首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   368篇
  免费   21篇
  国内免费   51篇
测绘学   16篇
大气科学   12篇
地球物理   20篇
地质学   40篇
海洋学   298篇
综合类   28篇
自然地理   26篇
  2023年   6篇
  2022年   12篇
  2021年   9篇
  2020年   15篇
  2019年   11篇
  2018年   11篇
  2017年   10篇
  2016年   20篇
  2015年   7篇
  2014年   12篇
  2013年   31篇
  2012年   13篇
  2011年   5篇
  2010年   13篇
  2009年   12篇
  2008年   20篇
  2007年   39篇
  2006年   28篇
  2005年   23篇
  2004年   16篇
  2003年   20篇
  2002年   14篇
  2001年   7篇
  2000年   14篇
  1999年   11篇
  1998年   10篇
  1997年   11篇
  1996年   13篇
  1995年   4篇
  1994年   6篇
  1993年   5篇
  1992年   8篇
  1990年   1篇
  1989年   2篇
  1987年   1篇
排序方式: 共有440条查询结果,搜索用时 16 毫秒
41.
2018年4月在“福建省长乐海蚌资源增殖保护区”及其邻近海域布设31个站位进行底拖网调查取样,共鉴定渔获物45种,其中甲壳类21种、贝类14种、鱼类10种。西施舌(Mactra antiquata)的站位出现率为38.71%。站位的质量密度平均为975.68 kg/km2(0~23 384.53 kg/km2),个数密度平均为2.983×104个/km2(0~59.607×104个/km2)。西施舌种群在渔获物中的IRI值高达 1 556.5 \[仅次于截形白樱蛤(Psammacoma gubernaculum)],为优势种,表明西施舌在保护区生物资源组成中占有重要的地位;保护区内西施舌的站位出现率及质量密度明显高于区外,表明西施舌资源主要集中分布于保护区范围内;西施舌种群的年龄组分布较广,年龄结构较为合理。大部分西施舌均达到性成熟年龄和大小,表明资源具有可持续发展的潜力。与2006年和2012年的调查结果相比,保护区2018年西施舌资源比2012年有显著提高,分布范围亦有明显扩大,但尚未恢复到2006年的总体水平。综上可以看出,西施舌资源保护区范围总体界定合理,西施舌资源保护显见成效;“福建省长乐海蚌资源增殖保护区”范围内不同区块实行不同程度的禁捕、限捕管控,2011年以来累计放流西施舌种苗253.07万粒,对于西施舌资源恢复和养护管理是有效的举措。  相似文献   
42.
华贵栉孔扇贝两种壳色群体生长和消化酶活力比较   总被引:2,自引:0,他引:2  
以华贵栉孔扇贝桔黄和紫褐壳色个体为亲本建立两个壳色群体,从壳长、壳高、壳宽和体重几方面进行生长比较,并对两个群体做了淀粉酶和纤维素酶活力比较。结果表明:桔黄壳色群体的平均壳长、平均壳高、平均壳宽和平均体重显著大于紫褐壳色群体(P<0.05)。在24℃、27℃、30℃和33℃条件下,桔黄壳色群体的淀粉酶和纤维素酶活力显著高于紫褐壳色群体(P<0.05);不同壳色群体生长与其淀粉酶和纤维素酶活力有关。  相似文献   
43.
基于部件可靠度和多阶段动态库存理论,分析了备件需求与库存管理。提出了基于部件可靠度的动态需求模型;以部件的寿命服从指数分布为例,结合实际生产中备件的使用情况,通过确定备件储备的时间点,动态确定各周期备件的需求量。在确定备件动态需求量的基础上,采用多阶段动态库存理论确定了订货次数和每次订货数量,使库存量更符合实际和趋于合理。通过算例分析,给出了最小库存费用,表明基于部件可靠度和多阶段动态库存的备件管理具有一定的实际意义。  相似文献   
44.
于2013-2014 年在三沙湾选择增殖放流区Ⅰ区和Ⅱ区, 开展了2 批次的缢蛏增殖放流, 计 放流缢蛏苗数量约3747 万粒。采用对缢蛏生长、生物量动态变化和滩涂底质主要因子跟踪监测的方 法, 开展对缢蛏增殖放流效果的评估。研究结果, 2 个增殖放流区的缢蛏密度和缢蛏生物量都比对照 区有极显著提高, 共收获65 吨缢蛏大规格苗, 增殖放流产出比为1: 8.0-9.0, 获得了一定的资源增 殖效果和经济效益。放流期间, 滩涂底泥中的氧化还原电位、硫化物和有机碳等浓度在增殖区Ⅰ区、 Ⅱ区和对照区之间都无显著差异(P>0.05), 但, 底泥中Ⅰ区、Ⅱ区的氧化还原电位平均值分别比对照 区提高了26%和18%, 硫化物浓度平均值分别比对照区降低了45%和34%, 有机碳浓度平均值分别 比对照区下降了5.8%和6.8%, 表明, 缢蛏增殖放流获得了一定的改良底质环境效果。  相似文献   
45.
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.  相似文献   
46.
Measuring and forecasting recruitment are central to the understanding and management of fish stocks. Kainge et al. (2013) studied the effect of spawning stock size and environmental fluctuations on the recruitment levels of the Cape hake Merluccius capensis in Namibia. However, their study contains some flaws that undermine the conclusion that Cape hake recruitment is under the influence of upwelling in summer. Until those flaws are properly addressed, this conclusion, in our view, should be treated with caution.  相似文献   
47.
Seven microsatellite markers were used to evaluate the genetic diversity and differentiation of seven stocks of Litopenaeus vannamei, which were introduced from Central and South America to China. All seven microsatellite loci were polymorphic, with polymorphism information content(PIC) values ranging from 0.593 to 0.952. Totally 92 alleles were identified, and the number of alleles(Na) and effective alleles(Ne) varied between 4 and 21 and 2.7 and 14.6, respectively. Observed heterozygosity(Ho) values were lower than the expected heterozygosity(He) values(0.526–0.754), which indicated that the seven stocks possessed a rich genetic diversity. Thirty-seven tests were detected for reasonable significant deviation from Hardy-Weinberg equilibrium. Fis values were positive at five loci, suggesting that there was a relatively high degree of inbreeding within stocks. Pairwise Fst values ranged from 0.0225 to 0.151, and most of the stock pairs were moderately differentiated. Genetic distance and cluster analysis using UPGMA revealed a close genetic relationship of L. vannamei between Pop2 and Pop3. AMOVA indicated that the genetic variation among stocks(11.3%) was much lower than that within stocks(88.7%). Although the seven stocks had a certain degree of genetic differentiation and a rich genetic diversity, there is an increasing risk of decreased performance due to inbreeding in subsequent generations.  相似文献   
48.
Korea's coastal and offshore fisheries have experienced reduction in their catch in the early 2000. The amount of catch from coastal and offshore fisheries dropped from 1.7 million tons in 1986 to 1 million tons in 2004. To address such catch reduction, fish stock enhancement programs have been constantly developed and implemented. However, as fish stocks have been estimated to decrease since 2000 in spite of various management measures, the Korean Government genuinely acknowledges the necessity to enhance fisheries productivity through the recovery of depleted fish stocks. Based on such acknowledgement, a fish stock rebuilding plan (FSRP) combined with conventional fish stock enhancement programs was established in 2005. For stocks which have shown drastic decrease, a FSRP was set up and promoted. So far, 10 FSRPs have been established and operated, and plan is being made to expand them to 20 species by 2012. The result of pilot projects shows that stocks have been increasing after the introduction of FSRPs. For instance, the catch per unit effort (CPUE) of sandfish in the East Sea has increased from 0.44 in 2005 to 0.78 in 2011. Consequently, fishing income has increased by 15%. The key lessons learned during the implementation of FSRP are: the causes for the decrease of stock vary and are complicated and it is necessary to adjust and eliminate some conventional policies that could have unforeseen negative impacts on fish stocks. The FSRP-based fisheries management policy in Korea carries great significance, for it has changed the focus of policy from simply maintaining the status quo to stock recovery and it allows relevant stakeholders to get actively involved in the procedures of establishing and promoting the plan, leading to effective implementation of the plan. The current FSRP is operated with species, but if it can be gradually expanded to encompass the whole ecosystem, it will greatly contribute to more effective management and fish stock recovery for all species in offshore and coastal waters surrounding Korea.  相似文献   
49.
罗非鱼4个选育群体遗传结构SSR分析   总被引:2,自引:0,他引:2  
采用SSR分子标记分析了吉富品系罗非鱼的两个家系(GF1和GF2)以及奥利亚罗非鱼(Fo)和奥本尼罗罗非鱼(Fn)群体的遗传结构。结果显示,扩增后等位基因数为3~8个,随引物不同而异,14对引物共扩增60个等位基因,扩增片断大小在102~267 bp之间。微卫星分析表明,奥本尼罗罗非鱼(Fn)的平均观测杂合度(0.764 3)和平均期望杂合度(0.519 6)均最高,吉富尼罗罗非鱼(GF1)平均多态信息含量(0.419 5)最高;吉富尼罗罗非鱼(GF2)平均观测杂合度(0.614 2),奥利亚罗非鱼(Fo)的平均期望观测度(0.446 6)、平均多态信息含量(0.359 2)最低;因此,吉富鱼(GF1)的遗传多样性最高,奥利亚的遗传多样性最低。Hardy-Weinberg平衡遗传偏离指数(D)奥利亚(Fo)和尼罗(Fn)(0.463 4和0.478 9)明显高于吉富的两家系(0.234 1和0.250 0)。Fo与GF2间遗传距离(0.477 0)最大;而GF1和GF2间的遗传距离(0.302 7)最小,遗传相似系数(0.607 3)最大,可推断新一代吉富罗非鱼与本地选育群体有相对较远的亲缘关系,更具杂种优势。  相似文献   
50.
本文采用物质流分析(Material Flow Analysis, MFA)的方法, 构建了铝全生命周期动态物质流分析框架, 自上而下的追踪了1949—2021年中国铝的流量、存量和供需情况。结果表明: 1)1949—2021年中国铝土矿铝物质累计流入量5.86亿t, 国内开采与进口各占一半左右。实际铝物质累计消费量3.74亿t。生命周期内铝损失严重, 铝物质损失量2.39亿t。2)2021年人均铝物质社会存量208.9 kg/人, 与同经济发展时期的北美、日本基本一致。3)1990年以来中国再生铝产量快速增长, 累计0.77亿t, 以国内生产为主, 占比76.7%。随着含铝产品到达寿命及未来铝需求量的持续走高, 将迎来含铝产品的报废时代。建议从全生命周期的角度出发, 减少铝在各个环节的损失量; 合理布局再生铝产业, 提高再生铝的利用, 优化铝供应结构, 促进铝工业低碳转型, 支撑“双碳”目标的实现。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号