Historic fur returns from Hudson's Bay Company posts in northwestern Canada reveal periodic oscillations in mink (Neovision vision) harvests lagging 2–3 years behind oscillations in muskrat (Ondatra zibethicus) harvests, as would be expected in a predator–prey interaction. Toward central and eastern Canada, the strength of the interaction between time series of harvests of minks and muskrats weakens and the lag between fluctuations of these 2 species decreases to 1 and 0 years, respectively. We tested the hypothesis that this gradient in mink–muskrat interactions is the result of decreased dependency of minks on muskrats in areas where minks have access to more alternate prey. We tested 2 predictions: species richness of mink prey is greatest in eastern Canada and decreases to the west, and percent muskrats in the diets of minks decreases as species richness of mink prey increases. Contrary to the 1st prediction, we found that species richness of mink prey in Canada is highest in south-central Canada. Consistent with the 2nd prediction, percent occurrence of muskrats in the diets of minks was much lower in areas with greater species richness of mink prey. Local species richness of mink prey therefore could influence the degree of specialization of minks on muskrats, but may be insufficient to explain the geographic pattern in the lag between muskrat and mink harvests in eastern Canada.