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   1194篇
  免费   55篇
  国内免费   122篇
测绘学   23篇
大气科学   3篇
地球物理   195篇
地质学   59篇
海洋学   882篇
综合类   107篇
自然地理   102篇
  2024年   6篇
  2023年   19篇
  2022年   37篇
  2021年   26篇
  2020年   30篇
  2019年   36篇
  2018年   21篇
  2017年   47篇
  2016年   35篇
  2015年   29篇
  2014年   41篇
  2013年   119篇
  2012年   46篇
  2011年   60篇
  2010年   47篇
  2009年   70篇
  2008年   59篇
  2007年   83篇
  2006年   67篇
  2005年   58篇
  2004年   47篇
  2003年   39篇
  2002年   42篇
  2001年   33篇
  2000年   39篇
  1999年   41篇
  1998年   27篇
  1997年   31篇
  1996年   28篇
  1995年   18篇
  1994年   14篇
  1993年   20篇
  1992年   16篇
  1991年   7篇
  1990年   10篇
  1989年   1篇
  1988年   3篇
  1986年   1篇
  1985年   4篇
  1984年   5篇
  1983年   2篇
  1982年   2篇
  1981年   3篇
  1980年   2篇
排序方式: 共有1371条查询结果,搜索用时 15 毫秒
91.
维生素D(VD)是一类具有抗佝偻病作用的类固醇激素,其中维生素D3(VD3)是VD在动物体内的主要存在形式.作为生命活动必不可少的一类物质,VD发挥着广泛且重要的生理学作用.VD在动物体内的代谢过程及相关的代谢酶具有较高的进化保守性,同时也受到多种因素的调控影响.由于鱼类在生活环境方面与陆生脊椎动物存在显著差异,VD在...  相似文献   
92.
综述了我国及广东省十多年来鱼虾种苗人工放流及人工鱼礁的投放情况,结合广东省广大海区的水温、盐度、pH值、海流、营养盐含量情况及天然饵料的种类、分布等,提出了除在大亚湾已设置了水产增殖站以外,还应该在广东东部的南澳和西部的湛江各设一个增殖站,会收到更大的效果。人工鱼礁应总结过去投放的经验,加强礁区管理,逐步解决投资与收益的关系问题,并进一步扩大投放人工鱼礁的地点和密度,把广东海区切实建成理想的海洋牧场。  相似文献   
93.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin.  相似文献   
94.
The paper reviews the recent advances in studying grouper nutrition requirement for the development of cost-effective and environmentally friendly artificial diets. It consists of seven parts: protein and amino acid, lipid and essential fatty acid, carbohydrate, vitamin, mineral, alternative protein source, broodstock and larval nutrition. The review provides some basic information for further investigation of nutrient requirements of groupers.  相似文献   
95.
为了解初中毕业生地震科普知识的掌握情况,自行设计了武汉市初中毕业生地震科普知识调查问卷,对100名武汉市初中毕业生进行现场调查。调查结果为良、中、差等级分别占4%、18%、78%。 说明武汉市初中毕业生地震科普知识掌握情况令人堪忧,应尽早加强中学生地震科普知识教育, 以提高全民防震减灾意识。  相似文献   
96.
This feeding trial was conducted to evaluate the potential of poultry by-product meal (PBM) as a protein source in the culture ofLitopenaeus vannamei. Seven isonitrogenous and isoenergetic diets were formulated to near to commercial diet with about 40% protein and 7.5% lipid. Fish meal was replaced by 0, 30%, 40%, 50%, 60%, 70% and 100% of PBM (diets 1-7). The diet with 100% fish meal was used as a control (diet 1). Post-larvae were reared in an indoor semi-closed re-circulating system. Each dietary treatment was tested in 4 replicate tanks (260 L) of 40 shrimp, arranged in a completely randomized design. The shrimps were hand-fed for three times a day to near-satiation (0700, 1200 and 1800) for 60d. Percentage weight gain, survival, feed conversion ratio (FCR), protein efficiency ratio (PER) and body composition of shrimps were measured. There were no significant differences (P>0.05) in growth performance among shrimps fed diets 1-5 (0-60% fish meal replacement). However, shrimps fed diet 7 (100% fish meal replacement) had significantly lower (P<0.05) growth than those fed diets 1-5 (0-60% fish meal replacement). Shrimp fed diets 2-4 (30%-50% fish meal replacement) showed significantly higher growth than those fed diets 6 and 7 (70% and 100% fish meal replacement, respectively). Survival ranged from 94.7% to 100.0% and did not differ significantly (P>0.05) among different experimental diets. No differences in body composition were found among shrimps fed different diets. These results showed that up to 70% of fish meal protein can be replaced by PBM without adversely affecting the growth, survival, FCR, PER and body composition of Litopenaeus vannamei.  相似文献   
97.
Dam removal is becoming an effective approach for aquatic biodiversity restoration in damming river in order to balance the aquatic ecosystem conservation with large-scale cascade damming. However, the effects of dam removal on fish communities in Asian mountainous rivers, which are dominated by Cypriniformes fishes, are still not well known. To determine whether dam removal on a mountainous river benefit restoration of fish diversity, we investigated the response of fish assemblage to dam removal using a before-after-control-impact design in two tributaries of the Lancang River(dam removal river: the Jidu River, and control river: the Fengdian River). Fish surveys were conducted one year prior to dam removal(2012) and three years(2013–2015) following dam removal. We observed rapidly and notably spatio-temporal changes in fish biodiversity metrics and assemblage structure, occurring in the Jidu River within the first year after dam removal. Overall, fish species richness, density and Shannon-Wiener diversity all increased immediately in above-and below-dam sites, and maintained a stable level in subsequent years, compared to unchanged situation in the control river. All sites in the Jidu River experienced shifts in fish composition after dam removal, with the greatest temporal changes occurred in sites below-and above-the former dam, resulting in a temporal homogenization tendency in the dam removed river. These findings suggest that dam removal can benefit the recovery of habitat conditions and fish community in Asian mountainous rivers, but the results should be further evaluated when apply to other dammed rivers since the dam age, fluvial geomorphology and situation of source populations could all affect the responses of fish assemblages.  相似文献   
98.
应用以芽孢杆菌为主导菌的微生物复合制剂进行分解养鱼池有机污泥的试验,菌剂合活菌数为109个/g.用量为1.5~4.5mg·L-1。经一个月的试验,池底原有厚3~5cm的有机污泥被分解,并对鱼类有明显的促生长作用。说明应用以芽孢杆菌为主导菌的微生物复合制剂净化养殖池塘底质是一种有效的措施。  相似文献   
99.
Liisa Rohweder 《GeoJournal》2004,60(2):175-181
Environmental education in business school context has a significant effect on the way companies view environmental problems. Business schools train the future decision makers who can, with their own attitudes and practical actions, influence the contribution of business to the process of ecologically sustainable development. However, several studies show that no systematic thinking has been undertaken within business education as to what the educational prerequisites of the process of ecologically sustainable development are. In this article, I will firstly analyze by means of environmental education how business polytechnics in Finland have taken into consideration ecologically sustainable development. Secondly, I will raise the starting points for the development of the implementation of environmental education as part of the business schools▪ educational plans. Integrating environmental education into the vocational courses is identified as being one of the most effective ways of increasing the effectiveness of environmental education. Thirdly, the emphasis is on identification of barriers of integrating environmental education into the educational plans of business polytechnics. Theoretically, the article is attached to the objectives and learning concepts of environmental education and to educational planning theory in general. The empirical data are based on interviews I conducted 1999–2000 in all Finnish polytechnics offering business education. My article increases theoretical and practical understanding of how environmental education should be implemented in business education and how the current situation could be developed. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
100.
利用包头市区1951—2010年历年7月气温、降水、日照、天气现象及1991—2010年历年7月自记风等气象资料进行统计分析,揭示包头市区历年7月基本气候特征。分析结果表明:7月份是包头市区一年中最热的月份,97%年份的最高月平均气温、87%年份的最高月平均最高气温、60%的年份的年极端最高气温出现在7月份,46%年份的月最多大雨以上降水日数、60%的最多月雷暴日数、40%年份的最多月降水量和冰雹日数均出现在7月份。7月中旬前后副热带高压雨带逐渐北抬至河套地区,暖湿气流强盛,开始进入主汛期,高温、降水和强对流天气明显增多,它们的到来不仅给"中运会"带来影响,还会给人们的生产和生活带来灾难。通过总结7月份包头市区气候状况,提出重点关注和防御的重要天气过程,以此达到警示和预防的作用。  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