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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

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June 2005

Aquatic invaders

How big is the threat?

Ron Martin

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Aquatic invasive species (sometimes referred to as alien or exotic species) are receiving increasing attention because of the threats they pose to our waters. But how great is the urgency to act? Are we in any immediate danger of having our lakes and rivers overrun by alien species? To answer these questions we need to look at how these species are getting here and what actions are being taken to alleviate future threats.


Ballast water from ships is a primary pathway for introducing exotic species into the Great Lakes. Foreign freighters enter the Great Lakes through the St. Lawrence Seaway and legally discharge hundreds of thousands of gallons of ballast water each year. At least 180 foreign species have already entered the Great Lakes, the majority of them in the last 40 years. Despite Coast Guard regulations in place since 1993 that require ballast water to be exchanged on the high seas, new species keep arriving in the Great Lakes at the rate of one every eight months. There is widespread recognition in the scientific community that the current regulations are ineffective.

Tougher new laws and funding to combat future invasions have been stalled in Congress since 2003. Regional efforts by the Great Lakes Panel on Aquatic Nuisance Species and the Council of Great Lakes Governors to push for passage of the legislation have been unsuccessful.

Recent invaders to Lake Michigan, zebra mussels (Dreissena polymorpha) and round gobies (Neogobius melanostomus), have posed the greatest threat to the lake's ecosystem. Zebra mussels are very efficient filter feeders that have significantly reduced populations of phytoplankton (microscopic algae) and zooplankton (tiny animals) that are the dietary staple of larval and juvenile fish.

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A fixed plate sampler is designed to give zebra mussel veligers a place to "land" and colonize. This one looked like it worked. © Kurt Welke

zebra mussel.jpg - 5720 Bytes A zebra mussel. © DNR Photo

Zebra mussels are threatening the food web that has evolved over thousands of years. For instance, small shrimp-like organisms called Diporeia are a key food source for native whitefish. Diporeia numbers have been substantially reduced in recent years and the zebra mussels have been blamed. This also is bad news for forage fish that feed on Diporeia and for top-level predators like the trout and salmon that feed on the forage fish.

Increasing water clarity that results from zebra mussels filtering the water may be a perceived benefit, but there is a downside. Sunlight can penetrate deeper, creating more places for attached algae to grow and reach nuisance levels. In recent years we have seen an increase in stringy algae, Cladophora, that have washed up on Lake Michigan beaches, creating unsightly and smelly conditions.

Round gobies are a strange-looking bottom-dwelling fish that came in via the ballast water of ships over a decade ago. They are very aggressive and eat the eggs of sport fish, such as smallmouth bass, trout and sturgeon. The gobies out-compete native species like the mottled sculpin for food and spawning sites. They are already the most abundant fish in the nearshore areas of Lake Michigan and are considered a nuisance by anglers. Given their aggressive nature and explosive growth, gobies can adversely affect the valuable sport fish populations in Lake Michigan.

There are a number of other invasive species that inhabit Lake Michigan including the sea lamprey (Petromyzon marinus), alewives (Alosa Pseudoharegus), ruffe (Gymnocephalus cernuus), white perch (Morone americana), three-spined stickleback (Gasterosteus aculeatus), the Quagga mussel (Dreissena bugensis), Eurasian watermilfoil (Myriophyllum spicatum), and the spiny (Bythotrephes cederstroemi) and fishhook (Cercopagis pengoi) waterfleas. The cumulative impact on Lake Michigan is not good. As more exotic species are introduced, there is greater stress placed on native species and an increased likelihood of an ecological breakdown.

So what are the prospects for future invasions and the likelihood that new invaders as devastating as the zebra mussel or round goby will be introduced? It is quite large given the loopholes in the current ballast water regulations. Over 80 percent of the ships arriving from foreign ports are exempt from ballast water regulations because they report no ballast water on board. These supposedly empty ballast water tanks actually carry sludge and water that harbor foreign organisms that are often washed out at ports.

An immediate threat are the Asian carp (in particular the silver Hypophthalmichthys molitrix and bighead Hypophthalmichthys nobilis). They compete directly with native fish and decrease the natural biodiversity. In waters of the Lower Missouri and middle Mississippi River where they have become firmly established, Asian carp constitute 90 percent of fish abundance. They are moving towards Wisconsin waters from the Illinois River and the Mississippi River. Asian carp are perpetual eating machines that escaped from southern fish farms in Arkansas during the last decade. They have been making their way northward at the rate of about 50 miles a year.

The only thing that is keeping Asian carp from moving into Lake Michigan is an electrical barrier system on the Chicago Sanitary and Ship Canal. But even the new barrier, which cost $9.1 million, does not a guarantee that the carp will not eventually enter Lake Michigan. For example, flooding on the Des Plaines River could carry water overland into the Sanitary and Ship Canal upstream of the electrical barrier. Furthermore, there are no assurances that the new barrier will not break down in the future. Obviously, contingency plans will need to be put in place.

The Asian carp are also moving up the Mississippi River. Other than one bighead carp that was caught in Lake Pepin in 2003, there have been no sightings of the Asian carp in the Wisconsin waters of the Mississippi River. A joint feasibility study with Minnesota, Iowa, Illinois, Wisconsin, the U.S. Fish and Wildlife Service, and the U.S. Army Corps of Engineers recommended installing two barriers on the Mississippi River at Lock & Dam 11 (near the Wisconsin border) and at Lock & Dam 14 or 15 (in Iowa) by 2006. The cost to build the two barriers will depend on the sites and types of technologies selected. Any technologies employed will definitely be challenging to build and potentially more costly than the barrier on the Chicago Sanitary and Ship Canal. The design will need to allow the free flow of native fish up and downstream for spawning while blocking movement of the Asian carp upstream.

Any barrier will be designed to repel the fish, not kill them. If they do get into Lake Michigan or the Upper Mississippi River, it will only be a matter of time before they make it into other Wisconsin waters. The fear is that these fish, which can grow up to 100 pounds, will seriously alter the ecology of our waters. And once they are here, there will be no way to eradicate them.

There are still other pathways through which unwanted organisms can enter state waters. These include aquaculture, the aquarium and pet trades, the sale and distribution of bait, horticultural practices, commercial barge traffic, fish stocking, plant nurseries and recreational boating. Not all exotics introduced via these pathways are harmful; some actually can be beneficial. The Department of Natural Resources, in partnership with the Wisconsin Council on Invasive Species, is assessing the potential threat of invasive species and the various pathways of introduction. They will also be examining the effectiveness of current regulations and identifying gaps in current laws. The Department of Natural Resources will be developing handbooks on procedures/protocols to help guide actions related to these methods of transport.

There is no way to completely close the door on all the pathways of introduction. But it is certainly possible to greatly reduce the likelihood that new species will be introduced and that existing nuisance species will be transported between waterbodies. This can only occur if public and private entities unify in their fight against aquatic invasive species. A good model is the regional Great Lakes states' approach to sharing information and education.

With passage of the state's last biennial budget some assistance is available to help control new infestations. Through an annual appropriation of $500,000, cost-share grants to local communities can be used to control pioneer infestations of aquatic invasive species before they become fully established.

So what is the prognosis with regard to aquatic invasive species? Clearly invasive species pose one of the most severe environmental challenges we face today. But the fact remains that the majority of regional waters in the Great Lakes basin and most of Wisconsin's inland waters remain free of invasive species. All levels of government – federal, state and local – as well as local citizens need to collectively join forces to solve this problem. It is everyone's problem. The keys to success are adequate resources to effectively combat the problem and demonstrating the political will to act.

Ron Martin is DNR's aquatic exotic species program coordinator.

Resources for being part of the solution