The zebra mussels came in on foreign freighters who then pumped them out into the lake with the ballast water. They really did clear up the water because they are filter feeders, but it was feared they would upset the ecological balance by displacing domestic species. Their effect has been mixed, with benefits to various fish species who feed on the mussels, and plant life flourishing in the brighter light of the clearer water.
Fishing in Lake Erie has been great for the last twenty years or more. Aside from the threat of Asian carp which haven't got there yet, the biggest problems these days are algal blooms creating dead zones in summer, and layers of toxic waste in the lakebed. Even the Cuyahoga River has a healthy diverse fish popluation.