The phrase “you are what you eat” can be traced to French food commentator Jean Anthelme Brillat-Savarin in 1825. Brillat-Savarin published a book whose translated title was, “The Physiology of Taste: Meditations on Transcendental Gastronomy,” in which he states: “Tell me what you eat, and I will tell you what you are.”

It’s doubtful Brillat-Savarin could have predicted the phrase would become a cornerstone in a chemical analysis used to understand the feeding patterns of fish. Even so, the phrase serves as a useful starting point to understand the intricacies of an analysis used to understand fisheries throughout Wyoming.

You are what you eat applies to the way fish integrate the food they eat into tissue growth. Each prey item a fish eats has its own chemical identity that relies on ratios of stable isotopes. Simply put, a stable isotope is a version of an atom that can persist in nature. Isotopes that are more stable tend to be more common in nature and organisms typically evolve to use them most readily. It passes through cellular processes like photosynthesis or metabolism most efficiently because of this. Conversely, organisms are not always well-equipped to use less abundant isotopes. Think of these isotopes as being a bit sticky. They are harder to use in many biological processes. 

Scientists are able to compare abundance ratios of the most common isotopes to the less common ones to track food web interactions. Because of the stickiness of less common stable isotopes, these ratios change in predictable ways as atoms move from the atmosphere, to waterways, to plants/primary producers, to small fish and to large fish. Scientists across the world have worked to map these ecosystem-level changes in isotope ratios. Once that map is developed for an ecosystem, an individual fish’s tissue can be analyzed for isotope ratios, which can show researchers where that fish fits into the big food web picture.

The Wyoming Game and Fish Department has been working on several stable isotope analysis projects in partnership with research labs at the University of Wyoming and its Stable Isotope Facility. The following projects are providing insights into food web and ecosystem interactions of fish statewide.

Walleye consumption of stocked trout in North Platte reservoirs

Nathan Jaksha, a graduate student in the lab of William Fetzer at the University of Wyoming, is working with Game and Fish to use stable isotopes to understand the impact walleye have on stocked trout in Alcova and Pathfinder reservoirs. Predatory interactions between walleye and trout can make managing these two species in a single lake or reservoir challenging. Jaksha, working with a team of Game and Fish fisheries biologists in Casper, was tasked with understanding these interactions through the use of stable isotopes.

Jaksha completed fieldwork in 2021–23 and collected walleye and stocked trout from the reservoirs. He analyzed samples for their sulfur stable isotope ratios. Sulfur provides insights into the consumption of stocked fish because fish reared in a hatchery are typically fed food with ocean-sourced protein that is rich in a particular stable isotope of sulfur with a marine signature. This food chemically tags a fish as being raised in a hatchery since marine sulfur isotopes cannot have originated in Wyoming. This tag, which manifests as a high-sulfur ratio value, persists for the first several weeks or months after stocking until eating natural food in the environment begins to dilute and eventually erases the unique signal of the hatchery.

Jaksha submitted muscle and egg samples from walleye to the UW Stable Isotope Facility for analysis, and the data he got back was fascinating. 

“When we looked at the results from muscle tissues, walleye collected in the summer had high sulfur values in their muscle, indicating they were eating the trout that were stocked during the spring and kokanee salmon that were stocked in the summer,” Jaksha said. “But when we looked at walleye collected in late fall and winter, their muscle tissue showed almost no trace of the elevated sulfur levels of stocked fish, despite additional trout stocking events that took place in the fall. However, when we analyzed the eggs collected from those same winter-collected walleye, they had very high sulfur values.”

Reasoning through these trends points to a clear cause of the pattern. During spring and summer, walleye dedicate most of the energy they consume to growing muscle and body fat, so that is where the elevated sulfur values of stocked trout went. However, in the fall and winter, walleye are preparing to spawn, and most of the energy they consume goes toward reproduction – growing eggs. This indicated that walleye were eating stocked trout and gave insights into how these fish allocate their energy resources toward growth throughout the year.

These results tell Game and Fish that walleye likely have a large impact on stocked trout in Alcova and Pathfinder reservoirs. Jaksha is finalizing his data analysis, and his results will be used to guide future management strategies. 

Tracing the birthplace of rainbow trout

A question that often plagues fisheries managers is how to balance fish stocking with naturally reproducing populations. Stocking often is used as a method to recover a fish population or introduce a new sportfish into a location. However, stocking is an expensive and time-intensive process. At some point, many species start to reproduce naturally in a system and that natural reproduction may negate the need to continue stocking. But how do managers know when that balance shifts?

A few labs across the world are using a new research method that sparked an idea in Wyoming. This technique uses the eye lenses of fish to understand their history. The eye lenses develop similarly to layers of an onion. Distinct layers are formed as the fish grows, and each layer captures a snapshot of isotope ratios of the food that was used to fuel the fish’s growth at that time. As a result, you can collect an adult fish, peel layers of the eye lenses off and analyze each layer to understand what the fish ate throughout its life. Since we know fish raised in hatcheries are fed food that chemically tags them as being different from fish in the wild, could we use eye lenses to understand if a fish caught as an adult was born in a hatchery or the wild? If so, this could be a method to understand the success of naturally reproducing fish to inform whether continued stocking is necessary.

To test these questions, Game and Fish Casper Fisheries Biologists Jeff Glaid and Matt Hahn collected rainbow trout from the North Platte River near Glenrock. In this area stocked fish are marked with a clipped pelvic fin. Clipping fins to mark stocked fish means they can be identified later, but it is a time and labor-intensive process that eye lens analysis could possibly replace. For now, the trout with clipped fins allowed Glaid and Hahn to collect some adult fish they knew were stocked and some that were wild-reproduced. Having these identities would allow researchers to confirm if eye lens stable isotope analysis could detect the same birthplace as indicated by the fin clip.

Researchers at UW led by Fetzer peeled the eye lenses of these fish and submitted layers to the Stable Isotope Facility to be analyzed for their sulfur isotope ratios. 

“The rainbow trout with clipped fins, that we knew were stocked, had very high sulfur values in the innermost layers of the eye lens, representing the time they were raised in the hatchery,” Fetzer said. “Then they went through a transition period, corresponding with when they were stocked, where their sulfur values decreased until they matched those of the naturally reproduced fish. Those naturally reproduced fish maintained relatively low sulfur values throughout their whole lives.”

These results demonstrated managers can collect adult fish and use the innermost layers of the lens to determine if the fish was raised in a hatchery or not. This new method may be faster and more cost-effective than traditional methods like fin clipping. Following these results, Game and Fish began collecting eye lenses from kokanee salmon in Rob Roy Reservoir near Albany in the Laramie Region to implement the same birthplace analysis. That project is ongoing and will provide insights into the success of naturally reproducing kokanee.

Response of native fishes to an invasive species

Fisheries managers face the continuous challenge and threat of aquatic invasive species. Some AIS have well-documented impacts like zebra mussels siphoning plankton out of the water and leaving less food for native species. Zebra and quagga mussels get a lot of attention for disrupting aquatic food webs and human infrastructure, but there are many other AIS whose impacts are less understood.

One found in several waters around Wyoming is the brook stickleback. These little fish are native to the central and northern United States and Canada, but their distribution has expanded. They were first documented in Wyoming in 1993. These fish are only a few inches long but are known as aggressive players in the food web, leading to concerns they may outcompete native fish for food.

Jake Ruthven, at the time a graduate student at UW and now a fisheries biologist for Game and Fish in Lander, worked with Annika Walters and Game and Fish personnel in the Laramie and Cody regions to investigate whether brook stickleback were disrupting the feeding habits of native fish in Wyoming. Ruthven used carbon and nitrogen stable isotopes to figure out where in the food web brook stickleback appeared and whether native fish were displaced by the invasive species.

“Even though brook stickleback were eating a lot of the same food items as native species, the native fish didn’t seem to alter their own feeding behavior because of it,” Ruthven said. “Generally speaking, if brook stickleback were competing with native fish for food, we would have expected to see a shift in the diet of native fish to avoid that competition. Not seeing that trend indicates that, as long as there is plenty of food to go around, our native Wyoming fish aren’t being substantially affected by the presence of brook stickleback.”

Ruthven’s work was good news to fisheries managers. Small fish tend to get less attention from researchers and the public, but they play a critical role in biodiversity and often serve as food web links between primary producers and predators. Protecting small, native fish is a priority for fisheries managers at Game and Fish. Ruthven’s work suggests that while we do not want to see the species spread any farther, brook stickleback may pose a lesser threat than other aquatic invasive species in the state.

Stable isotope analysis has been around for decades, but advancements in technology and in the understanding of ecological patterns of isotope ratios continue to improve our ability to answer fisheries management questions using this technique. These ongoing and recently completed projects showcase a few of the many ways that stable isotope analysis can contribute to better understanding fisheries, leading to strong management decisions that will improve Wyoming’s fisheries into the future.

— Caroline Rosinski is the Game and Fish information and education specialist in the Laramie Region. She received her master’s degree at the University of Wyoming where she worked on various stable isotope analysis projects.