The lake had been considered suspect since May, when larvae were found but no adults.
The small black-and-white striped mussels are native to eastern Europe and voracious eaters that gobble up plankton many native freshwater fish need to survive. The mussels can also attach themselves to boat motors, bait buckets and dam mechanisms and cause damage to motors and facilities.
Game and Parks officials worry that boats will be transporting the mussel to other bodies of water in the region. Boaters are urged to clean, drain and dry watercraft to prevent the spread.