No description available.

The objective of this study was to understand the pattern of variation in group structure and feeding behaviour of wintering Common Goldeneyes ( Bucephala clangula) on the Saint-Lawrence River, by comparing two major habitats for this species in the province of Quebec, one in freshwater and one in saltwater. Groups of goldeneyes were smaller in Montreal than in the estuary and consisted of twice as many males. Individuals spent a greater proportion of time feeding in Montreal than in the estuary. At Montreal, goldeneyes spent more time feeding under water. Temporal variables had a major influence on group structure and on diving time. At Montreal, time of day explained most of variation whereas in the estuary, tide was more important.

No description available.

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.