全文获取类型
收费全文 | 311090篇 |
免费 | 17209篇 |
国内免费 | 673篇 |
专业分类
林业 | 24453篇 |
农学 | 14212篇 |
基础科学 | 2999篇 |
45706篇 | |
综合类 | 41817篇 |
农作物 | 17317篇 |
水产渔业 | 18174篇 |
畜牧兽医 | 132378篇 |
园艺 | 6520篇 |
植物保护 | 25396篇 |
出版年
2020年 | 2974篇 |
2019年 | 3783篇 |
2018年 | 4541篇 |
2017年 | 5021篇 |
2016年 | 5139篇 |
2015年 | 4500篇 |
2014年 | 5877篇 |
2013年 | 16483篇 |
2012年 | 8211篇 |
2011年 | 10680篇 |
2010年 | 8244篇 |
2009年 | 8699篇 |
2008年 | 9980篇 |
2007年 | 9162篇 |
2006年 | 9381篇 |
2005年 | 8684篇 |
2004年 | 8453篇 |
2003年 | 8050篇 |
2002年 | 7226篇 |
2001年 | 8190篇 |
2000年 | 7843篇 |
1999年 | 6783篇 |
1998年 | 4089篇 |
1997年 | 4122篇 |
1996年 | 3966篇 |
1995年 | 4801篇 |
1994年 | 4158篇 |
1993年 | 3880篇 |
1992年 | 5630篇 |
1991年 | 5817篇 |
1990年 | 5484篇 |
1989年 | 5560篇 |
1988年 | 4961篇 |
1987年 | 5046篇 |
1986年 | 5130篇 |
1985年 | 5214篇 |
1984年 | 4612篇 |
1983年 | 4249篇 |
1982年 | 3325篇 |
1981年 | 3172篇 |
1980年 | 3123篇 |
1979年 | 4104篇 |
1978年 | 3527篇 |
1977年 | 3198篇 |
1976年 | 3073篇 |
1975年 | 3067篇 |
1974年 | 3383篇 |
1973年 | 3377篇 |
1972年 | 3062篇 |
1971年 | 2981篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
This study examines how appetite and growth regulation of Atlantic salmon are affected by low dissolved oxygen (LO) and dietary digestible energy levels (DE: high [HE] vs. low [LE]). Long‐term exposure to LO resulted in a reduced feed intake, growth, digestible protein and fat retention efficiencies and increased feed conversation ratio and plasma ghrelin concentrations (p < .05) compared to high dissolved oxygen (HO). Pair‐feeding of rations based on the feed intake of the LO groups, but fed at HO, resulted in a 50% growth improvement in HE diet groups. This suggests that the poor growth under LO was not entirely caused by the reduced feed intake. Salmon adapted to LO by increased haemoglobin concentrations, while osmoregulation was affected by increased plasma chloride concentrations (p < .05). Plasma ghrelin concentration was unaffected by DE (p > .05). Growth regulation was affected by the HE diet, with increased liver and muscle growth hormone receptor ghr1 mRNA (p < .05), regardless of oxygen level. The growth depression due to low oxygen appears to be related to higher metabolic costs, while higher DE upregulates the GH‐IGF system at the ghr1 level and found to be beneficial for growth, feed intake, oxyregulation and osmoregulation under hypoxia. 相似文献
92.
Chemical composition of Lippia spp. essential oil and antimicrobial activity against Aeromonas hydrophila 下载免费PDF全文
Cláudia Majolo Silvia Imaculada Barros da Rocha Edsandra Campos Chagas Francisco Célio Maia Chaves Humberto Ribeiro Bizzo 《Aquaculture Research》2017,48(5):2380-2387
Phytotherapy can replace antibiotic administration as an alternative to control Aeromonas hydrophila, one of the main bacteria involved in the aetiology of farmed fish diseases. Given that plants of the Lippia spp. genus show biological potential for antimicrobial activity, this study evaluated the chemical composition of essential oils extracted from Lippia alba, Lippia origanoides and Lippia sidoides and their activity against A. hydrophila. The oils were obtained by steam distillation in a Clevenger‐type apparatus and their composition determined by gas chromatography and mass spectrometry (CG/MS). Antibacterial activity was assessed by calculating the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) using broth microdilution method. The main compounds identified were geranial (25.4%) and neral (16.6%) in L. alba oil, carvacrol (40.4%) and p‐cymene (11.4%) in L. origanoides oil and thymol (76.6%) and ortho‐cymene (6.3%) in L. sidoides oil. The three Lippia species showed bacteriostatic and bactericidal action against A. hydrophila, with MICs and MBCs ranging from 1250 to 5000 μg mL?1. Of the species tested, the best performance was obtained with essential oil of L. sidoides. 相似文献
93.
Tracking escaped Atlantic cod (Gadus morhua L.) aggregated around Norwegian farms: Considerations for management strategies 下载免费PDF全文
R. D. Hedger R. M. Serra‐Llinares P. Arechavala‐Lopez R. Nilsen P. A. Bjørn I. Uglem 《Fisheries Management and Ecology》2017,24(4):265-273
To examine site fidelity and potential for ecosystem impacts, 20 escapee cod (Gadus morhua L.) from fjord‐based cod farms were tagged with acoustic transmitters, released in the vicinity of the same farm, and their subsequent behaviour and survival monitored using a fixed array of acoustic receivers. Most individuals demonstrated high fidelity to their farm where tagged, and this increased during the polar night. Only five individuals (25%) were occasionally detected at another farm (4 km distance), and only three cod (15%) migrated out of the fjord during the 6‐month study period. Vertical movements were consistently greater in the hours around noon than at night, a behaviour inconsistent with that observed for wild fish. A high mortality rate (40%) was observed during the first 4 months post‐release, and actual mortality may have been even higher, limiting the potential for negative ecosystem effects from cod escapees. Findings from this study are used to discuss the development of recapture strategies for minimising potential impacts from cod escapees to ensure sustainable farming activity. 相似文献
94.
Enhancing capacity for freshwater conservation at the genetic level: a demonstration using three stream macroinvertebrates 下载免费PDF全文
- Species diversity is declining more rapidly in freshwater ecosystems than in any other, but the consequences for genetic diversity, and hence evolutionary potential, are poorly understood. In part this reflects limited use and development of modern molecular tools and genetic approaches to address conservation questions in rivers, lakes and wetlands. As widespread, diverse and functionally important organisms, freshwater macroinvertebrates are ideal candidates for genetic approaches to reveal, for example, the conservation consequences of demographic histories and past disturbances. However, the availability of microsatellite markers for this group is very limited.
- Using next generation sequencing, microsatellite markers were developed for Isoperla grammatica (Poda, 1761), Amphinemura sulcicollis (Stephens, 1836) and Baetis rhodani (Pictet, 1843) to enable conservation genetic investigations of these widespread invertebrate species. Fifty‐two robust microsatellite loci were developed (18, 21 and 13 per species), all with high levels of allelic diversity (7–27, 3–16, 5–13 alleles per loci, respectively).
- These tools will allow assessment of genetic structure, dispersal and demographic resilience in these model species as a function of environmental change and variation, thereby aiding freshwater monitoring and conservation. The authors urge further capacity building to support genetic applications to the conservation biology of other aquatic organisms.
95.
Sirirat Chatvijitkul Claude E. Boyd D. Allen Davis Aaron A. McNevin 《Journal of the World Aquaculture Society》2017,48(1):7-19
Global averages were obtained for amounts of energy, land, water, wildfish, nitrogen, and phosphorus embodied in aquaculture feed ingredients. These data allowed amounts of these embodied resources to be calculated for typical feed formulations for channel catfish, Ictalurus punctatus; hybrid catfish, I. punctatus♀ × I. furcatus♂; Vietnamese catfish, Pangasius spp.; Atlantic salmon, Salmo salar; rainbow trout, Oncorhynchus mykiss; tilapia, Oreochromis spp.; whiteleg shrimp, Litopenaeus vannamei; and black tiger shrimp, Penaeus monodon. Embodied resource use per m.t. of feed varied among species: energy, 4.90–12.48 GJ/m.t.; land, 0.082–0.312 ha/m.t.; water, 502–1227 m3/m.t.; wildfish, 0–2880 kg/m.t.; nitrogen, 3.08–8.63 kg/m.t.; phosphorus, 1.16–5.62 kg/m.t. These calculations did not account for variations in site‐specific factors related to embodied resources and feed composition and use. But they suggest that reducing feed conversion ratio (FCR) by 0.1 unit for the seven species (species groups) could potentially reduce feed use by around 1.1 million tonne (Mt) while conserving 9.8 million GJ of energy, 270,000 ha of agricultural land, 1.4 billion m3 of freshwater, and 1.24 Mt of wildfish. Reduction of the FCR is a powerful means of lessening farm‐level production costs and negative impacts of feed production and use. 相似文献
96.
Combining fishers’ knowledge and cost‐effective monitoring tools in the management of marine recreational fisheries: A case study of the squid and cuttlefish fishery of the Ría of Vigo (NW Spain) 下载免费PDF全文
A new methodology based in the use of fishers’ knowledge and cost‐effective tools to obtain information about marine recreational fisheries (MRF) is presented. The squid and cuttlefish fishery of the Ría of Vigo (NW Spain) was selected because it is managed in a data‐poor environment. In‐depth interviews (57) were conducted with fishers, collecting ecological and socio‐economic information. A cartography of fishing grounds based on their knowledge was obtained, while the intensity of effort and catches was mapped by the monitoring of two vessels with low‐cost GPS data loggers. The 102 shore anglers and 248 recreational boats catch 8 t/year of European squid Loligo vulgaris and 11 t/year of common cuttlefish Sepia officinalis (11% of total catches on these species in the area). Shore anglers fish from 11 ports, while boat fishers use 14 fishing grounds (covering 30 km2). Most of the catches (86%) are landed by boats, and their CPUE is higher in the outer part of the Ría of Vigo. The use of fishers’ knowledge and cost‐effective monitoring is encouraged to obtain information for the management of MRF. Given the economic contribution of MRF (260,000 €/year in direct expenses), this activity should be considered in the regulations. 相似文献
97.
Impact of sampling and storage technique,and duration of storage,on the composition of fresh grass when analysed using near‐infrared reflectance spectroscopy 下载免费PDF全文
The objective of this study was to examine the effect of sampling technique (pluck or cut), storage duration (immediate analysis, 24‐h or 48‐h), storage temperature (ambient or chilled) and storage conditions (air present, air excluded or breathable) on the composition of fresh grass sampled from a sward managed to simulate grazing. Treatments were repeated across four sampling dates, with grass samples stored in grip seal bags prior to analysis using near‐infrared reflectance spectroscopy. Grass sampled by ‘pluck’ had a higher crude protein and ME content, and a lower acid detergent fibre (ADF) content, compared to that sampled by ‘cut’. Grass stored for 48 h had a lower water soluble carbohydrate (WSC) and ME content and a higher ADF content than for immediate analysis. Samples stored for 24 h did not differ from immediate analysis. Grass stored at ambient temperature had a lower WSC and ME content compared to immediate analysis. Grass stored under ‘breathable’ conditions had a lower ME content and higher ADF content than immediate analysis or samples stored with air present or air excluded. It is recommended that grass for analysis should be sampled by cutting, stored chilled (4°C) in a sealed bag to minimize exposure to oxygen and analysed within 24 h of harvest. 相似文献
98.
terraGIS – a web GIS for delivery of digital soil maps in cotton‐growing areas of Australia 下载免费PDF全文
Effective management of soil requires the spatial distribution of its various physical, chemical and hydrological properties. This is because properties, for example clay content, determine the ability of soil to hold cations and retain water. However, data acquisition is labour intensive and time‐consuming. To add value to the limited soil data, remote sensing (e.g. airborne gamma‐ray spectrometry) and proximal sensing, such as electromagnetic (EM) induction, are being used as ancillary data. Here, we provide examples of developing Digital Soil Maps (DSM) of soil physical, chemical and hydrological properties, for seven cotton‐growing areas of southeastern Australia, by coupling soil data with remote and proximal sensed ancillary data. A greater challenge is how to get these DSM to a stakeholder in a way that is useful for practical soil use and management. This study describes how we facilitate access to the DSMs, using a simple‐to‐use web GIS platform, called terraGIS. The platform is underpinned by Google Maps API, which is an open‐source development environment for building spatially enabled Internet applications. In conclusion, we consider that terraGIS and the supporting information, available on the sister web page ( http://www.terragis.bees.unsw.edu.au/ ), allow easy access to explanation of DSM of soil properties, which are relevant to cotton growers, farm managers, consultants, extension staff, researchers, state and federal government agency personnel and policy analysts. Future work should be aimed at developing error budget maps to identify where additional soil and/or ancillary data is required to improve the accuracy of the DSMs. 相似文献
99.
N.H. Sissener B.E. Torstensen M.A.G. Owen N.S. Liland I. Stubhaug G. Rosenlund 《Aquaculture Nutrition》2017,23(4):865-878
Atlantic salmon (Salmo salar) were fed five graded levels of eicosapentaenoic acid (EPA, 20:5n‐3) and docosahexaenoic acid (DHA, 22:6n‐3), from 1.4 to 5.2% of total fatty acids (FA, 5–17 mg kg?1 feed), and grew from ~160 g to ~3000 g, with the period from 1450 g onwards conducted both at 6 °C and at 12 °C. All fish appeared healthy, and there were no diet‐related differences in haematological or plasma parameters, as well as intestinal histological or gut microbiota analysis. Fish reared at 6 °C had higher accumulation of storage lipids in the liver compared to fish reared at 12 °C. Liver lipids also increased with decreasing dietary EPA + DHA at 6 °C, while there was no such relationship at 12 °C. Gene expression of SREBP1 and 2, LXR, FAS and CPT1 could not explain the differences in liver lipid accumulation. In liver polar lipids, DHA was found to be reduced when dietary EPA + DHA was <2.7% of FAs, while the level of EPA in the membranes was not affected. In conclusion, reducing dietary EPA + DHA from 5.2 to 1.4% of total FAs had a minor impact on fish health. Temperature was the factor that most affected the liver lipid accumulation, but there was also an interaction with dietary components. 相似文献
100.
Otolith chemistry indicates walleye movement and entrainment in a large serial reservoir system 下载免费PDF全文
Understanding fish movement in impounded river systems is important for fisheries management. Otolith chemistry was used to examine walleye Sander vitreus (Mitchill) intra‐ and inter‐reservoir movement in four Missouri River impoundments. Age‐0 individuals were reclassified with 75%–93% accuracy to known natal sites, allowing for reliable evaluation of movement patterns of age‐1 and older fish. Nearly half of walleye occupied the same location annually (i.e. site residency) between 2009 and 2010. In 2011, during the largest flood on record since 1898, downstream movement (49% of age‐1 and older fish) exceeded upstream movement (7%) and site residency (33%) across the study area. After the flood, most walleye moved downstream within reservoirs (45%) or were site residents (37%). Entrainment occurred most frequently during the flood year and was proportionally greatest in downstream reservoirs. Otolith chemistry is useful for understanding walleye movement and entrainment and is a tool for fisheries management with applications such as informing reservoir water releases, harvest regulations and habitat protection and rehabilitation within and outside the Missouri River reservoir system. 相似文献