A review of the assessments of the major fisheries resources in South Africa

The waters off South Africa's coastline boast a rich mix of commercially fished species. Quantitative assessments of these marine resources have developed from simple methods first applied in the 1970s, to models that encompass a wide range of methodologies. The more valuable resources have undergone regular assessments in recent decades, with frequencies closely related to the management approach employed for each fishery. Many of these assessments form the operating models used to simulation-test candidate management procedures. This paper provides a comprehensive review of the assessments of 11 of the most important fisheries resources in South Africa. Some assessments use simple biomass dynamics models, whereas others are a hybrid of age- and length-based models, each designed to model the specific characteristics of the resource and fishery concerned. Many of the assessments have been disaggregated by species/stock and/or area as related multispecies/stock/ distribution hypotheses have arisen. This paper explores the similarities and differences in the data available and the methods applied. The review indicates that, whereas the status of three of these resources cannot be estimated reliably at present, the status of six resources is considered to be reasonable to good, whereas that of abalone Haliotis midae and West Coast rock lobster Jasus lalandii remains poor.     

Proposal to Use MAGDAS/CPMN to Study the Equatorial Electrojet: A Philippine Contribution to the International Heliophysical Year

As a Philippine contribution to the International Heliophysical Year, we propose to use the MAGnetic Data Acquisition System/Circum Pan-Pacific Magnetometer Network (MAGDAS/CPMN), installed by the Space Environment Research Center (SERC), Kyushu University along the 210° magnetic meridian and the magnetic equator, to study the equatorial electrojet (EEJ) and counter electrojet (CEJ). Through this installation, it is made possible to observe geomagnetic field variations in real time. By utilizing this network of ground-based instruments, we hope to elucidate their regular day-to-day and seasonal variabilities and variations during magnetic storms and substorms. We also want to study the behavior of this ionospheric current system before, during, and after the occurrence of an earthquake.     

鲁西新太古代济宁群含铁岩系形成时代——SHRIMP U-Pb锆石定年

济宁群以隐伏的地质体分布于华北克拉通鲁西地区,以往被划归古元古代。近年来在济宁群的钻探和地球物理研究中,发现存在较大规模条带状硅铁建造,其形成时代引起地学界极大关注。本文对济宁群变质碎屑沉积岩和长英质火山岩进行了锆石SHRIMP年龄测定。含砾绿泥绢云千枚岩的碎屑锆石年龄主要集中在～2.7Ga,可靠的最年轻碎屑锆石年龄为2.61±0.01Ga。变质长英质火山岩岩浆锆石年龄为2.56±0.02Ga。研究表明,济宁群形成于新太古代晚期,而不是以往认为的古元古代。华北克拉通条带状铁建造主要形成于新太古代晚期。济宁群在岩石组合、变质作用方面与鞍山、冀东以及五台地区同时代含硅铁建造的表壳岩系有差别,但是与本溪南芬铁矿的岩石组合有相似之处。这些可能为我国的隐伏太古宙BIF铁矿的研究与勘探提供了重要依据。     

The structure and sequence of geological events in the Basement Complex of the Ibadan area,Western Nigeria

The Basement Complex in Western Nigeria in general, and in the Ibadan area in particular, is composed primarily of a banded gneiss in which hornblende-biotite rich bands alternate with quartz-oligoclase rich bands. The banded gneiss, which originated as part of a sedimentary sequence, contains large lenses of granite gneiss and thin intercolated layers of quartzite and amphibolite.Two distinct major structural events can be clearly identified in the early geological history of the Ibadan area. Detailed field studies suggest that the formation of the Ibadan Granite Gneiss, which had yielded an Eburnean Rb-Sr isochron age, was associated with the later of these events. Five phases of dyke or vein formation, two of which pre-date the formation of the Granite Gneiss, have also been identified giving an overall sequence of geological events the first of which may correspond to the beginning of the Liberian orogeny, around 3000 m.y. ago, and the last of which reflects the waning of the Pan-African thermo-tectonic event about 500 m.y. ago.     

Deformation of the African plate as a consequence of membrane stress domains generated by post-Jurassic drift

As plates move across the surface of the earth they change their curvature in response to the changing curvature of the earth and as a consequence membrane stresses are generated. At any particular point within a plate the stress generated will be proportional to the change in radius of curvature at that point.The rate of change of curvature of the geoid is latitude dependent, consequently the calculation of latitude-generated stress (σ_γ) is relatively straightforward. For a point moving from an initial latitude of γ₀ to a latitude of γ:σ_λ∝ (sin²λ₀?sin²λ)By determining the stress generated at a sequence of points within a plate during set time intervals, membrane stress domains can be defined and used to predict the way in which a plate would deform.In West Africa two phases of compression (Coniacian-Santonian and Palaeocene-Eocene) have been observed within the Lower Benue Rift, followed by a phase of extension (starting about 25 m.y. ago and continuing to the present day) across the Cameroun Volcanic Line. Membrane stress domain theory offers an explanation not only for the timing of all three phases of deformation but is also closely in agreement with the best available information for the extent of deformation.The Red Sea Rift and the rift between Madagascar and the rest of the African continent were both initiated in domains of very high membrane stress generation.     

Constraints on the age of Lake Nyos, Cameroon

The upper 40 m of Lake Nyos are retained by a weak natural dam which, if it were to fail, would not only devastate the area hit by the 1986 gas disaster but would also cause a serious flood to surge down the Katsina Ala into Nigeria. The age of the pyroclastic cone, of which the dam is the last remnant, is therefore of great practical importance to the people of the area. If the pyroclastic cone is only a few hundred years old, as some have suggested, then it is eroding away quickly and the dam must surely fail in the very near future. If, on the other hand, it is many thousands of years old, then there is less immediate cause for concern.The age of the pyroclastic cone can be constrained in three ways:(1) Two samples of basalt, one from the dam itself and one from a lava flow which post-dates the pyroclastic cone, have both yielded K–Ar ages in excess of 100,000 years.(2) Photographic evidence indicates that there has been no detectable change (>2 m) to the width of the dam since 1958. This constrains the average erosion rate and suggests that the pyroclastic cone is at least 4000 years old.(3) Cores from sediment deposited after the level of a small lake to the northeast of Lake Nyos was raised by a debris slide from the pyroclastic cone, contain no volcanic ash. A sample from the base of this sequence has yielded a radiocarbon age of 2700 years.The Lake Nyos dam must therefore be, at the very least, a few thousand years old and although its general stability must give serious cause for concern there is no reason to suspect that the rate at which it is currently eroding away is of itself sufficient to pose an immediate threat.