New Wisconsin tool helps keep stocked walleye out of the mouths of invasive smelt

By on September 25, 2014

Wisconsin’s fisheries managers have a new tool in their fight against an invasive species that has already spoiled walleye fisheries in a few inland lakes and could spread to hundreds more.

The tool will help managers make sure the young walleye they’re stocking will plainly not fit inside of their invasive predators’ mouths.

Rainbow smelt have already invaded 24 Wisconsin lakes, collapsing walleye recruitment in Long Lake, Sparkling Lake, Dead Pike Lake and others. Another 550 lakes across the state could potentially support a smelt invasion, according to Zach Lawson, a fisheries biologist with the state’s Department of Natural Resources.

“With a lot of these smaller lakes in northern Wisconsin especially, these rainbow smelt get in and wreak havoc on the native fish community, and in particular walleye,” he said.

Lawson said biologists don’t have a great handle on all the ways smelt bring down walleye, but they do have some insights. It looks like young walleye and smelt compete for the same plankton food sources, and adult walleye that eat too much smelt may develop a vitamin deficiency that hampers reproduction. And smelt eat newly hatched walleye.

A smelt caught dead with a young-of-the-year walleye stuck in its throat (Credit: Zach Lawson)

That last effect is where the new tool may help managers exert some control. In lakes where smelt have invaded and walleye have declined, the state depends on stocking to preserve the species while providing valuable fisheries. But stocking doesn’t always work, especially if the planted fish are small enough for smelt to eat right away.

The tool — a mathematical model — lets managers plug in information on the size of smelt in a particular lake. It turns that into an estimate of how large stocked walleye will need to be to make sure they don’t immediately become lunch.

Lawson developed the tool while a student at the University of Wisconsin-Madison’s Center for Limnology. He and his colleagues collected smelt from three Wisconsin lakes and measured their lengths and gapes — “the largest internal dimension of the mouth.” By establishing how those two measurements are related to each other in enough fish, they could reasonably say smelt of a particular length have a mouth of a particular size.

Next, they developed a similar relationship between the length and body depth of walleye fingerlings. The tool pulls it all together: If the lake’s smelt are this long, here’s how long the walleye need to be before they physically won’t fit inside the smelts’ mouths.

“We can say 95 percent of the smelt are not going to be able to consume these walleyes that we’re going to put in,” Lawson said.

In addition to helping ensure more successful stockings, the tool could also save the state a little money. Raising fish is expensive, and pinpointing the size at which hatchery walleye are ready means the fish won’t spend more time growing at the facility than necessary.


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