Harvestable Surplus

[Note: The following was first published in the January 2017 Alberta Outdoorsmen.]

Copyright © 2017 Don H. Meredith, All Rights Reserved.

Setting bag limits for fish and game is a complicated business. Balancing the wants of anglers and hunters against the sustainability of fish and wildlife populations takes a lot of research, experience and wisdom to get it right. And you can be sure that whatever a manager decides will not satisfy all, or even some of the people, all the time.


Hunting opportunity is an important element in establishing the harvestable surplus.

One of the factors a biologist considers is the “harvestable surplus,” or the number of individuals that can be harvested from a fish or game population without affecting the long-term stability of that population. For example, let’s say a certain wildlife management unit (WMU) has a population of 1000 mule deer that have survived the winter: 800 does and 200 bucks, or a ratio of 4:1. Of the 800 does, the 480 adults give birth to 450 fawns in the coming spring (some does were not pregnant or lost their fetuses during the winter). This raises the overall population to 1450 deer. Since half of the new fawns are bucks, the overall doe to buck ratio is 1025 females to 425 males, or 2.4:1.

Because it is spring, a lot of food is being produced in forest and field to feed all. However, predators (e.g., coyotes, wolves, cougars) are also raising their young and hunt fawns (among other animals) to feed their growing families. They also take a few adult deer that were weakened by the winter, or are too old, or are just unlucky. As well, some deer die from collisions with vehicles on the highway that crosses the WMU, and still others die of disease. The result is the population is reduced to 1200 by the end of summer; of which 900 are does and 300 are bucks (3:1).

Table 1. Hypothetical Mule Deer Population Fluctuations, Pre-hunt

  No. Does No. Bucks Total
Post Winter Population 800 200 1000
Spring Births +225 +225 +450
Spring/Summer Mortality -125 -125 -250
Pre-hunt Population 900 300 1200

Now, as the wildlife biologist managing this WMU, you know that 1000 mule deer (4:1 doe to buck ratio) surviving the winter is the optimum number to sustain the population. But how many mule deer licences should you make available during the hunting draw for this WMU? Simple math will tell you there are 200 more animals than the winter carrying capacity will allow to survive. So, are 200 deer the harvestable surplus?

Well, maybe. Several other factors come in to play. They include:

  1. Statistically, 150 of those deer would be does and 50 would be bucks. Do you allow a doe season, or hunt just bucks?
  2. Not every hunter who is drawn is going to score a deer. The game harvest surveys for this WMU tell you that about 45% of hunters will be successful killing a mule deer. So, do you increase the number of licences to compensate?
  3. Predators, vehicle collisions, disease and starvation will have an impact on the population. Do you compensate for that?

Buck or Doe
Traditionally, buck-only seasons were put into place to protect the reproductive capacity of the population. One buck can impregnate several does, so reducing the number of bucks (to a point), while protecting the does will not significantly affect the number of fawns born the following spring. Doe seasons are put into place to reduce that reproduction, perhaps in reaction to a population that is threatening to exceed the habitat’s carrying capacity. As well, a wildlife manager considers hunter opportunity. If the deer population appears healthy enough, the manager might consider allowing a few doe tags to increase that opportunity.

In my example, if the ideal doe-buck ratio is 4:1, then 100 does and 100 bucks need to be removed to come to that ratio (800 does, 200 bucks). But should hunting do all the removing?

Hunter Success
The success of hunters is an important consideration. It can vary widely among different game animals, the WMU and the year. That’s why accurate game harvest surveys are important. In my example, the number of licences could be twice the number of deer to be removed by hunting.

“Natural” Mortality
Predators are going to take a number of deer throughout the year. The amount will vary with the number of deer and predators, snow conditions, etc. As well, the vehicles on the highway are going to kill deer. Disease is ever present and tends to kill those animals already weakened by injury or starvation. When calculating harvestable surplus, it is assumed the animals harvested will reduce the number killed by predators, vehicles, disease and starvation. This is called “compensatory mortality.” Now, if that substitution is one-for-one, then a manager could call the harvestable surplus the full 200 in my example. However, it is not a one-for-one trade. Animals are still going to die from other than human hunting.

So, in my rudimentary example, a manager might decide there will be 10% mortality from natural causes to the pre-hunt population of 1200 mule deer, or 90 does and 30 bucks. That leaves 10 does and 70 bucks to be harvested, or a harvestable surplus of 80 animals. Thus, the manager might propose issuing licences for 20 does and 140 bucks.

Table 2. Hypothetical Mule Deer Population, Determining Harvestable Surplus

  No. Does No. Bucks Total
Pre-hunt Population 900 300 1200
Natural Winter Mortality -90 -30 -120
Hunting Mortality -10 -70 -80
Post Winter Population 800 200 1000

The Reality
Now, my example is totally hypothetical and deer biologists might dispute my figures. But I believe it illustrates some of the issues game managers face. First, they need reasonably accurate population estimates. This requires costly aerial surveys, usually flown in the winter, and in times of budgetary restraint, only flown over “priority” areas.

Second, they need an estimate of the amount of natural mortality that occurs, as well as the birth rate for the population. These are not easy figures to obtain without expensive studies. What biologists often do is go to the scientific literature and determine these figures from studies that were made in their areas in the past, or from adjacent regions.

In lieu of specific information, game biologists will make educated guesstimates about what might be going on in the WMU, based on 1) hunter harvest reports, 2) their own observations, and 3) the observations of others, including hunters and landowners. As one professor told our class many decades ago, the most effective wildlife biologists are those who have worked in their areas a long time and are familiar with the wildlife and the local people who have stakes in that wildlife. Knowledge, wisdom and intuition play big roles.

Other factors are also in play. Regional and provincial goals for species could override what the local biologist might want. And one cannot forget the role of politics. No matter what a manager or a deputy minister recommends, politicians can override all—sometimes based on the concerns of one constituent.

Much of what I’ve said here could be applied to fisheries management. Indeed, fisheries biologists might have been the first to use the term “harvestable surplus” to describe sustainable fishing management. The concept is the same but obviously the management differs. What is important is that there is a harvestable surplus in a healthy fish population. The challenge is reasonably harvesting that surplus.

Comments are always welcome (below).


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About Don Meredith

I am a writer and biologist living in Alberta, Canada. I wrote a monthly column for the Alberta Outdoorsmen magazine, and have published articles for several other magazines.
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