全文获取类型
收费全文 | 16746篇 |
免费 | 822篇 |
国内免费 | 23篇 |
专业分类
林业 | 1145篇 |
农学 | 485篇 |
基础科学 | 142篇 |
2871篇 | |
综合类 | 2697篇 |
农作物 | 846篇 |
水产渔业 | 1422篇 |
畜牧兽医 | 6152篇 |
园艺 | 470篇 |
植物保护 | 1361篇 |
出版年
2023年 | 101篇 |
2022年 | 123篇 |
2021年 | 280篇 |
2020年 | 354篇 |
2019年 | 368篇 |
2018年 | 346篇 |
2017年 | 355篇 |
2016年 | 365篇 |
2015年 | 304篇 |
2014年 | 422篇 |
2013年 | 769篇 |
2012年 | 905篇 |
2011年 | 1219篇 |
2010年 | 671篇 |
2009年 | 633篇 |
2008年 | 1136篇 |
2007年 | 1074篇 |
2006年 | 1057篇 |
2005年 | 1072篇 |
2004年 | 1000篇 |
2003年 | 1015篇 |
2002年 | 888篇 |
2001年 | 187篇 |
2000年 | 172篇 |
1999年 | 186篇 |
1998年 | 160篇 |
1997年 | 143篇 |
1996年 | 128篇 |
1995年 | 145篇 |
1994年 | 142篇 |
1993年 | 119篇 |
1992年 | 109篇 |
1991年 | 92篇 |
1990年 | 116篇 |
1989年 | 90篇 |
1988年 | 79篇 |
1987年 | 76篇 |
1986年 | 76篇 |
1985年 | 80篇 |
1984年 | 80篇 |
1983年 | 84篇 |
1982年 | 57篇 |
1981年 | 48篇 |
1980年 | 61篇 |
1979年 | 51篇 |
1978年 | 56篇 |
1977年 | 49篇 |
1976年 | 52篇 |
1975年 | 59篇 |
1974年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Trout and charr, members of the salmonid family, have high conservation value but are also susceptible to anthropogenic threats in part due to the specificity of their habitat requirements. Understanding historical and future threats facing these species is necessary to promote their recovery. Of freshwater trout and charr in the Canadian Rocky Mountain region, westslope cutthroat trout (Oncorhynchus clarkii lewisi), bull trout (Salvelinus confluentus; a charr species) and Athabasca rainbow trout (Oncorhynchus mykiss) are of conservation concern. And indeed, range contractions and declining populations are evident throughout much of their ranges. Range contraction was most evident in the southern Alberta designatable unit (DU) of westslope cutthroat trout. Diminished populations were also evident in the downstream watersheds of the Alberta bull trout range, and throughout the Athabasca rainbow trout range. We assessed historical and future threats to evaluate the relative importance of individual threats to each DU and compare their impact among species. Individual threats fall into the broad categories of angling, non-native species and genes, habitat loss and alteration, and climate change. Severity of each threat varies by DU and reflects the interaction between species’ biology and the location of the DU. Severity of threats facing each DU has changed over time, reflecting extirpation of native populations, changes in management and industry best practices, expansion of non-native species and progressing climate change. The overall threat impact for each DU indicates a high probability of substantial and continuing declines and calls for immediate action. 相似文献
2.
Variability in the performance of juvenile Chinook salmon is explained primarily by when and where they resided in estuarine habitats
下载免费PDF全文
![点击此处可从《Ecology of Freshwater Fish》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Paul Chittaro Lyndal Johnson David Teel Paul Moran Sean Sol Kate Macneale Richard Zabel 《Ecology of Freshwater Fish》2018,27(3):857-873
Estuarine habitats provide rearing opportunities for the juvenile life stage of anadromous fishes. Because survival is positively correlated with juvenile performance, these estuarine habitats play an important role in population abundance and productivity. To provide information for the recovery of several depressed stocks of Chinook salmon in the Columbia River Basin, we sought to identify the factors that explain variability in performance. Using otolith‐derived estimates of juvenile somatic growth rate as an index of recent performance, we observed a negative nonlinear relationship between growth rate and day of year, and a decreasing and increasing trend of growth rate over the 8 years of this study and distance from the river mouth respectively. Using a generalised linear modelling approach, we found that variability in juvenile somatic growth rate was best explained by where and when individuals were collected, their body size, contaminant loads, stock of origin, and whether a fish was hatchery produced or unmarked. Lastly, we argue that a considerable improvement to the growth rate of juveniles in estuarine habitats is physiologically possible. The results of this 8‐year study provide a baseline of the performance of juvenile Chinook salmon to evaluate habitat restoration programs and to compare against future anthropogenic conditions. 相似文献
3.
Freshwater fishes are threatened globally, and often too little is known about threatened species to effectively guide their conservation. Habitat complexity is linked to fish species diversity and persistence, and degraded streams often lack habitat complexity. Beaver Castor spp., in turn, have been used to restore streams and increase habitat complexity. The northern leatherside chub Lepidomeda copei is a rare, small‐bodied, drift‐feeding minnow that has anecdotally been observed to use complex habitats associated with beaver dams in the western United States. To investigate this anecdote, we conducted fish and habitat surveys, the latter focusing on quantifying habitat complexity, in a sub‐basin of the Upper Snake River Basin in the USA. Complementary generalised linear model and path analyses revealed that northern leatherside chub occurred more often at sites with complex streamflows, and streamflows were more complex when beaver dams were present and pools were deeper. Northern leatherside chubs were also more likely to occur when temperatures were warmer, aquatic macrophytes were abundant and stream channels were narrow and deep. The linkage between chubs, complex streamflows and beaver dams needs to be evaluated more broadly to completely understand its role in the rangewide status of the species. However, it does suggests that increased use of beaver reintroductions and dam analogues for stream restoration could be a boon for the northern leatherside chub, but such efforts should be monitored to determine their effectiveness to help adapt beaver‐based restoration approaches to best benefit the species. 相似文献
4.
5.
Kevin R. Stuart Arnold G. Eversole David E. Brune 《Journal of the World Aquaculture Society》2001,32(1):105-111
Abstract.— The freshwater mussel Elliptio complanata was provided water containing green algae and cyanobacteria delivered from the Partitioned Aquaculture System (PAS) at eight flow rates to determine algal filtration rates as mg of particulate organic carbon (POC)/kg wet tissue weight per h. The dominant taxon in cyanobacterial waters was Microcystis while the dominant taxa in green algal waters were Scenedesmus and Ankistrodesmus . The cell counts of Scenedesmus and Ankistrodesmus were the only algal taxa that were significantly different between the incoming water and water filtered by mussels. Filtration rates of POC obtained from green algal water were significantly greater than from cyanobacteria-dominated waters at all flow rates. A significant increase in mean filtration rate was observed as flow rates increased. The filtration rate of green algae increased as POC concentration increased, peaking at 28 mg C/L. A maximum filtration rate was not observed with cyanobacterial waters. 相似文献
6.
7.
8.
9.
Karen R. Muñana Patricia J. Luttgen Mary Anna Thrall Thomas W. Mitchell David A. Wenger 《Journal of veterinary internal medicine / American College of Veterinary Internal Medicine》1994,8(2):117-121
Seven Domestic shorthair cats with a lysosomal storage disorder analogous to human Niemann-Pick disease type C, from a breeding colony were studied to characterize the neurological manifestations of this disorder. Affected cats were identified by means of liver biopsies at 4 to 6 weeks of age. Neurological examinations were performed at 2 week intervals from the onset of clinical signs. All cats displayed signs referrable to the cerebellum, with a subtle intention tremor noticed initially at 8 to 12 weeks of age; the disease was rapidly progressive. The tremor became more pronounced, menace response was lost, and severe dysmetria and ataxia developed. Three cats also had signs referrable to other areas of the central nervous system. Cats died or were euthanized between 12 and 43 weeks of age. Pathological findings included accumulation of substrate within neurons throughout the central nervous system, and axonal spheroid formation. The clinical and pathological findings in these cats are comparable to those in the human form of the disease. 相似文献
10.
Anthony S. Moore BVSc MVSc Gregory K. Ogilvie David M. Vail 《Journal of veterinary internal medicine / American College of Veterinary Internal Medicine》1994,8(5):343-344
Twenty-five dogs with malignant lymphoma refractory to chemotherapy were treated with actinomycin D at a median dose of 0.7 mg/m2 (range, 0.5 to 0.9 mg/m2 ) every 3 weeks. The dogs treated had received between 2 and 8 chemotherapeutic agents (median 7), for a median of 266 days before being treated with actinomycin D. For 23 of the 25 dogs, previous chemotherapy included doxorubicin. No dog responded to actinomycin D chemotherapy. 相似文献