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The environmental impacts of artisanal fishing gear on coral reef ecosystems were studied in the multi-gear fishery of southern Kenya to evaluate which types of gear have the greatest impact on coral reef biodiversity. The gear types studied were large and small traps, gill nets, beach seines, hand lines and spear guns. Levels of coral damage, proportion of juvenile fish and discards, size and maturity stage at first capture were quantified and compared amongst the gear types. Results indicate that fishers using beach seines, spears and gill nets cause the most direct physical damage to corals. Spear fishers showed the highest number of contacts to live corals per unit catch followed by fishers using gill nets (12.6 ± 1.8 and 5.9 ± 2.0 coral contacts per kg fish caught per trip respectively). Apart from discarding 6.5% of their daily catch in the sea, as it was too small, beach seine fishers also landed the highest percentage of juvenile fish (68.4 ± 15.7%), a proportion significantly higher (p < 0.001) than in any other gear. The size and maturity stage at first capture for 150 of 195 species caught by all gear types was well below the lengths at which they mature. For example, 100% of Lethrinus xanthochilus, 99% of Lethrinus nebulosus and 94% of Lethrinus harak caught were juveniles. Across all gear types, 50.1 ± 22.7% of the catch consisted of juvenile fish, indicating serious growth overfishing. Field assessment of levels of coral density showed that fishing grounds where beach seines were still in use had a significantly lower density than where beach seining was not used. This correlation is likely to arise in part because seines cannot be used in the most coral rich areas, and in part because coral loss is a consequence of seine use. On a per gear basis therefore, beach seines had the most impact on coral reef biodiversity. This study emphasizes the need to enforce restrictions on destructive gear and mesh sizes. 相似文献
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海洋地震勘探过程中,水中放置的空气枪与船体之间须保持一定安全距离,否则引起船载设施和仪器剧烈抖动,影响船舶设施和仪器的安全运行,甚至更严重情况下,有可能造成船体的损坏.利用海水压力脉冲与船体受到的冲击压力关系,结合建立的空气枪震源子波模型模拟近场震源子波,可计算出船体与空气枪间的安全距离.研究结果表明,根据震源子波模型模拟的近场子波主峰值,总容量150in3G枪、300 in3相干G枪及760 in3相干G枪的安全距离分别为3.90、6.57 m和9.13 m.由此可知,空气枪与船体间距离只有大于安全距离,才能避免船体大的振动或设备免遭损害,保证海洋地震勘探有效、安全的进行. 相似文献
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阿拉斯加德纳里国家公园公路沿线冰椎和融化失稳问题的减灾备选方案 总被引：1，自引：0，他引：1
TedS.Vinson 《冰川冻土》2005,27(1):17-32
Only one access road leads into Denali Park. The serviceability and safety of this gravel road is obviously of paramount importance to the National Park Service (NPS). Since the late 1950s and mid1960s major icings and a landslide, respectively, have occurred along the Denali Park access road. During the summer of 1990 the landslide activity intensified. The central section of the Park through which the access road traverses is designated as a wilderness area. Consequently, off road field exploration required to quantify the hazards and remediation activities that may be proposed to mitigate icings and stabilize the landslide, are severely restricted and closely scrutinized by the NPS. The results of an evaluation of (1) the current state-of-the-practice to control icings, and (2) thaw stabilization techniques that could be appiled to the northwest corner of the landslide are presented herein. The recommendations which followed, respecting the wilderness area designation for the Park, are also presented. 相似文献
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海洋天然气水合物三维地震与海底地震勘探中的震源技术研究 总被引：2，自引：0，他引：2
三维地震与海底地震勘探技术愈来愈广泛应用于海洋天然气水合物调查中.为了获取高品质的纵波、转换横波等地震信息,揭示天然气水合物地层的速度结构异常,地震震源是决定调查成功与否的关键技术之一.本文对激发频宽、输出、气泡效应等震源特性及组合技术进行了综合研究,设计了一种新型的GI枪点震源系统,并于2006-2009年期间在南海北部某海域进行一系列试验.试验效果的综合对比表明:震源优化技术的应用明显提高了地震纵波的地层穿透深度,并改善了海底地震仪(OBS)纵波及转换横波的接收效果. 相似文献
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The welding was one of the technologies to manufacture manned cabin, which was a significant part of the submersible. To improve the procedure, horizontal EBW was employed on TC4 ELI alloy with 60 mm thickness, and the microstructures and properties of the joints were investigated. The crystals and microstructures of the welds were homogeneous from the top to bottom, and the microhardness of the weld was equivalent, which was harder than that of the base metal. The tensile properties of the joints were homogeneous along the penetration. The tensile strength and yield strength of the joints were higher than those of the base metal, and the percentage elongation after fracture was 80% of the base metal, which indicated the strength mis-match as overmatching. The impact toughnesses of the welds were equivalent along the penetration, which reached 80% of the base metal. The joint specimens were bended to 180° with no cracks. The investigation verified the excellent procedure and properties of horizontal EBW, which would make China become one of the countries with the capacity of manufacturing submersible cabin. 相似文献
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In order to study the excitation of large-volume airgun source with different combinations in Hutubi, Xinjiang,China,we conducted a targeted experiment. The method of time-frequency analysis is used to study the signals recorded by a seismometer on the shore of the excited pool, and it is concluded that different gun combinations will lead to different frequency of bubble pulse signals. Besides, linear combination method is used to analyze the signal-to-noise ratios of signals excited by different gun combinations which was recorded by seismic stations around the airgun source. In order to improve the signal-to-noise ratios, it is more effective to increase the activation energy (the number of excited guns at the same time) than to stack the excited signals with smaller energy repeatedly. 相似文献