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Pretty Good Yield Metaanalysis and sustainability

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Alec MacCall 2000 (Mote Symposium) Lets admit we don't want or know the true optimum. We want good fisheries performance, not 100% - but maybe a B grade 80% or better ... – PowerPoint PPT presentation

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Title: Pretty Good Yield Metaanalysis and sustainability


1
Pretty Good YieldMeta-analysis and sustainability
Ray Hilborn Ram Myers Carl Walters
un-indicted co-conspirators
2
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
3
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

4
Steepness
  • key parameter in spawner recruit relationship -
    is it important to retain spawning stock to
    provide recruitment
  • primary determinant in region of sustainability
  • Myers has assembled 700 data sets from stock
    assessments (with all flaws) from around the world

5
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6
Steepness by family
7
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

8
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9
Without salmonids
10
Some results for different species groups based
on life history traits
11
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
  • 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)

12
Basic runs
  • Survival 0.8
  • fished 1 year before mature - knife edged
  • first vulnerable at 1/3 of maximum weight
  • deterministic results
  • explore effect of steepness

13
Yield vs h.r. for different steepness
14
Yield vs. depletion (B/B0)
15
Depletion vs h.r.
16
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

17
Pretty good yield PGY
18
PGY is a big place!
19
PGY Harvest rate/natural mortality
20
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

21
Impact of stochastic recruitments.8 z.7
22
Results of recruitment variance
  • Doesnt have much effect on potential yield,
    optimum harvest rate or optimum stock size

23
Impact of autocorrelations.8 z.7 sigmaw.6
24
Autocorrelation results
  • Higher autocorrelation leads to optimum harvest
    rates being a little lower, but not a big effect

25
Moving selectivity ogive
26
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!

27
Where vulnerability comes in relation to growth
(von B k)
28
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

29
Steepness by family
30
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

31
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

32
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

33
Final conclusions
  • Good biological yield can be obtained over a
    broad range of fishing policies, either stock
    size or harvest rate
  • 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

34
A proposal for moving forward from BMSY
  • For each stock specify
  • a lower threshold where definite specified catch
    reductions will kick in (as per D. Gilbert)
  • Make this in terms of measurable quantities such
    as surveys, CPUE or a weighted average
  • Above this threshold stakeholders specify
    decision rules - Mfish reviews these to make sure
    they also meet sustainability goals

35
Lower threshold
  • Based on best scientific knowledge including
    historical trends in abundance, meta-analysis
    etc.
  • Will be admittedly arbitrary, but based on
    available knowledge.

36
Specific examples of lower thresholds
  • Campbell SBW - Use 1991 biomass as reference
    point for threshold
  • SNA1 - use historical low abundances
  • Rock lobster - historical low abundance levels
    for rebuilt stocks
  • New fisheries - fraction of initial biomass
    using high target fraction (50 perhaps)

37
Rules above threshold
  • Stakeholders given wide freedom
  • Rules would normally be data based including not
    only biological data (surveys, CPUE), but also
    economic data such as price, exchange rates etc
  • Could allow for short term updates pre-season
    upward or downward revisions of TACC based on
    stakeholder agreement
  • the rules could specify the range of TACC that
    would be allowable given the data

38
Final Note
  • With respect to overfishing
  • NMFS estimates we lose 14 of potential US yield
    due to overfishing
  • That is a B or B in my book (86)
  • Maybe we are putting our energies into the wrong
    problem institutions, incentives and
    overcapitilization
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