Beneath the surface: Understanding fish populations in streams and rivers

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Beneath the surface: Understanding fish populations in streams and rivers

For United States Geological Survey (USGS) research ecologist, Dr. Mary Freeman, studying the creatures that live beneath the surface of streams and rivers is like peeking into another world. Freeman’s research, which investigates how human activities affect the species that live within them, will help those worlds survive changing climates and human development.


“I conduct research in river and stream systems that is responsive to management needs, mostly managing the human impact on streams and rivers to allow native creatures to persist. It’s what really fascinates me—the questions of why each species exists where it does, why it acts the way it does, and how it functions within the system,” Freeman said.


One of Freeman’s recent projects is studying the way fish use habitat within restored streams. Stream restoration is an activity undertaken to mitigate for habitat altered by development elsewhere. However, there hasn’t been much research done on how or why some restored streams support more native species than others.


“In a restored stream, the channels follow a predictable pattern and mostly people abandon the old stream channel, dig a new channel and then redirect the stream there. We know that fish live within them—you can catch them—but no one has looked at how they actually use the novel habitats in these created environments,” Freeman explained.


She’d like to help uncover a small piece of that puzzle. While traditional sampling methods—using a net to catch fish, identifying species, recording data such as condition and size, and then releasing them—give scientists an idea of which species are present, Freeman strives to improve estimates of population size over time using quantitative methods.


“There always are biases—you’re trying to count moving things that you can’t see until you pull up the net, and some species are harder to catch than others. We want to know how these species are changing relative to how we’re managing environmental or climate change, but it’s impossible to know if we don’t have accurate estimates of their populations. That aspect of quantitative methods is quite fun to me. I enjoy asking the question: how do we actually estimate population size and population change?” Freeman said.


By creating more accurate models—that account for some of the biases inherent in sampling streams and rivers—Freeman hopes to be able to answer these questions and more.


This lifelong quest to uncover the mysteries that lie beneath the surface of the region’s rivers and streams began during her undergraduate studies at the University of Georgia.


“In 1979, I fell under the spell of Don Scott, who was at one time the director of biological sciences here. He knew everything—all the fishes and invertebrate species in the rivers and streams. All the dragonflies. And I thought—I want to know everything too. The world got a little bigger for me then,” Freeman said.


After forty years of research, she still hasn’t satisfied the curiosity she feels about the hidden lives of aquatic species.