Our Living Resources — Non-native Species 



445 



The zebra mussel (Dreissena polymorpha) is 

 a European species that was accidentally 

 introduced into North America. It has had a 

 tremendous impact on freshwater ecosystems of 

 the United States and Canada. Since the zebra 

 mussel was first discovered in Lake St. Clair in 

 1988, it has spread to each of the Great Lakes 

 and to the major river systems of central and 

 eastern United States. Communities along the 

 affected lakes and rivers rely on these waters for 

 drinking, industrial water supplies, transpoila- 

 tion, commercial fishing and shelling, and 

 recreation. Rapidly expanding populations of 

 zebra mussels could ultimately affect many of 

 these activities, in addition to changing the 

 structure of the ecosystem. 



By firmly attaching to hard surfaces, zebra 



mussels have closged water-intake 



and 



-&&-" — — pipes 



fouled hard-shelled animals such as clams and 

 snails. In addition, zebra mussels have reduced 

 plankton populations as colonies of mussels fil- 

 ter large volumes of water for food (e.g., 

 Holland 1993), potentially depleting food 

 resources of larval and planktivorous fishes 

 such as smelt, chub, and alewife (Alosa pseiido- 

 harengus). Transfer of suspended material to 

 the lake bottom in mussel waste products also 

 leads to increased water clarity (Reeders et al. 

 1992) and increased growth of aquatic plants, a 

 phenomenon already observed in some of the 

 shallower harbors of Chicago. Although clear 

 water is often considered aesthetically pleasing, 

 this clarity indicates that drastic changes have 

 occurred at the base of the food web and that 



energy flow through the ecosystem has been 

 altered. 



The first live zebra mussel was discovered in 

 Lake Michigan near Chicago in 1989. We doc- 

 umented the subsequent establishment of the 

 zebra mussel in southern Lake Michigan by 

 monitoring larval and adult zebra mussels in 

 1991-93. Monitoring was conducted primarily 

 along the Illinois and Indiana shorelines: limit- 

 ed sampling occurred along the southern 

 Wisconsin shoreline. We also quantified the 

 initial effects of the invasion on water clarity 

 and native fauna. 



Zebra Mussel Densities 



Larval zebra mussels were present at all 

 sampling locations during 1991-93; however, 

 the number of sampling locations decreased 

 from 8 to 3 over the 3 years. Peak numbers were 

 collected each year at Burns Harbor, Indiana, 

 where the highest average density was 37,044 

 veligers/nr'' n,049/ft-'')^in 1991; 74,493/m-^ 

 (2.109/ft-'') in 1992; and 42,099/m-'' (1,192/ft'') 

 in 1993. 



Attached zebra mussels were found in quite 

 low numbers (less than 130/m- or 14/ft-) in 

 1991 at one Wisconsin and four Illinois loca- 

 tions sampled by divers. The maximum density 

 in 1991 (up to 2,389/m- or 222/ft-) was record- 

 ed on concrete blocks in the intake channel of 

 an Indiana power plant inaccessible to divers. 

 By 1992, sampling at 2 Wisconsin and 4 Illinois 

 sites revealed that the population had exploded, 

 with a minimum averase densitv of 57,1 15/m- 



Zebra Mussels 



in 



Southwestern 



Lake 



Michigan 



by 



Tammy Keniry 



J. Ellen Marsden 



Illinois Natural History 



Survey 



Zebra mussel {Dreissena polymor- 

 pha) on fragile papershell mussel 

 ( Leptodea fragilis). 



Passage of the Nonindigenous Aquatic 

 Nuisance Prevention and Control Act of 

 1990 called for a national program to control 

 and reduce the risk of further introductions 

 of nonindigenous aquatic nuisance species. 

 This legislation specifically addressed the 

 non-native zebra mussel (Dreissena polx- 

 morpha), which is expected to affect two- 

 thirds of the nation's waterways. 



The zebra mussel, a European species, 

 was first discovered in Lake St. Clair in June 

 1988 and is now well established in North 

 America. Zebra mussel introductions through 

 ballast water may be responsible for many 

 other introductions to the Great Lakes as well. 



Aside from economic impacts, there 

 could also be severe biological impacts. 

 Plankton populations are directly affected by 

 zebra mussels because of the tremendous fil- 

 tering capacity of large mussel colonies; this 

 could potentially shift sy.steni energetics and 

 reduce available food resources for higher 

 organisms. Biologists in the Great Lakes 

 region believe that zebra mussels have 

 already had an effect on the ecology of Lake 



Invasion of the Zebra 



Mussel in the United 



States 



by 



Amy J. Benson 



Charles P. Boydstun 



National Biological Service 



St. Clair (Griffiths 1993); increased water 

 clarity there potentially could cause a shift in 

 the fish species composition. There has also 

 been a detrimental effect on native mussel 

 populations in Lake Erie since the arrival of 

 zebra mussels (Masteller and Schloesser 

 1991 ). Native freshwater mussels are affect- 

 ed when zebra mussel larvae settle and 

 attach on native mussels, covering them so 

 completely that they can no longer carry out 

 life processes. In addition, zebra mussels 

 reduce the amount of food and possibly oxy- 

 gen available to native mussels. 



One important part of the nonindigenous 

 program is to monitor the zebra mussel's 

 distribution and provide technical assistance 

 to other federal agencies, states, and the pri- 

 vate sector. The National Biological 

 Service's Southeastern Biological Science 

 Center (SBSC) in Gainesville. Florida, mon- 

 itors the zebra mussel as part of this pro- 

 gram. By using the zebra mussel as a proto- 

 type species, personnel at SBSC also began 

 developing a national geographic informa- 

 tion system (GIS) to organize a coherent set 

 of nonindigenous aquatic species data. 



Federal, state, and pri\'ate cooperators 

 supplied us with information, resulting in 

 the most complete digital data set of zebra 

 mussel sightings in North America 

 (Boydstun and Benson 1992). The locations 

 of sightings were then entered into a data 

 base. Since July 1991, between the United 

 Stales and Canada we have collected more 

 than 1,000 records of zebra mussel occur- 

 rences going back to their discovery in 1988 

 in Lake St. Clair. 



