by Mike Thompson,
FWP Wildlife Biologist
Seeley Swan Pathfinder Game Range Column
March 4, 1999
Reader response to last week's article was overwhelming. Before I receive another card or letter (and, by the way, that would be the first card or letter) let me stem the tide by finishing my analysis of the Blackfoot-Clearwater elk population. This week, I'll focus on numbers of bull elk, both past and predicted. Hopefully, Gary has consented in reprinted last week's graph for your reference.
If you study the graph, you'll find major inconsistencies in the relationship between numbers of calves counted in one year and numbers of spike bulls counted in the next. For example, the graph shows that our largest calf crop in Game Range records (1989) only led to a mediocre recruitment of spikes in 1990. In fact, the recruitment of spikes was higher in 1996, even though it followed one of the poorest calf crops we've seen (1995). Wide fluctuations in annual hunting harvests of spikes probably account for most of the variation in spike recruitment.
Based on an average of data from 1989-1999, we can expect to recruit 36% of our January calf count as spikes in the following winter. That's the factor I used to predict spike numbers in relation to numbers of calves I expect to see on the Game Range in the years 2000-2005.
This "36%" figure makes sense if you assume that 50% of the calves are males, and 28% of those males die between their first and second Januarys of life (i.e., in hunting season). This mortality rate for spike bulls is lower than the estimate Mark Hurley obtained by recording mortality rates for radioed spikes (41%) from 1988-1990. Of course, Mark's data applied only to a sample of 17 animals, and his study is almost ten years old, so it may be a stretch to apply Mark's data to our situation today.
I also think there are more calves in the population than we actually count. When I used to work in the Elkhorn Mountains, near Helena, I spent a lot of time classifying cows and calves from aircraft and through a spotting scope on the ground. Generally, it seemed that I could pick out an extra ten calves per 100 cows from the ground, when there was time to look elk over carefully. I was reminded of this last January, after classifying 33 calves per 100 cows in a group of about 180 elk that I watched closely from the ground, after having obtained a ratio of 23 calves per 100 cows from the helicopter during the annual Game Range survey. We get a good look at cows and calves from the helicopter, but guesswork increases when elk are on the move and milling around in groups of a hundred or more. Mortality of spike bulls could actually be close to 40% if we have more calves than we claim in our helicopter surveys.
On the average, about 25% of the spikes we count in winter will be counted as two-year-olds or raghorns, in the following winter. This suggests a 75% loss of bulls between their second and third Januarys, including the hunting season when they first achieve brow-tined bull status. This figure is higher than the 60% mortality that Mark Hurley observed in a sample of 20 two-year-old bulls, and may be influenced by the fact that some two-year-olds disperse to other populations during this period of life. Also, we may be misclassifying some two-year-olds as mature bulls rather than raghorns.
We know there is considerable annual variation in the proportion of mature bulls (three years old or older) that migrate all the way to the winter range and are counted in our surveys. In our radio-tracking days, it was not unusual for bulls captured on the Game Range to occasionally winter in the Bob Marshall Wilderness, or on higher ridgelines north of the Game Range, during years when early winter weather was mild. In fact, I'd suggest that our record high count of 67 mature bulls on the Game Range in 1998 was as much of a fluke as the surprisingly low count of 25 this year. It's not hard to believe that many bulls didn't migrate far this year, and didn't show up in our helicopter survey, due to the unusually mild weather in early winter. But, I also suspect that an exceptionally high proportion of mature bulls may have moved to the Game Range promptly at the beginning of last winter. I'm not claiming that last winter was severe, but I wonder if bulls that survived the previous harsh winter were especially prone to "get a move on" to the winter range in 1997-98. For the purposes of predicting mature bull numbers in the next millennium, I used an average factor of 1.9 mature bulls for every raghorn counted in the previous year.
Considering the fact that the calf crop in 1999 will first show up in our counts of mature bulls in the year 2002, you get the idea that lag time plays an important role in bull elk management. We've been riding the high calf crops of 1989-1993 (and maybe 1994) for our high counts of mature bulls in 1995-1998. It's this lag time that allowed us to enjoy good bull:cow ratios throughout the period when our elk populations were otherwise hamstrung by the winter of 1996-97. Lag time also means that we won't feel the full impact of that hard winter on numbers of mature bulls until the winter of 2000-2001.
It's fun to apply this information into the future to plot strategies for bull elk management. Notice that FWP plans a peak in overall population numbers in the year 2002, but total numbers of bulls will not peak until the next year, when spikes are recruited from the calf crop from the year before. In 2004, the second year of a planned population decline, total bull numbers will probably still be higher than they were during the population peak in 2002, and numbers of mature bulls probably won't peak until 2005. By the time we see a decline in mature bulls, we'll be enjoying a fresh flush in numbers of spikes.
If you've worked through these admittedly dry articles over the past two weeks, you've probably come to the same conclusion as me. It's interesting to review past and current trends in the Blackfoot-Clearwater elk population, but it's more fun and useful to use that information for planning where we can go in the future. Based on data collected from 1989-1999 on the Game Range, we are justified in believing that we can rebuild mature bull numbers through 2005, once we ride out a temporary decline over the next two years.