Permeability of three-strand electric fences by black bears and grizzly bears

Abstract

Electric fencing has been used to deter bears in North America for several decades. Producers have turned to a design of a minimum of five-wire electric fence as their primary solution to reduce livestock depredation and to reduce raids of chicken houses and beeyards. However, these designs are expensive and reducing the number of wires used in a design to three wires would be beneficial. Scientific evaluations of the efficacy of three-wire electric fencing at deterring bears is lacking. In 2015 and 2016, I conducted a study in the Blackfoot Valley of Montana to evaluate the efficacy of rapid-deployment electric fencing designs in deterring bears from baited enclosures. Baited enclosures of two fencing configurations were established at 20 sites in the study area. Each enclosure was systematically energized and unenergized for 3-day periods; and passage into the enclosure was monitored with trail cameras to provide information on effectiveness and permeability. I recorded 134 visits by bears to fenced enclosures during the study seasons of 2015 and 2016. Of these visits, 78 occurred in 2015 and included 57 black bears and 21 grizzly bears. Fifty-six visits occurred in 2016, including 34 black bears and 22 grizzly bears. Black bears and grizzly bears were successful at passing the short fence 48% (95% CI: 32.0 -- 63.6) and 23% (5.0 -- 53.8) of the time, respectively, when it was not energized. When the short fence was energized, black bears were 7% (0.2 -- 33.9) successful in passing, whereas grizzly bears were successful in 25% (5.5 -- 57.2) of attempts. When not energized, both species successfully passed the tall fence design in 58% (95% CI: 27.7 -- 84.8) of attempts. Black bears and grizzly bears successfully entered energized enclosures with tall fences in 30% (95% CI: 13.2 -- 52.9) and 0% (95% CI: 0.0 -- 45.9) of attempts, respectively. Both fence types deterred bears from entering baited enclosures and all fences allowed less than perfect access when unenergized, suggesting that even minimalistic configurations of electric fences may act as barriers to black and grizzly bears. Further study evaluating the effects of rapidly increasing construction of electric fencing is needed to assess landscape level effects on bear movement and habitat selection.