Results for the Reference-case abalone spatial- and age-structured assessment model for Zones A, B, C and D in 2009
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2009
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University of Cape Town
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Abstract
A summary is presented of the results obtained from the Reference-case model described by Plagányi (2008) that was fit to Zones A, B, CNP, CP and D in combination (hereafter referred to as the “combined ABCD model”). The full details of the spatial- and agestructured production model (ASPM) are provided in Appendices 1 and 2. The Reference-case model (when the updated and extrapolated “old” CPUE series is used in the model fitting procedure) estimates a pristine spawning biomass, sp B0 (in tonnes) with 90% Hessian-based confidence intervals shown in brackets, of 9 876 (5 985; 13 767), 5 902 (5 449; 6 355), 7 462 (7 177; 7 747) and 10 439 (6 562; 14 316) for Zones A, B, C and D respectively. The 2010 (inshore+offshore) spawning biomasses (and associated 90% confidence intervals) of abalone in Zones A, B, C and D are estimated at ca. 29 % (23%; 35%), 26 % (19%; 32%), 6% (3%; 9%) and 11 % (8%; 14%) respectively of their preexploitation levels. The “nonpoached” CNP and “poached” CP areas of Zone C are estimated at ca. 11 % and 6 % respectively with the inshore region particularly depleted: the model predicts almost no remaining abalone in the inshore area of Zone D. Equivalent estimates for Zones A and B are 15% and 19%. The model estimate of the proportion poached from Zone A is 0.68 (90% Hessian-based confidence interval 0.58 – 0.77). Natural mortality is reasonably estimated (e.g. 0.32 yr-1 for age 0 and 0.13 yr-1 for age 15+) and in Zones C and D, the additional mortality estimated for 0-yr old abalone (due to the ecosystem-change effect) corresponds to near zero current annual survival rates. Poaching is severely impacting the resource, with Zone A particularly impacted in recent years. The combined Zones A-D model-predicted 2009 poaching estimate is 939 MT and corresponds to the assumption that, on average, 14% of all poached abalone are confiscated.
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Reference:
Brandão, A., & Butterworth, D. S. (2009). Results for the Reference-case abalone spatial and age-structured assessment model for Zones A, B, C and in 2009. Marine and Coastal Management document: MCM.