Browsing by Subject "Assessment model"

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    Results for the Reference-case abalone spatial- and age-structured assessment model for Zones A, B, C and D in 2009
    (2009) Brandão, Anabela; Butterworth, Doug S
    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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    Open Access
    Results of sensitivity tests applied to the South African squid stock assessment model, including for harvesting strategies with a constant annual catch component for small scale fishers
    (2012) Glazer, Jean Patricia; Butterworth, Doug S
    Currently a Bayesian approach is applied in assessing the status of the South African squid resource, Loligo reynaudii. The underlying model is biomass-based and a year is split into two time periods to better model the dynamics of the stock and the two fisheries (jig and trawl) that exploit it. This paper reports results from various sensitivity tests conducted on the existing model.