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.     

Temporal behaviour of the thermal model of hard X-ray bursts

A simple, analytic model is presented of a hot (10⁸ K), thermal hard X-ray source, continuously heated, bounded by ion-acoustic conduction fronts, and expanding in a loop. The model is used to investigate the assumption, made in some published comparisons of this model with data, that the rise time of the X-ray emission is approximately given by the loop length divided by the ion-sound speed appropriate to the peak temperature. It is found that a freely-expanding source does not behave in this way; instead, the rise time is symptomatic of the timescale for primary energy release. If the energy release rate does not fall significantly before the source fills the loop, however, then this assumption may be approximately satisfied, if a condition on the temporal behaviour of the energy release is satisfied.Finally, some remarks on the relative timing of temperature and emission measure peaks are made, and possible further applications mentioned of the results presented herein.     

Do fast protons and α particles have the same energy distributions in solar flares?

Nuclear de-excitation γ-ray lines yield diagnostic information on ion acceleration in solar flares. Deductions using these lines of flare site ion distributions generally assume that all accelerated ion species have energy distributions of the same form. Particularly high total energy contents for fast ions result. Here we show how this assumption may be relaxed. Sufficiently precise measurements of a key set of lines may be combined to deduce separately the distributions of the fast protons and α-particles that produce the narrow de-excitation lines. We apply the resulting procedure to sets of line fluences obtained previously from SMM/GRS data. Interpretations of these data appear plausible in which comparatively small numbers of α particles are accelerated into energy distributions significantly harder than those found for protons.     

Turbidite facies in an ancient subduction complex: Torlesse terrane,New Zealand

The Torlesse terrane of New Zealand is an ancient subduction complex consisting of deformed turbidite-facies rocks. These are mainly thick-bedded sandstone (facies B and C) with subordinate mudstone (facies D and E), comparable to inner- and middle-fan deposits of a submarine fan. Strata were deposited in trench-floor and trench-slope settings that received sandy sediment from slope-cutting submarine canyons. The dominance of sandstone suggests that some mudstone may have been selectively subducted. Construction of a detailed sediment dispersal model is not possible because tectonic deformation has largely destroyed original facies relationships and paleocurrent patterns.     

DATA ANALYSIS WITH MINIMAL ASSUMPTIONS

The computer has made it possible to scrutinize data rapidly by means of graphics. This should be doneprior to the application of any model to the data, since the model must be validated before using it asa means of analyzing the data. The procedure is illustrated in terms of two examples of real experimentaldata.     

The measurement of the volatile organic fraction of the TOC in seawater

A method based on a dynamic head-space extraction and high-temperature oxidation of the trapped organic matter is described for the determination of the volatile organic carbon (VOC) concentration in seawater. The VOC concentrations in various geographic regions have been measured and calculated as a fraction of the total organic carbon (VOC/TOC). The VOC concentrations ranged from 15–60 μg C/liter in most oceanic areas. This volatile component was found to be a small fraction (1.5–6%) of the TOC. The VOC concentrations are highest in coastal regions and in the surface zone while the VOC/TOC ratio is relatively constant with depth and location. The volatile organic components may be important to the cycling of organic matter in sea water and the dynamics of this role have been postulated.     

Return current and collisional effects in nonthermal electron beams with pulsed injection

The evolution of a pulse of nonthermal electrons, with a truncated power-law spectrum, injected into a uniformly dense coronal plasma, is studied by the 1-D particle simulation of return current and direct collisional effects. The beam pulse injection profile varies gradually enough for total current neutrality, and a steady-state Ohm's law for the return current, to apply at each point and time. Quasi-linear relaxation of the beam is deliberately ignored in order to isolate the modification of return current effects by pulsed injection.When parameters are such that return current losses dominate, the resulting energetic electron trajectories differ very markedly between the pulsed injection case and the continuous injection case for the same beam current at injection. In general, the energy losses to return current dissipation are lower in the pulsed case. This is due to the rapid fall in the local beam current as the pulse disperses in space. Conditions necessary for the beam to comprise a series of pulses sufficiently separated for dispersion effects to be significant are also discussed.On leave from the Astronomical Institute of the Czechoslovak Academy of Sciences, 25165 Ondejov, Czechoslovakia.     

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.