Consequences of changing climate for North Atlantic cod stocks and implications for fisheries manage

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Consequences of changing climate for North
Atlantic cod stocks and implications for
fisheries management Keith BranderICES/GLOBEC
Coordinator
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Cod stocks included in the analysis of recruitment
NE Arctic
Baltic
Faroe
Iceland
W Baltic
NorthSea
W Scotland
2J3KL
Georges Bank
IrishSea
4X
CelticSea
4VsW
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Why use NAO?

• Well studied climate indicator
• Range of biological effects
• Values are timely and free
• NAO does not have local values
• With T and other hydroclimatic variables you
  have to select a specific value
• There is a geographic pattern of NAO effects on
  T, cloud, wind, precipitation

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Classic surface signature
Correlation between winter NAO (DJFM) and sea
surface temperature (SST) in March to May
1948-2000
SST values are from NCEP/NCAR reanalysis
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Decadal mean NAO values
2004
Further reading on how the NAO is coupled to
global climate processes http//www.cgd.ucar.edu/
jhurrell/press.htmltrs
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Stock and recruitment data for 13 cod stocks
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Fitting stock-recruit relations (1)

• Ricker function relating stock (SSB) to
  recruitment (R)
• R a SSB exp(-b SSB) Eq. 1
• Parameters redefined to curves maximum point,
  SSBmax, Rmax
• R exp(Rmax/ SSBmax) SSB exp(-SSB/SSBmax) Eq.
  2

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Fitting stock-recruit relations (2)

• add NAO term (3rd parameter)
• R exp( Rmax/ SSBmax) SSBexp(-SSB/SSBmax)
  exp(cNAO) Eq. 3
• (this eq. has been used in several studies)

exp(cNAO) gt1
exp(cNAO) lt1
2 parameter and 3 parameter fit to the data using
a Markov Chain Monte Carlo (MCMC) algorithm
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Akaike Information Criterion (AIC) used to
compare the 2 parameter and 3 parameter fits. ?
AIC values marked show statistically
significant improved fit when third parameter is
added.
Values of c
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The symbols (, , 0, -, -) indicate the sign and
strength of the relationship between NAO and cod
recruitment in the 3-parameter Ricker model.
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What processes could be at work?

• NAO ? Temperature ? growth ? survival ?
  recruitment(many papers deal with this)
• NAO ? plankton production ? survival ?
  recruitment(Brander, Dickson and Shepherd 2001)
• NAO ? spawning conditions ? survival ?
  recruitment (Baltic inflows Andersen et al.
  this Symposium)

There are many possible processes and they may
not be the same for all stocks.
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Conclusions

• 4/13 cod stocks show significant effects of the
  NAO on recruitment. Changes in R and SSB since
  1960 are partly due to the NAO
• Geographic influence of the NAO on recruitment
  for all stocks is consistent with influence on
  physical factors
• Medium and long term strategies for fisheries
  management include explicit or implicit
  assumptions about future states of the NAO

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Effects on North Sea cod(Brander and Mohn
CJFAS,in press)
Low NAO
Target and limit reference points for fisheries
management are strongly influenced by the
assumptions which are made about future levels of
NAO. (Ignoring the issue is making an implicit
assumption about future NAO levels.)
High NAO
Low NAO
High NAO
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But is the S/R model adequate?

• R exp( Rmax/ SSBmax) SSBexp(-SSB/SSBmax)
  exp(cNAO)

A multiplier which is independent of SSB
Can we test the hypothesis that the environmental
effect is independent of SSB?
Time series for individual stocks are too short
and noisy, but the European shelf stocks south of
62o respond to the NAO in the same way, so a
joint analysis can be undertaken.
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Divide data (SSB, R and NAO) into 3 categories to
overcome scale differences and carry out
frequency analysis. Rothschild B.J. and Mullen
A.J. 1985. The information content of
stock-and-recruitment data and its non-parametric
classification. Journal du Conseil International
pour l'Exploration de la Mer 42 116-124
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Joint frequency analysis (200 values) and ?2p
that R is independent of NAO lt0.001 at low SSB
lt0.1 at med SSB gt0.5 at high SSB NAO
has a strong effect on recruitment when SSB is
lowEnvironmental effect is not independent of
SSB
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Why should environmental effects be stronger at
low SSB?

• Fewer age classes and fewer old fish at low SSB
• Spawning distribution may be reduced at low SSB

Marteinsdottir G. and Thorarinsson,K. 1998.
Improving the stock-recruitment relationship in
Icelandic cod (Gadus morhua L.) by including
age-diversity of spawners. Canadian Journal of
Fisheries and Aquatic Sciences 55 1372-1377.
Begg,G.A. and Marteinsdottir,G. 2002.
Environmental and stock effects on spatial
distribution and abundance of mature cod (Gadus
morhua). Marine Ecology Progress Series 229
245-262
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Conclusion

• The effect of environmental variability on
  European cod recruitment is not adequately
  represented by a standard Ricker S/R model with a
  multiplicative term.

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What are the management implications?

• Even stronger reason to avoid low SSB
• If low SBB occurs then recovery is very dependent
  on favourable environmental conditions
• New non-linear models are needed to explore
  medium and long-term consequences

We are already witnessing strong environmental
effects on stocks with low SSB
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Any Questions?
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