Pretty Good Yield

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Pretty Good Yield

• FISH 458/556

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Maximum sustainable yield (MSY) "A quantity
that has been shown by biologists not to exist,
and by economists to be misleading if it did
exist. The key to modern fisheries
management.John Gulland
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Pretty Good Yield

• Alec MacCall 2000 (Mote Symposium)
• Lets admit we dont want or know the true optimum
• We want good fisheries performance, not 100 -
  but maybe a B grade 80 or better
• Wouldnt we all be happy if our fisheries
  management got a B grade?
• Take Poll A B C D or F

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Steepness

• key parameter in spawner recruit relationship -
  is it important to retain spawning stock to
  provide recruitment
• primary determinant of the region of
  sustainability
• Myers has assembled 700 data sets from stock
  assessments (with all flaws) from around the world

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Steepness by family
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Ecological considerations of steepness

• Depends on strong density dependence
• Species that are highly limited by habitat
  protection from predators
• Clupeids and salmonids both pelagic species
  have low steepness
• Gadids and pleuronectids, generally demersal, are
  more habitat dependent

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Without salmonids
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Some results for different species groups based
on life history traits
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Potential problems with steepness analysis

• Errors in variables biasing steepness too high
  (Ludwig and Walters)
• these data sets generally have lots of contrast
  and this bias is not too severe if it exists at
  all
• Selection bias for productive stocks
• Data are not data, but model outputs
• Many examples of low steepness may actually be
  environmental changes where reduced recruitment
  caused reduced spawning stock (Gilbert CJFAS
  paper)

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Basic runs

• North Atlantic cod type life history
• Survival 0.8
• fished 1 year before mature - knife edged
• first vulnerable at 1/3 of maximum weight
• deterministic results
• explore effect of steepness

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Yield vs h.r. for different steepness
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Yield vs. depletion (B/B0)
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Depletion vs h.r.
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Pretty good yield PGY

• Instead of looking for the optimum lets look for
  the range where we get pretty good yield -- here
  I use 80 of MSY
• This occurs over a broad range of both stock
  sizes and harvest rates

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Pretty good yield PGY
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PGY is a big place!
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PGY Harvest rate/natural mortality
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Now we systematically explore other factors

• Stochasticity
• autocorrelation of residuals
• age of first vulnerability compared to age of
  first maturity
• where maturity and vulnerability occur relative
  to maximum body weight

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Impact of stochastic recruitments.8 z.7
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Results of recruitment variance

• Doesnt have much effect on potential yield,
  optimum harvest rate or optimum stock size

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Impact of autocorrelations.8 z.7 sigmaw.6
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Autocorrelation results

• Higher autocorrelation leads to optimum harvest
  rates being a little lower, but not a big effect

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Moving selectivity ogive
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Selectivity ogive

• Shown was the number of years vulnerable to
  fishing prior to breeding
• this is very important - almost as important as
  steepness
• This is Myers explanation for the collapse of the
  Northern Cod
• And incidentally why fishing spawning
  aggregations may be a GOOD practice!

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Where vulnerability comes in relation to growth
(von B k)
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Parameter used is the von Bertalanffy k

• The lowest k has the lowest potential yield, this
  corresponded to maturity/vulnerability at 1/6 of
  maximum body weight
• The next k was the base case with maturity at 1/3
  of maximum body weight

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Steepness by family
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Conclusions

• The two most important factors are steepness and
  where the selectivity ogive comes in relation to
  maturity
• PGY occurs over a broad range
• For the types of steepness found in clupeids,
  gadids and pleuronectids PGY is obtained anywhere
  from 15-55 of virgin stock size

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Comments on overfishing definitions

• Overfishing definitions that have been adopted in
  many jurisdictions will fall within the range of
  PGY - that is the stocks are within the range of
  good biological management yet classified as
  overfished

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Where we want to be

• PGY is obtained over a broad range of stock sizes
• For economic reasons we would generally want to
  be at the high stock size - we should probably
  use 50 of virgin stock size as a target for new
  fisheries - not 30-40
• BUT if we find ourselves at 15 or 20 of virgin
  stock size, we may be giving up little if any
  yield, and there may be little economic reason to
  rebuild

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Final conclusions

• Good biological yield can be obtained over a
  broad range of fishing policies, either stock
  size or harvest rate
• This is all ignoring any interactions with other
  species
• Management plans (whether rebuilding or not) can
  take into account the biology of the species and
  then search for policies that produce PGY and are
  economically viable