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   4328篇
  免费   750篇
  国内免费   530篇
林业   14篇
农学   91篇
基础科学   43篇
  159篇
综合类   849篇
农作物   81篇
水产渔业   4023篇
畜牧兽医   324篇
园艺   4篇
植物保护   20篇
  2024年   40篇
  2023年   77篇
  2022年   92篇
  2021年   180篇
  2020年   253篇
  2019年   321篇
  2018年   231篇
  2017年   335篇
  2016年   267篇
  2015年   197篇
  2014年   234篇
  2013年   385篇
  2012年   271篇
  2011年   307篇
  2010年   222篇
  2009年   194篇
  2008年   179篇
  2007年   201篇
  2006年   167篇
  2005年   170篇
  2004年   150篇
  2003年   128篇
  2002年   97篇
  2001年   139篇
  2000年   117篇
  1999年   94篇
  1998年   53篇
  1997年   76篇
  1996年   82篇
  1995年   71篇
  1994年   62篇
  1993年   54篇
  1992年   45篇
  1991年   28篇
  1990年   20篇
  1989年   31篇
  1988年   12篇
  1987年   11篇
  1986年   13篇
  1985年   1篇
  1981年   1篇
排序方式: 共有5608条查询结果,搜索用时 31 毫秒
51.
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   
52.
应用以葡萄球菌A蛋白协同凝集试验(SPA-COA)为主要检测手段的鱼病诊断箱,于1995 ̄1997年在吉林,辽宁等7省进行现场检测试验。结果,61个渔场11个品种的516尾病鱼有菌生长的285尾,经SPA-COA鉴定阳性数为239尾,占有菌生长的83.86%,12个渔场5个品种1216尾假定健康鱼有菌生长的为28尾,经SPA-COA鉴定阳性数为22尾,占有菌生长的78.57%,认为本诊断箱适用于基  相似文献   
53.
54.
SUMMARY: Experimental infection of Tetrahymena pyriformis was conducted on ornamental fishes with skin wounded by acetic acid treatment. Among fishes used in this experimental infection, guppy Poecilia reticulata , pristella Pristella maxillaris , neontetra Paracheirodon innesi and cherry barbs Puntius titteya proved to be sensitive to challenge, while medaka Oryzias latipes , dwarf gourami Colisa lalia , goldfish Carassius auratus , platy Xiphophorus maculatus and angelfish Pterophyllum scalare were resistant. In catfish Corydoras aeneus infection was unsuccessful. Exposure to Tetrahymena at ≥100 cells/mL at 25 to 30°C and at pH of 6.0 to 8.0 resulted in the successful infection of some sensitive fish species. Histological and bacteriological observations suggest that deeply destroyed skin tissues from the acid-treated method are the primary factors in successful infection.  相似文献   
55.
56.
本研究建立了快速检测鱼源沙门氏菌的BA-ELISA工作程序。该程序灵敏度为105个/mL,可经受108个/mL杂菌的干扰,对含有10个以上沙门氏菌的接种标本经21~23h两步增菌后即可检出,可在25~27h内报告结果,比常规分离培养方法缩短3~5d。从6个品种的498尾淡水鱼和7个品种的160尾海水鱼,共采集标本778份,以常规分离培养法检出29份阳性标本,分离率为3.7%。对包括该29份分离阳性标本在内的184份鱼源标本实施BA-ELISA的结果表明,两法的符合率为96.73%,BA-ELISA法的敏感性为100%(29/29),特异性为96.1%(149/155)。独立性检验表明,两法检测结果间有极显著意义的一致性(P<0.01)。血清学分型表明,分离的菌株全部为鼠伤寒沙门氏菌(1,4,5,12:i:1,2)。  相似文献   
57.
水产动物微颗粒饲料加工工艺研究进展   总被引:1,自引:0,他引:1  
水产动物幼体微颗粒饲料是近些年来国内外研究开发的热点,文中综述了国际上较为流行的几种微颗粒饲料造粒工艺,包括微黏合工艺、微包膜工艺、聚合反应工艺、相分离技术造粒工艺、物理机械法造粒以及混合法造粒工艺。微颗粒饲料制粒工艺是微颗粒饲料产业乃至水产育苗行业的关键技术,其进一步发展有赖于制粒工艺技术研究和水产动物幼苗营养及消化生理研究的综合进展。  相似文献   
58.
Abstract –  The native and non-native freshwater fish species richness as well as the proportion of native fish species to overall fish species were examined to test a hypothesis of a spatial linkage between the distributions of dams and non-native fishes based on survey data collected in rivers and streams in Hokkaido, Japan. Non-native fish species have steadily increased in Hokkaido since the mid 1970s, as indicated by the decreasing proportion of native fish species per fish survey. Non-native species richness was significantly higher in areas above dams (i.e., reservoirs and their inlet streams) compared to areas without dams. As a result, the proportion of native fish species was lower above dams. This proportion was lower in developed and agricultural areas compared with forest areas, although both native and non-native species richness were higher in areas with development and agriculture. Non-native cyprinids, catfish and snakehead occurred primarily in a particular river basin with the largest drainage area, the longest history of reclamation and highest human population in the island. Non-native salmonids on the other hand, occurred in forest areas at higher elevations and their occurrence was associated with the existence of dams. There was a spatial linkage between the distribution of dams and non-native species, although it was applicable only to salmonids. The management of dams and reservoirs needs a strategy to prevent further expansion of exotic salmonids such as rainbow and brown trout. These species can be a significant threat to native Japanese salmonids as a result of various biological mechanisms.  相似文献   
59.
Economic profitability of Nile tilapia production in Kenya was analysed using a model that simulated individual fish growth and took fish population dynamics in the pond into account. The results suggest that the currently practiced mixed‐sex tilapia culture is economically unsustainable. It is suggested that research and extension efforts be geared towards developing monosex Nile tilapia production systems. Nile tilapia culture with African catfish predation should be viewed as an intermediate step towards all‐male Nile tilapia culture. This will allow accumulation of both physical and human capital to support all‐male tilapia culture. Under all‐male culture, economic returns are high enough to justify investment in Nile tilapia culture using borrowed capital. However, the success of monosex culture will depend on the availability and affordability of quality fingerlings and low‐cost fish feeds. The results have a wide application in Sub‐Saharan Africa where mixed‐sex Nile tilapia culture is common.  相似文献   
60.
Abstract –  We studied the migratory behaviour of armado, Pterodoras granulosus , in the Paraná River Basin of Brazil, Paraguay and Argentina, during 1997–2005. This species invaded the Upper Paraná River after upstream dispersal was facilitated when Itaipu Reservoir inundated a natural barrier. Fish were tagged ( N  = 8051) in the mainstems of the Yacyreta and Itaipu reservoirs, bays of major tributaries, the Paraná River floodplain above Itaipu Reservoir, and below dams. In all, 420 fish were recaptured of which 61% moved away from the release area. Fish moved a maximum of 215 km (mean 42), and at a maximum rate of 9.4 km·day−1 (mean 0.6). Of the 256 armados that moved away from the release site, 145 moved upstream towards unimpounded stretches of the Paraná River and 111 moved downstream into the reservoir and bays of its tributaries (maximum 150 km). Based on the observed migratory movements, we suspect that most of the reproductive output originates in tributaries to the reservoirs. The ability of this species to expand its range presents a conundrum by pitting fishery management interests against conservation needs. Maintenance of the important armado fisheries depends on the ability of the species to migrate freely to use spawning and nursery areas in reservoir tributaries and floodplains. However, its ability to migrate long distances can allow this non-native species the opportunity to invade most of the Upper Paraná River.  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