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John Hampton & Shelton Harley SPC Oceanic Fisheries Programme
Issues with the Bigeye Tuna Recruitment Trend and MSY-Based Stock Status Determination John Hampton & Shelton Harley SPC Oceanic Fisheries Programme
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2010 Base Case Recruitment Estimates
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Equilibrium Recruitment
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Impact on Stock Status Equilibrium recruitment is lower than recent average, higher than historical MSY is << recent catches or the catches that would result at FMSY SB remains near to or above SBMSY even though F >> FMSY for more than 10 years
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Issues Is the estimated recruitment pattern real, e.g. caused by an environmental regime shift? If yes, the issue is: Which regime do we use to characterize equilibrium recruitment for MSY-based stock status determination – early, late or average? If no, and we accept that the trend results from an estimation bias, the issue is: How do we eliminate that bias, and its resulting impacts on MSY-based stock status determination
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1. Accept the Estimated Recruitment Pattern
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1. Accept the Estimated Recruitment Pattern
Issue: What to use to characterize equilibrium recruitment for stock status determination?
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SRR SRR (base case)
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SRR SRR
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Stock Status – SB vs SBMSY
SRR
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Stock Status – SB vs SBMSY
SRR
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“Kobe” Plots
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Management Quantities
Quantity SRR SRR Steepness 0.98 0.87 SBMSY 155,500 t 315,600 t SB2008/SBMSY 1.167 0.496 F2008/FMSY 1.554 1.827 MSY 73,440 t 129,040 t Recent catch 134,200 t
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2. Recruitment Pattern is Due to Estimation Bias
Causes of the estimated recruitment pattern Steps necessary to remove the trend Impact on MSY stock status determination
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Recruitment and PS Catch
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Longline CPUE and PS Catch
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Recruitment Trend Primary cause is relatively flat longline CPUE during and after the period when juvenile PS catch increased The model’s only possible response is to increase recruitment to explain both the increase in juvenile catch while maintaining a flat LL CPUE Noting that LL catchability is assumed to be constant over time and M(age) is fixed
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Pre-1990 LL Effort R3,R4 Removed
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Pre-1990 LL Effort R3,R4 Removed
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Pre-1990 LL Effort R3,R4 Removed
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Management Quantities
Quantity 2010 Base No LL effort R3, R4 Steepness 0.98 0.88 SBMSY 155,500 t 299,000 t SB2009/SBMSY 1.167 0.87 F2009/FMSY 1.554 1.37 MSY 73,440 t 105,000 t Recent catch 134,200 t
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Conclusions If we accept the estimated base case recruitment pattern, should compute MSY-based stock status based on recent phase Restricting SRR estimation window to recent 20 years results in steepness estimate approaching the prior mode, higher equilibrium recruitment, higher SBMSY, lower recent SB/SBMSY and higher MSY Probably a better estimate of current stock status!
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Conclusions If we do not accept the estimated base case recruitment pattern, removing pre-1990 LL effort in R3 and R4 eliminates the 2-phase pattern This also results in steepness estimate approaching the prior mode, higher equilibrium recruitment, higher SBMSY, lower recent SB/SBMSY and higher MSY
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Suggestion Provide SC7 with both the standard base case and the run removing LL effort in R3 and R4 as a key sensitivity analysis For the base case, and assuming that the 2-phase recruitment pattern persists in the 2011 assessment, provide management quantities based on SRRs computed for (a) the whole time series and (b) post-1990
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