Biological invasions threaten the stability and biodiversity of freshwater ecosystems worldwide. The impacts of an invading species often vary across systems, making their prediction difficult. When data from multiple invaded sites are available, statistical models can be developed to correlate an invader's distribution and abundance with local environmental variables; such models could then provide managers with useful tools to help prioritize efforts to control the invader. The introduction of the zebra mussel (Dreissena polymorpha) and quagga mussel (D. bugensis) to North America ranks among the most ecologically and economically disruptive aquatic invasions ever documented. While some attempts have been made to predict zebra mussel occurrence and abundance, none have been made for quagga mussels. Furthermore, few studies have been based on river systems, which possess the bulk of North American freshwater biodiversity. I related zebra and quagga mussel occurrence and biomass to physical habitat variables (calcium concentration, substrate size and depth) in the St. Lawrence River. I then developed predictive models of abundance for each species from combinations of these variables. Each variable explained a significant amount of variation in mussel biomass, but different combinations of variables were obtained for each species. Although these models do not account for all of the variation in abundance, they do provide a useful basis for predicting dreissenid distribution and abundance in other invaded river systems.

Biological invasions are a global phenomenon that can threaten native species and disrupt ecosystem processes. Exotic species also impact ecosystems in less conspicuous ways by provoking native species to alter their foraging behaviour. Subtle impacts such as diet shifts are frequent, and can have consequences for food web dynamics and the fitness of native predators. Diet shifts involving the consumption of exotic species require native predators to recognize, capture and handle novel prey. In this thesis, I document a diet shift in the St. Lawrence River involving a common native fish and Eurasian mussels that invaded the river in the early 1990s. I conducted diet analysis of yellow perch (Perca flavescens) at multiple sites in the upper St. Lawrence River and discovered that they consumed substantial quantities of zebra and quagga mussels (Dreissena spp.) in the Soulanges Canal, an artificial waterway west of Montreal. This was unexpected because perch lack adaptations for crushing molluscs. This foraging innovation was not observed at the same site in the early 1990s or at other sites at any time. Mussel shells were weaker at this site, probably because of exposure to calcium-poor water. This suggests that water chemistry mediates yellow perch predation on mussels. This study provides an example of diet shifts involving exotic prey and illustrates the influence of abiotic factors on species interactions.

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