Rendement maximal durable (RMD) définition, estimation ... Biseau RMD-PE-2015.pdf · 4. MSY -...

Rendement maximal durable (RMD) définition, estimation

Maximum sustainable yield (MSY)

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Maximum sustainable yield (MSY)definition, estimation

Alain Biseau, Ifremer, France

PE 23 février 2015

�Not only a scientific issue but a political decision/commitment (Johannesburg, 2002 …)

�Not new (1945), but not considered for years since the priority was to avoid/stop depletion/collapse (overfishing): precautionary approach (first step) :

��safe biological limits

0. MSY introduction

PE 23 février 2015

��safe biological limits

Blim: spawning biomass under which the risk of lowrecruitment is high

Bpa: safety margin to avoid Blim

Flim: fishing mortality leading to BlimFpa

Blim Bpa

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� Precautionnary approach :at Bpa (and Fpa) : within safe biological limits[CFP 2002]

� sustainable …… but usually far below maximisation / optimisation

0. MSY introduction

PE 23 février 2015

� MSY approach :second step [CFP 2014]: maximisation of the yield(from a stock, an ecosystem…)

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MSY definition:

“ The highest theoretical equilibrium yield that can be continuously taken (on average) from a stock under existing (average) environmental conditions without affecting significantly the reproduction process” (FAO glossary)

1. MSY - definition

PE 23 février 2015

theoritical, on average, under existing environmental conditions

… and depending on the selection pattern

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1. MSY definition

60

80

100

Cap

ture

Y

MSY

600

800

Bio

mas

se B

Maximum yield

No fishing :Virgin population

5PE 23 février 2015

0.0 0.5 1.0 1.5

0

20

40

60

Pression de peche

C

aptu

re Y

Fmsy Fext 0

200

400 Bio

mas

se B

Bmsy

Fishing pressure

OverfishingCollapse

�At equilibrium (i.e. everything else unchanged, includingrecruitment), proxies of FMSY : Fmax, F0.1, F35%....

2. MSY estimation (F)

0.20

0.25

Y/R

(under current exploitation pattern)Rendement et Biomasse par recrue

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Yield and biomass per recruit

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.00

0.05

0.10

0.15

0.20

Pression de pêche

Y

/R (

___

)

FmaxF0.1 current

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B/R

(---

)

B/R

B35%

F35%


Fishing pressure

� From simulations: Taking into account uncertainties (and variation in recruitment), simulations of the impact of various F values on future yield and biomass

Pick up the F value that gives:-the highest long term yield


and -a probability for B being below Blim < 5%


On EC request, ICES estimated a range of F around Fmsy that gives a yield no less than 95% of MSY, and for which the probability of the resulting SSB being < Blim is < 5%


FMSY

F tq prob(B<Blim)=5%

8PE 23 février 2015(ICES, WKMSYREF3- 2014)

The precautionnary condition [Prob(B<Blim)<5%] may result in a narrower range of F


FMSY

F tq prob(B<Blim)=5%

9PE 23 février 2015(ICES, WKMSYREF3- 2014)

�BMSY value can easily be estimated for one stock in isolation assuming ‘everything else unchanged’

�Biomass = BMSY if fishing mortality = FMSY for (some) years

2. MSY estimation (B)

100

MSY

B

Spawning biomass


0.0 0.5 1.0 1.5

0

20

40

60

80

100

Pression de peche

C

aptu

re Y

Fmsy Fext 0

200

400

600

800

Bio

mas

se B

Bmsy

FMSY

BMSY

However, this value is probably not the one that would beachieved in reality : it will change when other stocks (predadors or preys) reach MSY as well


Biomass at Fmsy according to natural mortality

(and if environmental conditions change)


�‘all other thing kept unchanged’ hardly ever occurs in reality� even at a constant F, SSB will fluctuate due to naturalfactors� current F having been far above FMSY, for most stocks, anticipating what would be the fluctuations of SSB at FMSY wasconsidered not wise� need contemporary data with fishing at F to experience

2. MSY estimation (B): B MSY vs MSY-B trigger

� need contemporary data with fishing at FMSY to experience the range of fluctuations in SSB, before giving A value as a management reference point for BMSY


�use a preliminary value : MSY-Btrigger, and update/revise it regularly (until ‘true’ BMSY can be estimated)�MSY-Btrigger = lower bound limit of the confidence intervalaround the current estimate of BMSY

= threshold under which management action is required

2. MSY estimation (B): B MSY vs MSY-B trigger


�develop multispecies model to take species interactions into account.

Example in the Baltic:


BMSY value for cod= 115 ktons if stock considered in isolation, = 200 ktons if species interactions are taken into account


(ICES, 2013)

�� Any change in the selection pattern leads to a change in MSY

3. Discussion : back to definition

2000

2500

Catch

sele

ctiv

ity

Catch according to selectivityMSY


0

500

1000

1500

20

13

20

15

20

17

20

19

20

21

20

23

20

25

20

27

20

29

20

31

20

33

20

35

(tonnes)

Fsq

Sq Sel0

Sq Sel1

Incr

easi

ngse

lect

ivity

�� Any change in the selection pattern leads to a change in MSY and therefore in the values estimated for FMSY et BMSY

3. Discussion : back to definition

F /F

Biomass according to selectivity

sele

ctiv

ity

BMSY

FMSY/Fcurrent

1.421.000.80


Incr

easi

ngse

lect

ivity

0

50

100ga

in/p

erte

long

ter

me

(%)

1234

Equilibrium yield

MSY

Age 1st capture

PE 23 février 2015

-100

-50

0

gain

/per

te lo

ng t

erm

e (%

)

Fishing pressure +-

Fmsy Fmsy Fmsy

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Fcurrent

MSY higher, FMSY higherwhenselectivityimproves

Looking for the maximum of the maxima… combining changes in the selection pattern and fishing effort [assuming everything else unchanged]

MSY

PE 23 février 2015

Fishing effortSelectivity

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4. MSY - Conclusions

�Estimation of FMSY from simulations (analytical assessment) :F giving the long term yield with prob(B<Blim) < 5%.

�The use of a range around FMSY is a management decision

� BMSY value estimated today would probably not be the value whichwill be ‘observed’. Use MSY-Btrigger for the time beeing (as a first step), revise its value periodically

�Any environmental change or modification of the selection pattern must lead to a revision of the MSY reference points

�‘On average’ means that MSY may not be achieved in all years, but in 50%


« To be at MSY » what does it mean?:

�� A fishing mortality = or < FMSY ?or�� A biomass = or > BMSY ?

Actually both in the long term (equilibrium):


Actually both in the long term (equilibrium): Fishing at FMSY for (some) years leads to Biomass = BMSY

From a management point of view : FMSY


MSY approach :

To reconcile conservation and exploitation


Thanks !


�� Assuming ‘everything else unchanged’, including recruitment, then FMSY=Fmax the fishing mortality leading to the maximum yield


Yie

ld

Fmax


Yie

ld

Fishing effort

� Fmax sometimes difficult to estimate


???

Thus need for looking for F0.1, F35%....

???

Fmax


�F0.1 = F such as the marginal gain in yield is 10% of what it is at the origin


F0.1


Rendement maximal durable (RMD) définition, estimation ... Biseau RMD-PE-2015.pdf · 4. MSY -...

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