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   2014篇
  免费   196篇
  国内免费   84篇
林业   53篇
农学   46篇
基础科学   20篇
  45篇
综合类   439篇
农作物   625篇
水产渔业   945篇
畜牧兽医   68篇
园艺   9篇
植物保护   44篇
  2024年   18篇
  2023年   26篇
  2022年   141篇
  2021年   179篇
  2020年   97篇
  2019年   104篇
  2018年   58篇
  2017年   73篇
  2016年   107篇
  2015年   113篇
  2014年   169篇
  2013年   162篇
  2012年   146篇
  2011年   160篇
  2010年   129篇
  2009年   95篇
  2008年   86篇
  2007年   69篇
  2006年   73篇
  2005年   54篇
  2004年   44篇
  2003年   30篇
  2002年   22篇
  2001年   44篇
  2000年   28篇
  1999年   12篇
  1998年   6篇
  1997年   10篇
  1996年   12篇
  1995年   5篇
  1994年   6篇
  1992年   5篇
  1990年   3篇
  1989年   2篇
  1987年   2篇
  1986年   1篇
  1973年   2篇
  1962年   1篇
排序方式: 共有2294条查询结果,搜索用时 31 毫秒
91.
92.
我国海洋牧场建设回顾与展望   总被引:7,自引:12,他引:7  
杨红生 《水产学报》2016,40(7):1133-1140
海洋牧场是基于海洋生态学原理和现代海洋工程技术,充分利用自然生产力,在特定海域科学培育和管理渔业资源而形成的人工渔场。海洋牧场的特点包括以增加渔业资源量为目的、具有明确的边界和权属、苗种主要来源于人工育苗或驯化、通过放流或移植进入自然海域、主要以天然饵料为食和对资源实施科学管理等。早在1965年,我国学者曾呈奎先生就原创性地提出在海洋中通过人工控制种植或养殖海洋生物,建设"牧场"的理念。上世纪八十年代以来,我国海洋牧场经历了从增殖放流、人工鱼礁建设到系统化的海洋牧场发展过程,在取得巨大成就的同时,也面临着海洋牧场的涵义应用过于宽泛、缺乏统筹规划和科学论证、忽视海洋牧场生态作用以及忽视项目评估和系统管理等问题。展望未来,我国的海洋牧场建设应在"生态优先、陆海统筹、三产贯通、四化同步、创新跨越"的发展理念指导下,加强海洋牧场建设的宏观引导、推动海洋牧场体系化建设、实施海洋牧场企业化运营,在获得经济效益的同时,实现产业繁荣和保持健康生态系统的和谐统一。  相似文献   
93.
Petrosky CE, Schaller HA. Influence of river conditions during seaward migration and ocean conditions on survival rates of Snake River Chinook salmon and steelhead.
Ecology of Freshwater Fish 2010: 19: 520–536. © 2010 John Wiley & Sons A/S Abstract – Improved understanding of the relative influence of ocean and freshwater factors on survival of at‐risk anadromous fish populations is critical to success of conservation and recovery efforts. Abundance and smolt to adult survival rates of Snake River Chinook salmon and steelhead decreased dramatically coincident with construction of hydropower dams in the 1970s. However, separating the influence of ocean and freshwater conditions is difficult because of possible confounding factors. We used long time‐series of smolt to adult survival rates for Chinook salmon and steelhead to estimate first year ocean survival rates. We constructed multiple regression models that explained the survival rate patterns using environmental indices for ocean conditions and in‐river conditions experienced during seaward migration. Survival rates during the smolt to adult and first year ocean life stages for both species were associated with both ocean and river conditions. Best‐fit, simplest models indicate that lower survival rates for Chinook salmon are associated with warmer ocean conditions, reduced upwelling in the spring, and with slower river velocity during the smolt migration or multiple passages through powerhouses at dams. Similarly, lower survival rates for steelhead are associated with warmer ocean conditions, reduced upwelling in the spring, and with slower river velocity and warmer river temperatures. Given projections for warming ocean conditions, a precautionary management approach should focus on improving in‐river migration conditions by increasing water velocity, relying on increased spill, or other actions that reduce delay of smolts through the river corridor during their seaward migration.  相似文献   
94.
A H Dye 《African Zoology》2013,48(2):201-205
Diurnal vertical migrations of meiofauna were observed in an estuarine sand flat and these were related primarily to desiccation and temperature. The migrations, which occurred in the top 10 cm of the sediment, had a mean range of 5 cm and were most strongly exhibited by the interstitial flatworms, polychaetes and oligochaetes, followed by the nematodes and harpacticoid copepods. It is concluded that vertical migrations are restricted to areas experiencing some degree of desiccation and would not be significant in waterlogged areas such as mud flats.  相似文献   
95.
Platelet aggregation before and after administration of 0.5 mg/m2 of vincristine (VCR) was evaluated in 7 dogs with spontaneously occuring lymphoma. Aggregation on platelet-rich plasma separated from blood collected in 3.8% sodium citrate was performed using adenosine diphosphate (ADP), arachidonic acid (AA), and collagen (COL) as agonists. The slope for aggregation in response to ADP was significantly lower after administration of VCR (P = .032). Maximal aggregation after administration of VCR was significantly lower in response to ADP, COL, and AA (P = .03, P = .04, and P = .03, respectively).  相似文献   
96.
基于颠茄发根的外源基因表达系统的建立   总被引:1,自引:0,他引:1  
杨春贤  陈敏  廖志华  谌容  阳义健  张磊 《园艺学报》2006,33(5):1103-1105
 用携带植物高效表达载体pCAMBIA1304 + “解除武装”的重组C58C1工程菌转化颠茄无菌苗真叶, 发根诱导率达100%。PCR检测证实发根型质粒pRiA4和表达型质粒pCAMBIA1304 +均能整合到颠茄发根基因组内, 共转化率达30%。建立了基于颠茄发根的高效外源基因表达系统, 为将托品烷类生物碱东莨菪碱的生物合成关键酶基因导入颠茄, 实现其次生代谢工程及开展分子育种奠定了基础。  相似文献   
97.
The effect of variations in the amount or quality of food provided on the amino acid profile of larvae, was tested in two marine fish species, the Senegal sole (Solea senegalensis) and the sea bass (Dicentrarchus labrax). The objective was to assess whether such nutritionally suboptimum dietary patterns, which simulate situations that may occur in hatcheries, result in variations in the concentration of one or more amino acids that could be used as indicators of nutritional condition of larvae. Restriction in the normal food ration by 60% had no clear effect on the amino acid profile of sea bass, while the concentration of some amino acids (Arg, Ala and Phe) showed significant variations in sole. Feeding on artificial diets, which have shown their inadequacy a priori, resulted also in no significant effect on the profile of sea bass, but in a dramatic modification of the amino acid profile in sole. In addition, changes in the amino acid profiles considered as a whole were evaluated using the cluster analysis instead of paired comparisons between amino acid concentrations in each treatment. The analysis clearly separated profiles of larvae fed restrictedly or inadequately from their controls, irrespective of the species. This demonstrated the potential value of evaluating whole changes in amino acid profile as nutritional indicator. Besides, it was demonstrated that sensitivity of larvae to nutritional changes that may be reflected in the amino acid profile is greatly conditioned by the species and developmental stage.  相似文献   
98.
99.
中的鞘氨醇类代谢产物   总被引:2,自引:0,他引:2  
从中国香港红树林的两株南海海洋真菌(2526#和1850#)的代谢产物中分离到4个化合物,通过完整的波谱数据分别解析为鞘氨醇类化合物N-(2’,3‘-二羟基二十四碳酰基)-1,3-二羟基-2-氨基十八烷(A)、n-(2’,3‘-二羟基二十六碳酰基)-1,3-二羟基-2-氨基十八烷(B)、2‘-dehydroxycerebroside D(C)和cerebroside D(D)。化合物A和B未见文献报道。  相似文献   
100.
Souza WM  Stinghen AE  Santos CA 《Fitoterapia》2004,75(7-8):750-753
The alkaloidal fraction from Himatanthus lancifolius barks demonstrated a broad-spectrum in vitro antimicrobial activity for most of the Gram (+) and Gram (-) tested microorganisms.  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