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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

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June 2001

Tales from the exotics battlefront

Four stories of exotics gone awry.

Natasha Kassulke


Table of contents
How tiny smelt wage war on walleyes
The exotic that would feed the world got away
Bad times loom where the rustys roam
Sea lamprey and alewife packed a powerful punch

How tiny smelt wage war on walleyes

While other exotic introductions to Wisconsin have received much press and public attention, an exotic called rainbow smelt is turning up in inland lakes in the shadows of the media spotlight.

Vilas County is one example where rainbow smelt, has reduced native fish such as walleye over the past 10 years.

Table of Contents

Smelt, also called "frost fish" or "ice fish" for their silvery color, can be a tasty treat when rolled in corn meal and fried until golden brown. But on the loose in many Wisconsin waters, it is anything but appetizing.

"Because of rainbow smelt, six lakes in Vilas County have lost a naturally reproducing walleye population," notes Steve Gilbert, the DNR fisheries biologist for Vilas County.

Many people are surprised to learn that rainbow smelt is exotic to Wisconsin. Smelt is a marine fish native to the north Atlantic coast of North America. There also are a few freshwater smelt that are native to several inland lakes in Maine.

In the early 1900s, smelt from a freshwater source in Maine were stocked in Crystal Lake, Mich. The fish made their way into nearby Lake Michigan and soon were showing up in all the Great Lakes. The first finding of rainbow smelt in Wisconsin was in Little Sturgeon Bay (Door County) in 1928. Through accidental or intentional efforts, they have been spreading inland ever since. In fact, the spread of rainbow smelt to inland lakes may be due in part to anglers who put live smelt in bait buckets where eggs and milt mixed, fertilized, and then were "stocked" when the angler dumped the contents into the wild. Such dumpings are illegal and since smelt are classified as "rough fish," they cannot be transported live within the state without a permit from the Department of Natural Resources.

"We are just now realizing all the adverse impacts that this species is having on our native fish communities," Gilbert notes.

The reasons walleye are susceptible to smelt are two-fold. First, walleye hatch about the same time as smelt do.

"The young walleye then move to the middle of the lake to eat zooplankton," Gilbert notes. "Circumstantial evidence indicates that adult smelt feed on little walleye."

In addition, young walleye compete with young smelt for the same zooplankton food source.

"There isn't much left at the dinner table when smelt are done," Gilbert notes.

Sparkling Lake in Vilas County shows how drastically smelt change fish diversity in a lake. The lake had a history as a good natural walleye fishery. Smelt were discovered there in low numbers in 1981 and as their numbers increased, walleye populations started to fail. Stocking walleye, which had not been necessary since the late 1950s, was resumed in 1997 to offset the effects of the smelt. But Gilbert notes that the stocking has only had limited success.

Currently, there is no easy way to remove smelt from a lake without harming the rest of the fishery, Gilbert says. Thus, public education is the best tool to prevent rainbow smelt from spreading in Wisconsin. In addition to walleye, populations of native fishes such as cisco (lake herring), whitefish, lake trout and yellow perch have declined because of smelt.

"We encourage people to use the same steps for rainbow smelt that they use to help stop the spread of other new exotics such as ruffe and gobies," Gilbert says. "People need to be careful not to transport them to uninfested waters."

The exotic that would feed the world got away

Izaak Walton extolled the virtues of carp in 1653 as "The Queen of Rivers."

In 1882, Dr. Increase Lapham wrote: "The day will come when the people of the state (Wisconsin) will thank the men who have introduced and planted this extra fine species of fish (carp)."

But in Wisconsin, the carp's reign as queen of the rivers has been questioned, and the thanks that Lapham predicted has yet to be heard.

In fact, if Walton and Lapham could see us now, and witness the havoc carp have caused in some Wisconsin waters, they might be eating their words.

Three invaders: carp, purple loosestrife and Eurasian water milfoil. © DNR Photo
Three invaders: carp, purple loosestrife and Eurasian water milfoil. © DNR Photo

The common carp is native to Asia but was introduced in Wisconsin 121 years ago as a food source. Since then, carp have become a nuisance especially in southern Wisconsin. Carp is the main fish species in some waters out-competing native game fish for food, spawning area and habitat.

Early efforts to help carp thrive in one state were aimed at putting an end to starvation in Wisconsin," explains Steve Gilbert, the DNR fisheries biologist for Vilas County and a fisheries historian.

As many as 35,000 carp were placed into Wisconsin waters in 1890 and distribution continued until 1895, when the program was discontinued.

Problems associated with carp were recognized as early as 1901. Fisherman considered them a nuisance. Excessive carp populations uproot native underwater plants, attack plant species such as cattails and resuspend sediments making the water cloudy.

Laura Stremick-Thompson, a DNR fisheries biologist, notes that carp especially are abundant in large, shallow lakes and streams in southern and central Wisconsin such as the Horicon Marsh.

The Horicon Marsh in Dodge County includes the 20,976-acre Horicon National Wildlife Refuge and the 10,967-acre state Horicon Marsh Wildlife Area.

"In this geographic area," Stremick-Thompson notes, "that habitat has been so modified from its original form by agriculture, dams and other human interactions that we created the ideal carp habitat. This is a shallow, wide-water marsh.

Carp survive very well in the state's warm waters with little concern for low oxygen content, pollution or sudden temperature changes. The carp's ability to tolerate low oxygen levels often leaves them one of the last survivors in oxygen depleted waters. Competition exists between young largemouth bass and carp of all ages for food and habitat.

"Carp have contributed greatly to the marsh's lack of native vegetation and desirable fish species," Stremick-Thompson notes.

Part of the marsh's long-term wildlife and fishery habitation project includes carp control, which is overseen by a coalition of the Department of Natural Resources, U.S. Fish and Wildlife Service and a citizen advisory Horicon Marsh Management Committee.

The fish toxicant, rotenone, was used at Horicon Marsh last year to kill thousands of carp. Stremick-Thompson notes that before the treatment, the marsh's fish population consisted of 48.7 percent bullhead, 48.5 percent carp and just 2.5 percent desirable species such as northern pike, walleye and panfish.

By killing the carp, it was hoped that submerged plants would grow and provide food for waterfowl, plus shelter and spawning areas for invertebrates and fish.

Historical carp controls have included seining, erecting barriers to prevent carp from entering new waters, using electrical current and chemical treatment.

"The fishery has improved since the rotenone treatment, but not as much as we had anticipated," Stremick-Thompson notes. "But it is a slow process and the goal is to keep carp at an acceptable level."

Following the chemical treatment, work began to restock the marsh with predatory fish that could act as a biocontrol for carp. Forage species are being stocked to establish a food source for other fish species such as northern pike and smallmouth bass. Follow-up chemical treatments will be used in certain areas of the marsh when carp spawn to further target the adult population.

"I think of carp as a symptom of a larger problem," Stremick-Thompson notes. "A symptom of a really degraded ecosystem. And many people don't know that carp are an exotic, they just know that they are a nuisance."

Bad times loom where the rustys roam

Doug Jensen, the Exotic Species Information Center coordinator for the University of Minnesota Sea Grant, knows it can be tough to identify a rusty crayfish. That's why Minnesota has special regulations to prevent their spread. Live crayfish taken from a waterbody can only be used as live bait in that same waterbody in Minnesota. Selling live crayfish for bait or aquarium use is illegal.

The concern is that the "rusty" upsets the ecological balance in infested waterbodies in Minnesota and Wisconsin.

A rusty crayfish has 'rust spots' on each side of its carapace (outer covering). Rusties each just about anything and use their claws to uproot vegetation. © DNR Photo.
A rusty crayfish has 'rust spots' on each side of its carapace (outer covering). Rusties each just about anything and use their claws to uproot vegetation. © DNR Photo.

Rusty crayfish are native to streams in Ohio, Kentucky and Tennessee, but populations have rapidly expanded since the 1960s in the lakes and streams throughout northern Wisconsin.

Jensen notes that 42 waterbodies in Minnesota are infested with the rustys and over 110 lakes and rivers in Wisconsin.

"It's interesting because when we talk about exotics such as the round goby and spiny water flea, they came from another continent," Jensen notes. "But rustys are native to other parts of the Great Lakes, and have become an invader in other parts of the country."

Rusty crayfish reduce aquatic vegetation and deprive native fish of cover and food. They eat just about anything and use their claws to uproot vegetation making waters murky and decreasing plant growth, which also leads to potential shoreland erosion.

"Rustys mature when they are small (about 1 3/8 inches) and eat more than native crayfish species," Jensen explains. "In fact, they can eat twice as much because they have high metabolisms."

Other potential problems with rustys are that they eat fish eggs, raid fish nets, and displace native crayfish.

Some cabin owners on heavily infested Wisconsin and Minnesota lakes have stopped swimming because they fear stepping on them and being pinched.

"They look ominous and have been known to target people's toes," Jensen notes.

Rusty crayfish have robust claws and dark, rusty spots on each side of their carapace (outer covering). Their claws tend to be grayish-green to reddish-brown and smoother than most other crayfish.

As proof that the railroad tracks go both ways when it comes to exotics and their overseas travels, rustys also have found their way from North America to Europe and are causing havoc for Scandinavian crayfish populations.

Unfortunately, there is little that can be done to eradicate rustys once they are established. They are edible, but so small that even crayfish lovers consider them a lot of work for a forkfull.

"There's no silver bullet technology or management strategy to eradicate them right now," Jensen notes. "That's why it's so important to prevent their spread."

Sea lamprey and alewife packed a powerful punch

The fish community in Lake Michigan – and to a lesser extent Lake Superior – has changed dramatically over the last 100 years largely due to the introduction of exotic species.

One of the greatest impacts on the Great Lakes has been the eel-like sea lamprey, which is native to the Atlantic Ocean. It made its way past Niagara Falls via the Welland Canal by the 1920s, and colonized Lake Michigan in the 1930s and Lake Superior in the 1940s.

The effect of the sea lamprey on lake trout, whitefish and other large bodied species was devastating, explains Bill Horns, a DNR Great Lakes fisheries biologist.

For example, before the lamprey invasion of Lake Superior, the lake trout harvest averaged about 4.5 million pounds; by 1960, it was less than 500,000 pounds. The lamprey is a parasitic species that feeds on body fluids of other fish by attaching itself to a fish with its sucker-like mouth and rasping a hole in the body. Fish that survive those attacks often have scars to prove it. But, only one in seven fish survives a lamprey attack. Each adult lamprey can consume 40 pounds of fish a year. Along with harvest by commercial fishing and habitat degradation, lamprey contributed to the extermination of lake trout in all the Great Lakes, except Superior.

In response to the lamprey invasion, the Great Lakes Fishery Commission was created in 1954 and given authority to implement a binational sea lamprey control program.

The commission developed a program that uses barriers to block the migration of adult sea lamprey upstream to spawn, uses a selective lampricide (known as TFM) to kill larval sea lamprey, sterilizes males, and is studying the use of pheromones to lure adult lamprey into traps.

By the 1960s, the sea lamprey control program had reduced sea lamprey abundance by 90 percent to the point where large fish such as lake trout, salmon, burbot and whitefish once again thrived in the Great Lakes. This opened the door to fish stocking and resurgence of sport and commercial fisheries. Yet, this parasite still takes about half as much of the lean lake trout from Lake Superior as do sport and commercial fishing, emphasizing the need to continue sea lamprey control. Sea lamprey control costs taxpayers $15 million annually.

Alewifes enters the Great Lakes through rivers and canals. The Lake Michigan population exploded in the 1960s and '70s, resulting in huge die-offs. © DNR Photo
Alewifes enters the Great Lakes through rivers and canals. The Lake Michigan population exploded in the 1960s and '70s, resulting in huge die-offs.
© DNR Photo

Like the lamprey, alewives gained access to Lake Michigan through the Welland Canal. The first documented report was in 1949.

Because lamprey had collapsed the lake trout populations in Lake Michigan in the 1950s, there were no predators to control alewives. In Lake Michigan, the alewife population became too large to support in the 1960s and 70s, resulting in huge die-offs. Some will recall beaches littered with piles of silvery dead alewives in places such as Milwaukee. The abundant alewives may have hurt some native species including yellow perch.

In 1966 fish biologists turned to coho salmon and stocked them in Lake Michigan as alewife predators, and followed by stocking chinook salmon, brown trout and rainbow trout.

"It's an example of controlling an exotic with another exotic," Horns notes. "Alewives are held in check primarily by the chinook salmon, which is also not native to Lake Michigan."

Natasha Kassulke is Associate Editor of Wisconsin Natural Resources magazine.