Post by ridge on May 5, 2017 1:09:28 GMT -5
Modeling Disease Mitigation
Harvest Strategies
The Concerned Sportsmen of Michigan
Jim Sweeney
Introduction
The increasing presence of CWD and Bovine Tuberculosis in the free ranging
deer herd in Michigan poses a significant threat to the future of deer hunting,
as well as having a tangible negative economic impact on Michigan. One means
of limiting the negative impact of these diseases is to implement harvest
strategies designed to maximize the mitigating impact that regulations can
have, with the goal of decreasing the potential spread of these diseases
geographically, as well as limiting the prevalence rates within the herd.
Modeling is designed to illustrate what some potential impacts of various
harvest strategies may be, to better understand how and why some strategies
may be more effective in mitigating disease than others may be.
These models are not intended to predict actual results, there are too many
unrelated factors involved to be able to incorporate all of them into a model,
for predictive purposes. Instead, the modeling is designed to show how specific
changes may alter specific outcomes within the harvest.
Bovine Tuberculosis Modeling
There has been much discussion on social media recently over suggestions that
Mandatory Antler Point Restrictions should be implemented as part of disease
mitigation efforts, based on the assumption that such regulations would
increase the harvest of antlerless deer. This idea is partially based on the
claim that the bulk of diseased animals within the herd, infected with diseases
like CWD and TB, are in the female cohort, due to generally unbalanced adult
sex ratios. The originator of this idea has stated that there are many, many
more infected females in the herd than males and supported this assumption by
suggesting that the ratio of older females to males in the herd may be as much
as 80 to 1 in some age classes during some times of the year.
In order to assess the accuracy of that claim and to get a better idea of how
the demographic make-up of TB within the deer herd actually looks, we
decided to model the ratio of infected females and males within the herd at
different adult sex ratios.
The prevalence rate for yearling females in this model is hypothetical, any rate
that is used for modeling purposes would have the same result, as the
important factor is the proportion of all of the other age classes testing
positive, in relation to the yearling female age class. For example, a yearling
buck is 1.10 times more likely to test positive, based on DNR data, than a
yearling female. So to estimate the prevalence rate for a yearling buck for
modeling purposes, you multiply the hypothetical prevalence rate of the
yearling doe, in this case .005 x 1.10 = .006, which is the hypothetical
prevalence rate for yearling bucks in this model. It would not matter if you
used another figure for the yearling female prevalence rate, as long as you are
using the same proportional figures based on the available DNR data, the
proportional results of the model will be the same.
2
This model uses age class distributions taken from 2015 DNR check station data
from two NLP counties, Montmorency County, which is subject to Hunters
Choice Antler Restrictions and Roscommon Co. which does not have mandatory
APR's in effect. It should be noted that check station data likely does not
accurately reflect the actual age distribution within the herd, particularly
where any form of antler restrictions are in effect.
There was a significant difference in the distribution between different age
classes in the male cohort, between the two counties, which we would
attribute to the impact that HC regulations have had on increasing the buck
age structure in Montmorency Co. There was only minor variation in the age
class distribution within the female cohort, in both counties, which was
anticipated, as APR's do not generally influence the age structure of the female
cohort. For the purposes of this model, fawns of both genders were excluded
due to extremely low prevalence rates in that age class.
This model indicates that as Buck to Doe ratios diverge, the majority of
diseased animals shifts from the male cohort to the female cohort and that the
degree of disparity between the two is proportional to the degree of disparity
in adult sex ratio's.
Figure 1
Figure 2
3
Antler Restrictions are designed to create adult sex ratio's that are more
closely balanced, resulting from the yearling age class of bucks being protected
from harvest. Typical pre-hunting season adult sex ratios are generally
somewhere in the 1:2.5 range in non-APR counties and are closer to 1:2 in
counties under APR's. Post season ratios are more unbalanced, due to a greater
percentage of males being harvested, which will push ratio's into the 1:3.5
range.
According to this model, in a typical herd, with a 1:2.5 sex ratio, for every TB
infected adult male in the herd there will be 1.14 TB infected adult female, in
the TB zone currently and 1.33 infected adult females in counties without an
APR in place.
So at that sex ratio, there are more diseased females in the herd but not by
very many. For example, if there were 10 infected adult males in a herd of
1000 deer, there would be 11 infected adult females in the TB zone and 13
infected adult females in a non-APR county.
Based on this model, it looks like there are slightly greater numbers of TB
infected female deer in the herd at typical adult sex ratios but that the
majority shifts as sex ratios become more balanced. As far as evidence
supporting the claim that there are many, many more TB infected females,
while a subjective statement, it does not appear to be supported by the results
of this model.
CWD Modeling
Next we will look at what the potential impact that shifting harvest pressure
between the female and the male cohorts within the herd may have in terms of
removing the greatest number of diseases animals from the herd, which is an
important goal of disease mitigation plans.
For this we use a different model which illustrates the impact of these
changes. We look at three different proportions of females to males within the
harvest and one which includes an APR. These are not intended to reflect
actual harvest proportions seen currently, they are intended to show the
hypothetical outcomes resulting from shifting the proportions within the
harvest.
For the purpose of this model, fawns of both sexes are excluded. The fixed
total harvest of 25% of the herd is based on the approx. percentage of deer
removed from the Michigan herd annually. The CWD prevalence rates used are
based on average CWD prevalence rates from Wisconsin 2002 - 2009. The
yearling/adult harvest ratio used for males is 58/42 based on 2015 Ingham Co.
check station data. The yearling/adult harvest ratio used for females is 30/70,
based on 2015 Ingham Co. check station data. These models are based on a
1:2.5 adult sex ratio. The presumed yearling buck protection rate used is 70%
based on the rate indicated for a 4 pt. MAPR in the SLP, as indicated in the
4
MDNR Moritz protection rate tables. No impact was factored in for bucks older
than yearlings being protected by an APR.
Figure 3
The first graphic shows the outcome of a harvest strategy based on an adult
harvest composed of 40 % does and 60% bucks. This strategy projects 43
infected deer being removed from the herd.
Figure 4
Figure 4 shows the outcome when the harvest is balanced at 50% adult does
and 50% adult does. The projected number of infected deer falls to slightly to
41.
5
Figure 5
Figure 5 illustrates the shift that occurs as the harvest focus is shifted towards
a greater proportion of adult females. The total number of infected deer
removed under a strategy resulting in the harvest being 60% adult does and 40%
adult bucks drops to 39.
Figure 6
Figure 6 indicates the further shift that occurs when adding a mandatory APR
to a female focused harvest strategy featuring 60% adult female harvest and
40% adult male. The addition of the APR further reduces the number of
infected deer removed to 37, a 14% reduction compared to the 40/60 male
focused harvest strategy without an APR.
The results of this model suggest that a male focused harvest strategy without
a mandatory APR would be the most successful in terms of removing the
6
greatest number of CWD infected animals from the herd, of the harvest
strategies modeled.
The reason for this is due to the higher prevalence rates found in adult males
compared to adult females, for both CWD and Bovine TB. Despite the fact that
at typical sex ratios there may be slightly more infected female deer in the
herd, due to elevated prevalence rates, weighting the harvest towards males is
more likely to remove greater numbers of infected deer, due to harvest
efficiency
Target Identification and untargeted culling
Because the prevalence rates are substantially higher in males once you get to
the 2.5 age class and older, you generally have to shoot twice as many adult
males to remove the same number of diseased animals. This is based on the
premise that you can't identify which animals are diseased prior to shooting
them, which is referred to as untargeted culling. So if the goal is to lower the
herd prevalence rate by employing a female focused harvest strategy, first you
have to convince hunters to shoot a female instead of a male and then you
have to get them to shoot twice as many deer in order to get the same return,
in terms of number of diseased animals removed
Anther factor that comes into play is the fact that many hunters have a hard
time distinguishing fawns of either sex from adult females. Since fawns have a
very low prevalence rate for disease, killing a female fawn instead of an adult
doe is counterproductive to removing diseased animals from the herd. Killing a
male fawn also does not help lower prevalence rates or remove disease but it's
beneficial in terms of reducing yearling dispersal the following year, so at least
it's not counterproductive to disease mitigation efforts.
By the time you factor in the impact that killing fawns has on diluting the
disease removal occurring from female focused harvest, the ratio needed is
probably closer to 2.5 female deer per 1 antlered male, to remove the same
number of diseased animals. This would suggest that female focused harvest is
an inefficient means of reducing the number of diseased animals in the herd.
Harvest Fatigue
In addition to the decreased harvest efficiency of infected animals resulting
from a female focused harvest strategy, there are several other factors that
also can influence the viability of different harvest strategies.
One of the factors that has been seen in Wisconsin and in some other areas
where female focused harvest strategies have been attempted, for purposes of
disease mitigation, is hunter harvest fatigue.
In a presentation discussing "lessons learned" from Wisconsin's CWD mitigation
efforts, Dr. Julie Langenberg, who at the time was the Chief Veterinarian for the
WDNR, stated that "Wisconsin hunters and other key stakeholders are unwilling to
harvest enough deer for significant population reduction".
7
She further stated " more then half are concerned that control efforts will harm
deer populations and they do not support drastic herd reductions".
Her quote illustrates the tendency among hunters to slow down or stop completely
harvesting does when they start to believe that the deer population is being
lowered to the point that it will impact their hunting.
The female focused harvest strategy modeled above shows the results of what
might occur with a hypothetical harvest proportion of 60:40 does to bucks but the
model does not take into account the social consideration of whether or not it
would even be feasible to get hunters to shift harvest proportions to such an
extent.
From a practical standpoint, buck focused harvest strategies are both easier to
implement and are more likely to result in greater numbers of diseased animals
being removed from the herd, due to harvest efficiency. Hunters have no
problem identifying which bucks to remove. If they see antlers, Bang! Pretty
simple. You don't have to encourage hunters to shoot bucks, they will do that
on their own, you just need to keep out of their way. In fact, they will be
perfectly willing to purchase additional antlered licenses in a disease
mitigation zone, if you make them available, providing revenue for mitigation
efforts. To encourage substantial increases of antlerless harvest, under a
female focused harvest strategy in a disease zone, you probably will have to
either deeply discount tags or give them away, resulting in a loss of revenue.
Yearling Dispersal
Leading biologists and disease experts and a variety of state game agencies,
including the Michigan DNR, have suggested that the mechanism of yearling buck
dispersal increases the risk for the spread of communicable diseases like CWD an
bTB. This is a well accepted concept and a number of states including Missouri,
Minnesota and Arkansas have removed APR's that protect yearling bucks and
increase the number which disperse, in areas where CWD is present, to try and
minimize the risk of it's spread.
Most APR's in Michigan offer protection to approx. 70% of yearling bucks. The
concept of QDM is built on the premise that protecting younger bucks, often with
an APR, will allow them to advance to older age classes, so it's contradictory to
suggest, as some on social media have done, that removing such protections will
have no impact on how many yearling bucks are killed as yearlings, which in turn
has a direct bearing on the number of bucks dispersing.
Another incorrect assumption promoted on social media is that disease mitigation
only occurs if yearling bucks are killed prior to dispersing. Some have suggested
that killing yearling bucks after dispersal is "too late".
From an epidemiological standpoint, that premise is false, which is easily
demonstrated with a simple analogy. Anyone who has been a parent knows
8
elementary age kids spread germs readily and that classrooms can be hot zones for
spreading colds.
Ideally, if a child has the flu, a parent would not send them to school. That would
be analogous to killing an infected yearling buck prior to dispersal occurring.
Unfortunately, a lot of parents send sick kids to school. If that sick child sits in
the class room and spreads the disease to an average of 1 other child for every day
they attend school while sick, it's abundantly clear that the longer that they keep
coming to school while sick, the more children will be infected, who in turn infect
other children, causing a multiplier effect. If the teacher notices that the child is
sick after the first hour or two and sends them home, (analogous to killing an
infected yearling buck after dispersal has occurred), then the incidence of the
disease spreading in the classroom is substantially reduced.
The idea that as soon as that child steps into the classroom, "it's too late" to stop
the child from spreading germs, is obviously a false one. The suggestion that
having the sick child (or the infected yearling buck) continue to attend school for
another 10 months while sick, ignores the potential that exists for the child to
continue to spread his germs, infecting other children, during that time period.
The claim that focusing harvest pressure on yearling bucks is "too late" because
most yearlings have already dispersed by the time fall hunting season occurs, is
also highly questionable. The fact is that a substantial number of yearling bucks
have not dispersed prior to the beginning of fall hunting seasons in October and as
such are vulnerable to harvest prior to dispersal. In the QDMA Chesapeake Farms
study authored by Dr. Jonathon Shaw, Shaw stated that approx. 25% of the
yearling bucks in that study dispersed during the spring fawning season (Apr. -
June) at 10 - 12 months of age and 75% dispersed during the fall hunting season
(Sep - Nov.) at 15 - 17 months of age.
While killing yearling bucks prior to dispersal is certainly preferable, killing
yearling bucks which have already dispersed is another vital piece of the CWD
mitigation puzzle, as the longer those bucks survive in their new range as "sparks",
the more contact they have with other deer, the more Prions they shed while
feeding in food plots, the more rubbing and scraping they engage in and the more
potential there is for them facilitating CWD becoming entrenched in a new area.
This graphic from the Wisconsin DNR shows a typical dispersal pattern. In this
graphic, the red star indicates where this particular buck was radio collared as an
8 month old button buck on February 4th. Around October 20th, he started the
dispersal process and left his natal range. In Michigan, he would have already
been vulnerable for harvest for almost a month prior to dispersing, (unless
protected by an APR). Over the course of the next month he continued to disperse
farther from where he was born. He was harvested (indicated by the red X) on
November 20th after having dispersed around 3 miles from where he had spent
most of his life.
If he had been infected with CWD, any contacts that he had with other deer and
any Prion shedding that could occur, ended with his harvest.
9
Imagine the difference in potential disease impact, if this yearling had been
protected by an APR and had continued to disperse and spread disease for another
10 months, participating in breeding, joining bachelor groups, defecating in food
plots where other deer are feeding, engaging in scraping & rubbing, etc. To
suggest that there is no benefit to removing a potentially infected yearling buck
from the population during or after dispersal has already occurred, ignores reality.
Figure 7
It has also been suggested that focusing on herd reduction by employing a female
focused harvest strategy will reduce dispersal because after a number of years,
theoretically fewer numbers of male fawns will be born, resulting in there being
fewer to disperse as yearlings. There are two primary problems which this
essentially ignores, however. It completely ignores the immediate negative
impact, in terms of the spread of disease, which results from protecting yearling
bucks with APR's, as they will survive to disperse and spread disease or survive
after having dispersed. Under such a model much higher numbers of yearling
bucks would disperse than would occur under a male focused harvest plan without
regulatory protections for yearling bucks. You may eventually lower the number
of yearlings dispersing on an annual basis but during the 4 or 5 years that it takes
to realize any actual population reductions, 70% of the protected yearlings are
dispersing and the disease has spread over a much larger geographic region, while
you are waiting for the impact of fewer yearlings due to herd reduction. From a
practical disease mitigation standpoint, that is not a trade off worth making.
10
Deer Population and Recruitment
Female focused harvest strategy models should address the potential impact that
herd reduction may have on recruitment rates in some portions of the state.
Reducing the number of females in the herd can actually increase the deer
population, depending on carrying capacity and recruitment rates prior to herd
reduction. Models that assume no increase in recruitment resulting from female
focused harvest, may be valid in some portions of the state but may also be flawed
in the NLP & UP.
Recruitment refers to the number of additional deer added to the herd every year
by the beginning of hunting season. The number of offspring that adult does have
and whether or not doe fawns breed is highly variable and depends on the carrying
capacity of the area and the availability of food (which can also be tied to
population density). As the deer population is reduced, there is more food
available for the remaining does, which can change their reproductive capacity.
This is natures way of dealing with threats such as inclement weather, which can
reduce deer populations. As the population decreases, more fawns will be born
resulting in higher recruitment rates. As the population grows, there is less food
available, resulting in decreased reproductive capacity, resulting in fewer fawns
being born and lower recruitment rates. These two charts illustrate these points.
Figure 8
In Figure 8, "K" refers to carrying capacity, the bell curve represents recruitment.
As you can see, the highest recruitment rate is produced at 50% of carrying
capacity (K/2). So if you reduce the population by 50%, if it was at carrying
capacity, the recruitment rate will be at it's highest capacity. As herd size
increases from that point, recruitment decreases, resulting in less deer added to
the herd every year than at MSY (maximum sustainable yield). As herd size
decreases from K/2, reproductive capacity remains high but the increasingly
smaller number of does available to breed results in fewer fawns being born,
reducing recruitment.
11
The premise that herd reduction will result from APR's increasing female harvest
assumes that there will be no changes to recruitment rates resulting from herd
reduction. It would have you assume that the carrying capacity is fixed relative to
the herd population. That would not be the case in the majority of Michigan,
where carrying capacity is directly linked to a number of limiting factors, food
availability being a primary one. In those areas, it is entirely likely that female
focused harvest strategies, which reduce the number of does, may actually result
in larger herd sizes the following year, due to increased recruitment.
Figure 9
Figure 9 shows the inverse relationship between female reproductive capacity and
the population density of does. At higher populations, fawns does don't breed, as
they are not able to attain the minimum body weight that is required to stimulate
estrus as a fawn. As food availability increases, as the result of herd reduction
efforts, more doe fawns will begin to breed, increasing recruitment. The same is
true with adult does, as food supplies increase, more of them have twins or
triplets instead of singletons, resulting in increased rates of recruitment.
From a disease mitigation standpoint, the optimum realistic herd size in relation
to carrying capacity is one in which most doe fawns do not breed and a majority of
adult does are having singletons, yet the herd is maintained under K (carrying
capacity). This will limit recruitment while keeping the herd from growing in size
and will also keep hunters in the field as there will not be the hunter harvest
fatigue that occurs from female intensive harvest plans, such as occurred in
Wisconsin.
APR's Increasing Antlerless Harvest
There is a presumption that APR's will divert harvest pressure to antlerless deer
due to there being a reduced number of legal antlered bucks being available to
harvest, due to 70% of the yearling age class being protected by APR's. It's also,
however, presumed that such a diversion will only last as long as it takes for the
buck age structure to advance sufficiently so that the same number of antlerled
targets exist as prior to the APR's being put in place. That typically takes 1 - 2
years, as most 2.5 year old bucks will be legal for harvest under APR's. HC
regulations have the same impact in advancing the buck age structure, only it
takes several years longer before the number of legal targets increase to where
they were prior to the regulations. If the buck age structure has already advanced
under either form of APR's, any intended diversion to harvesting antlerless deer is
unlikely.
12
Conclusion
In our opinion, based on the data resulting from these models and the other
information provided within this report, the most efficient and effective
harvest strategy for disease mitigation in Michigan, best designed to stop the
spread of diseases like CWD and bTB and limit herd prevalece rates, would be a
majority male focused harvest strategy that harvests around 70% of the
yearling buck age class, keeping the overall buck age structure young,
populated mostly by 1.5's and 2.5 year olds and which harvests at least 30% of
the breeding doe population to keep the herd static.
Additional harvest focus on button bucks, by educating hunters on why they
should be targeted in disease zones, should also be encouraged . The DNR did a
good job of educating hunters on identifying button bucks, which resulted in a
significant decrease in the percentage harvested, now what is needed is to
reverse that trend and refocus harvest pressure on them in disease zones. The
plan should also include utilizing sharpshooters to eliminate entire doe family
groups in the immediate areas where CWD positive deer are found, as called
for in the current CWD plan.
Female focused harvest strategies, both with and without the addition of
mandatory APR's, are both inefficient in removing diseased animals from the
herd and also have the potential to increase the geographic spread of the these
diseases by increasing the level of dispersal of yearling males. The uncertainty
of whether such strategies will actually reduce overall herd populations,
resulting in a positive benefit is more than outweighed by the certainty that
such strategies will have a tangible negative impact resulting from increasing
the number of yearling bucks surviving to disperse, as well as increasing the
cohort within the herd with the highest prevalence rates.
Harvest Strategies
The Concerned Sportsmen of Michigan
Jim Sweeney
Introduction
The increasing presence of CWD and Bovine Tuberculosis in the free ranging
deer herd in Michigan poses a significant threat to the future of deer hunting,
as well as having a tangible negative economic impact on Michigan. One means
of limiting the negative impact of these diseases is to implement harvest
strategies designed to maximize the mitigating impact that regulations can
have, with the goal of decreasing the potential spread of these diseases
geographically, as well as limiting the prevalence rates within the herd.
Modeling is designed to illustrate what some potential impacts of various
harvest strategies may be, to better understand how and why some strategies
may be more effective in mitigating disease than others may be.
These models are not intended to predict actual results, there are too many
unrelated factors involved to be able to incorporate all of them into a model,
for predictive purposes. Instead, the modeling is designed to show how specific
changes may alter specific outcomes within the harvest.
Bovine Tuberculosis Modeling
There has been much discussion on social media recently over suggestions that
Mandatory Antler Point Restrictions should be implemented as part of disease
mitigation efforts, based on the assumption that such regulations would
increase the harvest of antlerless deer. This idea is partially based on the
claim that the bulk of diseased animals within the herd, infected with diseases
like CWD and TB, are in the female cohort, due to generally unbalanced adult
sex ratios. The originator of this idea has stated that there are many, many
more infected females in the herd than males and supported this assumption by
suggesting that the ratio of older females to males in the herd may be as much
as 80 to 1 in some age classes during some times of the year.
In order to assess the accuracy of that claim and to get a better idea of how
the demographic make-up of TB within the deer herd actually looks, we
decided to model the ratio of infected females and males within the herd at
different adult sex ratios.
The prevalence rate for yearling females in this model is hypothetical, any rate
that is used for modeling purposes would have the same result, as the
important factor is the proportion of all of the other age classes testing
positive, in relation to the yearling female age class. For example, a yearling
buck is 1.10 times more likely to test positive, based on DNR data, than a
yearling female. So to estimate the prevalence rate for a yearling buck for
modeling purposes, you multiply the hypothetical prevalence rate of the
yearling doe, in this case .005 x 1.10 = .006, which is the hypothetical
prevalence rate for yearling bucks in this model. It would not matter if you
used another figure for the yearling female prevalence rate, as long as you are
using the same proportional figures based on the available DNR data, the
proportional results of the model will be the same.
2
This model uses age class distributions taken from 2015 DNR check station data
from two NLP counties, Montmorency County, which is subject to Hunters
Choice Antler Restrictions and Roscommon Co. which does not have mandatory
APR's in effect. It should be noted that check station data likely does not
accurately reflect the actual age distribution within the herd, particularly
where any form of antler restrictions are in effect.
There was a significant difference in the distribution between different age
classes in the male cohort, between the two counties, which we would
attribute to the impact that HC regulations have had on increasing the buck
age structure in Montmorency Co. There was only minor variation in the age
class distribution within the female cohort, in both counties, which was
anticipated, as APR's do not generally influence the age structure of the female
cohort. For the purposes of this model, fawns of both genders were excluded
due to extremely low prevalence rates in that age class.
This model indicates that as Buck to Doe ratios diverge, the majority of
diseased animals shifts from the male cohort to the female cohort and that the
degree of disparity between the two is proportional to the degree of disparity
in adult sex ratio's.
Figure 1
Figure 2
3
Antler Restrictions are designed to create adult sex ratio's that are more
closely balanced, resulting from the yearling age class of bucks being protected
from harvest. Typical pre-hunting season adult sex ratios are generally
somewhere in the 1:2.5 range in non-APR counties and are closer to 1:2 in
counties under APR's. Post season ratios are more unbalanced, due to a greater
percentage of males being harvested, which will push ratio's into the 1:3.5
range.
According to this model, in a typical herd, with a 1:2.5 sex ratio, for every TB
infected adult male in the herd there will be 1.14 TB infected adult female, in
the TB zone currently and 1.33 infected adult females in counties without an
APR in place.
So at that sex ratio, there are more diseased females in the herd but not by
very many. For example, if there were 10 infected adult males in a herd of
1000 deer, there would be 11 infected adult females in the TB zone and 13
infected adult females in a non-APR county.
Based on this model, it looks like there are slightly greater numbers of TB
infected female deer in the herd at typical adult sex ratios but that the
majority shifts as sex ratios become more balanced. As far as evidence
supporting the claim that there are many, many more TB infected females,
while a subjective statement, it does not appear to be supported by the results
of this model.
CWD Modeling
Next we will look at what the potential impact that shifting harvest pressure
between the female and the male cohorts within the herd may have in terms of
removing the greatest number of diseases animals from the herd, which is an
important goal of disease mitigation plans.
For this we use a different model which illustrates the impact of these
changes. We look at three different proportions of females to males within the
harvest and one which includes an APR. These are not intended to reflect
actual harvest proportions seen currently, they are intended to show the
hypothetical outcomes resulting from shifting the proportions within the
harvest.
For the purpose of this model, fawns of both sexes are excluded. The fixed
total harvest of 25% of the herd is based on the approx. percentage of deer
removed from the Michigan herd annually. The CWD prevalence rates used are
based on average CWD prevalence rates from Wisconsin 2002 - 2009. The
yearling/adult harvest ratio used for males is 58/42 based on 2015 Ingham Co.
check station data. The yearling/adult harvest ratio used for females is 30/70,
based on 2015 Ingham Co. check station data. These models are based on a
1:2.5 adult sex ratio. The presumed yearling buck protection rate used is 70%
based on the rate indicated for a 4 pt. MAPR in the SLP, as indicated in the
4
MDNR Moritz protection rate tables. No impact was factored in for bucks older
than yearlings being protected by an APR.
Figure 3
The first graphic shows the outcome of a harvest strategy based on an adult
harvest composed of 40 % does and 60% bucks. This strategy projects 43
infected deer being removed from the herd.
Figure 4
Figure 4 shows the outcome when the harvest is balanced at 50% adult does
and 50% adult does. The projected number of infected deer falls to slightly to
41.
5
Figure 5
Figure 5 illustrates the shift that occurs as the harvest focus is shifted towards
a greater proportion of adult females. The total number of infected deer
removed under a strategy resulting in the harvest being 60% adult does and 40%
adult bucks drops to 39.
Figure 6
Figure 6 indicates the further shift that occurs when adding a mandatory APR
to a female focused harvest strategy featuring 60% adult female harvest and
40% adult male. The addition of the APR further reduces the number of
infected deer removed to 37, a 14% reduction compared to the 40/60 male
focused harvest strategy without an APR.
The results of this model suggest that a male focused harvest strategy without
a mandatory APR would be the most successful in terms of removing the
6
greatest number of CWD infected animals from the herd, of the harvest
strategies modeled.
The reason for this is due to the higher prevalence rates found in adult males
compared to adult females, for both CWD and Bovine TB. Despite the fact that
at typical sex ratios there may be slightly more infected female deer in the
herd, due to elevated prevalence rates, weighting the harvest towards males is
more likely to remove greater numbers of infected deer, due to harvest
efficiency
Target Identification and untargeted culling
Because the prevalence rates are substantially higher in males once you get to
the 2.5 age class and older, you generally have to shoot twice as many adult
males to remove the same number of diseased animals. This is based on the
premise that you can't identify which animals are diseased prior to shooting
them, which is referred to as untargeted culling. So if the goal is to lower the
herd prevalence rate by employing a female focused harvest strategy, first you
have to convince hunters to shoot a female instead of a male and then you
have to get them to shoot twice as many deer in order to get the same return,
in terms of number of diseased animals removed
Anther factor that comes into play is the fact that many hunters have a hard
time distinguishing fawns of either sex from adult females. Since fawns have a
very low prevalence rate for disease, killing a female fawn instead of an adult
doe is counterproductive to removing diseased animals from the herd. Killing a
male fawn also does not help lower prevalence rates or remove disease but it's
beneficial in terms of reducing yearling dispersal the following year, so at least
it's not counterproductive to disease mitigation efforts.
By the time you factor in the impact that killing fawns has on diluting the
disease removal occurring from female focused harvest, the ratio needed is
probably closer to 2.5 female deer per 1 antlered male, to remove the same
number of diseased animals. This would suggest that female focused harvest is
an inefficient means of reducing the number of diseased animals in the herd.
Harvest Fatigue
In addition to the decreased harvest efficiency of infected animals resulting
from a female focused harvest strategy, there are several other factors that
also can influence the viability of different harvest strategies.
One of the factors that has been seen in Wisconsin and in some other areas
where female focused harvest strategies have been attempted, for purposes of
disease mitigation, is hunter harvest fatigue.
In a presentation discussing "lessons learned" from Wisconsin's CWD mitigation
efforts, Dr. Julie Langenberg, who at the time was the Chief Veterinarian for the
WDNR, stated that "Wisconsin hunters and other key stakeholders are unwilling to
harvest enough deer for significant population reduction".
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She further stated " more then half are concerned that control efforts will harm
deer populations and they do not support drastic herd reductions".
Her quote illustrates the tendency among hunters to slow down or stop completely
harvesting does when they start to believe that the deer population is being
lowered to the point that it will impact their hunting.
The female focused harvest strategy modeled above shows the results of what
might occur with a hypothetical harvest proportion of 60:40 does to bucks but the
model does not take into account the social consideration of whether or not it
would even be feasible to get hunters to shift harvest proportions to such an
extent.
From a practical standpoint, buck focused harvest strategies are both easier to
implement and are more likely to result in greater numbers of diseased animals
being removed from the herd, due to harvest efficiency. Hunters have no
problem identifying which bucks to remove. If they see antlers, Bang! Pretty
simple. You don't have to encourage hunters to shoot bucks, they will do that
on their own, you just need to keep out of their way. In fact, they will be
perfectly willing to purchase additional antlered licenses in a disease
mitigation zone, if you make them available, providing revenue for mitigation
efforts. To encourage substantial increases of antlerless harvest, under a
female focused harvest strategy in a disease zone, you probably will have to
either deeply discount tags or give them away, resulting in a loss of revenue.
Yearling Dispersal
Leading biologists and disease experts and a variety of state game agencies,
including the Michigan DNR, have suggested that the mechanism of yearling buck
dispersal increases the risk for the spread of communicable diseases like CWD an
bTB. This is a well accepted concept and a number of states including Missouri,
Minnesota and Arkansas have removed APR's that protect yearling bucks and
increase the number which disperse, in areas where CWD is present, to try and
minimize the risk of it's spread.
Most APR's in Michigan offer protection to approx. 70% of yearling bucks. The
concept of QDM is built on the premise that protecting younger bucks, often with
an APR, will allow them to advance to older age classes, so it's contradictory to
suggest, as some on social media have done, that removing such protections will
have no impact on how many yearling bucks are killed as yearlings, which in turn
has a direct bearing on the number of bucks dispersing.
Another incorrect assumption promoted on social media is that disease mitigation
only occurs if yearling bucks are killed prior to dispersing. Some have suggested
that killing yearling bucks after dispersal is "too late".
From an epidemiological standpoint, that premise is false, which is easily
demonstrated with a simple analogy. Anyone who has been a parent knows
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elementary age kids spread germs readily and that classrooms can be hot zones for
spreading colds.
Ideally, if a child has the flu, a parent would not send them to school. That would
be analogous to killing an infected yearling buck prior to dispersal occurring.
Unfortunately, a lot of parents send sick kids to school. If that sick child sits in
the class room and spreads the disease to an average of 1 other child for every day
they attend school while sick, it's abundantly clear that the longer that they keep
coming to school while sick, the more children will be infected, who in turn infect
other children, causing a multiplier effect. If the teacher notices that the child is
sick after the first hour or two and sends them home, (analogous to killing an
infected yearling buck after dispersal has occurred), then the incidence of the
disease spreading in the classroom is substantially reduced.
The idea that as soon as that child steps into the classroom, "it's too late" to stop
the child from spreading germs, is obviously a false one. The suggestion that
having the sick child (or the infected yearling buck) continue to attend school for
another 10 months while sick, ignores the potential that exists for the child to
continue to spread his germs, infecting other children, during that time period.
The claim that focusing harvest pressure on yearling bucks is "too late" because
most yearlings have already dispersed by the time fall hunting season occurs, is
also highly questionable. The fact is that a substantial number of yearling bucks
have not dispersed prior to the beginning of fall hunting seasons in October and as
such are vulnerable to harvest prior to dispersal. In the QDMA Chesapeake Farms
study authored by Dr. Jonathon Shaw, Shaw stated that approx. 25% of the
yearling bucks in that study dispersed during the spring fawning season (Apr. -
June) at 10 - 12 months of age and 75% dispersed during the fall hunting season
(Sep - Nov.) at 15 - 17 months of age.
While killing yearling bucks prior to dispersal is certainly preferable, killing
yearling bucks which have already dispersed is another vital piece of the CWD
mitigation puzzle, as the longer those bucks survive in their new range as "sparks",
the more contact they have with other deer, the more Prions they shed while
feeding in food plots, the more rubbing and scraping they engage in and the more
potential there is for them facilitating CWD becoming entrenched in a new area.
This graphic from the Wisconsin DNR shows a typical dispersal pattern. In this
graphic, the red star indicates where this particular buck was radio collared as an
8 month old button buck on February 4th. Around October 20th, he started the
dispersal process and left his natal range. In Michigan, he would have already
been vulnerable for harvest for almost a month prior to dispersing, (unless
protected by an APR). Over the course of the next month he continued to disperse
farther from where he was born. He was harvested (indicated by the red X) on
November 20th after having dispersed around 3 miles from where he had spent
most of his life.
If he had been infected with CWD, any contacts that he had with other deer and
any Prion shedding that could occur, ended with his harvest.
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Imagine the difference in potential disease impact, if this yearling had been
protected by an APR and had continued to disperse and spread disease for another
10 months, participating in breeding, joining bachelor groups, defecating in food
plots where other deer are feeding, engaging in scraping & rubbing, etc. To
suggest that there is no benefit to removing a potentially infected yearling buck
from the population during or after dispersal has already occurred, ignores reality.
Figure 7
It has also been suggested that focusing on herd reduction by employing a female
focused harvest strategy will reduce dispersal because after a number of years,
theoretically fewer numbers of male fawns will be born, resulting in there being
fewer to disperse as yearlings. There are two primary problems which this
essentially ignores, however. It completely ignores the immediate negative
impact, in terms of the spread of disease, which results from protecting yearling
bucks with APR's, as they will survive to disperse and spread disease or survive
after having dispersed. Under such a model much higher numbers of yearling
bucks would disperse than would occur under a male focused harvest plan without
regulatory protections for yearling bucks. You may eventually lower the number
of yearlings dispersing on an annual basis but during the 4 or 5 years that it takes
to realize any actual population reductions, 70% of the protected yearlings are
dispersing and the disease has spread over a much larger geographic region, while
you are waiting for the impact of fewer yearlings due to herd reduction. From a
practical disease mitigation standpoint, that is not a trade off worth making.
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Deer Population and Recruitment
Female focused harvest strategy models should address the potential impact that
herd reduction may have on recruitment rates in some portions of the state.
Reducing the number of females in the herd can actually increase the deer
population, depending on carrying capacity and recruitment rates prior to herd
reduction. Models that assume no increase in recruitment resulting from female
focused harvest, may be valid in some portions of the state but may also be flawed
in the NLP & UP.
Recruitment refers to the number of additional deer added to the herd every year
by the beginning of hunting season. The number of offspring that adult does have
and whether or not doe fawns breed is highly variable and depends on the carrying
capacity of the area and the availability of food (which can also be tied to
population density). As the deer population is reduced, there is more food
available for the remaining does, which can change their reproductive capacity.
This is natures way of dealing with threats such as inclement weather, which can
reduce deer populations. As the population decreases, more fawns will be born
resulting in higher recruitment rates. As the population grows, there is less food
available, resulting in decreased reproductive capacity, resulting in fewer fawns
being born and lower recruitment rates. These two charts illustrate these points.
Figure 8
In Figure 8, "K" refers to carrying capacity, the bell curve represents recruitment.
As you can see, the highest recruitment rate is produced at 50% of carrying
capacity (K/2). So if you reduce the population by 50%, if it was at carrying
capacity, the recruitment rate will be at it's highest capacity. As herd size
increases from that point, recruitment decreases, resulting in less deer added to
the herd every year than at MSY (maximum sustainable yield). As herd size
decreases from K/2, reproductive capacity remains high but the increasingly
smaller number of does available to breed results in fewer fawns being born,
reducing recruitment.
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The premise that herd reduction will result from APR's increasing female harvest
assumes that there will be no changes to recruitment rates resulting from herd
reduction. It would have you assume that the carrying capacity is fixed relative to
the herd population. That would not be the case in the majority of Michigan,
where carrying capacity is directly linked to a number of limiting factors, food
availability being a primary one. In those areas, it is entirely likely that female
focused harvest strategies, which reduce the number of does, may actually result
in larger herd sizes the following year, due to increased recruitment.
Figure 9
Figure 9 shows the inverse relationship between female reproductive capacity and
the population density of does. At higher populations, fawns does don't breed, as
they are not able to attain the minimum body weight that is required to stimulate
estrus as a fawn. As food availability increases, as the result of herd reduction
efforts, more doe fawns will begin to breed, increasing recruitment. The same is
true with adult does, as food supplies increase, more of them have twins or
triplets instead of singletons, resulting in increased rates of recruitment.
From a disease mitigation standpoint, the optimum realistic herd size in relation
to carrying capacity is one in which most doe fawns do not breed and a majority of
adult does are having singletons, yet the herd is maintained under K (carrying
capacity). This will limit recruitment while keeping the herd from growing in size
and will also keep hunters in the field as there will not be the hunter harvest
fatigue that occurs from female intensive harvest plans, such as occurred in
Wisconsin.
APR's Increasing Antlerless Harvest
There is a presumption that APR's will divert harvest pressure to antlerless deer
due to there being a reduced number of legal antlered bucks being available to
harvest, due to 70% of the yearling age class being protected by APR's. It's also,
however, presumed that such a diversion will only last as long as it takes for the
buck age structure to advance sufficiently so that the same number of antlerled
targets exist as prior to the APR's being put in place. That typically takes 1 - 2
years, as most 2.5 year old bucks will be legal for harvest under APR's. HC
regulations have the same impact in advancing the buck age structure, only it
takes several years longer before the number of legal targets increase to where
they were prior to the regulations. If the buck age structure has already advanced
under either form of APR's, any intended diversion to harvesting antlerless deer is
unlikely.
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Conclusion
In our opinion, based on the data resulting from these models and the other
information provided within this report, the most efficient and effective
harvest strategy for disease mitigation in Michigan, best designed to stop the
spread of diseases like CWD and bTB and limit herd prevalece rates, would be a
majority male focused harvest strategy that harvests around 70% of the
yearling buck age class, keeping the overall buck age structure young,
populated mostly by 1.5's and 2.5 year olds and which harvests at least 30% of
the breeding doe population to keep the herd static.
Additional harvest focus on button bucks, by educating hunters on why they
should be targeted in disease zones, should also be encouraged . The DNR did a
good job of educating hunters on identifying button bucks, which resulted in a
significant decrease in the percentage harvested, now what is needed is to
reverse that trend and refocus harvest pressure on them in disease zones. The
plan should also include utilizing sharpshooters to eliminate entire doe family
groups in the immediate areas where CWD positive deer are found, as called
for in the current CWD plan.
Female focused harvest strategies, both with and without the addition of
mandatory APR's, are both inefficient in removing diseased animals from the
herd and also have the potential to increase the geographic spread of the these
diseases by increasing the level of dispersal of yearling males. The uncertainty
of whether such strategies will actually reduce overall herd populations,
resulting in a positive benefit is more than outweighed by the certainty that
such strategies will have a tangible negative impact resulting from increasing
the number of yearling bucks surviving to disperse, as well as increasing the
cohort within the herd with the highest prevalence rates.
