Geostatistical modelling of the spatial life history of post-larval deepwater hake Merluccius paradoxus in the Benguela Current Large Marine Ecosystem

Optimal and sustainable management of fish resources cannot be ensured without a thorough understanding of the migration patterns and population (demographic stock) structure. Recent studies suggest that these aspects of the economically and ecologically important deepwater hake Merluccius paradoxus are not reflected in the current assessment and management practices for the Benguela Current Large Marine Ecosystem. In this study, we compiled data from multiple demersal trawl surveys from the entire distribution area and applied state-of-the-art geostatistical population modelling (GeoPop) to estimate growth rate, mortality, and spatial and temporal distribution patterns of M. paradoxus. The data and the model enabled us to follow temporal and spatial changes in the distribution and infer movements from the recruitment/nursery areas, through the juvenile phase and the adults’ migration to the spawning areas outside/upstream of the nursery areas. The results indicated one primary recruitment/nursery area on the west coast of South Africa and a secondary less-productive recruitment/nursery area on the south coast near Port Elizabeth. Juveniles initially migrated away from the main recruitment area, followed by natal homing by larger individuals. This pattern was highly consistent through the time-series of the study. This perception of a, primarily, panmictic population that performs transboundary migrations between Namibia and South Africa corresponds largely to the hypothesis and data plots given in recent studies. We recommend that fisheries assessment, advice and management take into consideration these aspects of the distribution and population (stock) structure of M. paradoxus.     

On the bremsstrahlung efficiency of nonthermal hard X-ray source models

It has often been stated, but never rigorously proven, that interpreting observed hard X-ray emission in terms of a thick-target source gives a lower limit to the flux of electrons which have to be injected into the source. The truth of this statement, for theburst-integrated emission, is rigorously established here. Also an explicit inversion for the injected electron flux in terms of the photon spectrum is given, for the case where all electrons traverse a single value of column density. This generalises the previous results for the thick- and thin-target limits.The use of the standard thick-target formalism for the interpretation of instantaneous (as opposed to burst-integrated) photon fluxes is also discussed. By considering the specific case of the thick-target trap model, it is shown that use of this formalism can either under-estimate or over-estimate the injected electron flux, at different times in the same event, but that integration of the inferred electron fluxes over the event nevertheless yields the true, burst-integrated electron flux.