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Harvest
• Harvested v. unharvested populations
– Why are some species not harvested?
– Why are some species harvested at different rates?
– Why does harvest not seem to affect some species?
Harvest
• Harvestable surplus (Leopold 1933)
– Originally only the “doomed” surplus
Harvest
• Assuming a surplus
– Overharvest
• Extinction
– Regulated hunting has never led to an extinction!
– Underharvest
• Environmental degradation (Think like a mountain!)
• Loss of recreational opportunities (value!)
Harvest
• Characteristics of harvested populations
– Population size
– Population stability
– Fecundity
– Life span
– Mortality from other causes
• Unknown effects
– Trophy’s smaller?
– Genetics?
Harvest
• Principles
– Yield*
– Diminishing returns
– Compensatory harvest mortality
– So which one happens?
• History, Trial & Error, and Human Dimensions
Harvest
• When populations are limited by some resource
(density-dependence/logistic growth; from Caughley
& Sinclair 1994)
– A population is harvested at its growth rate
– A population must be stimulated to produce a yield
– Harvesting trades off yield against population size
Harvest
• Sustained yield, harvest rate, & population size
Inflection Point (½ K)
Decelerating
Phase
Inflection Point (½ K)
dN/dt
N
K
Accelerating Phase
Time
N
K
Harvest
• Sustained yield, harvest rate, & population size
Surplus or Yield
dN/dt
Inflection Point (½ K)
K
N
no harvest sustainable
MSY
200
200
150
100
100
50
0
0
250
500
Population Size
750
0
1000
Harvest
Yield
potentially sustainable harvest
Harvest
• Harvestable surplus
– Sustained yield (SY)
– Maximum sustained yield (MSY)
– Optimal sustained yield (OSY)
Yield
200
MSY
100
0
Lower SY
0
250
Upper SY
500
Population Size
750
1000
Harvest
1200
Yield
Population Size + Recruits (Yield)
1050
Upper SY
75
900
750
MSY
100
600
450
Lower SY
75
300
150
0
0
250
500
750
Population Size
1000
Harvest
dN/dt
200
200
150
150
100
100
50
50
0
N
0
250
500
750
Population Size
0
1000
Harvest
Time
Yield
N
• When populations are not limited by resources
(exponential growth)?
Harvest
• Do we need strict regulations or is the
harvest self-regulating?
Hunter Effort
Total Harvest
Harvest Rate
– The Law of Diminishing Returns
Abundance
Abundance
Abundance
Harvest
• Additive v. Compensatory Mortality
120
# Dying/yr
100
Harvest
Predation
Starvation
Exposure
Accidents
Disease
80
60
40
20
H
un
te
d
C
B
H
un
te
d
A
un
te
d
H
U
nh
u
nt
ed
0
Harvest
• Additive v. Compensatory Mortality
– Temporal studies
B
Annual Survival Rate
Annual Survival Rate
A
C
Harvest Mortality Rate
Harvest Mortality Rate
c = critical threshold
Harvest
• Additive v. Compensatory Mortality
– Large scale experiments
– SHunting = SNo Hunting
– Late season harvest
Harvest
• Additive v. Compensatory Mortality
– Management implications
• Harvest as compensatory mortality
– Doomed surplus
– Harvestable surplus
Murray and Frye (1964)
Harvest
• When density independent factors are
controlling populations
– Harvestable surplus?
N
•K
Time
Harvest
• What if we don’t know enough?
• What if regulations are impractical?
Harvest
• Harvest Management (Ideal)
– Determine the status of the resource
– Determine the objectives & goals
– Establish management strategies
– Determine how closely the management
strategy achieved the objectives & goals
– Adjust management strategies
• Adaptive harvest management
Harvest
• Harvest Management
– Determine the status of the resource
• Research & monitoring
–
–
–
–
Mandatory reporting
Check stations
Surveys
Large scale studies
Harvest
• Harvest Management
– Determine the objectives & goals
• Biological, social, economic, & enforcement considerations
– Increase, decrease, maintain, or stabilize population size*
» Wildlife viewing, hunting, fishing, etc.
» Wildlife damage
» Wildlife disease (e.g., CWD, rabies)
» Habitat & community effects (e.g., brain worm)
» MSY or OSY
» Quality v. quantity
Harvest
• Managing for the hunter, fisher, or trapper
– Quality v. quantity
• E.g., QDM
– Deer, habitat, & experience
(U.S Fish & Wildlife Service)
(U.S Fish & Wildlife Service)
(U.S Fish & Wildlife Service)
Harvest
• Harvest Management
– Establish management strategies
• Seasons
– Opening day phenomenon
•
•
•
•
•
•
Bag limits
Methods
Sex & age taken
Permits
Areas
Effects on and of farming, timber, & non-consumptive
wildlife uses
• Safety
• Enforcement
Harvest
• Harvest Management
– Determine how closely the management
strategy achieved the objectives & goals
• Research & monitoring
–
–
–
–
Mandatory reporting
Check stations
Surveys
Large scale studies
Harvest
• Harvest Management
– Adjust management strategies & start again
• Adaptive management
• History, Trial & Error, and Human Dimensions
Harvest
• Harvests can stabilize populations
N
– A management tool
Time
Harvest
• Economics: Billions of dollars & hundreds
of thousands of jobs!
• Costs of not harvesting
Harvest
• Illegal taking (poaching)
– Equal to the legal harvest?
– Fair chase?
– This is not hunting!
– Implications
• Population & habitat management
– SY considerations
– Estimate illegal take
Harvest
• Differential Vulnerability
– Sex
– Age
• Mating habits
– Effects on sex ratios & age structure
• Productivity
Harvest
• Differential Vulnerability
– Flook (1970): Unhunted elk
• Sex ratio of newborn calves:
• Sex ratio of yearlings:
• Sex ratio of 2 year olds:
• Why?
– Fat, teeth, mating system
100:100
131:100
31:100
Harvest
• Differential Vulnerability
– Hunted black bears
• Sex ratio of population:
• Sex ratio of harvest:
72:100
145:100
• Why?
– Movements: male home range size = 30.8 km2
female home range size = 5.2 km2
– Harvest technique: 4% of harvest using bait is female
40% of harvest using dogs is female
– Harvest timing: denning
• Management
Harvest
• Differential Vulnerability
– Management implications
• Sex & age structure
• Mating habits
Harvest
• Federal v. state management
• Public v. private lands
– Private lands harvest management initiatives