Im pact of the Dreissena invasion on native unionid bivalves in the upper

Abstract: Introduced Eurasian mussels (Dreissena polymorpha and Dreissena bugensis) 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 MontrØal. Unionids suffered heavy (90100%) mortality at sites where Dreissena occurred in high densities (i.e., 4000 20 000/m2). Mean infestations (<100 dreissenids/unionid) were 10100 times lower than those reported for infested unionid populations in Lake Erie and Lake St. Clair, yet resulted in similarly high mortality levels. At two St. Lawrence River sites, significant declines in unionid density were first observed when mean infestations exceeded 10/unionid. North American data suggest that populations of unionids that carry, on average, a mass of Dreissena nearly equal to or greater than their own mass will become extirpated. We hypothesize that dreissenid infestation enhances unionid mortality primarily by interfering with normal activity (feeding, respiration, locomotion) in such a way as to cause the unionid to expend energy reserves required for surviving winter. Résumé: Au dØbut des annØes 1990 dans le cour supØrieur du Saint-Laurent, des moules eurasiennes (Dreissena polymorpha et Dreissena bugensis) introduites ont colonisØ des bivalves de la famille des unionidØs, indigŁnes dans ces eaux. De 1992 à 1995, nous avons ØtudiØ linfestation et ØvaluØ limpact des Dreissena chez les unionidØs de diverses stations situØes à proximitØ de lîle de MontrØal. Là oø la densitØ des Dreissena Øtait ØlevØe (cest à dire de 4000 à 20 000 m2), la mortalitØ a ØtØ trŁs forte chez les unionidØs (90100%). Les infestations moyennes (<100 Dreissena/unionidØ