Fish Eyes Provide a Look Into Their Eating Habits

California scientists found a “holy grail” technique for monitoring the health of fish populations: look them right in the eye.

A hatchery fish’s eye lens, as it appears under a microscope. (Credit: UC Davis)

(CN) — The newly published results of experiments on Californian freshwater fish demonstrate that scientists can reveal an individual fish’s life history, including its eating habits, just by taking a close look at its eyes.

“Even the nerdiest fish biologists say, ‘You can do what with fish eyes?’” said Rachel Johnson, study co-author and a research biologist at the University of California at Davis, in a statement. “This is an exciting new tool we can use to measure the value of different habitats and focus conservation work.”

Johnson and the rest of the team, led by UC Davis fish ecologist Miranda Tilcock, published their paper about the new method Thursday in the peer-reviewed scientific journal Methods in Ecology and Evolution.

The researchers gathered Chinook salmon — the largest species salmon in the Pacific, named for the groups of Chinookan-speaking indigenous people in the Pacific Northwest — from California’s Yolo Bypass, in the state’s Central Valley. They collected salmon from a river biome, a floodplain and a hatchery.

The authors’ eyes were on the fishes’ eyes, which they likened to tree rings: their eyeballs’ spherical lenses grow layer by layer from a central core. By taking a look at each layer’s chemical composition, the scientists found key insights into the fishes’ diets over time.

The scientists released a video of themselves using fine instruments to peel away a Chinook salmon’s eye lens, a process known as delamination. About 1.5 millimeters in diameter, the lens is already tiny; after dividing the lens into four thin layers, all that’s left is the eye core. Tilcock likened the process to “peeling the world’s tiniest onion.”

“It’s like a little diet journal the fish keeps for us, which is really nice,” Tilcock said in the statement.

The eyeball of a fish is rich in protein. As the fish eats over the course of its lifetime, isotopes from its foods bind to proteins in its eye, allowing scientists to read these diet journal entries via stable isotopic analysis.

Every chemical element has variants known as isotopes, and by examining the relative abundance of stable isotopes — that is, nondecaying or nonradioactive ones — in a sample, scientists are able to glean information about the material’s composition. In this case, the scientists could see how the traces of elements show how a fish’s diet changes over its lifespan.

The fish collected from the floodplain had many layers of eyeball lenses, indicating rapid development compared to the fish from the river and hatchery, for instance. This sort of information could be useful to hatcheries, wildlife biologists and others who monitor the health of captive and wild fish populations.

“This tool is not just unique to salmon in the Central Valley,” Tilcock said in the statement. “There are many migratory species all over the world that need freshwater habitat. If you can isolate their habitat and value for diet, you can quantify it for long-term success.”

Johnson, Tilcock and fellow co-author Carson Jeffres, another UC Davis ecologist, are working with California’s Department of Fish and Wildlife to study the life histories of fish by examining their eye lenses and otoliths, the organ that allows vertebrates including humans and fish to perceive acceleration and gravity.

“You use the otolith to trace the river or hatchery where a fish was born based on the unique geology and water chemistry of the tributaries in the San Francisco Bay watershed,” Johnson said in the statement. “Then you have the eye lens, which tells you where it’s eating to help identify floodplain habitats.”

With the new methods detailed in Thursday’s paper, the team can form a fuller picture of where salmon are moving and what they are eating during their lifetimes — and can compare habitats, food sources, pollutants and other material that might appear in the chemical composition of the fishes’ eyes.

“Now we have the tool we have been looking for to link juvenile floodplain benefits across the salmon life cycle to adulthood,” Jeffres said in the statement. “It’s the holy grail of measuring restoration success.”

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