PREDATION OF ZEBRA MUSSELS BY DIVING DUCKS - AN EXCLOSURE STUDY

Zebra mussels (Dreissena polymorpha) are a novel and abundant potential food source for several species of diving ducks in the Great Lakes region. Using predator exclusion cages, behavioral observations, and analyses of duck gizzard contents, we examined the predator-prey relationship between zebra mussels and their duck predators during the fall migratory period in Lake Erie at Point Pelee, Ontario. Diving ducks feeding on zebra mussels in the area reduced mussel biomass by 57% during the period of heaviest feeding, but had no measurable impact on mussel numbers. Birds were size-selective predators, preferentially taking medium and large mussels over the more common small ones, thereby altering the size structure of the mussel population. Ducks were abundant at Point Pelee only during late fall, and differences between cages and control areas had disappeared by the following spring. Overall, ducks had little lasting impact on mussel populations, but mussel abundance may have determined duck concentration in the area. Duck staging populations were higher in 1990, when mussels were more abundant, than in 1991, when mussels were much scarcer. Effective predator control of zebra mussels in the portion of the Great Lakes region that freezes over winter is unlikely because diving ducks are unable to overwinter in the area. However, mussels have become an important food source for diving ducks, and temporary reductions of mussel density through predation are likely in areas where ducks feed during migratory periods. Similarly, in regions where ducks are resident through winter, greater and longer lasting effects may be observed.