Reference-case 2008 assessment model for abalone in Zones A, B, C and D

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University of Cape Town

A summary is presented of the results obtained from the 2008 Reference-case model and two variants 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 Appendices 1 and 2. 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 three model versions presented in Table 1 are as follows: Model a) Concern was expressed by the AWG that the CPUPE trend for Zone B declines too steeply in recent years. This may be attributable to an incorrect partitioning of confiscated abalone between Zones A and B. Rather than estimating the amount poached in Zone B in recent years, this model combines the estimates of the amount poached from Zones A and B and then estimates a parameter that describes the proportion of this total that is taken from Zone A versus Zone B from 2000 onwards. The model estimated proportion poached from Zone A is 0.65 [90% Hessian-based confidence interval 0.55 – 0.75. This model version has been refined slightly from a preliminary version presented to the AWG in that parameter estimates for Zone A now result in an improved fit. Model b) The old Reference case model is similar but estimates the Zone B poaching amount (in numbers), that is assume to apply from 2005-2008. Model c) This model version used a single compartment per zone, rather than assuming inshore and offshore model regions. This model had the worst AIC value. The new Reference Case Model estimates a pristine spawning biomass, (in tonnes) with 90% Hessian-based confidence intervals shown in square brackets, of 9760 [6060; 13460], 5840 [5400; 6280], 7290 [7050; 7530] and 9650 [6440; 12850] for Zones A, B, C and D respectively. The current (inshore+offshore) spawning biomasses (and associated 90% confidence intervals) of abalone in Zones A, B, C and D are estimated at ca. 33 % [26%; 39%], 25 % [19%; 30%], 4% [1%; 6%] and 12 % [8%; 16%] respectively of their preexploitation levels. The “nonpoached” CNP and “poached” CP areas of Zone C are estimated at ca. 6 % and 3 % respectively with the inshore region particularly depleted: the model predicts zero remaining abalone in the inshore CNP, CP and Zone D areas. Equivalent estimates for Zones A and B are 21% and 22%. 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 Bsp 0 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 860 MT and corresponds to the assumption that, on average, 14% of all poached abalone are confiscated.