Study of Fish Food Holds Key to Stable Population

By Katharine Ott, Milwaukee Journal Sentinel

Jun. 26–For a small fish, finding its next meal is a monumental task.

The tiny, transparent zooplankters it hungers for float by among thousands of other inedible particles. A fish must recognize and capture the shrimp-shaped creature before it is eaten by another fish or moves too far away.

It’s like standing inside a Powerball drawing and reaching out and catching the No. 5 ball, says Rudi Strickler, a biology professor at University of Wisconsin-Milwaukee and a senior scientist at the Great Lakes WATER Institute.

A pioneer in this field of study, Strickler wants to know what attracts a fish to its prey.

“What is it about a small particle in this sea of gray that makes a fish say, ‘There’s my food’ ?” he said.

To find the answer, he and his team of graduate and undergraduate students are using three-dimensional animation tools to create computer-generated virtual prey.

Researchers first videotape live zooplankters in 3D.

The video is used as a model to create the virtual zooplankters.

These computer animations are shown to fish, and the fish choose which zooplankter they would like to eat.

The team begins the process by simulating the natural environment of zooplankters in an aquarium.

Then researchers use high-speed, infrared cameras to record their movements.

Side and front angles are used to create a three-dimensional image of a zooplankter swimming.

Instruments capture the infrared and plot the position of the zooplankter on a coordinate plane.

The coordinates of the zooplankter are entered into a software package similar to that used by Pixar Animation Studios, creators of the movie “Cars.”

The result is a 3D animation of a zooplankter swimming exactly as it does in an ocean, lake or pond.

Researchers can alter the shape, speed and swimming patterns of this virtual zooplankter.

For instance, researchers may speed up a zooplankter so that it moves twice as fast as before.

Videos of the original zooplankter and its speedy counterpart are played side-by-side on a screen that projects into a tank.

A fish is exposed to the two virtual zooplankters behind a glass wall and then released toward the screen.

Believing its next meal is right there, the fish chooses which zooplankter to pursue.

Researchers record if the fish swims to the slow zooplankter or the fast zooplankter.

Once the fish touches either screen it is rewarded with real food released into that side of the tank.

Repeated trials show that fish prefer faster-moving zooplankters with more spinning motions.

This information could be helpful to fishermen seeking a larger catch, but Strickler found something else. The faster-moving zooplankters were infected with bacteria. He’s now researching how the bacteria affect the movements of zooplankters.

In particular, he wonders if fish are targeting infected zooplankters because they are more appetizing or easier to catch.

If so, both human health and fish populations could be adversely affected.

Zooplankters can act as “fast taxis” for bacteria to climb up the food chain to humans.

They ingest the bacteria through the water they filter. Zooplankters and the fish that eat them end up carrying bacteria in large quantities.

A human eating an undercooked fish containing these bacteria could then become sick.

Another possible outcome is a reduction in the fish population. Polluted waters carrying heavy loads of bacteria make zooplankters easier prey for fish. Then fish deplete the zooplankter population; the fish population is reduced.

Graduate student Ai Nihongi summarized this scenario as follows: “No zooplankter, no Friday fish fry.”

The WATER Institute lab is studying zooplankters infected with vibrio cholerae, the bacteria that cause cholera in humans.

Initial experiments show that these zooplankters also move with a significantly higher frequency.

The high-speed, infrared cameras used in this research were designed by Strickler.

The threat of global warming is making it increasingly important to study aquatic ecosystems, said Edward Buskey, professor of marine science at University of Texas at Austin.

Zooplankters are the base of this ecosystem and, despite their microscopic size, warrant more study, he said.

Strickler believes this research is important because it investigates the mechanisms behind fish populations.

“What we really need to know,” he said, “is how does a fish get healthy? How does it stay healthy? What about the food of fish? And the food of the food of fish?”

—–

Copyright (c) 2006, Milwaukee Journal Sentinel

Distributed by Knight Ridder/Tribune Business News.

For reprints, email tmsreprints@permissionsgroup.com, call 800-374-7985 or 847-635-6550, send a fax to 847-635-6968, or write to The Permissions Group Inc., 1247 Milwaukee Ave., Suite 303, Glenview, IL 60025, USA.