Aggregative Response of Common Mergansers (Mergus merganser): Predicting Flock Size and Abundance on Vancouver Island Salmon Streams

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Abstract
The abundance of common mergansers (Mergus merganser) was monitored on four salmon-producing streams on the east coast of Vancouver Island, B.C., during the spring and summer of 1980–83. Overall, merganser abundance declined steadily from March through June but increased following recruitment of juvenile birds. Mergansers congregated on streams where juvenile salmonid migrations were enhanced by hatcheries or spawning channels. Movement between streams was evident from inverse trends in abundance on three adjacent streams differing in juvenile salmonid production. At least 8 of 12 mergansers resighted after being marked and released on the Big Qualicum River visited other nearby streams. Flock-size distributions predicted by an equilibrium arrival–departure model were consistent with those observed during May–mid-june, but not those during late June. Social interactions and reduced flight activity appeared to influence dispersion to a greater extent during late June to August so that assumptions of the model were violated. A similar model was used to predict aggregation patterns on hatchery streams from observed relationships between fish availability and frequencies of arrival and departure. The aggregation model provides a parsimonious explanation of merganser aggregation patterns during March–June and is a potentially useful tool for designing fish release schedules to minimize predation by mergansers.