Harlequin duck (Histrionicus histrionicus) density on rivers in southwestern British Columbia in relation to food availability and indirect interactions with fish

I investigated factors affecting harlequin duck (Histrionicus histrionicus) prey availability on breeding streams in southwestern British Columbia. I measured flow variability, prey availability, harlequin duck breeding density, and quantified fish communities on eight rivers in 2003 and 2004. I found that prey availability was strongly and negatively associated with flow variability. Harlequin duck density was positively associated with prey availability in both years. I found a negative relationship between harlequin ducks breeding density and an index of fish abundance, supporting the existence of a Behaviourally Mediated Indirect Interaction between harlequin ducks and fish, in which prey availability is reduced in fish-bearing streams because insects alter behaviour to reduce vulnerability to fish. This supports the hypothesis that fish introductions into previously fishless rivers has negatively affected prey availability on breeding streams. Such widespread introductions may be contributing to the current low productivity measured in the western North American harlequin duck population.