PREDICTING THE INTENSITY AND IMPACT OF DREISSENA INFESTATION ON NATIVE UNIONID BIVALVES FROM DREISSENA FIELD DENSITY

Introduced dreissenid mussels (Dreissena polymorpha and Dreissena buge nsis) foul native unionid bivalves by attaching to their shells in lar ge' clusters and may critically impair many North American unionids th at are already threatened by habitat degradation. Using literature and new field data, we examined patterns of Dreissena infestation on unio nids, and the relationships between Dreissena field density, infestati on intensity, and unionid mortality. Linear regression models showed t hat Dreissena field density strongly predicts (i) the proportion of un ionids colonized by dreissenids (r(2) = 0.90, p < 0.0001) and (ii) the mean number of dreissenids attached to unionids (r(2) = 0.81, p < 0.0 001). We fitted a compound Poisson model that accounts for dreissenid clustering and predicts both the proportion of colonized unionids and the mean infestation intensity as effectively as our empirically deriv ed models. The proportion of unionids colonized by Dreissena follows a saturation curve, increasing rapidly with Dreissena densities up to 2 00/m(2), and reaching a plateau at 70-80% colonization. Unionid mortal ity (reflected by the proportion of dead unionids) is strongly correla ted with Dreissena field density (r(2) = 0.82, p < 0.002) at densities above 1000/m(2). Our models predict that severe unionid mortality (>9 0%) occurs when Dreissena density and mean infestation intensity reach 6000/m(2) and 100 dreissenids/unionid.