Biogeography GEOG342

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Biogeography GEOG342 Harvesting populations See
Krebs Chapter 17
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Harvesting

• Natural populations exploited in forestry,
  livestock production, fishing, agriculture,
  wildlife management etc.
• Key issue in economic exploitation is how to
  maximize the off-take without damaging the
  resource for future harvests i.e. maximize the
  long-term harvest

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Harvesting

• Fisheries are most-studied system
• Many marine fisheries are rapidly dwindling
  because of overfishing e.g. Coho salmon off
  Oregon coast

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Harvesting

• The relevant aspect of a harvested population is
  its yield (a rate e.g. kg/yr or number
  caught/yr)
• Two components of the population can be
  recognized residual population that escapes
  capture stock that can be caught with current
  methods
• Stock is a balance
• Stock Existing stock (Growth
  recruitment)-(natural mortality fishing
  mortality)
• To maintain fishery pop in yr 1 must equal pop in
  yr2
• i.e. Growth recruitment natural
  mortality fishing mortality
• When fished, stock drops and fishing offtake is
  compensated for by increased recruitment and/or
  reduced mortality

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Harvesting

• Yield and stock are related

Logistic growth model of optimal fish catch
size To maintain maximum yield, keep population
at about 53 maximum density Not at maximum
density
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Harvesting

• dN / dt r N (K N)/K
• Where K is the upper asymptote or maximum value
  of N, the i.e. Karrying capacity
• Note (1 N/K) (K N) / K
• (K-N)/K is the unutilized opportunity for
  population growth.
• This is the differential form of the logistic
  curve
• (cannot be used to calculate N in this form).
• Integral form of logistic model
• Nt K / ( 1 e a-rt)
• Where a is an empirically derived parameter
  defining the position of the curve relative to
  the origin.

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Harvesting

• Rate of population increase r (K N)/K r -
  rN/K
• Modify this rate for fishing catch
• r (K N)/K r - rN/K q/XN
• Where q catchability (constant)
• X fishing effort (qX is mortality due to
  fishing)

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Harvesting