Scientists at the University of Wisconsin-Madison are conducting groundbreaking research on “lake mixing” as a tool to control fish species composition.
Deep lakes tend to stratify into two layers, the upper layer warm and the lower layer cold. Different fish species require different water temperatures, so mixing the lake can have major implications for some sensitive fish species.
Researchers are trying to kill rainbow smelt, an invasive species established in Lake Michigan that sometimes finds its way into inland lakes. It feeds voraciously on many native fish species, so it has a very detrimental effect on the food web of the entire lake. The fish needs cold water to survive, so the scientists are mixing the lack in an attempt to eliminate the cold water layer in stratified lakes.
“Nothing like this has been done before,” said Jake Vander Zanden, professor of limnology and zoology at UW-Maidosn, referring to mixing up the lake’s temperature zones.
Limnologists usually are not allowed to play around with the entire ecosystem of a lake; they are normally restricted to simply monitoring lakes and hoping to draw conclusions from the existing conditions. Yet poisoning an entire lake is the only alternative for serious smelt control. For obvious reasons, fishery managers would like an alternative to poison.
However, mixing a lake is no easy task. The researchers are using a device called a Gradual Entrainment Lake Inverter (GELI), which was invented by Jordan Read, a UW-Madison graduate in Environmental and Civil engineering. The GELI employs a series of trampoline sized bladders that float when filled with air and sink when deflated. One unit includes six of the bladders connected to air hoses, controlled by a central computer. The up and down motion of the bladders is what mixes the water profile.
The scientists are mixing Crystal Lake, near Boulders Junction, Wis. The lake was chosen both because it is small and has been studied by the UW-Madison Center for Limnology for over thirty years. It is well known and easily controllable relative to larger lakes.
Crystal Lake is stratified, yet the rainbow smelt is the only species present that requires cold water; warming of the cold layer will not kill any native species.
Crystal Lake is also nearby lakes that do not have the rainbow smelt, but could be accessible to the invasive fish if the water were to rise high during spring flooding. Lakes are very densely packed in the area, so the smelt could potentially spread fast.
There are almost 15,000 lakes in Wisconsin, and the smelt are only in about 25 of them. Therefore, fisheries managers are under pressure to keep the spread of smelt under control and scientists thought that lake mixing would be very effective in killing smelt.
Laboratory tests have shown that smelt will die if the water temperature raises around 20 degrees Celsius. The GELI devices have raised the temperature of the lake well above 20 degrees Celsius, and while some smelt did die, the experiment has not eradicated them from the lake.
“This was somewhat of a surprise, now we are raising the question, why didn’t [the smelt] die? The original question was, what will change within the ecosystem?” said Zanden. He said this is a perfect demonstration of why ecosystem science cannot rely on laboratory data.
Yet Zanden hypothesizes that the smelt may be stressed enough so that they will either die in the upcoming years or that they will not be able to reproduce. The lake mixing is slated to run for two more years, which may still prove fatal for the rainbow smelt. It is also very interesting and important for questions that go beyond smelt.
“In a sense, this is an analogue for global warming. The same kind of homogenization and warming are likely to happen in the future,” said Zanden.
Scientists will continue to intensively monitor fish populations using net capture and high resolution sonar after the experiment is over. They are hoping that this work will answer questions about how global warming could affect the ecology of Wisconsin lakes.
