Graduate student examines invasive fish species and their ecosystem impacts

A project designed to quantify the impact that introduced fish species can have on a lake’s ecosystem earned Marybeth Brey the top prize in the natural resources category of this spring’s Graduate Student Research Symposium at N.C. State University.

Scientists know that an invasive species can destroy habitat, push down populations of native species and alter an entire ecosystem. But what Brey, a Ph.D. student in biology, is trying to unravel is what happens when multiple species are introduced at the same time.

Specifically, do the effects on the food web add up in a linear way? Or is it more complicated than that? And based on what’s known about both the native and introduced species, can you predict what might happen?

Brey’s advisers are Drs. Derek Aday and Jim Rice of the College of Agriculture and Life Sciences’ Department of Biology. Her project focuses on Lake Norman, a large manmade lake north of Charlotte, and four fish – white perch, alewife, flathead catfish and spotted bass – that have been introduced there.

Fish are introduced into North Carolina’s lakes in two ways, Brey says: Sport fishermen intentionally add food fish, thinking they will increase the size and abundance of the fish they want to catch. And they unintentionally introduce new species when they dump buckets of live bait into the water.

For the past four or five years, Brey has been sampling in Lake Norman to find out which kinds of fish and how many live in different areas of the lake and how those populations have changed over the past four or five years.

She’s also been learning what those fish eat. To do so, she’s been using a technique known as direct diet analysis – basically, putting the fishes’ stomach contents under a microscope to see what they are eating, whether it be plants, zooplankton, insects, other fish or something else.

At the same time, Brey is considering overlap indices – quantitative measures of how much overlap there is between diets of the various fish.

“With overlap indices, we have this quantitative measure of what a fish is eating and how much overlap there is with what other fish are eating,” she says. “That’s very useful for comparing, for example, introduced and native or established species, so we can see how much they might share food resources.”

She’s also using stable isotope analysis, which involves sending pieces of muscle from the fish to a Cornell University lab to determine the relative amounts of nitrogen and carbon in them. That gives her an idea of the amounts of plants and other fish that the species in question consumes, and it tells her whether they are feeding near shore or out in open waters.

Finally, she’s putting all the data she’s collected over the years into the computer modeling program Ecopath with Ecosim in an attempt to make predictions about how future introductions might impact the population levels of the fish she’s studying.

The program has been used before to predict how fishing pressures will affect fisheries, Brey says, “but it’s rarely been used to ask questions about introduced species and specifically multiple introduced species.”

So far, she has found in Lake Norman that there’s been a rapid increase in the amount of white perch, introduced in the reservoir around 2001, and that the perch may be competing for food with black crappie, an established sport fish. She’s also seen that the forage fish alewife, another introduced species, have become an important part of the diet of several fish in the lake, but they aren’t increasing in number as much as the white perch.

Brey thinks that because the alewife seems to occupy a unique niche in the food web – eating fly larvae as they emerge in a particular area of the lake – they have been able to become established with few consequences to native prey species.

As she wraps up her dissertation in hopes of graduating in December, Brey is focusing on the modeling work and determining whether the approach she’s taking is an effective way to quantify the effects of introduced species on a reservoir food web.

Because she has funding from the Sport Fish Restoration Funds, which come from an excise tax on fishing rods, reels, creels, lures, flies and artificial baits, Brey hopes the research she’s done at N.C. State will help address sport fishermen’s questions about fish introductions and will contribute to fisheries management decisions.

— Dee Shore