Life History Patterns and Demography

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Life History Patterns and Demography
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Demography/Pop. dynamics

• Different species live out their lives on
  different time scales
• It is critical that we know these time scales in
  order to know
• 1)which organisms are most vulnerable to
  overexploitation
• 2) expected time horizons for population
  recovery

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Key Life History Parameters

• Charnov-Improves on r-K selection idea
• Life history characteristics are result of
  benefit-cost tradeoffs
• Result from optimization of genetic fitness
  (e.g. lifetime production of replacements for
  adults)

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Age at Maturity

• Usually measured as age at 50 maturity
• Highly correlated with longevity (max. age)
• Determines generation time
• time (decades for many rockfish) required for
  recovery from depletion

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Gonadosomatic Index

• Gonad wt./body wt.
• A measure of reproductive effort
• Negatively correlated with longevity
• Late maturing fish with low GSI are easily
  overfished, and slow to recover
• MPAs take a long time to produce results

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Generation Time

• The average age of mothers giving birth in a
  population with a stable age distribution
• population can be stable (r0), growing or
  declining, but proportion at each age is constant
• Determines expected time to full population
  recovery
• Determined by age at maturity and adult life span
  
• Adult life span 1/M

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Natural Mortality (M)

• M instantaneous rate of natural mortality
• dN/dt -MN where dN/dt is the number of deaths
  per year
• Can be estimated from a catch curve

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Demonstration of Mortality Calculation
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Guessing M

• Role in food chain
• few predators----low M
• notable exceptions tuna
• Body size
• large size----low M over animal kingdom
• notable exceptions tuna, thornyheads
• poor correlation within adult fish

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Guessing M (cont)

• GSI
• low GSI..low M
• explains 75 of variation in M for fish
• Environment
• low water temp, low oxygen..low M
• higher M in tropics, lower M in deep-slope
• exceptionslanternfish, yelloweye, thornyhead

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Using M to Estimate Pop. Growth rate

• Imagine a stable population (lingcod on a reef.
  Habitat limits pop. Size)
• Annual recruitment of adults losses due to adult
  mortality (RM)
• Assumes no density-dependent changes in mortality
• probably an underestimate of potential R when
  density is low (compensatory mortality)

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Estimating Pop Growth (roughly)

• RdN/dtNM
• N(t) N(0)exp(Mt)
• Pop. doubling time (t)
• ln(2)/M t

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Generation Time

• Sum (Age x proportion surviving to that age x
  fecundity at that age) / Sum (proportion
  surviving x fecundity)