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1.
Comparison of Drags on Fish Cages of Different Shapes 总被引:1,自引:0,他引:1
1 .Introduction Commercial fishfarming in net cages is becoming widespread around the world as an importantsource of food.In PRChina ,mostfishfarmingtakes placeincalmcoastal areas .Fishcages work wellat these sites becausethe environmental forces are usua… 相似文献
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本文介绍了车载式GPS道路修测系统的基本原理和体系结构,详细讲述了该系统在“中国公路网GPS测绘工程”的应用。“中国公路网GPS测绘工程”是由多家部门共同参与的大型道路测绘项目,目的是解决现有公路网数据库现势性差的问题。文中结合“中国公路网GPS测绘工程”重点阐述了修测系统中几个关键技术的实现方法,最后给出了修测系统在该工程中的使用效果。 相似文献
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QITI GUO 《Journal of Metamorphic Geology》1984,2(4):267-295
Multisystems of n+k (k > 3) phases are very complicated and knowledge of them has suffered as a result. The successful solution of the topological relationships in n+ 3 phase multisystems by Zen (1966, 1967) and Zen & Roseboom (1972) has aroused much interest regarding what will happen in a multisystem of more than n+ 3 phases. Since 1979, some important research results on this topic have been published. These results have expounded the substantial rules governing the appearance of phase relations in phase diagrams of n - k (k > 3) phase multisystems. The most significant conclusions include: (1) It is impossible to incorporate all the possible phase relations in an n+k (k > 3) phase multisystem in a single closed net. Therefore, it is no longer enough to use only a single closed net to depict the topological relations involved in these types of multisystems. Instead, one or more groups of closed nets, namely the complete system(s) of closed nets are necessary for this purpose. (2) A principle called the Combination Principle has been proposed and proved. It states: Any closed net of one n+k (k > 3) phase multisystem must be a combination of two or more distinct n+ 3 order submultisystem closed nets belonging to the given n+k phase multisystem, if it is not one of the n+ 3 order submultisystem closed nets itself. The combination principle provides both a theoretical basis and a practical method for the construction of closed nets and, hence, for the derivation of the real phase diagrams for any n+k (k > 3) phase multisystem. (3) A theorem on divariant-assemblage-characteristic-stability-polygons is also important to our understanding of the n+k (k± 3) phase multisystem closed nets. This theorem can be stated as follows: A divariant assemblage of an n+k (k± 3) phase multisystem will be stable in an l-polygon lacking diagonals in an appropriate set of closed-net-diagrams, and this l-polygon may be at least a triangle, and at most a k-polygon. In addition, the closed-net-diagrams of unary and binary n+ 4 phase multisystems derived respectively by Guo (1980b, 1980c, 1981a) and by Roseboom & Zen (1982) have also been summarized. The combination principle is applied to a practical petrological problem in this paper, dealing with 7 phases in the system FeO-Fe2O3-SiO2. 相似文献
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The current study provides long-term catch-rate, biological and feeding data for smooth hammerhead sharks, Sphyrna zygaena, caught in South Africa’s KwaZulu-Natal bather protection programme. In total, 2 512 S. zygaena were caught in net installations between 1978 and 2014, and 72 S. zygaena were caught on drumlines between 2007 and 2014. There was no significant log-linear year trend in the net catch rate over time (slope = 0.0054, t = 1.808, p = 0.07). However, there was a significant temporal increase in mean size of the captured sharks (slope = 0.0012, t = 3.502, p < 0.001). A quasi-Poisson generalised additive mixed model showed that increasing latitude, winter months, colder sea temperatures and the deployment of drumlines all had a significant positive effect on the catch rate of sharks in nets. The size frequency of the catch was unimodal, with significantly more females caught in the nets and more males on the drumlines. The majority (93.1%) of all sharks caught were immature and measured between 80 and 120 cm precaudal length. Teleosts and cephalopods dominated the sharks’ diet in terms of all dietary indices. The prey species consumed indicate that immature S. zygaena are feeding primarily within the pelagic zone of shallow coastal habitats. 相似文献
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Deborah L. Elliott-Fisk 《自然地理学》2013,34(4):299-323
The White Mountains, astride the California-Nevada stateline, are the highest and westernmost of the Great Basin ranges. This range was extensively glaciated during the Quaternary Period. Glacial landforms and scattered erratics were identified in the field using primarily morpho-and lithostratigraphic criteria, and mapped on aerial photographs and topographic maps. Topographic characteristics of the glacial deposits were analyzed using standard statistical procedures. A sequence of glacial deposits was identified in terms of six glacial stages, these glaciations named according to type site, and relative ages inferred. With the exception of perched Stage I (early) deposits along the range crest, reconstructed equilibrium-line altitudes and elevation of the glacier termini increase to the present, with glacier length and inferred size decreasing through time. Preliminary data suggest that weathering and pedogenesis are also progressive, though environmental gradients mask some of these distinctions between deposits. It is hypothesized that the White Mountains have a similar glacial chronology to the adjacent Sierra Nevada, but that with the intensification of the Sierran rainshadow during the Quaternary, the extent of glaciation in the White Mountains apparently decreased through time owing to regional tectonic uplift. This had led to the preservation of a more complete sequence of glacial deposits than in the Sierra Nevada, making the delineation of multiple mid-Quaternary events possible in many valleys. Further radiometric and chronometric dating of these deposits is in progress. [Key words: Glaciation, Quaternary, glacial geomorphology, White Mountains, California, Nevada. 相似文献
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Abstract A theoretical explanation is advanced consisting of a five stage process for the formation of polygonal ground which consists of stone borders forming regular hexagons and soil centres. One of these stages, namely the onset of convection in a porous soil between temperatures of 0°C and approximately 4-6°C, is studied analytically. Darcy's law is employed but variable permeability is allowed for and a parabolic density dependence on temperature is assumed. It is found that the theoretical predictions of the aspect ratio agree very well with field studies when a constant upper surface heat flux condition is imposed and an upwardly stratified permeability is chosen. Field study data, which agree very well with the theory, are reported in detail. 相似文献
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Recruitment of early life stages into estuaries is an integral part of the life cycle of many marine fish species. Although estuaries are naturally environmentally dynamic, they also are subject to anthropogenic disturbances, including land use and climate change, which may affect recruitment. Rhabdosargus holubi is an endemic marine-spawning species predominantly associated with freshwater-rich estuaries which serve as nursery areas for postflexion larvae and juveniles. This study assessed the effect of environmental variables on the dynamics of recruitment of R. holubi larvae and juveniles into the Swartkops and Sundays estuaries, South Africa. Over a period of two years, fyke nets were set at each estuary mouth to monitor movement into the estuaries at each tidal phase over a 24 h cycle during two sampling sessions per season. Rhabdosargus holubi larvae recruited into estuaries primarily in summer and autumn and during the ebb tide at night, while juvenile movements showed no pattern. Salinity, turbidity and temperature were significantly important factors affecting R. holubi recruitment, with pH having no significant effect. Turbidity and salinity are affected by rainfall and freshwater abstraction which may thus influence recruitment. Rhabdosargus holubi is dependent on estuaries, so a combination of future changes in turbidity, salinity and temperature due to predicted climate change may place populations at risk. 相似文献