The history of a fish can be traced through its eyes

Experts are describing how fish eyes reveal clues about their surrounding environment. The researchers analyzed the eye lenses of salmon in California’s Central Valley to gain a better understanding of how landscape disturbances impact fish.

While the same method has been used to examine fish in marine ecosystems throughout the ocean, the current investigation is the first to apply the technique to freshwater fish. 

The lenses of fish eyes are similar to tree rings in that they grow in layers throughout life, recording chemical signatures that store valuable information. In particular, data is recorded on the diets of fish in specific habitats.

Study lead author Miranda Bell Tilcock is an assistant specialist with the UC Davis Center for Watershed Sciences. “It’s like a little diet journal the fish keeps for us, which is really nice.” 

Tilcock explained that the study was like “peeling the world’s tiniest onion.” The experts used fine-tipped forceps to remove the layers until they reached a tiny ball, which is where the fish eyes first began to develop. 

The eyeballs of fish are very rich in protein. According to the scientists, the isotopic values in food webs bind to protein in the eye, leaving geochemical fingerprints that can be uncovered with advanced analysis.

The researchers studied juvenile chinook salmon living in three distinct food webs – river, floodplain, and hatchery – on the Yolo Bypass of California’s Central Valley.

Stable isotopes are forms of atoms that can be used to trace origins, diets, and migratory patterns of species. The team conducted stable isotope analyses on the eye lenses of an adult salmon to to determine which food webs and habitats the fish used at various stages over the course of life. 

During the 39-day study, fish on the floodplain grew quickly and accumulated additional laminae, or layers of lenses, compared to fish reared in the river or hatchery. 

“This tool is not just unique to salmon in the Central Valley,” said Tilcock. “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.”

The research has implications for managing floodplains, fish and natural resources and prioritizing habitat restoration efforts.

“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,” said study co-author Rachel Johnson. “Then you have the eye lens, which tells you where it’s eating to help identify floodplain habitats.”

“They really work together to present a fuller picture of how salmon move and what they eat as they use different mosaics of habitats across the landscape over their lifetime,” said study co-author Carson Jeffres. “Now we have the tool we have been looking for to link juvenile floodplain benefits across the salmon life cycle to adulthood. It’s the holy grail of measuring restoration success.”

The study is published in the journal Methods in Ecology and Evolution.

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By Chrissy Sexton, Earth.com Staff Writer