Invasive Species Zebra Mussels Threaten Water Quality, Fishing Industry

This is the second of two articles about water quality in Door County. Read the first here>>

The generations-old fishing boats are lined up quietly at the Weborg dock in Gills Rock, a town that usually harbors a bustling fishing community. What used to be a busy dock stands still in the late morning, and according to Mark Weborg, a life-long fisherman, a lot has changed.

“Fishing in Door County isn’t what it used to be – they’re coming up skinny or spotted and there are a lot less,” he said. “Yesterday we didn’t get one fish.” Local perch catches are low, trout and chubs have virtually disappeared, and whitefish are taking six years to reach the legal catch limit of 17 inches according to Weborg.

“Whitefish used to grow 17 inches in three years, now it’s taking twice as long,” he said.

Although fish populations may seem to have little to do with lake water quality at first glance, they reflect what has become a troubling question for our county – if the lake health is good, how do we explain how so many fish are dying or disappearing? The presence of invasive species may be one of the key answers for both the decline of lake water quality and the disappearing fish populations.

The Zebra Mussel

One of the best examples of the harmful effect of invasive species on Door County landscapes and habitats is the presence of the small white mollusks, zebra mussels, which have taken over the shorelines and the depths of the lake. These little shells, harmful to our bare feet as we walk along the shore and to the delicate fishing nets they get entangled in, are creating havoc in the lake.

“The zebra mussels have contributed to many problems,” Weborg said. “There are three times more algae because they’re clearing the water. They eat the main food supply for our fish – fresh water shrimp, and our fish end up eating the mussels which end up destroying their digestive tracts.”

Researchers and local activists seem to confirm Weborg’s diagnosis and point to the zebra mussel as an example of why we should try to limit the spread of invasives in our county. According to Bob Bultman, coordinator for the Door County Invasive Species Team (DCIST), “Zebra mussels screw up the balance of the lake.” He said that the introduction of a new species into the lake “created turmoil, disease, and major algae blooms.”

The arrival of the zebra mussel in the lake at the end of the ‘80s has had a large impact on both water quality and fish health. Though beach-goers praise the “Caribbean-clear” water of Door County, to many this is not a good sign. Zebra mussels are extremely prolific and resistant and have managed to push out most native mollusks in the lake. Because zebra mussels constantly filter the water, they allow more light to penetrate at greater depths and, according to the Great Lakes Water Institute, “Cladophora algae grows more predominantly.” The smelly algae then wash up on our shores and diminish the quality of the beachfront. Jerry Viste from the Door County Environmental Council (DCEC) warns us that we are only getting a glimpse of the problem.

“What we’re seeing on our shorelines is only one percent of what’s out there,” Viste said. Tim Weborg, who’s been fishing for 36 years, agrees that zebra mussels are contributing to the presence of “smelly slimy algae.” The Cladophora can prevent fishermen from being able to draw in their nets.

“That stuff is so heavy, sometimes we can’t wheel the nets in,” Tim Weborg said. Even more alarming, zebra mussels have been linked to the disappearance of freshwater shrimp, diporeia, the main food source for fish like whitefish or chubs. Weborg said, “There used to be a thousand of these small shrimp per yard but now there are none.” Charlie’s Smoke House in Gills Rock used to sell a variety of smoked fish, including whitefish and chubs, but now only a few precious pieces of salmon rest under the counter.

A photocopy of an article recently published by the Associated Press is glued to the glass next to a sign reading “Sorry No Chubs.” Its headline explains the lack of selection: “Lake Michigan fishermen are giving up on chubs.” Because of the lack of diporeia in the lake, fish are starving or disappearing. Chubs and whitefish are no longer an abundant local product. Because the fish are starving they have resorted to eating zebra mussels, which tear apart their digestive tracts and push out their intestines, eventually killing them.

What’s coming next?

Zebra mussels are not the only ones to have reached our lake, and on average a new invasive enters our lake every eight months, according to the Wisconsin Department of Natural Resources (WDNR). Because invasives adapt easily, and are more prolific and robust, they often out-compete local species. Their “generalized habitat and feeding requirements enable them to dominate fragile native species that have specialized needs” according to the Union of Concerned Scientists. One of the big threats drawing near is mud snails from New Zealand. They have been spotted in the Duluth Superior Harbor. Their rapid reproduction has severely impacted local trout populations by out-competing them for native insects, which serve as food for both species. According to Doug Jensen of the Minnesota Sea Grant Institute one can see “700,000 snails per square meter, which is comparable to if you had 585,000 snails in your bathtub.”

Other species that might be coming soon include killer shrimp and the Asian Carp. The Asian Carp is expected to have a disastrous impact. According to the WDNR, “In waters of lower Missouri and middle Mississippi River…Asian carp constitute 90 percent of fish abundance.”

A temporary electrical barrier has been set up on the Chicago Sanitary and Ship Canal to repel the carp before they reach the lake. This fall, for $9.1 million, a new barrier will be added to prevent this fish from entering Lake Michigan. But the WDNR said this is not a guarantee the carp will not eventually enter anyway. “The carp are big enough so we could electroshock the lake to get rid of them once they’re here, but that’s a very costly solution,” said Bultman.

And what about all the other species in the lake that might also disappear if such a solution were employed? The WDNR said most of the 180 invasive species in the Great Lakes have arrived during the last 40 years. Many invasives enter the lake on ballast water from ships that “legally discharge hundreds of thousands of gallons of ballast water each year.” Eighty percent of the ships are exempt from regulations because they report no ballast water on board. Regional organizations such as the Council of Great Lakes Governors and DCIST agree that tougher laws need to be put into place to enforce the regulations on ballast water control. However, Bultman said funds are often limited for the amount of work needed.

“The DNR does the best with what they have but there is basically no funding or mandating for this kind of work,” he said. Viste said no one seems to care until it is too late.

“People didn’t care about zebra mussels until the smelly Cladophora started washing up on our shores,” he said. “It’s the American way – to hell with it as long as we can make some money.” Tim and Mark Weborg said nothing can be done short of closing the St. Lawrence Seaway, which is letting so many foreign boats into the lake.”

“No one would ever do that because of the money – it’s big business,” Tim Weborg said. But Bultman said there is still hope to reduce the impact and spread of invasive species in Door County.

“It’s the reason to get up in the morning and fight to keep what we have,” he said. By establishing awareness about the risks invasive species present to the quality of our water and fish, projects like Clean Boats Clean Waters, the training of volunteer watercraft inspectors, and stricter laws on ballast water from foreign ships we may be able limit the harmful effects these foreign beings will have on the lake. For more information please visit the DCIST website: