PREDATION OF ZEBRA MUSSELS BY DIVING DUCKS - AN EXCLOSURE STUDY

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