Limnology in the Lab: Guppies Eating Swordfish, Bluegill Eating Shrimp

Down in the basement of the Water Sciences and Engineering Lab, right next door to us here at the Center for Limnology,  sits an aquatic ecosystem with one crazy food web.

Chelsea Blanke stands next to her experimental "habitat" of guppies, zebrafish, bluegill and minnows.  Photo: A. Hinterthuer
Chelsey Blanke stands next to her experimental “habitat” of guppies, zebrafish, bluegill and minnows. Photo: A. Hinterthuer

In the experimental habitat, four different species of fishes – zebrafish, guppies, bluegill and fathead minnows – are feasting on a scientifically measured diet of dried flakes of swordfish, walleye, perch and shrimp.
And those fish flakes, says Chelsey Blanke, a CFL graduate student working with professor Jake Vander Zanden, weren’t exactly something she could pick up from the pet store.
Food fit for a king. Or a guppy - jars of powdered swordfish, walleye, perch and shrimp.
Food fit for a king. Or a guppy – jars of powdered swordfish, walleye, perch and shrimp.

“I bought everything from the grocery store,” she says, “and did almost all of it in my kitchen. We had to peel one hundred pounds of shrimp, grind it up in my food processor and bake it in a sort of cookie,” she recalls. “It was kind of some strange cooking, you know?”
The purpose of these weird recipes (and the experiment), Blanke says, is to validate equations that scientists use to try to calculate trophic positions, or where an animal sits on the food web.
“This method has mostly been used in marine systems,” she says, “[where] it’s a really accurate method for predicting trophic levels. So we’re trying to bring the method to freshwater ecosystems.”
These may look like lowly minnows but, isotopically, at least, they may be at the top of their food chain.
These may look like lowly minnows but, isotopically, at least, they may be at the top of their food chain.

Right now, scientists can take a tissue sample of a saltwater fish, use some fancy machines to perform what’s called an “isotopic analysis,” and evaluate carbon and nitrogen in the amino acids present in the tissue. Comparing the composition of carbon and nitrogen in amino acids to other known samples can tell them if the fish is a top predator, or food for a bunch of other fishes, or somewhere in between.
It’s an indispensable tool in understanding freshwater ecosystems, since it’s a little difficult for aquatic scientists to sit down at the bottom of  a lake all day, observing fish in their native habitat and taking notes on who is eating whom!
In Blanke’s experiment, individuals from each species of fish are eating different diets – some guppies get swordfish, others get walleye, still others get perch and the rest are fed the shrimp. The same is true for the bluegill, minnows and zebrafish.
Feeding time. Blanke administers each species' unorthodox diet.
Feeding time. Blanke administers each species’ unorthodox diet.

In nature, a swordfish sits pretty high on the food web, so anything eating it is a top predator. Shrimp, on the other hand, occupy a lower tier, and walleye and perch are somewhere in between. Theoretically, Blanke says, when she analyzes each species’ tissue samples after the experiment, the isotopic signatures will tell her who was eating what.
“Essentially, we’re seeing if the equations are accurately predicting the trophic levels. The idea is that you should be able to go out and grab a fish from any lake, even one you’ve never studied before, and use this analysis to predict where [that fish] sits in the food web,” she says.
Blanke says this is one of the first times the analysis has been done for freshwater fisheries and hopes it can become a sort of foundational study, making it easier for researchers to piece together the intricate pieces of an ecosystem’s food web and better understand how our lakes, rivers and streams work.

All photos by Adam Hinterthuer

Fish Fry Day is when Wisconsin restaurants put fried fish on their menu, and we offer up delectable bits of fish info on our blog. Enjoy!

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