Title: Pretty Good Yield Metaanalysis and sustainability
1Pretty Good YieldMeta-analysis and sustainability
Ray Hilborn Ram Myers Carl Walters
un-indicted co-conspirators
2Maximum 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
3Pretty 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
4Steepness
- 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
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6Steepness by family
7Ecological 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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9Without salmonids
10Some results for different species groups based
on life history traits
11Potential 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)
12Basic 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
13Yield vs h.r. for different steepness
14Yield vs. depletion (B/B0)
15Depletion vs h.r.
16Pretty 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
17Pretty good yield PGY
18PGY is a big place!
19PGY Harvest rate/natural mortality
20Now 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
21Impact of stochastic recruitments.8 z.7
22Results of recruitment variance
- Doesnt have much effect on potential yield,
optimum harvest rate or optimum stock size
23Impact of autocorrelations.8 z.7 sigmaw.6
24Autocorrelation results
- Higher autocorrelation leads to optimum harvest
rates being a little lower, but not a big effect
25Moving selectivity ogive
26Selectivity 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!
27Where vulnerability comes in relation to growth
(von B k)
28Parameter 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
29Steepness by family
30Conclusions
- 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
31Comments 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
32Where 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
33Final 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
34A 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
35Lower threshold
- Based on best scientific knowledge including
historical trends in abundance, meta-analysis
etc. - Will be admittedly arbitrary, but based on
available knowledge.
36Specific 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)
37Rules 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
38Final 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