The Pew Oceans Commission

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Sources of Fish DeclineIV. Habitat disruption

• Breeding areas
• Larval development areas
• Bottom structure

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Not all patterns are negative
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Population processes aid to intelligent
management?

• 1830s concerns about fluctuations in catch in
  North Sea
• Disbelief that humans could cause this
• C. D. J. Petersen (Denmark) applied science
• mark-recapture to estimate population size
• collected data on age-dependent reproduction
• applied population model to predict connection
  between fishing mortality fish populations
• Criticized as irrational (not profitable)
• Tested during WW I and WW II

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What do we need to know?

• Demography the study of processes affecting
  populations
• Processes adding to populations
• births, immigration
• Processes subtracting from populations
• death, emigration
• Base number of individuals
• Whether the processes are constant through time
• could vary with season or other scales of time
• could vary with the density of individuals, which
  change over time

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What if processes are constant?

• Population size in the next generation will
  depend on the base and the difference between
  births and deaths
• assuming we have an isolated group of individuals
• Nt1 Nt b d

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Exponential growth
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What if processes vary with density?

• If populations get larger, what do you predict
  will happen to birth rates?
• If populations get larger, what do you predict
  will happen to death rates?
• How many individuals are added to the population
  when birth rates and death rates are equal?

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Logistic growth pattern
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Summary of population models

• The Logistic model of density dependence predicts
  maximal sustainable yield at ½ K
• S-shape curve of population growth may not be
  seen when
• The response to density lags changes in the
  environment
• For populations with large excesses of births
  (rgt2) and where generations are distinct

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Added realism individuals vary in b and d

• Size (or age) influences
• Reproductive capacity ( of offspring likely)
• The risk of being eaten by a predator
• The probability of being captured in a net
• Age-specific demographic processes
• Fecundity
• Survivorship

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Age-specific parameters

• Start with a bunch of ? individuals newly born (
  a cohort)
• Determine the number of individuals that survive
  to each successive age (x)
• Sx
• The probability of survival from birth to age,
  x lx
• The number of ? offspring produced per ?
  individual of age x mx

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Life Table collection of data on Sx, lx, mx

• We can then project how each cohort will
  contribute to the population through its lifetime
• Some values derived from a life table
• Net Reproductive Rate, R0 the number of ?
  progeny expected to accumulate during the entire
  lifetime of an average ?
• Intrinsic growth rate, r
• Reproductive Value (Vx) the expected number of
  future ? progeny for a ? of age x (relative
  to that of a newborn, R0)

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The real world is not a set of simple equations

• Randomness is a factor
• Deterministic models always follow the same
  path given the same conditions
• Stochastic models include chance
• How is this done?
• Use an average value for a parameter
• But for any generation, the value used can
  deviate somewhat from that average
• Coefficient of Variation and distribution
  define the limits of deviation

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Success of species-based management
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Changes in food webs target levels
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What are the connections between food web and
demographic approaches?

• What demographic parameters are influenced?
• Are models still useful and how?

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An alternative to capture fisheries