by Adam Hinterthuer
Last fall, under the cover of darkness, a small group of people decked out in chest waders and headlamps slowly backed a truck down a boat ramp just off of Highway 51 in the middle of the night. Working quickly, they began to unload their cargo into the dark water of Sparkling Lake. When the sun rose the next morning, the lake was home to nearly a thousand new cisco, a native species of fish that had disappeared from Sparkling Lake sometime back in the 1980’s.
This nocturnal crew wasn’t some fishy version of Johnny Appleseed. They were a team of researchers from TLS and the WDNR. And their goal was two-fold – to try to restore a native species of fish and to fight back an invasive species at the same time.
What they hope to do is find “a better way to manage a fishery,” says Joe Mrnak a graduate student at TLS. “Historically and even currently, management has taken a single species approach where [for example] walleye are declining, and we’re just going to stock more walleye in a lake,” says Mrnak.
Mrnak and his colleagues are taking a dual approach to restoring cisco populations. They remove invasive rainbow smelt in the spring and stock cisco in the fall. Since smelt inhabit the same waters and compete with cisco for food, their removal may ‘open’ the niche required for cisco to successfully move back in.
Over the last several decades, Mrnak explains, cisco populations in lakes across Wisconsin have struggled. Scientists believe that this problem is caused by warming water temperatures. Cisco are what’s known as a “cold water” species of fish and don’t do well in warm water conditions.
In addition to facing warming water temperatures, native cold-water loving cisco struggle to compete with the non-native species called rainbow smelt which have colonized select lakes in the Northern Wisconsin region. Originally brought to Lake Michigan by anglers wanting to recreate the Atlantic Coast smelt runs of their youth, rainbow smelt quickly spread throughout the region. This is a problem, Mrnak says, because smelt can tolerate a wide range of temperature conditions and, most problematically, “smelt are also extremely omnivorous. If it can fit in their mouth and it’s in front of their face, they’ll eat it.”
Rainbow smelt have, essentially, taken ciscoes’ place in the food web. The problem, Mrnak says, is that cisco “evolved with the other fish in the system to sort of play nice with each other,” while smelt did not. The end result is an ecosystem that is out of balance.
Cisco eat the tiny, free-floating animals called zooplankton that form the base of the lake’s food web and, in turn, are eaten by bigger fish. Smelt are just as likely to be a predator as they are prey and chow down on everything from zooplankton to the younger members of many species of fish.
Mrnak hopes that, by removing smelt as they are reintroducing cisco, they will be able to “reset” the lake’s food web and give cisco a fighting chance to take back their niche.
Mrnak and his team are also doing the same thing in nearby Crystal Lake, where rainbow smelt have caused a crash in yellow perch populations. The goal in both lakes is to have stable cisco populations that fill a more natural, and far less disruptive, role in the food web.
“The thought is that cisco will gain a significant foothold … and smelt will exist at low enough levels that their impacts will be minimized,” Mrnak says. “We’re not hoping to remove smelt completely from these systems, we’re trying to control them.”