PowerPoint-presentasjon

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Per Sandberg and Sigurd Tjelmeland
Harvest rules and recovery strategies The case of
Norwegian spring spawning herring
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Background
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Agreed harvest control rule
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Questions evaluated

1. Is the expected net economic revenue of this
   agreed harvest rule higher than the economic
   revenue from a so-called target escapement
   rule?
2. Which target fishing mortality would give the
   highest economic revenue?
3. Is the harvest rule performing better or worse
   than a target escapement rule in a period of
   recovery?

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Biological model and assumptions

• Model Seastar
• (ICES assessment model for Norwegian spring
  spawning herring).
• Prognostic simulation
• Period 50 years
• Number of replicates 500
• Beverton Holt stochastic recruitment (as in
  Seastar)
• Deterministic growth, natural mortality and
  exploitation pattern.

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Recruitment RmaxSSB/(Rhalf SSB)- where Rmax
and Rhalf is estimated from two subsets of
recruitment numbers as illustrated below. The
variance of the residuals is the basis for
stochasticity. 75 of prognostic recruitment is
drawn from the black subset and 25 from the red
subset.
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Economic model and assumptions

• Consequences for Norwegian fishermen only
• Fixed share of TAC to Norway
• Allocation of Norwegian quota on three vessel
  groups according to pre-agreed rule
• Price Either fixed or decreasing in harvest
• Variable costs Decreasing in harvest for all
  three vessel groups, decreasing in biomass for
  coastal vessels.
• Discount rate 5 per annum

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The expected NPV, ENPV of the Norwegian
harvest of Norwegian spring spawning herring in a
50- year period when applying constant escapement
from 1 to 7 million tonnes and the Ad Hoc Rule
established by the managers of the stock.
Discount rate is set to 5. Figures in million
NOK
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The expected NPV, ENPV of the Norwegian
harvest of Norwegian spring spawning herring in a
50- year period when applying constant escapement
from 1 to 7 million tonnes and the Ad Hoc Rule
established by the managers of the stock.
Discount rate is set to 5. Figures in million NOK
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Question 2 Which target fishing mortality would
give highest economic revenue?
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The expected NPV of the Norwegian harvest of
Norwegian spring spawning herring in a 10-year
period.
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Question 3 Is the agreed harvest rule performing
better or worse than a constant escapement
strategy in a period of recovery?

• Consequences for Norwegian fishermen
• Starting year 1975
• Simulation period 50 years
• Prices decreasing in harvest
• Discount rate 5
• Fixed shares of TAC to Norway, allocation on
  three Norwegian vessel groups with cost structure
  as identified above.

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Development in spawning stock. Left panel
Target escapement of 5 million tonnes. Right
panel Agreed harvest control rule.
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Expected NPV, expected SSB at the end of the
period and the probability of SSBlt2.5 million
tonnes during the period.
Harvest rule ENPV during period ESSB at end of period P(SSBlt2.5 m.t.) during period
Target escapement 5 m.t. 326 4,3 56
Agreed harvest rule 869 4,1 70
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Summing up

1. When prices are fixed, and adjustment costs in
   the fleet are zero, target escapement gives
   higher NPV than the agreed harvest control rule.
2. When prices are decreasing in harvest, this
   conclusion is reversed.
3. In the agreed harvest strategy, increasing the
   fishing mortality above 0.10 only gives a
   moderate increase in NPV but a substantial
   reduction in SSB

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Summing up (2)

• In a period of recovery, the consequences of a
  target escapement strategy and the agreed harvest
  rule are very different.
• A target escapement strategy would imply a
  moratorium on the fishery for a long period.
• The agreed harvest rule would allow a fishery
  also at low stock levels.
• Consequently, there is a tradeoff between stock
  rebuilding and NPV of the fishery.