IMPACT OF THE DREISSENA INVASION ON NATIVE UNIONID BIVALVES IN THE UPPER ST-LAWRENCE-RIVER

Introduced Eurasian musseIs (Dreissena polymorpha and Dreissena bugens is) colonized native unionid bivalves in the upper St. Lawrence River in the early 1990s. From 1992 to 1995, we examined the infestation and impact of Dreissena on unionids at several sites near the Island of M ontreal. Unionids suffered heavy (90-100%) mortality at sites where Dr eissena occurred in high densities (i.e., 4000 - 20 000/m(2)). Mean in festations (<100 dreissenids/unionid) were 10-100 times lower than tho se reported for infested unionid populations in Lake Erie and Lake St. Clair, yet resulted in similarly high mortality levels. At two St. La wrence River sites, significant declines in unionid density were first observed when mean infestations exceeded 10/unionid. North American d ata suggest that populations of unionids that carry, on average, a mas s of Dreissena nearly equal to or greater than their own mass will bec ome extirpated. We hypothesize that dreissenid infestation enhances un ionid mortality primarily by interfering with normal activity (feeding , respiration, locomotion) in such a way as to cause the unionid to ex pend energy reserves required for surviving winter.