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Per Sandberg and Sigurd Tjelmeland

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

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Per Sandberg and Sigurd Tjelmeland

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  1. Per Sandberg and Sigurd Tjelmeland Harvest rules and recovery strategies The case of Norwegian spring spawning herring

  2. Background

  3. Agreed harvest control rule

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

  5. 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. Biological model and assumptions

  6. Recruitment = Rmax*SSB/(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.

  7. 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 Economic model and assumptions

  8. The expected NPV, E[NPV] 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

  9. The expected NPV, E[NPV] 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

  10. Question 2: Which ”target fishing mortality would give highest economic revenue?

  11. The expected NPV of the Norwegian harvest of Norwegian spring spawning herring in a 10-year period.

  12. 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. Question 3: Is the agreed harvest rule performing better or worse than a constant escapement strategy in a period of recovery?

  13. Development in spawning stock. Left panel: Target escapement of 5 million tonnes. Right panel: Agreed harvest control rule.

  14. Expected NPV, expected SSB at the end of the period and the probability of SSB<2.5 million tonnes during the period.

  15. When prices are fixed, and adjustment costs in the fleet are zero, target escapement gives higher NPV than the agreed harvest control rule. When prices are decreasing in harvest, this conclusion is reversed. 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 Summing up

  16. 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. Summing up (2)

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