December 07, 2016
By Dr. James C. Kroll
I get a lot of management questions from the public, and many are on the general theme of deciding how many of which deer to shoot. Unfortunately, in most cases some key data prove to be missing. That both complicates and delays the process of improving the deer population in question.
For example, I recently got an inquiry from a retired Army officer. He's on a committee concerned about the high doe harvest on an Army post. Here's what he wrote about the situation:
"The post has been in a 'quality deer management' program since 2010, and it is producing results; the average age of our harvested bucks has climbed as predicted. Each season we have about 1,000 hunters hunting on approximately 140,000 acres, though land availability changes based on Army training requirements.
"The average harvest has been 1,300 deer per year since 2010, with does averaging 65 percent of total harvest. Our lactation rate, within our very limited check station data, fluctuates between 50 and 70 percent. It's been declining in the past couple years, with fewer fawns seen on trail cameras.
"The research I've done in the past two weeks points to a significant coyote problem. We have a large population of coyotes on post, with very few people who hunt them.
"I've hunted here since 2002 and have seen the deer population in serious decline; however, our post biologist does not want to reduce the doe harvest. He thinks killing more does will just improve opportunities to kill big bucks. What's your thought on doe harvest in an area heavily impacted by predation?"
On the surface, this inquiry would seem to include a good bit of data. And it certainly includes more than some laymen provide. But the reality is that much of what a manager really needs to know hasn't been provided.
Rarely do wildlife agencies address management beyond the basics of population size. Whether we're talking about a huge military post or a piece of private land, taking management a step farther takes solid data of many types:
For starters, what's happened to the post's habitat over the last two decades? This would reveal how much natural deer food is really available. There should be data on range appraisals of stocking level.
Estimates for fawn crop are simply that: for the fawn crop, not true recruitment. Herd recruitment is the number of fawns reaching one year of age, the true point of recruitment into the adult population. If we shoot 50 percent of "recruitment" as determined by the fawn crop, we undoubtedly are overharvesting the herd.
The old concept of compensatory response — that is, the more deer you harvest, the higher the fawn crop and recruitment will be — is now useless, and in fact downright dangerous. What is meant by that term came from a book, The George Reserve Deer Herd, written by Dale McCullough.
A year after I presented a paper at the Southeast Deer Study Group on a maximum sustained yield model for whitetails, Dale published his book using data from the George Reserve in Michigan. The theory was that if you hold the herd at about half carrying capacity, you can have a sustained yield harvest. It followed that elevating deer numbers above this level would result in a lower fawn crop and recruitment as the herd approached the habitat's carrying capacity.
The real problem with this model — like all others — is that it assumes a steady-state environment, which never has occurred in history. Reality now has caught up with the model's incorrect assumption.
The reason it's dubious at best is that population growth rate is determined by births plus immigration minus deaths plus emigration (deer leaving the area). What comes into the herd and what goes out will determine the rate of herd growth.
Theoretically, this is true only if we really know the values of each. During the restoration days of deer management, there were few predators, habitats were vibrant and dynamic and there were few disease problems. Today, we have declining habitat quality, an ever-increasing suite of predators (each of which has a predatory specialty), more frequent EHD outbreaks by two exotic strains of the virus, and other mortality factors.
In the "old days," mortality in most areas was only hunting/poaching mortality (both deer recovered and those lost after being shot). Today, it's hunting/poaching mortality plus predation plus disease plus nutrition-based mortality plus auto accidents plus unknown causes that have to be considered.
Unfortunately, there are few scientific data to provide estimates of anything except hunting mortality. Mortality is additive, so to only have hunting mortality estimates is inadequate for determining annual deer losses.
A little-considered value is the minimum effective population. This is the smallest number of deer that will allow the population to sustain itself when all factors are considered. A biologist who still operates on a compensatory response mentality is asking for a deer crash. I've seen more and more herds crash in the last few years, and the direct blame falls fully on the biologists who made their harvest decisions using unreliable data.
It's important to be able to evaluate the overall health of the deer on a property. That includes conducting health checks and the all-important comparison of average age-related weights versus standardized weights expected for each age class.
In the inquiry I received, it was reported that the average number of deer harvested annually from 2010-13 was 1,300: 769 antlerless and 531 antlered. That breaks down to 59.1 percent antlerless and 40.9 percent antlered.
But "antlerless" doesn't include only does; it means deer, including buck fawns, without antlers. Not knowing the buck fawn harvest or the harvest age structure, I have no idea of the true female harvest totals or percentage on the property in question.
The post's estimated deer population is reported as 5,000-7,000, with an estimated buck:doe ratio of 1:2. If true, that means there are somewhere between 3,300-4,700 females. Taking the average, some 4,000 does of many ages are out there trying to breed.
A 70 percent fawn crop would produce 2,800 new deer by October. There would have to be 46 percent recruitment to keep the herd steady and break even. With no additional age-specific mortality factored in, I'd be shocked if there's anywhere near that high a rate of recruitment.
So my answer to the officer's question simply was that, by definition, the people managing this herd haven't really been doing so. They apparently can provide no long-term analysis of what's happening to age structure or recruitment over time. Nor are they factoring in what's going on with habitat or nutrition quality, predation, disease, etc. They've simply been setting bag limits each season.
So what should they be doing?
(1) Conduct range evaluations to determine the true stocking level of the base.
(2) Conduct health checks each February to determine the reproductive effort. (This would include harvesting does to determine the percentage pregnant and average number of fetuses and to estimate conception dates.)
(3) Conduct trail camera surveys on a scientifically valid design twice annually — once in early fall, then again in spring — to determine fawn crop and recruitment, respectively. Incidental-sightings data also would be helpful.
That way, they'll have three checks on the productivity of their herd: the number of fetuses produced, the number of fawns surviving to fall, and the number of fawns recruited. Then they'll be able to assign identity to various limiting factors.
(4) Monitor the age structure of the herd by requiring every deer harvested to be checked for age, weight and (in does) lactation, and compare average weights by age to the expected values.
(5) Conduct predation studies and an index to EHD losses, as well as road kill data they probably are collecting.
(6) Involve the base's hunters in the above process and communicate with them frequently.
(7) Adjust annual harvest to fit the above information, in conjunction with this better-informed public.
That is whitetail management.