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Tuesday, May 21, 2024

UC Davis researchers use isotopes of fisheye lenses to analyze fish diet

UC Davis Center for Watershed Sciences peel back the layers of the history of fish

For the first time ever with a freshwater fish, UC Davis researchers analyzed the stable isotopes of fisheye lenses to determine the life history of the aquatic organism by its diet. The ultimate goal was to determine if restoration projects aimed at helping the salmon species in the California Central Valley are effective.

         Miranda Bell Tilcock, an assistant research specialist with the UC Davis Center for Watershed Sciences, explored this method as a side project to her master’s thesis, which focused on looking at the isotopic food web differences between a floodplain and river habitat.

         “Knowing what a fish is eating and knowing where they eat helps us understand what habitats they’re using at different critical times in their life-history [so] that we can make better management decisions,” Tilcock said.

         Chinook salmon populations in the California Central Valley have been declining due to the building of dams, climate change and habitat loss, according to Tilcock. The California State Water Projects’ infrastructure also has severe environmental impacts on migratory fish that use upper watersheds for spawning, according to Rachel Johnson, a research fisheries biologist with National Oceanic and Atmospheric Administration (NOAA) Fisheries and research associate with the UC Davis Center for Watershed Sciences.

         Johnson’s background includes using the chemistry of the bony parts of the fish to identify important habitats in various points of the fish’s life. The individual rivers where salmon are reared have different geology and a variety of water signatures, and the isotopes correlated to these differences get recorded in the ear bone of the fish, according to Johnson.

         The Yolo Bypass of California’s Central Valley, a floodplain, has the same water chemistry as the Sacramento River, so habitat mapping could not be done using the ear bone. Only the diets of the fish could be used to identify from where they have traveled, due to the different food sources from each location. According to Johnson, the Sacramento River contains scarce amounts of food and the floodplain contains a larger quantity of food and unique sulfur isotopes.

         These differences can be found in the isotopes in the fish’s eye lenses. The method of peeling the layers of the lens allows researchers to see isotopes collected in the eye back to when the fish was a juvenile. The data indicates the diet of the fish throughout its life history which helps researchers determine where the fish lived by comparing the isotopes in their eye to the isotopes found in various floodplains and rivers, according to Carson Jeffres, a research ecologist at the UC Davis’ Center for Watershed Science who learned how to peel the layers of the eye lens from Tilcock.

         “If you’ve ever separated an onion, it’s like doing that on a very small scale,” Jeffres said.

         Beginners start off with a larger fish eye, but Tilcock has practiced peeling smaller eyes that are just 1.5 mm in diameter. The assistant research specialist had no one to teach her the methods, so she taught herself.

         Tilcock discovered through trial and error that if the lens was too wet when the peeling was attempted, the layers would squish together and result in a smashed lens. If the lens was too dry, it would become brittle and be at risk of shattering or flying off the eye. It is normal for a beginner to smash a few eyes, but within a week or two they can master the technique, according to Tilcock.

         Jeffres brought this technique to Brazil when studying the lenses of fish in the Xingu River. The world’s fourth largest dam and hydroelectric facility were built on this river, and Jeffres’ goal was to use the eye lenses to quantify changes in the food web before and after the dam was built.

         During the five months Jeffres was in Brazil, he observed that different fish have a lot of variation in their eye lenses. Visual predators, such as tuna, have bigger eyes and therefore bigger lenses. Amazonian catfish have small eyes because they live in turbid waters and depend on their sense of smell to navigate their way. The Amazonian catfish, which is large in size, has a small eye lens, proving that the size of a fish is not correlated with the size of its eye.

         On a local scale, the fisheye lens is used to quantify the benefits of the inundated Yolo Bypass of California’s Central Valley. The flooding can expand juvenile salmon habitats, a vital restoration project that would have a greater impact than making a side-channel or revegetation, according to Johnson.

         Tilcock’s tools give researchers the opportunity to figure out the benefits of salmon and other fish species living in a floodplain by analyzing their diet and the locations where they have thrived to help future restoration projects go in the right direction.

         “I think that this project and its contribution that Miranda has put together is a really exciting example of innovation and the ability to develop new tools for conservation planning and habitat restoration,” Johnson said.
Written by: Francheska Torres —science@theaggie.org


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