Relations among fishers, snow, and martens: development and evaluation of two hypotheses

Literature about the fisher (Martes pennanti) contains the following contradictions: (1) the species is an old-growth specialist versus a forest generalist, and (2) it lives with marten (M. americana) with minimal interaction versus densities of the two species are inversely related. These contradictions beg the questions of what is fisher habitat and does habitat affect the interactions of the two Martes. These questions were examined by analyzing the distributions of fishers (n = 15,549) and martens (n = 40,516) harvested in Maine, 1980-1987. This period was chosen because it had relatively stable pelt prices that were believed to result in harvests reflecting population occurrences. The spatial distribution of mean harvests of fisher and marten were compared to each other, and to snowfall distribution and frequency throughout Maine, 1980-1987. Martens were common only in northwestern Maine and were associated with frequent (monthly average = 6.5) and deep (total monthly average greater than or equal to 48 cm) snowfalls, December-March. Fishers were rare in northwestern Maine but were common throughout the rest of the state where snowfalls were less frequent. We hypothesize that regular accumulations of deep snow reduce the fisher's fitness (via decreasing recruitment, survival, or both), resulting in a low abundance in northwestern Maine. In addition, we hypothesize that martens are rare in southern Maine due to competition from a dense fisher population. These hypotheses were evaluated by looking at patterns in age and recruitment ratios of fishers (n = 2,706) and martens (n = 5,572) harvested in core and non-core habitats for each species, 1980-1984. We found low indices of fisher recruitment (P 0.001) in the marten's core habitat consistent with the hypothesis that deep and frequent snowfalls limit fishers. Few adult martens were harvested (P 0.001) in the core habitats of fisher, consistent with our hypothesis that high fisher densities limit marten pop.