The Wisconsin Department of Natural Resources is three years into its first comprehensive study of the cisco, a slender white fish that scientists say is an important food for other fish and may serve as an early warning of problems in fisheries.
"Cisco are a key component to how a lot of lakes operate," said John Lyons, a DNR fisheries scientist on the study.
Lyons and two other Wisconsin scientists have chosen nearly 200 public and private lakes with good cisco habitat. They're trying to measure the abundance of the fish in Wisconsin's lakes where sometimes the only evidence comes from old photos, rumors or inadvertent catches from unrelated studies.
They've never focused solely on cisco, which some anglers call lake herring or tullibee. Results from the study will become a foundation scientists will use as they assess which fish are at risk of disappearing.
"We think the cisco is going to be one of the most vulnerable lake fish to climate change," Lyons said.
Scientists elsewhere in the region have over the last decade focused on the fish they say many younger anglers either don't know about or don't care about.
They aren't easy or fun to catch because they cruise in the middle of deep lakes eating plankton and don't put up a good fight when caught. But scientists say the fish are a crucial high-energy food for others.
The lead scientists in the project - Lyons, Jeff Kampa and Greg Sass - are cautious not to overstate early results. But they gave The Associated Press the findings from lakes already studied that show mixed results for Wisconsin's cisco.
A number of lakes in Waukesha, Waupaca, Bayfield and other counties that once showed evidence of the fish turned up nothing when the DNR cast a top-to-bottom gill net in those lakes since 2011.
The Chain O' Lakes in Waupaca and several lakes in Washburn, Vilas, Sawyer and other counties had medium yields during netting last July. Only six lakes had high yields, and 27 of the 99 lakes tested had none. Most of the remaining tests will be conducted in Vilas and Oneida counties later this year.
The scientists say invasive rainbow smelt and warmer lakes may have already snuffed out the cisco in some areas.
"But we can't say that they have disappeared because of climate change at this point," Lyons said.
Jeff Winters, a longtime Boulder Junction fishing guide, remembers when anglers caught and ate cisco in Vilas County. He still catches cisco by chance when fishing for walleye.
"Used to be a like a tradition around the fall where guys would get them and smoke them for the winter," said Winters, who doesn't eat fish. "That was a different breed of people back then."
Winters said he's seen cisco killed off during long, hot summers. Scientists say that's caused by increased activity at the bottom of lakes, taking up oxygen, and the top waters warming during prolonged spring and fall. That combination can leave the cisco with nowhere to go.
Minnesota scientists have found cisco may live in about 650 lakes there, but populations have rapidly declined since 1975, according to Pete Jacobson, a fisheries research supervisor with the Minnesota Department of Natural Resources.
Minnesota is moving toward finding a way to protect watersheds of 176 refuge lakes, all but six of which are in the state's northern, forested half.
A driving force behind the Minnesota study is that losing the cisco could mean smaller game fish.
"We usually see our best trophy walleye, northern pike and musky in lakes that have good cisco populations," Jacobson said.