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!