Exotic Species Concerns

Today, exotics, the term for organisms that have been introduced into habitats where they are not native, are considered to be among the most severe, world-wide agents of habitat alteration and degradation. Exotics are a major cause in the continuing loss of biological diversity throughout the world.

Introducing species, accidentally, from one habitat into another, where they have never been before, is risky business. Freed from the predators, parasites, pathogens, and competitors that have kept their numbers in check, species introduced into new habitats will often overrun their new home and displace native species. In the presence of enough food and favorable environment, their numbers will explode. Once, established, exotics rarely can be eliminated.

The recent development of fast ocean freighters has greatly increased the risk of new exotics in the Great Lakes and surrounding region. Ships take on ballast water in Europe for stability during the ocean crossing. This water is pumped out when the ships pick up their loads in Great Lake ports. Because the ships make the crossing much more quickly, and Great Lakes harbors are often less polluted, more exotic species are likely to survive the journey.

Historically, the abundance and diversity of fish species that populated the Great Lakes, including the Lower Green Bay and Lower Fox River Area, was very different from what it is today (WDNR 1988). Overfishing of the Great Lakes was evident before the beginning of the 20th Century, and it greatly reduced the native fish populations. The invasion of exotics such as carp, alewife, sea lamprey, white perch and rainbow smelt also reduced some native fish numbers. To rehabilitate the commercial fishing industry, man has attempted to rectify some of these problems and restore an ecological balance to the Great Lakes through sea lamprey control, fish stocking and commercial harvesting of exotics. However, the system remains dramatically altered in fish species composition. The diversity may actually be higher in some cases, but the exotics displace more desireable native species (Meyers, 1998).

Today, the unbalanced fish community of the inner Bay and Lower Fox River is characterized by low abundance and low diversity of top predators (such as northern pike, walleye, and smallmouth bass) and native forage species (the spottail shiner) combined with a presence of certain exotic species (carp, alewife, white perch and sea lamprey). Excess carp populations may also present other problems. These fish are suspected of adversely affecting the Bay's ecosystem by uprooting native underwater plants, possibly allowing invader species to grow and resuspending sediments which increases turbidity. Abundant carp populations are present from Little Lake Buttes des Morts dam to Wrightstown (Meyers, 1977; Ron Bruch, personal communication, 1992 survey). However, carp really only add to the problems of eutophication and lack of habitat in the inner bay. High phosphorus and sediment loadings lead to the excessive production of algae and excess sediment loadings prevent rooted aquatic plants from establishing in the inner bay (Bougie, 1998).

Common Carp

The common carp (Cyprinus carpio) is a fish species native to Asia. In 1880, 75 carp were obtained from the U.S. Commission and forwarded from the carp ponds at Washington D.C., to the Nevin Hatchery in Madison, Wisconsin. In 1881, 163 carp fingerlings in lots of 20-22 were distributed to individuals in Rock, Columbia, Fond du Lac, Sauk and Manitowoc counties. As many as 35,000 carp were placed into Wisconsin waters in 1890; distribution continued until 1895, when the program was discontinued. By that time plantings had been made throughout the state, including the northern and central counties of Barron, Douglas, Eau Claire, Langlade, Marathon, St. Croix, Ashland, Marinette, Polk, Price, Sawyer, Shawano, Taylor and Washburn (Becker 1983). Today, carp are present in 63 Wisconsin counties. As of the mid-1970s, carp were not present in Florence, Bayfield, Price, Iron, Vilas, Oneida, Lincoln, Langlade and Menominee counties (Hacker 1975). Carp are an abundant and characteristic species in large, shallow lakes and streams in southern and central Wisconsin. In the 1980s, carp were thought to be common in some northern Wisconsin Waters (Becker 1983).

Although carp are known to survive under a wide range of conditions, they prefer warm streams, lakes, and shallows containing an abundance of aquatic vegetation. Carp tolerate all bottom types and clear or turbid waters, but are not normally found in clear, cold waters or streams of high gradient. Carp adapt to a wider variety of conditions than almost any native North American fish. Habitat requirements permit carp to range the state's extensive warm waters with little concern for low oxygen content, pollution, or sudden temperature changes (Becker 1983). The carp's ability to tolerate low oxygen levels is well known and often it is one of the last survivors in oxygen depleted waters. This relates to its ability to use atmospheric oxygen (MacKay 1963).

Problems associated with the carp were recognized as early as 1901. Fisherman considered the species a nuisance. There are many illustrations of the loss of aquatic vegetation where carp are abundant. A 1944-45 study conducted in Madison, documented the aquatic vegetation destruction by carp. Carp were stocked at an equivalent of 532 kg/ha (475 lb/acre) in a pond with a dense growth of waterweed, sago pondweed, and lesser amounts of pickerel weed, coontail, and wildrice (Black 1946). Fifty-one days later the aquatic vegetation was reduced to a very critical stage; the wild rice had been uprooted within a few days of stocking, the sago pondweed was torn out quickly, and the coontail soon after; only the waterweed, heavily grazed, and the pickerel weed remained. When the pickerel weed was attacked, however, it was destroyed completely in less than two weeks (Becker 1983). Carp are also known to attack and uproot larger emergent plant species such as cattails.

Carp not only destroy aquatic vegetation stands they also resuspend sediments, which increases water temperature, lowers dissolved oxygen levels, and reduces light penetration. These abrupt changes in the aquatic ecosystem, reduce new plant growth, and deteriorate suitable habitat for other fish and aquatic organisms.

Competition exists between young largemouth bass and carp of all ages for the availability of food and in areas of spawning the largemouth is at a definite disadvantage (Sigler 1958, Mraz and Cooper 1957). Competition between the carp and the green sunfish for available habitat takes place because both fish frequent shallow water and occasionally compete for the same spawning area (Sigler 1958).

The contention that the presence of carp even in small numbers is detrimental to game fish is strongly disputed by the fact that nearly every lake in southern Wisconsin has carp present, yet game fish thrive except in those lakes where a dense carp population exists (Mraz and Cooper 1957). Nonetheless, there is constant pressure by the public to remove the carp, even from waters where their numbers are known to be small (Becker 1983).

Recommendation:

1. The Lower Fox River Basin Team should work to increase predator populations such as muskellunge and northern pike in the Lower Fox River (Type C).

Zebra Mussels

The zebra mussel (Dreissena polymorpha) is a tiny (1/8 - 2 inches) bottom-dwelling mussel native to Europe. The mussel gets its name from the striped shell. Zebra mussels were introduced into the Great Lakes system (Lake St. Clair) in 1985 or 1986 via ship ballast water. They have spread throughout the Great Lakes and are now found in Green Bay. Zebra mussels were first identified in the Wisconsin waters of Lake Michigan on a ship in Sturgeon Bay (WDNR observation and Green Bay Press-Gazette 1989). They appeared at Two Rivers and Kewaunee and in the Sturgeon Bay shipping canal in 1992 (Kraft 1992). The first report of zebra mussels in Door County north of the Sturgeon Bay canal occurred in October 1992 (Kraft 1992). The first indication of zebra mussel establishments in southern (lower) Green Bay was in 1991 (Kraft 1991). Zebra mussels currently inhabit all of Green Bay. In the spring of 1998, a DNR surveyors found attched zebra mussels on anchor lines in the Fox River at DePere. Problems associated with zebra mussels include: windrows of mussels covering shorelines of the lower bay, adhesion to docks, boat hulls and any water in-take structure (Bougie 1998).

A zebra mussel generates a tuft of fibers known as byssus, or byssal threads, from a gland in the foot. The byssus protrudes through the two halves of the shell. These threads attach to hard surfaces with an adhesive secretion that anchors the mussels in place. Although the mussels are small, they can cluster together to form colonies of thousands (30,000-70,000) of individuals per square meter (Ohio Sea Grant 1990). Any hard underwater surface such as rocks, docks, boat hulls, commercial fishing nets, buoys, water in-take pipes, and even other invertebrates can be covered by layers of mussels in a short time. Established zebra mussel colonies create a uniform gravel or cobble-sized substrate.

Most authorities consider the spread of zebra mussels across Wisconsin to be inevitable and the mussel will likely become a permanent part of the Great Lakes environment. The mussel is a prolific breeder (each mature female can produce 30,000 - 50,000 eggs per season). Zebra mussels can spread from the Great Lakes to inland water either as veligers (larvae) transported in boat live wells, bait buckets, or engine cooling water, or as juveniles and adults attached to boat hulls, engines, fish cages or other items. Zebra mussels become mature within the year and may live from four to six years.

The efficient feeding habits of zebra mussels have the potential to alter the entire ecology of many of our inland lakes. One zebra mussel can filter about a liter of water per day. However, when they are abundant, competition reduces filter capacity. Nearly all particulate matter, including plankton, is strained from the water. The mussels digest mostly algae. Uneaten plankton and other particulate matter is bound with mucous and deposited on the lake bottom. Thus, a considerable amount of material can be removed from the water column, making it unavailable for larval and juvenile fish and plankton-feeding forage fish. The mussels' feeding method might also affect water clarity in some areas.

For additional information on zebra mussels, contact the Wisconsin Department of Natural Resources, P.O. Box 7921, Madison, WI 53707. (608) 266-9270.

Recommendation:

1. If the Lower Fox Locks are reopened, the Lower Fox River Team should maintain the barrier at Rapids Croche Dam to clean each boat going upstream until such time when the zebra mussels are found upstream in Lake Winnebago (Type C).

Eurasian Water Milfoil

Eurasian water milfoil (Myriophyllum spicatum) is an exotic plant species from Europe that has invaded many lakes in Wisconsin. The plant is normally found only after it has established itself in a lake and become a nuisance. In the Midwest, Eurasian water milfoil has mainly affected surface water recreation. The plant can grow to depths of 15 feet or more. Once this growth peaks in July and August, dense floating mats of vegetation can form and interfere with boating, fishing and swimming. From an aquatic community perspective, Eurasian water milfoil out competes native plants thereby reducing diversity. It is not colonized or utilized by aquatic insects as readily as native species and it grows so thick that it impairs the ability of sight feeding fish to find prey.

If left unchecked, further spread of Eurasian water milfoil is likely. This plant can regenerate from broken fragments. Boats and trailers are the usual transport, though plant fragments can travel to other waterbodies in other ways (e.g. by waterfowl). Although removal of plant fragments from water craft will not eliminate the spread of milfoil, it is essential to limiting the number of infestations, thus slowing the spread.

It is unknown if Eurasian water milfoil inhabits the lower Bay and the Fox River in the Lower Fox River Basin.

Recommendation:

1. The Lower Fox River Basin Team should monitor Eurasian water milfoil to determine if it inhabits the lower Bay and Fox River during routine monitoring and post Milfoil Alert signs at boat launches to educate the public and prevent the spread of an exotic species (Type C).

For additional information on Eurasian water milfoil and its control, in the Green Bay area contact the Wisconsin Department of Natural Resources, P.O. Box 10448, Green Bay, WI 54307, (414) 492-5903 or 492-5905.

Purple Loosestrife

Purple loosestrife (Lythrum salicaria) is a wetland plant from Europe and Asia. It was introduced into the east coast of North America in the 1800s via ship ballast water. First spreading along roads, canals, and drainage ditches, then later distributed as an ornamental, this exotic plant is in 40 states and all Canadian border provinces (MDNR 1992).

Purple loosestrife invades marshes and lakeshores, replacing cattails and other native wetland plants. The plant can form dense, impenetrable stands that are unsuitable as cover, food, or nesting sites for a wide range of native wetland animals including ducks, geese, rails, bitterns, muskrats, frogs, toads, and turtles. Many rare and endangered wetland plants and animals are also at risk.

Loosestrife's numbers and rate-of-spread have increased rapidly in the Midwest during the past 10 to 15 years. As a result, not only is our diverse wetland vegetation (including rare and endangered plants) threatened, but also most wildlife that depend upon native vegetation for food and shelter. Once a wetland is dominated by loosestrife, traditional residents such as muskrat and waterfowl decline in numbers significantly. Others, such as marsh wrens and least bitterns, are displaced completely from the wetland (WDNR 1990). Once, loosestrife begins to invade, canopy closure is likely and a virtual biological Adesert@ develops underneath.

Purple loosestrife thrives on disturbed, moist soils, often invading after some type of construction activity. Eradicating an established stand is difficult because of an enormous number of seeds in the soil. One adult plant can disperse 2 million seeds annually (MDNR 1992). The plant is able to resprout from roots and broken stems that fall to the ground or into the water.

A major reason for purple loosestrife's expansion is a lack of effective predators in North America. This plant managed to escape the specialized insects and diseases that keep it in check in its native lands. Free from these natural controls, loosestrife gained a competitive edge over our native wetland plants. Several European insects that only attack purple loosestrife are being tested as a possible long-term biological control of purple loosestrife in North America. Chemical control is costly and requires long-term application. The use of nonspecific herbicides has detrimental effects on nontarget wetland plants.

Purple loosestrife is very prolific in the Atkinson's Marsh area near the petroleum tanks on the west shore of the Lower Fox River.

Brown County has a Purple Loosestrife Task Force which was formed several years ago. The task force identified areas (mainly in the Baird Creek Parkway) where the loosestrife was colonizing and contacted landowners to eradicate the exotic plants. A group of volunteers also assisted the landowner with the removal of the plants.

Aquatic Plant Management

Excessive and unsightly plants, exotic or native, growing where they are not wanted concern many Wisconsin citizens. In some lakes, these Aweeds@ impair fishing and other recreational uses. Yet the extreme opposite of excessive plant growth, a plant-barren lake bottom, is much worse. Aquatic plants play an essential and beneficial role in the life-support systems of most lakes. They produce oxygen and organic material, which help keep the lake and organisms in it alive. The leaves and stems of aquatic plants are home to insects and small attached plants. Plants provide spawning areas, food and protective cover for aquatic organisms. Their roots help stabilize lake bottoms and prevent shoreline erosion. Ducks, beaver and muskrat use plant roots, tubers and stems as food and building materials.

If excessive plant growth does become a concern in a lake, there are a variety of management techniques available, both mechanical and non-mechanical. Hand harvesting, raking, aquatic plant screens and mechanical harvesting have been successfully applied on Wisconsin lakes. These types of techniques are preferable to chemical herbicide application. Aquatic herbicides are not always selective. They can destroy valuable native vegetation along with less desirable plants and encourages exotics. Lake biologists are concerned about the long and short-term effects chemical may have on lakes. WDNR can provide fact sheets on a number of chemical herbicides and their alternatives.

When choosing a technique to manage aquatic plants, lake property owners and lake management organizations should select the technique that offers the best control with the least potential for disrupting the balance for the lake's ecosystem. WDNR's aquatic plant management specialists can help concerned organizations find the best solution for managing an individual lake. However, weed removal, chemical treatment and screens may provide only a temporary solution. The best way to thwart excessive plant growth is to cut off surplus nutrients and sediments flowing into a lake. Most lakes have so many nutrients in them already that stopping the inflow of nutrients will not cause change for many years. However, it will keep problems from worsening, and it helps protect any investment in short-term management.