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Post Hunt Deer Population Estimates
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Deer herd abundance is estimated annually with hunter-collected data and a mathematical model to get post hunt deer population estimates.
For additional Information….
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The Wisconsin DNR annually estimates the size of deer populations in each deer management unit (DMU). Post hunt population estimates are the starting point for setting antlerless quotas and harvest of antlerless deer is the primary way to manage deer herd abundance. For instance, in farmland management zones, harvesting approximately 25% of the antlerless deer will stabilize the population, while the population will tend to grow with a lower harvest rate and decrease with a higher harvest rate.
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Collection and analysis methods
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Mandatory registration of every deer harvested during the hunting season is the backbone of the state’s deer monitoring system. When hunters register their deer, information is collected on the date and place of harvest and the sex of the deer. The proportion of the adult buck population taken by hunters is relatively uniform from one year to the next. Under such stable conditions, managers have found that buck harvest trends closely track deer population trends.
Information from harvest registration and aging, along with other data, is used in a mathematical population model called the Sex-Age-Kill (SAK) formula. Information on the age composition of the buck harvest is used to estimate the percentage of adult bucks killed during the legal hunt. The SAK formula combines this estimate with information on the size of the buck harvest to estimate the size of the pre-hunt adult buck population. The adult buck population is then expanded to the entire population using estimates of the number of does per buck and the number of fawns per doe in the pre-hunt population. The overwinter deer population for each DMU is determined by subtracting the harvest from the pre-hunt population estimate.
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Deer population estimates from a DMU can be compared over time. Three-year running averages of population size have been calculated to help illustrate overall population trend. Changes in deer population estimates among years in the same DMU may reflect previous winter severity (in the northern DMUs, especially), amount of antlerless harvest, or variation in buck harvest rates. Variation in deer abundance across the state largely reflects variation in weather and habitat.
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Limitations and precautions
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While the length of the November gun season has not often changed in most of Wisconsin and hunting patterns and the proportion of the adult buck population taken by hunters is relatively stable, there is some year-to-year variation in buck harvest rates that affect SAK population estimates. Some of this variation is caused by shifts in opening dates of the November gun season (earliest date 17th, latest date 23rd) in relationship to the timing of peak breeding activity. Additionally weather variation during the 9-day gun season can alter deer and hunter behavior. Consequently, some of the annual variation in deer abundance estimates is the result of variation in buck harvest rates.
Sample sizes for some of the inputs of the SAK formula are limited. Consequently, it is necessary to pool data over multiple DMUs and/or years to produce annual deer population estimates for all DMUs. There can be considerable local variation in deer density within DMUs due to differences in deer habitat quality and local hunting pressure.
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The Wisconsin DNR continues to look for alternative ways to cost-effectively monitor changes in deer population size in DMUs. A better understanding of factors affecting buck harvest rates may improve the accuracy of harvest-based population estimates.
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Additional background materials related to this metric
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Harvest and hunter survey reports are available for viewing on the Wisconsin DNR website dnr.wi.gov keyword “wildlife reports”.
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