Harvest management quotas for fishers (Pekania pennanti) in some jurisdictions are estimated from the previous year’s harvest, and stem from the hypothesis that age ratios in the harvest are largely influenced by ‘top-down’ trapping pressure.  The influence of ‘bottom-up’ food supply on fisher harvest age ratios might be underappreciated, which could result in a misallocation of quotas in management planning. We assessed a variety of data sources to test the influence of bottom-up processes on fisher populations in Ontario, Canada. We found evidence that bottom-up trophic effects influence the harvested fisher age structure in some regions of Ontario. Evidence also suggests that harvest pressure had little top-down influence on age ratios over the course of our study, and that basing management strategies on this assumption may lead to unintentional overharvest in years of low productivity.  We suggest several trophic linkages with potential to facilitate fisher management, including connections among berry and seed crops, small mammals, and Northern saw-whet owls (Aegolius acadicus).

Our study took place in the Great Lakes-St. Lawrence East region of Ontario, which encompasses the southern range boundary of fishers within the province.  We received fisher harvest data from 11 fur management units (FMUs) within this region - Owen Sound, Huronia, Peterborough, Mazinaw, Bancroft, Carleton Place, Brockville, Cornwall, Pembroke, Parry Sound, and Bracebridge.  Following the harvest, fisher trappers were invited to submit skulls, jaws, or teeth to their local Ontario Ministry of Natural Resources and Forestry (OMNRF) office.  Wildlife managers then aged submissions by x-raying the teeth and measuring pulp cavity width (Dix and Strickland 1986).  We used these data to determine juvenile:adult female (J:AF) ratio for each FMU for each year.  Fishers were considered adults if they were >1.5 years of age. 

Red-backed vole abundance was determined from an ongoing annual trapping program in Algonquin Provincial Park, which has been described in several published articles (e.g., Fryxell et al. 1998, 1999, Falls et al. 2007, Denomme-Brown et al. 2020).  Small mammals were trapped once or twice a month, May-August, at 10 trap lines along the Highway 60 corridor of the park.  Each trapline was approximately 90 m long and contained 10 trapping stations spaced 10 m apart, each with two small (7.5 cm x 7.5 cm x 30.5 cm) Sherman live traps.  One Sherman and one Longworth trap (6.4 cm x 8.4 cm x 13.8 cm) were used for the years 2013 to 2016 inclusive.  Therefore, each line contained a total of 20 traps.  Traps contained a ball of polyester batting for use as bedding and were baited each evening with water-soaked sunflower seeds, oats, or peanut butter and checked the following morning.  Captured animals were identified to species and marked with a Monel ear tag.  We calculated species abundance as the number of captures per 100 trap nights (TN) and exponential rate of increase as the natural logarithm of the ratio of the abundance at time t and the abundance the previous year, at time t-1.  The annual mean (SD) trap nights for our 23 years of data was 4678 (±427).

We collected data on the productivity of mountain ash and oak from the OMNRF’s annual Ontario Wildlife Food Survey.  To complete this survey, OMNRF field personnel submitted estimated summer and fall fruit/seed productivity scores for a range of plant species from their region on a scale of 0-4, with 0 indicating no or almost no production and 4 indicating a bumper crop.  For the purposes of our study we considered only data from the Great Lakes-St. Lawrence East region of Ontario.  We excluded data from three districts within this region (Sault Ste. Marie, Wawa, and Aylmer) because they did not overlap with and were not adjacent to the FMUs included in our study.  There was variation in reporting strategy within and among districts. For example, some districts submitted multiple observations from specific locations within the district while others provided an overall estimate.  For comparability, mean annual productivity scores for each reporting area were weighted equally to calculate a mean overall productivity score for each species of interest over the entire study area.  

We estimated relative abundance of Northern saw-whet owls using data collected during Birds Canada Nocturnal Owl Surveys in the central region of Ontario, which is roughly bordered by the 47^oN and 44^oN lines of latitude.  Surveys were conducted annually on a single evening in April by volunteers following a predetermined 18 km route.  These routes were located along secondary roads and consisted of 10 stops spaced 2 km apart.  The protocol at each stop began with 2 min of silent listening for owl calls, followed by the playing of a series of boreal owl (Aegolius funereus) and barred owl (Strix varia) calls interspersed with silent listening periods.  Volunteers recorded all owl calls they heard at each station.  Birds Canada personnel then estimated an annual index for each owl species, including Northern saw-whet owl, using generalized additive models with smoothing terms for geographic location to account for spatial autocorrelation.  The response variable was the number of occupied stations on each route for a given year.  Models were created using the binomial version of the “gam” function, which is based on the methods of Hastie (1992) and found in the GAM package for R.  Using the indices produced by this calculation (N) we calculated northern saw-whet owl exponential rate of increase in year t using the formula ln(N_t/N_t-1).  

Fisher J:AF values for each FMU are presented in the columns labelled with the FMU names.

Fisher age ratios were calculated over the entire fisher trapping season (October-January).  Data is presented in the row corresponding to the year in which the trapping season began (i.e. J:AF data in the "1998" row corresponds to the 1998-1999 trapping season).

Blank cells indicate that data was not available.