Evaluation of a North Sea Cod recovery plan taking into account biological interactions (Alexander...
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Transcript of Evaluation of a North Sea Cod recovery plan taking into account biological interactions (Alexander...
Evaluation of a North Sea Cod recovery plantaking into account biological interactions
(Alexander Kempf, Morten Vinther, Jens Floeter, Axel Temming)
Introduction
Cod stock in the North sea is in serious problems:
- SSB is on a historical all time low well under Blim
- very low recruitment numbers in the last years
In recent years management measures have been applied to rebuild the cod stock:
- box closures
- effort reductions
- cut down of total allowable catch (TAC)
Latest proposal of European comission (Reg 2003/0090 (SNS)):
- effort reductions/ control
- Harvest Control Rules (HCR) for setting TAC
HCR rules for rebuilding cod stock (Article 6 and 7 of Reg 2003/0090 (SNS)):
a) If SSB is over Blim
1. Council decide by qualified majority on a TAC for the following year
2. Either the SSB reaches Bpa or SSB should increase by 30% per year
3. Fishing mortality F is not allowed to exceed Fpa
(Fpa for Cod in the North Sea = 0.65)
4. If SSB is predicted to exceed Bpa with rule number two, a TAC should be calculated that will lead to a SSB equal to Bpa
5. Except for the first year, the TAC is not allowed to increase or decreaseby more than 15% compared to the TAC one year before
6. 4. + 5. shall not apply, if Fpa (see 3.) would be exceeded!
b) If SSB is under Blim:
1. If SSB is predicted to be under Blim in the end of the year, the fishing mortality(F) should be reduced to a level, that SSB will pass Blim in the following year.
2. If fishing mortality must be reduced to a level, that the resulting TAC wouldbe less then 20% of past years TAC, the fishing mortality must be increased
that the resulting TAC is exactly 20% of past years TAC.
Questions to be adressed in the analysis carried out:
- can the mentioned HCR rules help cod to recover?
- what are the effects of a cod recovery on other species in a multispecies context?
- lead the allowance of biological interactions to other answers for the fisheries management than the usual single species assessment?
Material and Methods
SSB i-1 < B lim
yes noReduce F i such that SSB i=B limor set F i=0
Estimate F i such that SSB i = minimum (Bpa , SSB i-1 * 1.3)
TAC i>TAC i-1*1.15
Reduce F i such that TAC i=TAC i-1*0.85 Increase F1 such that
TAC i=TAC i-1*1.15
yes no
yes no
i >= 1
Calculations for one yeari=0
TAC i<TAC i-1*0.2
Increase F i such that TAC i=TAC i-1
noyes
noyes
Estimate TAC i and SSB i from F i
F i > FpaF i=Fpa
i=i+1
F i > FpaF i=Fpanoyes
Estimate TAC i and SSB i from F i
SSB i-1 < B lim
yes noReduce F i such that SSB i=B limor set F i=0
Estimate F i such that SSB i = minimum (Bpa , SSB i-1 * 1.3)
TAC i>TAC i-1*1.15
Reduce F i such that TAC i=TAC i-1* Increase F1 such that
TAC i=TAC i-1*1.15
yes no
yes no
i >= 1
Calculations for one yeari=0
TAC i<TAC i-1*0.
Increase F i such that TAC i=TAC i-1*0.2
noyes
noyes
Estimate TAC i and SSB i from F iEstimate TAC i and SSB i from F i
F i > FpaF i=FpaF i > FpaF i=Fpa
i=i+1
F i > FpaF i=Fpanoyes
Estimate TAC i and SSB i from F i
Article 6 and 7 in pseudo computer language:
(based on Morthen Vinther)
Theory of VPA:
VPA (Virtual Population analysis): Rather simple technique by which you back-calculate how many fish there must have been in the sea to account for the observed catches (virtual population) (Sparre 1983).
Cohort concept:
N
N
N
N
age class
year
0
1
2
3
last datayear
start
C
C
C
D
D
D
year of birth
C= numbers caught by fishing D= number of natural deaths N= stock numbers
Theory of VPA:
Two kinds of VPA´s:
SSVPA: no biological interactions are taken into account (e.g. predation mortalitiy)!
only a qualified guess for the coefficient of natural mortalitiy (M) M is constant over the hole calculation time period Z= F + M
MSVPA: predation mortality (M2) is quantified inside the model!
Z= F+ M2+ M1
Z = total mortalityF = fishing mortality M2= predation mortalityM1= residual natural mortality (starvation, diseases...)
VPA run with 4M (multi or single species)
inital stock numbersstock recruitment relationshipF in the first prediction year
suitabilities for calculation of M2
Multi and single species VPAkeyrun of year 2003 was used!
Make prediction for one year iusing 4M
Take output from 4M and applyHCR routines for estimating next years fishing mortality F
(R program)
Write input data for 4M prediction
Final prediction year? ENDyesno
Predictions using 4M (single and multi-species possible) as external procedure:
(based on Morthen Vinther)
i= i+1
Further notes on prediction runs:
- HCR rules were simulated to be in use from year 2003 on
- last prediction year was 2010
- included predator and prey species:
only predator predator and prey only prey
saithe cod herring
North Sea mackerel whiting sprat
western mackerel haddock N. pout
sea birds sandeel
raja radiata.
grey seals
horse mackerel
- Stock recruitment relationship:
- Reference point (Blim, Bpa etc.) adjustment for using single species reference points in a multi species assessment
Recruitment (Ricker relation or Arithmetic Mean)
Species Recruitment Omitted YearsCod Ricker 1969
Haddock AM
Whiting Ricker 1977-1980
Saithe Ricker
Herring Ricker
Sprat Ricker 1973 and 1975
Sandeel AM
Norway Pout AM
Sole Ricker
Plaice Ricker
Way of reference point adjustment:(based on Morthen Vinther)
Stock areas in single species and multispecies assessment differ:
Single species values for Bpa and Blim were reduced by the same percentage as catches from outside area IV contribute to the total catch.
Fpa was left unchanged
single species multi speciesNorth Sea (IV)
Kattegat and Skagerrak (III)
English Channel (VIId)
only North Sea (IV)
Simulation szenarios carried out among others (in total 17):
Szenario no.HCR applied
to speciessingle or multi species mode
Prediction value for F
1 none single F current, SSVPA
2 none multiF current, MSVPA
3 none single Fpa
4 none multi Fpa
5 Cod single F current, HCR
6 Cod multi F current, HCR
7Cod, Whiting,
Haddockmulti F current, HCR
Results
year
2000 2002 2004 2006 2008 2010 2012
SS
B [t*
10
^3
]
10
20
30
40
50
60
70
singlemulti
Blim
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3]
0
50
100
150
200
250
300
350
400
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
300
400
500
600
700
800
singlemulti
Blim
Bpa
Haddock Sandeel
Cod Whiting
Scenario with Fcurr and no HCR species:
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
150
200
250
300
350
400
450
500
550
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
20
40
60
80
100
120
140
160
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
150
200
250
300
350
400
450
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
400
450
500
550
600
650
700
750
800
singlemulti
Blim
Bpa
Cod Whiting
Haddock Sandeel
Scenario with Fpa and no HCR species:
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3]
0
100
200
300
400
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
0
200
400
600
800
1000
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3]
150
200
250
300
350
400
450
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
0
100
200
300
400
singlemulti
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3
]
0
200
400
600
800
1000
singlemulti
Blim
Bpa
Scenario with Fcurr and Cod as HCR species:
Cod Whiting
Haddock Sandeel
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3]
0
100
200
300
400
multi
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3]
0
100
200
300
400
multi
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*1
0^3]
0
100
200
300
400
500
600
700
multi
Blim
Bpa
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*10
^3
]
100
150
200
250
300
multi
Blim
Bpa
Scenario with Fcurr and Cod, Whiting and Haddock as HCR species:
Cod Whiting
Haddock Sandeel
Summary and discussion:
1. There are marked differences between single species and multi species assessment.
Reasons:
- SSVPA and MSVPA calculate different
F pattern
stock recruitment relationships.
Single species assessment can´t cover trends in predation mortality due toincreasing or decreasing predator stocks!
Summary and discussion:
2. Cod recovery has negative effects on other species when taking biological interactions into account (esp. whiting)
Recovery plans must include strategies for all species of economic and ecological interest not only for one single
species alone!
3. HCR seem to help cod to recover
But: In keyrun 2003 Grey gurnard was excluded!
Grey gurnard plays an important role:
- in keyrun 2002 actual responsible for about 60% of the predation mortality on young cod- in keyrun 2003 excluded from the model
Reason:
Cod can´t recover in the model through the high predation mortality caused by grey gurnard even if cod is fished with Fpa!
year
2000 2002 2004 2006 2008 2010 2012
SS
B [
t*10^3]
0
20
40
60
80
100
120
140
without grey gurnardwith grey gurnard
Blim
Bpa
Questions:
1. Only one stomach data set from 1991 available just an extreme situation was sampled?
overlap highly variable actual MSVPA set up ignore this!
2. Is the bad situation for cod caused by the Holling type II parameterisation?
positive prey switching (b= +0,5) no switching negative prey switching (b= -0.2)
0,00 5000,00 10000,00 15000,00 20000,00
prey number
0,00
1000,00
2000,00
nu
mb
er
eate
n
Final conclusions:
When thinking about recovery plans and HCR, biological interactions mustbe taken into account!
But:
Uncertainties in multi species modelling must be reduced to give certain and usefulanswers!
EU project BECAUSE try to solve these problems Website: www.rrz.uni-hamburg.de/BECAUSE
Thanks for your attention!