()t{i Liviiii; Resources — Inveitehrnles 



I HI 





2 1 



13 



3 1 1 



2 12 3 



5 2 3 2 



8 1 



1986 

 Lake St, Clair 



<1 

 2 4 

 1 



14 18 



«4 

 n 30 



22 



1961 

 Western Lake Erie 



1991 

 Western Lake Erie 



declined from eight to five between 1961 and 

 1982. The declining populations of native mus- 

 sels are attributed to pollution that originated 

 from tributary rivers of the lake prior to the 

 1970"s. In the mid-1970"s, pollution-abatement 

 programs were begun, and water and substrate 

 quality began to improve in western Lake Erie 

 by the mid-1980"s. By the late 1980's. environ- 

 mental quality improved dramatically and pol- 

 lution-sensitive indicators such as burrowing 

 mayflies {Hexagenia spp.) began to return to 

 western Lake Erie (Farara and Burt 1993; 

 Schloesser, unpublished data). 



Current Status 



In the early 1990"s, however, native mussel 

 populations declined dramatically in the Lake 

 Huron-Lake Erie corridor, despite improve- 

 ments in water and substrate quality (Fig. 2; 

 Table 2). In Lake St. Clair, substantial declines 

 of mussels were documented between 1990 and 

 1992. Numbers and species of mussels were 

 about half those found only 2 years earlier. 

 Most changes in mussel populations in Lake St. 

 Clair occurred in the southern portion of the 

 lake, where mussels are no longer found (Fig. 

 2). In Lake Erie, mussel populations virtually 

 disappeared in offshore waters between 1982 

 and 1991 (Fig. 2: Table 2). 



Recent changes in native mussel populations 

 in the Lake Huron-Lake Erie corridor are attrib- 



uted to mortality caused by the exotic zebra 

 mussel (Dreissena polymorpha); these exotics 

 attach to the surface of mussels in such high 

 numbers that native mussels are unlikely to be 

 able to breathe and eat (Fig. 3). Intensive sam- 

 pling indicated that native mussel populations 

 declined rapidly between September 1989 and 

 May-June 1990 (Fig. 4). Zebra mussels became 

 abundant the summer of 1989. when infestation 

 on clams increased from 24 mussels to 7,000 

 mussels per clam (Schloesser and Kovalak 

 1991; Nalepa and Schloesser 1992). 



Erosion caused by deforestation, poor agri- 

 cultural practices, and destruction of riparian 

 zones, and organic and inorganic pollution have 

 long been recognized as other causes for mussel 

 mortality (Williams et al. 1993). Our knowledge 

 of the zebra mussel, however, and its coloniza- 

 tion on native mussels indicates that native mus- 

 sel mortalities in the 1990"s are attributable to 



Fig. 2. .Average ( mean ) densities 

 (number/m-) of native mussels in 

 Lake St. Clair and western Lake 

 Erie of the Lake Huron-Lake Erie 

 corridor of the Great Lakes, 1 96 1 - 

 92. 



Fig. 3. Typical native mussel (Polamilus alatus] uncolo- 

 nized (left) and colonized (right) by the exotic zebra mus- 

 sel (Dreissena polymorpha) in the Lake Huron-Lake Erie 

 corridor of the Great Lakes. 



