FISHERIES_2020_MAR_SWG-DEM_03.pdf (700.43 kB)

Simple variants of the SA hake Reference Case Operating Model (assessment) to take account of Namibian catches of M. paradoxus

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posted on 05.02.2021, 10:38 by Doug Butterworth, Andrea Ross-Gillespie
Variants of the SA hake Reference Case Operating Model (RC OM) which take account of Namibian catches in a manner that corresponds to the extreme scenario of demographic panmixia of M. paradoxus between the SA and Namibian regions are evaluated. These are based on the Namibian data currently available, and make a range of assumptions about, for example, the species split of the historical Namibian hake catches. A key result is that allowing for the possibility that there is sharing of the M. paradoxus resource between South Africa and Namibia results in an estimated status (current to pristine spawning biomass ratio) for that species which is (often considerably) better than indicated by the assessment of SA hake in isolation (in the RC OM). This result appears reasonably robust, given the panmictic assumption. For less complete mixing, similar results would be expected, though the quantitative improvement for the SA M. paradoxus “component” compared to the RC OM result would be less. These results indicate that the assumptions underlying the analyses of Butterworth and Rademeyer (2020) are conservative, and serve to strengthen their conclusion that that the SA hake OMP2018 is sufficiently robust to secure avoidance of the adverse consequences (in resource conservation terms) which could result given a M. paradoxus stock which may be demographically shared (to some extent) with Namibia.

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Department of Mathematics and Applied Mathematics, University of Cape Town

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Marine Resource Assessment and Management Group (MARAM)

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