and lived on a map showing seven different color-coded sets of streams in the 90-by-150-mile Nushagak watershed.
The team says the research can be used to understand how changes in the region might impact the salmon.
“Disturbances to salmon populations can range widely from large-scale disturbances due to a rapidly changing climate to smaller-scale disturbances, such as habitat loss or contamination from industrial development of the freshwater streams that are the spawning grounds of salmon,” said the study’s lead author, Sean Brennan, a 2007 University of Utah biology graduate and now a post-doctorate student at the University of Washington