CHAMPAIGN, Ill., 6/11/20: Wildlife managers track animal groups to control populations and determine the number of permits provided to hunters and trappers each year. Whether data are taken from the forest or from hunter surveys, their accuracy is necessary to inform conservation, according to Javan Bauder, post-doctoral researcher at the University of Illinois’ Illinois Natural History Survey.
Studying bobcat populations in northern Wisconsin with data from 1973 to 2013, Bauder and colleagues determined that the abundance of bobcats, or population, should correspond with a catch-per-unit-effort (CPUE) index used to study trends in bobcat numbers. Through hunter surveys, researchers can easily and inexpensively calculate CPUEs using the number of bobcats harvested and the hours that it took for success.
Rather than relying on more laborious counting of individual live bobcats after dark, hunter information from mandatory reporting is used, though there are drawbacks that can produce variable results.
“Harvest-based indices are strongly dependent on hunter and trapper behaviors,” Bauder said. “A change in pelt prices may determine whether they trap or not. Also, weather conditions, gas prices, unemployment rates, and other factors influence hunters’ decisions to hunt, all of which have nothing to do with animal abundances in the field.”
CPUEs help to control for these confounding factors and are an ideal metric to compare with animal abundances. Bauder used data from post-season surveys from 1993 to 2013 that were provided to roughly 1,200 hunters and trappers who received hunting permits. The surveys focused on the number of hours or days that permit holders spent hunting.
The number of bobcats harvested divided by the number of permits for that year provides results that researchers and wildlife managers should compare with abundance numbers over time for better accuracy.
As numbers of bobcats declined from 2003 to 2014, the CPUEs correlated weakly with abundance measures. The reason may be that hunters and trappers are more selective in the bobcats they harvest so they can showcase more impressive trophy animals. A negative relationship between CPUEs and abundance provides a less accurate picture of how bobcat numbers change from year to year.
The study was published in the journal PLOS ONE and was funded by a Federal Aid in Wildlife Restoration grant.
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Media contact: Javan Bauder, 217-300-9561, jbauder@illinois.edu
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