Abalone spatial- and age-structured assessment model preliminary results for Zones A, B, C and D in 2008

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University of Cape Town

A summary is presented of the results obtained from the 2008 Reference-case model 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 age-structured production model (ASPM) are provided in previous documents (Plagányi and Butterworth 2007). The 2008 base-case model uses an updated CPUPE index, and differs from last year’s base-case in estimating one more parameter for Zone A (the historic catch multiplier). The results of an “Alternative Policing Efficiency index” model version are also presented, based on the “Pedro case” series as described in document SWG-AB17. Model base-case results estimate a pristine spawning biomass, B sp 0 (in tonnes), of 8185, 5735, 6765 and 9064 for Zones A, B, C and D respectively. The current (inshore+offshore) spawning biomasses of abalone in Zones A, B and D are estimated at ca. 33 %, 35 % and 13 % respectively of their pre-exploitation levels. The “nonpoached” CNP and “poached” CP areas of Zone C are estimated at ca. 5 % and 4 % respectively with the inshore region particularly depleted: the model predicts zero remaining abalone in the inshore CNP, CP and Zone D areas. Equivalent est8imates for Zones A and B are 16% and 32%. Natural mortality is reasonably estimated (e.g. 0.33 yr-1 for age 0 and 0.14 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 2008 poaching estimate is 580 MT and corresponds to the assumption that, on average, 23% of all poached abalone are confiscated.