The exploitation of Upogebia africana (Crustacea: Thalassinidae) for bait in the Knysna Estuary

The activities of people collecting Upogebia africana for bait at six popular collecting sites in the Knysna Estuary were monitored from February 1995 until April 1996. Three groups of bait harvesters were identified on the basis of their source of income: subsistence fishers who rely on bait collecting and fishing for their income; supplementary fishers who catch fish to supplement their income; leisure anglers who draw no income from fishing. Two groups of collectors were identified based on methods of collecting bait and fishing: leisure anglers who collect bait using a prawn pusher or pump and fish using a rod and tackle; non-leisure fishers who collect using tin cans and fish with hand or planted lines. The average harvest of bait per collecting trip by leisure anglers was 59 mud prawns, whereas non-leisure fishers took 101 animals, twice the legal limit. The numbers of bait collectors present per mud bank were found to be highest on public holidays ([horizontal bar over]X = 43.5) and higher during the summer holidays ([horizontal bar over]X = 16.5) than during the spring/summer ([horizontal bar over]X = 8.6) and autumn/winter ([horizontal bar over]X = 4.6) periods. Most collectors spent 11-30 minutes on the mud banks. It was estimated that 1.86 x 10[superscript 6] U. africana or about 740 kg (dry mass) was removed by bait collectors annually from the six bait-collecting sites studied. This represented about 8.5% of the mud prawn stocks at these sites and about 0.9% of the entire estuary stock. 85% of the mud prawns taken as bait was removed by 77% of the bait collectors who were the non-leisure fishers

An estimation of the standing stock and population structure of Upogebia africana (Crustacea: Thalassinidae) in the Knysna Estuary

Within the Knysna Estuary Upogebia africana has an extensive distribution from north of Leisure Isle to the Red Bridge, occupying 62% of the available intertidal zone. Six sites were surveyed within the above distributional range. U. africana was found from the high-water level (Spartina zone) to the shallow subtidal zone (to about 1.5 m depth). The density, biomass and population structure of the mud prawns were found to be highly variable within the estuary. Only small animals (carapace length –× = 10.9 mm) were found in the upper estuary at the Red Bridge. Although large animals were present, the Invertebrate Reserve had a low density and biomass of U. africana (– × = 11.7 m–2; 3.9 g.m–2 dry weight). By contrast a relatively inaccessible centre mud bank (Oyster Bank) in the middle reaches of the estuary had much larger populations (–× = 176.5 m–2; 65.3 g.m–2 dry weight). This site is, therefore, a natural mud prawn reserve. Within the intertidal zone, mud prawn density (74–76 m–2) and biomass (26–27 g.m–2 dry weight) was usually greatest in the Spartina and lower Zostera zones. Sex ratios throughout the estuary did not deviate significantly from 1:1

Orientational Ordering and Dynamics of Rodlike Polyelectrolytes

The interplay between electrostatic interactions and orientational correlations is studied for a model system of charged rods positioned on a chain, using Monte Carlo simulation techniques. It is shown that the coupling brings about the notion of {\em electrostatic frustration}, which in turn results in: (i) a rich variety of novel orientational orderings such as chiral phases, and (ii) an inherently slow dynamics characterized by stretched-exponential behavior in the relaxation functions of the system.Comment: 7 pages, 10 figure

Learning From Early Attempts to Generalize Darwinian Principles to Social Evolution

Copyright University of Hertfordshire & author.Evolutionary psychology places the human psyche in the context of evolution, and addresses the Darwinian processes involved, particularly at the level of genetic evolution. A logically separate and potentially complementary argument is to consider the application of Darwinian principles not only to genes but also to social entities and processes. This idea of extending Darwinian principles was suggested by Darwin himself. Attempts to do this appeared as early as the 1870s and proliferated until the early twentieth century. But such ideas remained dormant in the social sciences from the 1920s until after the Second World War. Some lessons can be learned from this earlier period, particularly concerning the problem of specifying the social units of selection or replication

A physically-motivated quantisation of the electromagnetic field on curved spacetimes

Recently, Bennett et al. [Eur. J. Phys. 37:014001, 2016] presented a physically-motivated and explicitly gauge-independent scheme for the quantisation of the electromagnetic field in flat Minkowski space. In this paper we generalise this field quantisation scheme to curved spacetimes.Working within the standard assumptions of quantum field theory and only postulating the physicality of the photon, we derive the Hamiltonian, H, and the electric and magnetic field observables, E and B, without having to invoke a specific gauge. As an example, we quantise the electromagnetic field in the spacetime of an accelerated Minkowski observer, Rindler space, and demonstrate consistency with other field quantisation schemes by reproducing the Unruh effect

Nucleon-nucleon cross sections in neutron-rich matter and isospin transport in heavy-ion reactions at intermediate energies

Nucleon-nucleon (NN) cross sections are evaluated in neutron-rich matter using a scaling model according to nucleon effective masses. It is found that the in-medium NN cross sections are not only reduced but also have a different isospin dependence compared with the free-space ones. Because of the neutron-proton effective mass splitting the difference between nn and pp scattering cross sections increases with the increasing isospin asymmetry of the medium. Within the transport model IBUU04, the in-medium NN cross sections are found to influence significantly the isospin transport in heavy-ion reactions. With the in-medium NN cross sections, a symmetry energy of $E_{sym}(\rho)\approx 31.6(\rho /\rho_{0})^{0.69}$ was found most acceptable compared with both the MSU isospin diffusion data and the presently acceptable neutron-skin thickness in $^{208}$Pb. The isospin dependent part $K_{asy}(\rho _{0})$ of isobaric nuclear incompressibility was further narrowed down to $-500\pm 50$ MeV. The possibility of determining simultaneously the in-medium NN cross sections and the symmetry energy was also studied. The proton transverse flow, or even better the combined transverse flow of neutrons and protons, can be used as a probe of the in-medium NN cross sections without much hindrance from the uncertainties of the symmetry energy.Comment: 32 pages including 14 figures. Submitted to Phys. Rev.

Isospin splitting of the nucleon mean field

The isospin splitting of the nucleon mean field is derived from the Brueckner theory extended to asymmetric nuclear matter. The Argonne V18 has been adopted as bare interaction in combination with a microscopic three body force. The isospin splitting of the effective mass is determined from the Brueckner-Hartree-Fock self-energy: It is linear acording to the Lane ansatz and such that $m^*_n > m^*_p$ for neutron-rich matter. The symmetry potential is also determined and a comparison is made with the predictions of the Dirac-Brueckner approach and the phenomenological interactions. The theoretical predictions are also compared with the empirical parametrizations of neutron and proton optical-model potentials based on the experimental nucleon-nucleus scattering and the phenomenological ones adopted in transport-model simulations of heavy-ion collisions. The direct contribution of the rearrangement term due to three-body forces to the single particle potential and symmetry potential is discussed.Comment: 8 pages, 10 figure

The application of ultrasonic NDT techniques in tribology

The use of ultrasonic reflection is emerging as a technique for studying tribological contacts. Ultrasonic waves can be transmitted non-destructively through machine components and their behaviour at an interface describes the characteristics of that contact. This paper is a review of the current state of understanding of the mechanisms of ultrasonic reflection at interfaces, and how this has been used to investigate the processes of dry rough surface contact and lubricated contact. The review extends to cover how ultrasound has been used to study the tribological function of certain engineering machine elements