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Otoliths: A Window into a Salmon’s Migration Journey

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What are otoliths?

Otoliths are small bone-like structures that grow inside the inner ear of many fish species, including some of the native fishes that live in the Yuba River, like the Chinook salmon. Otoliths function much like the human inner ear, helping fish find their balance in the water. They also contribute to a fish’s hearing and sense of water depth.  

Otoliths grow incrementally each day throughout a fish’s life. Each day’s growth becomes a layer that looks much like the rings of a tree. Daily layers are so small that they must be viewed under a microscope. Fish also grow annual bands that are dictated by the changing seasons. Scientists can use daily and annual bands to determine the age of fish, but that’s just the tip of the iceberg. Scientists can utilize otoliths to learn a lot of other important information about a fish’s life, too. 

Otoliths are located in small pouches near the brain of bony fish. Photo courtesy of George Whitman, UC Davis.

How do scientists use otoliths?

Otoliths can reveal fish migration paths 

As otoliths build layers each day, they absorb chemical cues from the surrounding environment. Since California is one of the most geologically diverse places in the world (meaning we have a huge array of different types of rocks and sediments occurring naturally throughout our state), each river has a unique chemical signature based on the composition of the rocks over which it flows. 

As a result, each otolith ring acts as a tiny data point, recording where the fish was located on any given day in its life. From this, we can determine in which river a fish was born, how long (in days) it spent in that river, and where it made pit stops (or extended stays) on its migration out to the ocean. 

Scientists can use otoliths to track where fish were throughout their lives – like a tiny GPS device embedded in a fish’s head! Understanding fish migration is especially important for our native anadromous (migratory) fish species, such as the threatened Central Valley spring-run Chinook salmon. 

An adult Central Valley Chinook salmon otolith. Photo courtesy of Kimberly Evans, UC Davis.
A juvenile otolith (left) compared to tree rings (right). Otolith photo courtesy of George Whitman, UC Davis. Tree ring photo courtesy of the New York Times.

Otoliths reveal beneficial habitats for fish 

The width of each otolith ring relates directly to how quickly the fish is growing. A larger width between rings indicates that the fish has grown quickly. 

By examining the otolith’s chemical signature alongside the width of the ring, scientists can discover which environments encouraged more rapid growth. This is information is crucial. Salmon that grow faster are larger when they get to the ocean, and salmon that are larger when they get to the ocean are more likely to survive and return to spawn. 

What can otoliths tell us about the Yuba River?

Lower Yuba River at Hammon Bar
A juvenile Central Valley Chinook salmon otolith viewed under a microscope. Photo courtesy of George Whitman, UC Davis.

Otoliths can unlock information that helps us solve complex problems and answer equally complex questions. 

For example, we can use otoliths to understand the impact of fish habitat restoration projects on the Lower Yuba River. To do this, scientists can collect otoliths from before and after a restoration project has been completed. Using this information, they could examine how fast fish grew after hatching in the Yuba, and how long they stayed before migrating out through the Feather River.  

If the salmon possess wider growth rings from their time in the restored areas, we can assume that the improved habitat provides ample food for juvenile salmon. Similarly, if salmon spend longer amounts of time in the restored areas, this would suggest that fish prefer the restored area to other neighboring habitats. 

Scientists can also learn about environmental stressors from any scars present on an otolith. Scientists can use scars to better understand information about flows, water temperatures, and other conditions to learn the source of the stress and its impact on migration behavior. In a highly regulated river like the Yuba, this data can be used to design ideal flow and temperature plans for salmon.  

Lastly, we can use otoliths to examine major impacts to salmon over long periods of time. For example, we can assess the impact of dams on our salmon populations by comparing historical otoliths to current otoliths. Historical otoliths can often be found in indigenous archaeological sites, which create an excellent cache of this priceless information. 

As long as the otoliths are intact, they can be studied, even if they are hundreds of years old. Historical otoliths can tell us what the environment was like for salmon during their lifetime, allowing us to infer how dams, infrastructure, and other anthropogenic disturbances have impacted salmon over the years. 

Why is the Yuba so important for salmon?

The Yuba River has been a stronghold for threatened fish, including the Central Valley spring-run Chinook salmon, a subspecies of the Chinook salmon, which is the largest salmon species in the world. California rivers and river ecosystems are critically endangered due to water management practices compounded by recurring drought. In fact, the Yuba was listed as #5 on the America’s Most Endangered Rivers list, compiled by American Rivers.  

Salmon face numerous other threats. Water temperatures are rising to lethal levels for salmon. Historic mining practices have dramatically altered salmon spawning grounds and nursery habitats. Dams are preventing Yuba salmon from accessing 97% of their historic habitat. 

The antiquated infrastructure and water management system in California has proven to degrade the environment and to endanger native fishes. Simply changing water management is not enough. To save the salmon, we can no longer work within this broken system; we must rethink and repair it. 

SYRCL, through our Yuba Salmon Now campaign, works tirelessly to liberate salmon from the threat of extinction and to restore them to ecological prominence within our state’s natural heritage.

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