Black Bear Population Monitoring in Wisconsin

Currently, all Department bear research and management activities are directed towards managing bear populations within zones and developing harvest strategies to maintain populations at or near prescribed goals. Bear registration information plus ages of harvested bears, reproductive histories, population models, and bait station surveys are the major components of the monitoring program. Population goals for each zone are based on habitat suitability and human tolerance levels.

Population Model

Information on abundance of bears is needed for making sound harvest management decisions. Because black bears primarily use forested habitats, it is not possible to count them directly. Other methods of determining abundance, such as mark-recapture techniques, are costly to apply over large areas on a repeated basis so they are not often used for annual monitoring of bear populations. Population indices such as bait station surveys provide information on trends in bear populations (increasing or decreasing), but provide no information on actual numbers of bears. Therefore, WDNR uses a population model to develop estimates of bear population size, but also incorporates additional field research and mark/recapture studies into the model as data is acquired.

The bear population model used in Wisconsin was originally developed in Minnesota by Garshelis and Snow (1988). The model incorporates information about population parameters that affect population size in known ways. These include sex and age composition of harvested bears, age-specific reproductive rates, and rates of natural mortality. The size and composition of harvests is known from mandatory harvest registration. Estimates of rates of reproduction and natural mortality are based on bear research conducted in Wisconsin and neighboring states. The model, which combines estimates of these parameters, is used to estimate population size by fitting the simulated population trends to the observed trends in the bait station index. The model is then used to assess the effects of various harvest strategies.

Bait Station Surveys

Bear bait station surveys are conducted annually in the counties comprising Wisconsin's northern and central bear range. These surveys provide an index of population trends for comparison with and validation of population estimates calculated by the population model. Each survey transect consists of 50 bait stations (2 lb. bags of fat securely wired to trees) that are placed at 0.5 mile intervals along roads. Bait stations are checked 7 days later to determine the percentage visited by bears. The survey is conducted June 15 - July 15 to correspond with the breeding season when most bears are within their established home ranges.

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The average visitation rate along transects from all counties is calculated each year. Three-year running averages are then calculated to index population trends. Using this 3-year average reduces the influence of random annual fluctuations in visitation rates resulting from variable sample sizes and fluctuations in availability of natural foods.

Questions for Wildlife Management