WILL A WELL-MIXED, WARMER LAKE DOOM INVASIVE FISH?

From UW Madison

MADISON - The rainbow smelt, an invasive fish that threatens native species
such as walleye and perch, may soon be feeling the heat - literally.

In an experiment that could show the way to evicting the unwanted fish from
Wisconsin lakes, University of Wisconsin-Madison scientists and engineers
hope to experimentally warm Crystal Lake in Vilas County in an effort to
selectively wipe out the smelt. Using a device known as a GELI, an apparatus
that looks like a submerged trampoline, the researchers will mix the waters
of the 83-acre lake to warm the cool, deeper waters where the rainbow smelt
thrive.

"As far as I know, this is a completely new idea," according to UW-Madison
researcher Steve Carpenter, a world authority on lakes and a leader of the
new study along with civil and environmental engineering Professor Chin Wu.

"For it to work, one needs rather special circumstances," Carpenter explains.
"The species you want to eliminate must be intolerant of warm water, and the
warm water must not harm the native species that you wish to keep. That is
the case in Crystal Lake and perhaps some other lakes in northern Wisconsin
that have been invaded by smelt."

The idea, according to engineering graduate student Jordan Read, is to use
the GELI - which is propelled up and down in the water column using
compressed air and pushes water much like the bell of a jellyfish - to warm
the deeper waters of the lake by a few degrees to a temperature the invasive
fish is unable to tolerate.

"The main goal of the project is to mix the water column to the point where
the deeper cold water habitat refuge for smelt is gone," says Read.

Using the device, the Wisconsin researchers will warm Crystal Lake by about 6
degrees Fahrenheit, bringing the average July temperature of the lake to
nearly 66 degrees Fahrenheit, a temperature intended to make things
uncomfortable for the invasive fish.

The rainbow smelt is a native of the northeast coast of the United States and
was brought to the American Midwest in the 1920s as a potential food source
for walleyes, one of Wisconsin's most prized game fish. But the smelt spread
to lakes Michigan and Superior and is now finding its way to many of
Wisconsin's smaller inland lakes.

"Rainbow smelt are delicious, and many people know them as fried smelt," says
Carpenter. "They are also voracious predators that gobble up juveniles of
many fish species. They are particularly effective at eating walleye
juveniles, and walleyes are often eliminated from inland lakes that are
invaded by rainbow smelt."

The idea behind the Crystal Lake experiment, says Read, the graduate student
directing the fieldwork, is to determine if artificially mixing the lake and
warming its deeper waters will cause thermal stress for the smelt. The
hypothesis, he explains, is that increased temperature will either kill the
smelt outright or stress them to the point that survival and reproduction
rates are greatly reduced. "The goal is to alter the thermal habitat the fish
needs to survive," he notes of the experiment.

"Fish and most other aquatic invaders take on the temperature of their
environment," says Carpenter, who explains that the rise in temperature
should not harm native species such as walleye and bass. "If you warm the
lake above the upper lethal temperature that the invaders will tolerate, they
will die off."

The GELI is a radical departure from traditional methods of mixing lakes,
says Read. Ordinarily, to alter the water temperature of a body of water the
technique of choice is aeration, where compressed air is circulated through
the water. The GELI technology could also potentially be used to restore
oxygen to small areas of oxygen-depleted water.

"Preliminary measurements found the GELI technology to be much more efficient
in comparison with traditional aeration techniques," says Read, of the
8-meter diameter membrane fitted with a hose-like collar which is alternately
filled and emptied of air to raise and lower it in the water column. What's
more, the GELI technology is more environmentally friendly as, unlike
aeration, it does not stir up sediments.