Doug Leier, Published August 21 2012
Leier: Wildlife surveys are estimates, not counts
I have joked with many hunters over the years that North Dakota really does have two seasons – in-season and preseason.
As the long preseason progresses, and numbers from the spring waterfowl index or pheasant crowing counts are crunched and dispersed, hunters begin more intensive daydreaming about how their own season will play out.
But numbers and surveys without some explanation are like sitting down at a cafe without a menu. You really have no idea what you’ll be getting. Here’s a little insight into how survey numbers work.
For starters, there is a difference between how we try to estimate human and wildlife populations. Every 10 years the United States carries out a census during which an attempt is made to count every person in the country.
This is not the case with most wildlife surveys in North Dakota.
For example, an accurate census or actual count of sharp-tailed grouse in North Dakota is virtually impossible, due to their high numbers, broad range, and the fact they are hidden in grass much of the time. Besides, it’s probably not necessary to know if we have 500,000 sharptails or 612,576.
Instead of an actual count, wildlife surveys typically provide a population index. For instance, the spring mule deer index is mule deer per square mile surveyed. The pheasant brood index is broods per mile of survey route. One fish population index is fish per net hour.
By themselves these indexes don’t mean much, but when survey results from year to year are compared, trends start to develop. Biologists can then say certain populations are up or down from last year, and higher or lower than a long-term average.
An index is a statistically accepted method as long as the survey is similar from year to year. That’s why the spring pheasant crowing count, for example, takes place during the same time frame and along the same routes from year to year. If the routes changed from year to year, and one year surveyors started each route at sunrise and the next year they all started their routes at noon, the results would not be comparable.
Conducting surveys and compiling their related indexes is a science, but that doesn’t mean that all hunters will experience season results that are in line with survey results. After all the numbers are crunched, wildlife populations still may vary depending on locale and species.
Take the spring crowing counts for pheasants. The statewide index, which includes the average of all survey routes, increased 10 percent from 2011. That doesn’t mean pheasant hunters will shoot 10 percent more roosters this fall.
Remember that the statewide index is an average and the big picture will also contain many small areas where local weather conditions or habitat changes will yield a bird population – some lower, some higher – that is not in line with statewide predictions.
It’s those unknown variables that makes us anxious to move beyond the preseason and get the “in-season” started.
Leier, a biologist for the North Dakota Game and Fish Department in West Fargo, can be reached at email@example.com
Leier’s blog can be found online