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   291篇
  免费   26篇
  国内免费   1篇
林业   17篇
农学   8篇
基础科学   1篇
  32篇
综合类   40篇
农作物   7篇
水产渔业   51篇
畜牧兽医   136篇
园艺   18篇
植物保护   8篇
  2024年   1篇
  2023年   1篇
  2022年   5篇
  2021年   6篇
  2020年   14篇
  2019年   9篇
  2018年   15篇
  2017年   14篇
  2016年   8篇
  2015年   13篇
  2014年   11篇
  2013年   8篇
  2012年   10篇
  2011年   25篇
  2010年   15篇
  2009年   13篇
  2008年   15篇
  2007年   18篇
  2006年   24篇
  2005年   19篇
  2004年   14篇
  2003年   11篇
  2002年   20篇
  2001年   2篇
  2000年   3篇
  1998年   3篇
  1997年   3篇
  1993年   1篇
  1992年   2篇
  1991年   1篇
  1990年   2篇
  1989年   3篇
  1988年   1篇
  1987年   1篇
  1986年   1篇
  1985年   1篇
  1976年   1篇
  1974年   1篇
  1972年   1篇
  1967年   1篇
  1966年   1篇
排序方式: 共有318条查询结果,搜索用时 31 毫秒
51.
52.
OBJECTIVE: To evaluate cardiovascular and respiratory effects and pharmacokinetics of a 24-hour intravenous constant rate infusion (CRI) of dexmedetomidine (DMED) during and after propofol (PRO) or isoflurane (ISO) anaesthesia in dogs. STUDY DESIGN: Prospective, randomized, cross-over study. ANIMALS: Ten healthy adult Beagles. METHODS: Instrumented dogs received a DMED-loading bolus (25 microg m(-2)) at time 0 followed by a 24-hour CRI (25 microg m(-2) hour(-1)), with PRO or ISO induction/maintenance of anaesthesia during the first 2 hours (PRO and ISO treatment groups, respectively). Cardiovascular, respiratory, blood gas, airway gas, serum chemistry variables and DMED plasma concentration data were collected at -15, 5, 15, 30, 45, 60, 90 and 120 minutes. A number of cardiorespiratory and tissue oxygenation variables were calculated from the above data. After the 2-hours of anaesthesia, heart and respiratory rates and electrocardiograms were recorded and DMED plasma concentrations were determined for up to 26 hours. RESULTS: Vasopressor effects and the decrease in heart rate (HR) and cardiac index induced by DMED were greater for PRO than ISO, but were within clinically acceptable ranges. Adequate oxygenation was maintained above the critical O(2) delivery level. The overall incidence of unfavourable arrhythmias was low and tended to vary inversely with HR. Mean DMED plasma concentration ranged from 0.23 to 0.47 ng mL(-1) for both groups during the 24-hour CRI with a mean elimination half-life of approximately 0.46 hour. CONCLUSION AND/CLINICAL RELEVANCE: DMED CRI resulted in typical alpha(2)-agonist induced haemodynamic changes with minimal respiratory effects, and appeared to be an efficacious adjunct during and after PRO or ISO anaesthesia in healthy dogs.  相似文献   
53.
54.

Context

Strategic placement of fuel treatments across large landscapes is an important step to mitigate the collective effects of fires interacting over broad spatial and temporal extents. On landscapes where highly invasive cheatgrass (Bromus tectorum) is increasing fire activity, such an approach could help maintain landscape resilience.

Objectives

Our objectives are to 1) model and map fire connectivity on a cheatgrass-invaded landscape, as well as the centrality of large cheatgrass patches, in order to inform a landscape fuel treatment (i.e., a network of greenstrips); and 2) evaluate the modeled greenstrip network based on changes to cheatgrass patch centrality.

Methods

Our analysis covers 485-km2 on the Kaibab National Forest in Northern Arizona. We apply a circuit-theoretic model of fire connectivity between all pairs of large cheatgrass patches. Based on these results, we calculate a measure of centrality for each patch to inform fuel treatment placement. We evaluate the modeled greenstrip network by comparing the pre- and post-treatment centrality of each patch.

Results

After modeling fire connectivity across the landscape, we identify 25 of 68 large cheatgrass patches with relatively high centrality. When we simulate greenstrips around these focal patches, model results suggest that they are effective in reducing the centrality for at least 19 of the 25 patches.

Conclusions

Fire connectivity models provide robust network centrality measures, which can help generate multiple, landscape fuel treatment alternatives and facilitate on-the-ground decisions. The extension of these methods is well suited for landscape fuels management in other vegetation communities and ecosystems.
  相似文献   
55.
56.
Fisheries Science - This study presents age-based life history information on three small-bodied species targeted in the American Samoan fishery: Chlorurus japanensis palecheek...  相似文献   
57.
By‐catch in marine fisheries, particularly those using pelagic and demersal longlines, is a major driver of declines in abundance of sharks and rays around the world. A wide variety of by‐catch reduction devices (BRDs), that is, modified gears designed to reduce incidental captures of a variety of marine species while maintaining target catch rates, have been proposed, but the extent to which BRDs actually reduce the risk of catching sharks and rays remains unclear. We performed a meta‐analysis of 27 publications that reported the capture of sharks and rays and, in some cases, of targeted teleosts in longline gear deployed with and without BRDs. The risk of shark and ray capture differed between types of BRDs, but only one BRD type, longlines raised off the bottom, reduced by‐catch significantly. Circle hooks did not reduce the risk of capturing sharks and rays but might improve discard survival and are inexpensive, which might make them effective in reducing the detrimental effects of longlining on these species. In addition to being generally ineffective, some devices, such as electropositive and magnetic repellents, are expensive and have inherent construction drawbacks that are likely to make them unsuitable for commercial use. Overall, most BRDs did not affect the likelihood of catching targeted teleosts, but a substantial number of studies did not adequately assess target catch. We identified two poorly studied classes of BRD gear (i.e. raised demersal longlines, and monofilament nylon leaders), which represent promising directions for future research.  相似文献   
58.
Aquaculture is viewed as a potential mechanism to meet the growing demand for seafood around the world. The future of bivalve shellfish aquaculture in the U.S. hinges on sustainable practices on the part of industry and a more consistent regulatory regime. Bivalve shellfish aquaculture is a recent practice relative to its history in other countries, beginning in the late 1800s along the U.S. West Coast where it is now well established with farm raised product utilizing land-based hatcheries and grow-out directly in numerous estuaries. Bivalve shellfish aquaculture can be viewed as a disturbance which modifies the estuarine system in three ways: 1) changes in material processes — bivalves process food and produce wastes; 2) addition of physical structure — aquaculture introduces the cultured organisms and in some cases a physical anchoring structure; and 3) pulse disturbances like harvest and bed maintenance disturb sediments, remove species in addition to the cultured organisms themselves, and change resource or habitat availability. In U.S. West Coast estuaries, water column and sediment nutrient concentrations are relatively high and influenced by large tidal exchange and proximity to deeper nearshore ocean waters where upwelling controls production during summer months. Bivalves are unlikely to influence material processes except at local bed scales in these systems, although estuary-wide effects could appear as the fraction of cultured area rises or in poorly flushed bays. Bivalve culture clearly modifies estuarine habitat at local community and at landscape scales and effects are most often evaluated against existing structured habitat in the form of submerged aquatic vegetation. Individual activities act as pulse disturbances and the recovery of eelgrass (Zostera marina) to pre-disturbance levels is variable (< 2 to > 5 years). The extent of disturbance depends on the aquaculture practice and the distribution of eelgrass reflects a balance of space competition, pulse disturbance and recovery, and is therefore at dynamic equilibrium on aquaculture beds. Structure provided by aquaculture appears functionally similar to eelgrass for small benthic infauna and mobile epibenthic fauna while use of aquaculture as habitat by larger more mobile invertebrates and fish depends on mobility and varies with life-history stage and taxon being evaluated. Scale seems a very important management consideration and further research at estuarine landscape scales, especially for habitat use by important invertebrates and fish, may prove useful in designing and implementing best management practices. Though local and short term effects from aquaculture are clearly evident in U.S. West Coast estuaries, bivalve aquaculture does not remove area from the estuary or degrade water quality like other anthropogenic influences, and thus has not been implicated in shifts to alternate states or reduced adaptive capacity of the larger ecological system.  相似文献   
59.
Many studies have evaluated the adequacy of alternate ingredient diets for Atlantic salmon, Salmo salar, mainly with focus on fish performance and health; however, comprehensive analysis of fillet quality is lacking, particularly for salmon fed these diets in recirculation aquaculture systems (RAS). To this end, a study was conducted comparing fillet quality and processing attributes of postsmolt Atlantic salmon fed a fishmeal‐free diet (FMF) versus a standard fishmeal‐based diet, in replicate RAS. Mean weight of Atlantic salmon fed both diets was 1.72 kg following the 6‐mo trial and survival was >99%. Diet did not affect (P > 0.05) processing and fillet yields, whole‐body proximate composition(fat, moisture, protein), fillet proximate composition, cook yield, fillet texture, color, or omega‐3 fatty acid fillet content, including eicosapentaenoic acid and docosahexaenoic acid levels. Whole‐body ash content was greater in salmon fed the FMF diet. The FMF diet resulted in a wild fish‐in to farmed fish‐out ratio of 0:1 per Monterey Bay Aquarium's Seafood Watch criteria due to its fishmeal‐free status and use of lipids from fishery byproduct. Overall, fillet quality and processing attributes were generally unaffected when feeding a diet devoid of fishmeal to postsmolt Atlantic salmon cultured in RAS. [Correction added on 7 September 2017, after first online publication: the P value in Abstract has been changed from “P < 0.05” to “P > 0.05”.].  相似文献   
60.
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