1. 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

2. 2010 Base Case Recruitment Estimates

3. Equilibrium Recruitment

4. 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

5. 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

6. 1. Accept the Estimated Recruitment Pattern

7. 1. Accept the Estimated Recruitment Pattern
Issue: What to use to characterize equilibrium recruitment for stock status determination?

8. SRR
SRR (base case)

9. SRR
SRR

10. Stock Status – SB vs SBMSY
SRR

11. Stock Status – SB vs SBMSY
SRR

12. “Kobe” Plots

13. 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

14. 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

15. Recruitment and PS Catch

16. Longline CPUE and PS Catch

17. 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

18. Pre-1990 LL Effort R3,R4 Removed

19. Pre-1990 LL Effort R3,R4 Removed

20. Pre-1990 LL Effort R3,R4 Removed

21. 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

22. 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!

23. 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

24. 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