Otir Living Rt'sounes — Nan-native Species 



447 



(5,306/ft-) near Glencoe. Illinois. The iiiaxi- 

 miiin average density in 1992 was 267.885/m- 

 (24.885/ft-) at a Waukegan site that 1 year pre- 

 viously had only 25 mussels/m- (2/ft-) 

 (Marsden et al. 1993). Densities at two Illinois 

 locations remained high in 1993. with average 

 densities of 224.428/m- (20.838/ft-) "at 

 Waukegan and 52.428/m- (4.87()/t't-) at Lake 

 Forest. 



High reproductive success during 1991 was 

 clearly responsible for the huge increase in the 

 number of attached mussels during 1992. It is 

 interesting that although 1992 levels of repro- 

 duction were generally twice as high as in 1991 . 

 the population increase did not continue in 1993 

 at the two locations sampled. 



Water Clarity 



Water visibility (using a secchi disk) 

 increased from a maximum depth of 4 m ( 13 ft) 

 in 1990. to 6 m (20 ft) in 1991. to 10 m (33 ft) 

 in 1992. Water remained clear in 1993. with a 

 maximum depth at disappearance of 9.5 m (31 

 ft). At the site for which data are most consis- 

 tently available (Waukegan). minimum water 

 visibility measurements during 1991-93 were 

 higher than any measured values during 1990. 

 This trend should be interpreted with caution 

 given the natural variability in water clarity val- 

 ues. The data suggest, however, that the water 

 clarity of southern Lake Michigan may be 

 increasing due to colonization of the lake by 

 massive numbers of zebra mussels. This trend 

 has been documented in other recently colo- 

 nized lakes, such as Lake Erie (Leach 1992). 



Impacts on Snails 



Most native snails we collected were colo- 

 nized by one or more zebra mussels. Sragiiivola 

 was the most common genus collected in non- 

 quantitative samples. In 1991, 72% of these 

 snails had attached zebra mussels, with an aver- 

 age of 1.6 mussels per snail. By 1992, 999^ of 

 Stagnicola were fouled, with the average num- 

 ber increasing to 3.7 zebra mussels per individ- 

 ual snail. Eliinia snails dominated the quantita- 

 tive samples from rocky areas. In 1992, 99% of 

 94 Eliiiiiu were fouled with mussels; in 1993 

 divers failed to find any live Ellniia at the 

 Waukegan reef. 



Conclusions 



In the Great Lakes and associated river sys- 

 tems, populations of native clams are threatened 

 because of the colonization of their shells with 

 massive numbers of zebra mussels (Mackie 

 1991). Our data indicate that snails are also 

 being used as substrate for mussel attachment in 

 Lake Michigan. As grazers, snails are an impor- 



tant part of the bottom community. They are 

 also a source of food for tlshes such as yellow 

 perch { Perca flavescens), sunfish, and whitefish 

 (Scott and Grossman 1973). Given the limited 

 knowledge of the role of snails in Lake 

 Michigan and other large lakes, it is not possi- 

 ble to fully anticipate the effects of reduced or 

 decimated snail populations. 



The rapid increase in zebra mussel densities 

 we observed in the open waters of the lake was 

 retlected in their colonization of municipal and 

 industrial water-intake pipes. In 1991 and 1992 

 facilities drawing raw water from Lake 

 Michigan began treatment programs to reduce 

 infestation of intake pipes. The cost of retro- 

 fitting plants in Chicago and northern Illinois 

 shoreline communities had totaled $1,778,000 

 by 1992 (Nelson 1992). This value does not 

 include chemical costs, or increased personnel 

 costs as workers dealt with mussel-related prob- 

 lems. In addition to economic costs of retro- 

 fitting and chemical treatments. Lake Michigan 

 has an increased ecological risk of accidental 

 chemical spills or leakages. 



Zebra mussels also affect the aesthetic and 

 recreational value of the lake. Boat owners are 

 concerned about zebra mussels fouling boat 

 hulls and engine cooling systems, and 

 windrows of broken shells have begun to appear 

 along Lake Michigan beaches. 



The economic impact of zebra mussels is not 

 limited to industrial and recreational interests, 

 however. Native clams from the Illinois River 

 are shipped to Japan for use in the cultured pearl 

 industry; in 1991 the value of this resource was 

 $1.4 million annually. The infestation of clams 

 by zebra mussels has increased dramatically, 

 resulting in significant clam mortality. 

 Commercial shelling on the Illinois River was 

 recently banned, following a drop in harvest 

 from over 454,000 kg ( 1 million lb) in 1991 to 

 67,646 kg (149,000 lb) in 1993 (Don Duffeil, 

 Illinois Department of Conservation, personal 

 communication). 



Zebra mussels are a permanent addition to 

 the Lake Michigan ecosystem and connected 

 waters. Chemical and mechanical controls for 

 zebra mussels are only useful in localized areas 

 such as intake pipes and other artificial struc- 

 tures, but not in the open waters of the lake. 

 Ultimately, zebra mussel populations will 

 exceed the capacity of the environment to sup- 

 port them, after which their numbers will likely 

 decline. Native predators such as freshwater 

 drum (Aplodinotiis gninniens), diving ducks, 

 and crayfish may also keep mussel populations 

 in check in some areas. The adverse effects of 

 zebra mussels on human activities and native 

 aquatic species will never be totally eliminated, 

 but eventually they may become a more tolera- 

 ble nuisance. 



