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From Lakeland Times

Winning One Battle on Exotic Lake Species

Rainbow Smelt and Rusty Crayfish Study Showing the Way

by John Bates, for the University of Wisconsin Trout Lake Station

From its brilliant surface, no one would imagine that Sparkling Lake in Vilas County has been a lake under siege. But scratch the surface, and two exotic species come into view that have dramatically altered the lake's ecosystem. The invaders - rusty crayfish and rainbow smelt - have pushed the once plentiful yellow perch and cisco to near extinction, prevented walleye from reproducing, and decimated beds of aquatic plants.

Researchers from the University of Wisconsin Trout Lake Station and the Wisconsin DNR have initiated a study to understand the dynamics of lakes like Sparkling, how people influence them, and how exotic invasions can be reversed. Some of the answers, believes University of Wisconsin-Madison graduate student Brian Roth, may be in their latest round of research
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"Rusty crayfish appeared in northern Wisconsin lakes in the late 1950's, and were first seen in Sparkling Lake around 1975," Roth observes. "Rusties have continued to invade other lakes and streams since 1983 when they were finally banned as bait from all Wisconsin waters, but most of the damage had been done by then."

Studies over the past two decades found that rusty crayfish are bigger, more aggressive, and hatch out earlier than northern Wisconsin's two native crayfish, the benign fantail and blue crayfish. Thus, rusties often become dominant when they are introduced, and with a rapacious appetite for aquatic plants, soon devastate plant beds that historically served as food, cover, and hatching and spawning sites for aquatic insects, invertebrates, and small fish. "In high densities, rusty crayfish can become the equivalent of clearcutting loggers," notes Roth.

To make matters worse, rusties also eat fish eggs and invertebrates like snails along the lake-bottom.

In Sparkling Lake, rusties mowed down the plant beds to the point where small fish like bluegills and pumpkinseed that rely on plants for cover and spawning were almost entirely removed. Their loss caused top predators like bass, walleye, and muskellunge, which strongly prefer a diet of small fish to crayfish, to decline. The end result was radically altered fish populations in Sparkling.

To further complicate matters, rainbow smelt were introduced into Sparkling Lake around 1981, likely from anglers washing their nets out in the lake on their way home from fishing the annual spring spawning run in Lake Superior. "Smelt eggs stick to everything," says Roth, "and nets full of spawning fish are equally full of eggs."

Smelt dramatically affected yellow perch and walleye in Sparkling, but just how remains a mystery. Roth says there are two hypotheses: "One, smelt hatch before walleye and perch do, and can monopolize zooplankton resources such that juvenile walleye and perch die out. Two, smelt young are simply more effective at preying on zooplankton and are able to outforage young perch and walleye, causing them to die out." The bottom line is that both walleyes and perch were nearly extinct in Sparkling prior to the study. "If the DNR or tribes didn't stock walleye on the lakes where smelt have been introduced, anglers wouldn't be able to catch walleyes after ten years, because none would be there," Roth notes.

The smelt's impact on the native cisco in Sparkling was equally great. Smelt directly predate upon cisco, and by 1990, cisco were also nearly extinct in Sparkling.

Surprisingly, smelt have failed to appear on the radar screen of most Northwoods residents who worry more about Eurasian milfoil and other exotics. But anglers realize, says Roth, "If you want a walleye population, keep the smelt out. On nearly all the lakes where we have good records, walleyes have declined after smelt have gotten in, as have yellow perch."

What, then, to do? The key is that adult walleyes eat smelt, while adult bluegills and smallmouth and rock bass eat the smaller, young crayfish. Roth believes that Sparkling Lake can reach a stable state where the predators can control the smelt and crayfish if three things occur:
1- the adult smelt and crayfish populations can be dramatically reduced
2- the aquatic plant beds can be restored
3- the predator fish populations - the walleye and bluegills in particular - can be restored.

Enter Steve Gilbert, DNR fisheries biologist for Vilas County. For many years, Gilbert has been doing fall fish surveys of Sparkling Lake by electroshocking the entire shoreline and islands. "We had excellent young-of-the-year walleye recruitment in the 1970's and early 1980's, but by the 1990's, we found virtually no walleye young except during years where we stocked. In 2003, we found only one individual young-of-the-year."

Gilbert estimates that Sparkling's total smelt population reached 1.2 million in the 1990's, and the walleye young didn't stand a chance. The DNR stocked walleye fingerlings in 1997, 1998, 2001, and 2002, but the survival rate was absolutely dismal - in fact, zero survived in 2002. So, Gilbert began stocking larger walleyes that averaged about 8 inches long, and found that nearly 2/3 survived. He stocked 2,500 of these in 2002, and 3,000 in 2003. Some of the 2002 fish are already sexually mature, and they're all eating juvenile smelt

But word gets around about bountiful walleyes, so another piece to solving Sparkling's fish puzzle had to be put into place. Gilbert and Roth realized anglers could alter the whole experiment by harvesting the stocked walleye and the repopulating bluegills, so the DNR has placed stringent regulations on the number and size of fish that can be taken, and are encouraging catch-and-release as much as possible for now. "A key issue is that people take out the big walleyes as soon as they reach 15 inches, and it's the big walleyes you have to keep in the system. They're the ones that will eat the most smelt," says Roth.

Roth and Gilbert also realized that without reducing the smelt population, the DNR would have to stock walleye in perpetuity because none of their young would survive.

Enter Katie Hein, a graduate student at UW-Madison, and undergraduate Stacy Lishcka. Their job has been to capture adult rusty crayfish and smelt in order to "flip" the system into a something as close to a crayfish-and-smelt-free environment as possible. The results have been impressive.

In one week in the spring of 2002, they caught 170,000 smelt, a whopping 1.5 metric tons. In 2003, the catch was only 45,000, because most of the smelt spawned before the ice went out. In 2004, Hein and Lishcka outsmarted the early spawners by chopping through the ice to place their fyke nets, and they hauled in nearly 90,000 smelt. Summer estimates of the total smelt population in the lake have dropped from the original estimate of 1.2 million, to 750,000 in 2001, and to 370,000 in 2003 (numbers for 2004 are still being crunched). Roth and company believe with continued netting and the successful stocking of walleye that the smelt are on their way out, but it may take another ten years.

Summer crayfish trapping yielded 11,000 in 2001, 38,550 in 2002, 22,585 in 2003, and only 9,098 in 2004. Catch rates are clearly declining, as is the size and age of the catch. Hein and Lishcka are gathering many more females and juveniles than in earlier catches, a clear sign that they're making a major dent in the population.

General observations of the aquatic plant beds all indicate that they are regenerating, and if predators can become abundant enough, it's expected that their consumption of young crayfish will keep the rusties under control.

The final problem to face is that research monies run out next year, and Sparkling Lake is only one of a number of lakes with crayfish and smelt invasions. The researchers fervently hope that lake associations or other organizations will take what the study has shown and implement similar harvesting and stocking techniques to bring their lakes back into a natural balance.
Vilas County, with its nearly 1300 high-quality lakes covering almost 15% of its land surface, would seem to be the last place in Wisconsin to have serious fishery issues. But exotic species are equal opportunity invaders.

Roth sums it up: "It's all a matter of perspective. If all people want is some really big walleye for about five years, they can let the smelt go. If people don't care about aquatic plants and bluegills, they can let the rusty crayfish go. But I don't think that's what people want to do if they care about their kids and grandkids catching fish."

For more information on the studies conducted by the UW Trout Lake Station, visit their Web site at http://limnology.wisc.edu