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Photo: J.-D. Lebreton. Harvesting and Control. Harvest :. Number removed is the parameter of interest. How many deer can be taken from management unit?. Control:. Number remaining is the parameter of interest. Maintaining herd size of wild horses. Concept of Sustainable Harvest.

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Photo: J.-D. Lebreton

Harvesting and Control


Number removed is the parameter

of interest

How many deer can be taken

from management unit?


Number remaining is the parameter

of interest

Maintaining herd size of wild horses

Concept of Sustainable Harvest

Consider the logistic model

dN/dt = RN*(K - N)/K

Nt+1 = Nt + RNt(1-Nt)/K

Relationship of harvest

size to population size

What’s going on?

The maximum per

capita growth rate

occurs when ….

The Maximum Sustained Yield

“The largest harvest rate that can be

imposed without causing population

to decline”

For the logistic model:

MSY occurs at 1/2 K.

The value is RK/4

Logistic-based Harvest Models

dN/dt = rN(1-N/K) – L

L=loss from harvest

Fixed-Quota Harvest Model:

L = some constant

Fixed-Effort Harvest Model:

dN/dt = rN(1-N/K)-(E*C*N)

where E = effort, C = catchability, E*C*N = L

Implications of Fixed-Quota Harvest

What’s happening here




Note: trend of pop declining to extinction is

indistinguishable from pop decline to ½ K

Implications of Fixed-Effort Harvest




A yield

curve with

varying effort

is useful—

On board

Limitations of Logistic-based Harvest Models

  • Environmental variability

  • Estimation of carrying capacity

  • Estimating population parameters

  • More complex relationships

    • age/stage structure

Structured Population Models for Harvest

  • Motivation:

  • Many harvest strategies select specific

  • age/stage class

  • Implications of life history strategy

  • Experimental work frequently done

  • on invertebrates

Effects of Age/Stage

on Harvest

Harvest of youngest

age class:

1. Reduced total popn size

2. Altered popn structure

Before harvest


  • Total yield inc to 90%

  • harvest rate

  • -yield/ind increased

  • (compensation)

Total yield

Ind. yield

4. Increase in life expectancy

Additive versus Compensatory Mortality

Importance of



Two main goals:

  • Limit populations of desired species

  • Eradicate unwanted populations

  • (usually exotic species)




Consider the logistic model once again…

How could control work?

Lethal control of animal populations

Issues and Biological Considerations

Fertility Control

Two goals:


limitation of populations


prevent reproduction

Population-level implications?

Issues with wildlife contraception





  • Both harvest and control draw heavily

  • on population ecology of target species

  • Harvest and control primarily differ in

  • objective not in scientific principles

  • Ethical/societal issues of major importance

  • (this is as much about policy as it is

  • about science)

Ungraded Homework Assignment

How do the goals of control and

harvest differ?

What biological principles do they

both rely upon?