Dreissenid mussels in the Laurentian Great Lakes are preyed on by fish and diving ducks, but predators’ roles in structuring Great Lakes’ mussel populations are unclear. In Europe, predation impacts on zebra mussels are most pronounced on lakes where waterfowl winter. Using cages which excluded either fish and diving ducks or only diving ducks, we examined effects of these predators on both zebra mussels and quagga mussels (Dreissena polymorpha, D. bugensis) at Nanticoke, Lake Erie, where ducks both stage and winter. We arrayed cages at two sites, one within, and one adjacent to, a winter ice-free hole created by a generating station’s coolant water discharge. At the ice-free site, predation by diving ducks reduced numbers of both mussel species substantially, and especially numbers of larger mussels. At the ice-covered site, residual effects of predation by ducks the preceding fall depended on rock size: numbers of large mussels declined on large rocks. Predation by fish reduced quagga but not zebra mussel abundance at the ice-covered site. Effects appeared to be most pronounced on the smallest mussel length class, but characterizing length-class preferences was difficult because effects on large mussels were sensitive to rock size: large mussels were removed from small rocks. At both sites, prior correlations between the number of quagga mussels on a rock and the size of that rock weakened or broke down completely following predation. Our results extend previous work on zebra mussels to include the quagga mussel, and show experimentally the potential of both predator groups, but especially wintering diving ducks, to affect local density, demography, and substratespecific occupancy of dreissenid mussels.