Power Analysis of Bait Station Surveys in Idaho and Washington

We evaluated statistical power for detecting trends of specified magnitude in visitation rate for American black bear (Ursus americanus) bait stations in Idaho and Washington. We found evidence for lack of independence due to multiple visits when bait stations were 0.8 km apart and no evidence for this with stations 1.6 km apart. Based on the variability observed in Idaho, we assessed power for several sets of criteria. The minimum criteria were a relative decline of 50% over 3 years at á = 0.20 and power = 0.80. These criteria were met for many of the Idaho surveys, but were generally not met in Washington. More stringent criteria of a decline of 25% over 3 years at á = 0.10 and power = 0.90 were not met in either state. The initial visitation rate had a predominant influence on power, and in areas such as western Washington, where visitation was low but bear populations thought to be substantial, an effective monitoring program is contingent on improving the visitation rate through changes in survey methods. For long-term monitoring (5, 10, or 20 years), we estimated sampling requirements for declines of 50%, 25%, and 10% with á = 0.10 and power = 0.90 and estimated the costs of this sampling. Due to the inherent variability of bait station surveys, substantial sampling is required for detecting trends, and this method is likely to be cost effective only where visitation rates are relatively high. Although power analysis appears to be objective, determining the values for parameters used in its calculation is quite subjective and the results should be interpreted accordingly. Ursus 12:227â€"236