全文获取类型
收费全文 | 17450篇 |
免费 | 78篇 |
国内免费 | 2篇 |
专业分类
林业 | 3693篇 |
农学 | 1308篇 |
基础科学 | 140篇 |
2869篇 | |
综合类 | 914篇 |
农作物 | 2114篇 |
水产渔业 | 1885篇 |
畜牧兽医 | 1519篇 |
园艺 | 1139篇 |
植物保护 | 1949篇 |
出版年
2024年 | 7篇 |
2023年 | 13篇 |
2022年 | 11篇 |
2021年 | 30篇 |
2020年 | 35篇 |
2019年 | 42篇 |
2018年 | 2771篇 |
2017年 | 2722篇 |
2016年 | 1206篇 |
2015年 | 98篇 |
2014年 | 42篇 |
2013年 | 70篇 |
2012年 | 851篇 |
2011年 | 2203篇 |
2010年 | 2141篇 |
2009年 | 1286篇 |
2008年 | 1366篇 |
2007年 | 1641篇 |
2006年 | 95篇 |
2005年 | 153篇 |
2004年 | 147篇 |
2003年 | 195篇 |
2002年 | 100篇 |
2001年 | 18篇 |
2000年 | 58篇 |
1999年 | 10篇 |
1995年 | 5篇 |
1994年 | 4篇 |
1993年 | 18篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 8篇 |
1989年 | 11篇 |
1988年 | 15篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1980年 | 4篇 |
1978年 | 7篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1971年 | 5篇 |
1969年 | 5篇 |
1968年 | 5篇 |
1967年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
111.
Kyoko Suzuki Tomoyasu Yoshitomi Yoichi Kawaguchi Masaki Ichimura Kaneaki Edo Tsuguo Otake 《Fisheries Science》2011,77(3):313-320
Sakhalin taimen Hucho perryi populations have decreased drastically to near extinction. It is urgent to establish an effective conservation strategy based
on an understanding of the characteristics of migration and habitat use of this species. We examined the migration history
of anadromous Sakhalin taimen captured off the Sarufutsu coast, northern Hokkaido, Japan, using otolith Sr:Ca ratios and also
examined the relationship between their otolith Sr:Ca ratios during freshwater and seawater residence in a rearing experiment.
Otolith Sr:Ca ratios of some fish from the Sarufutsu coast showed freshwater levels (0.5–4.0 × 10−3) near the core, which thereafter increased to brackish water levels (4.0–6.0 × 10−3), and then to seawater levels (6.0–10.0 × 10−3) in the outermost regions. Those findings indicate that specimens from the Sarufutsu coast migrated to the brackish water
region or the sea and spent most of their lives there. The anadromous migration pattern including the timing of downstream
migration seems to be flexible among individuals in the species. They migrate between freshwater and seawater or brackish
water several times during their lives, showing extensive habitat use. It is essential to secure the continuity among the
freshwater, brackish water, and seawater areas for their effective conservation. 相似文献
112.
Monosex male culture of C. quadricarinatus is proposed for increasing yields because males reach a larger size at harvest than females. An experiment was done to evaluate
the effects of androgenic gland extract and temperature on sex reversal and somatic growth in early juvenile females. Females
were exposed to the following experimental conditions: C: control food and temperature maintained at 26 ± 1°C; HT (high temperature):
control food and temperature maintained at 28.5 ± 1°C; VHT (very high temperature): control food and temperature maintained
at 31 ± 1°C; AG: food with enriched with androgenic gland (1/10 dose for each juvenile per day) and temperature maintained
at (26 ± 1)°C. The juveniles were weighed, sexed, and growth increment (GI) and growth rate (GR) were calculated twice a month.
The HT and AG groups differed from the C group in GI and GR indicating a greater somatic growth. At the end of the experiment,
the HT and AG groups had similar weight but only the HT group had enhanced oocyte diameter, with some vitellogenic oocytes
compared to the C group. In the HT group, there was significant sex reversal, as indicated by development of male and intersex
male characteristics in different individuals. Both temperature and AG diet have a high potential for culture of this species
because of their enhancement of somatic growth; higher temperature increases the proportion of males, a desirable characteristic
of cultured populations. 相似文献
113.
The ontogenetic development of the digestive enzymes amylase, lipase, trypsin, and alkaline phosphatase and the effect of
starvation in miiuy croaker Miichthys miiuy larvae were studied. The activities of these enzymes were detected prior to exogenous feeding, but their developmental patterns
differed remarkably. Trypsin activity continuously increased from 2 days after hatching (dah), peaked on 20 dah, and decreased
to 25 dah at weaning. Alkaline phosphatase activity oscillated at low levels within a small range after the first feeding
on 3 dah. In contrast, amylase and lipase activities followed the general developmental pattern that has been characterized
in fish larvae, with a succession of increases or decreases. Amylase, lipase, and trypsin activities generally started to
increase or decrease at transitions from endogenous to exogenous feeding or diet changes, suggesting that these enzymatic
activities can be modulated by feeding modes. The activities of all the enzymes remained stable from 25 dah onwards, coinciding
with the formation of gastric glands and pyloric caecum. These results imply that specific activities of these enzymes underwent
changes due to morphological and physiological modifications or diet shift during larval development but that they became
stable after the development of the digestive organs and associated glands was fully completed and the organs/glands functioned.
Trypsin and alkaline phosphatase were more sensitive to starvation than amylase and lipase because delayed feeding up to 2 days
after mouth opening was able to adversely affect their activities. Enzyme activities did not significantly differ among feeding
groups during endogenous feeding; however, all activities were remarkably reduced when delayed feeding was within 3 days
after mouth opening. Initiation of larvae feeding should occur within 2 days after mouth opening so that good growth and survival
can be obtained in the culture. 相似文献
114.
115.
Methods of assessing extinction risk in marine fishes 总被引:1,自引:0,他引:1
Nicholas K Dulvy Jim R Ellis Nicholas B Goodwin Alastair Grant John D Reynolds & Simon Jennings 《Fish and Fisheries》2004,5(3):255-276
The decline and disappearance of species from large parts of their former geographical range has become an important issue in fisheries ecology. There is a need to identify which species are at risk of extinction. The available approaches have been subject to considerable debate – particularly when applied to commercially exploited species. Here we have compiled methods that have been used or may be used for assessing threat status of marine organisms. We organize the methods according to the availability of data on the natural history, ecology and population biology of species. There are three general approaches to inferring or assessing extinction risk: (i) correlative approaches based on knowledge of life histories and ecology; (ii) time‐series approaches that examine changes in abundance; and (iii) demographic approaches based on age‐ or stage‐based schedules of vital rates and fisheries reference points. Many methods are well suited to species that are highly catchable and/or have relatively low productivity, but theory is less well developed for assessing extinction risk in species exhibiting narrow geographical distributions or ecological specialization. There is considerable variation in both definitions of extinction risk and the precision and defensibility of the available risk assessment methods, so we suggest a two‐tiered approach for defining and assessing extinction risk. First, simple methods requiring a few easily estimated parameters are used to triage or rapidly assess large numbers of populations and species to identify potentially vulnerable populations or species. Second, the populations and species identified as vulnerable by this process can then be subject to more detailed and rigorous population analysis explicitly considering sources of error and uncertainty. 相似文献
116.
Yasuro Iwashita Nobuhiro Suzuki Takeshi Yamamoto Jun-ichirou Shibata Keiichirou Isokawa Ang H Soon Yuto Ikehata Hirofumi Furuita Tsuyoshi Sugita Takanobu Goto 《Fisheries Science》2008,74(5):1083-1095
The supplemental effects of cholyltaurine and soybean lecithin to a defatted soybean meal (SBM)-based fish meal-free diet
on the hepatic and intestinal morphology of rainbow trout Oncorhynchus mykiss were investigated. A fish meal-based diet (FM), three fish meal-free diets supplemented either with soybean lecithin (SL),
bovine gall powder (BG) or cholyltaurine (C-tau) and an unsupplemented fish meal-free diet (NFM), were given to fish (−11
g) for 10 weeks. The growth was the lowest in fish fed diet NFM, intermediate in fish fed diet SL and the highest in fish
fed diet FM, BG and C-tau. Hepatocytes of fish fed diet NFM were atrophied, and the distal intestine of these fish showed
abnormal features: disintegrated microvilli and fatty degeneration in epithelial cells and increased amount of connective
tissue in the submucosa. Hepatic and intestinal histological features in fish fed diets SL, BG and C-tau were similar to those
in fish fed diet FM. These results indicate that soybean lecithin, bovine bile salts and cholyltaurine have similar effects
on normalizing the hepatic and intestinal morphologies of rainbow trout fed the SBM-based diet, although growth promotion
effect was limited in the soybean lecithin. 相似文献
117.
Sung-Yong Oh Choong Hwan Noh Rae-Seon Kang Chong-Kwan Kim Sung Hwoan Cho Jae-Yoon Jo 《Fisheries Science》2008,74(4):846-852
ABSTRACT: Compensatory growth, feeding rate, feed efficiency and chemical composition of juvenile black rockfish (mean weight 1.43 g) were investigated for 35 days after a 14-day feed deprivation treatment under four feeding conditions: one group continuously fed (control) and the other three groups fasted for 5 days (F5), 10 days (F10) and 14 days (F14). All fasted fish were re-fed from day 15. Only F5 achieved the same body weight as the control, indicating that complete compensation occurred in F5. The specific growth rate (SGR) of F5 was the highest at day 21 and then decreased thereafter, showing higher values than the control at days 21, 28 and 42. In contrast, although SGRs of F10 and F14 were higher than that of the control during the whole refeeding period except day 21, they did not catch up the control in body mass, indicating that only partial compensation occurred in F10 and F14. The feeding rate (FR) of all groups except F14 changed in a pattern similar to SGR (Spearman's rank correlation, r s > 0.9), suggesting that SGR varied depending on FR. Similar feeding efficiencies (FEs) were found in the four groups and they did not vary significantly during the whole refeeding period, suggesting that FE was not the factor affecting SGR. At day 14, the ratios of lipid to lean body mass in F10 and F14 were lower than those in the control and F5, and there was no difference between the control and F5. At day 49, however, only F14 showed a lower value than the other three groups, and there was no difference among the three groups. These results indicate that juvenile black rockfish fasted for 5–14 days can exhibit compensatory growth after refeeding, but timing and degree vary depending on the duration of feed deprivation. 相似文献
118.
Brian R. Miranda Brian R. Sturtevant Jian Yang Eric J. Gustafson 《Landscape Ecology》2009,24(5):587-598
We demonstrate a method to evaluate the degree to which a meta-model approximates spatial disturbance processes represented
by a more detailed model across a range of landscape conditions, using neutral landscapes and equivalence testing. We illustrate
this approach by comparing burn patterns produced by a relatively simple fire spread algorithm with those generated by a more
detailed fire behavior model from which the simpler algorithm was derived. Equivalence testing allows objective comparisons
of the output of simple and complex models, to determine if the results are significantly similar. Neutral landscape models
represent a range of landscape conditions that the model may encounter, allowing evaluation of the sensitivity and behavior
of the model to different landscape compositions and configurations. We first tested the model for universal applicability,
then narrowed the testing to assess the practical domain of applicability. As a whole, the calibrated simple model passed
the test for significant equivalence using the 25% threshold. When applied to a range of landscape conditions different from
the calibration scenarios, the model failed the tests for equivalence. Although our particular model failed the tests, the
neutral landscape models were helpful in determining an appropriate domain of applicability and in assessing the model sensitivity
to landscape changes. Equivalence testing provides an effective method for model comparison, and coupled with neutral landscapes,
our approach provides an objective way to assess the domain of applicability of a spatial model. 相似文献
119.
Maintaining connectivity among local populations in a fragmented landscape is crucial for the survival of many species. For
isolated habitat patches, stochastic fluctuations and reduced gene flow can lead to high risk of extinction. The connectivity
of the landscape is especially crucial for the carabid species living in the fragmented forests of the Bereg plain (NE Hungary
and W Ukraine) because a highway will be constructed through the plain. Our purpose is to (1) evaluate the impacts of three
possible highway tracks, (2) suggest a solution that is realistic with less impact on connectivity than other plans and (3)
discuss how to decrease the disadvantageous effects of each track. Our results, based on a network analysis of landscape graph
of patches and ecological corridors, indicate that the intended highway could have deleterious consequences on forest-living
carabids. Relatively simple actions, like the establishment of stepping stones, could compensate for the loss of habitat connectivity
and promote the survival of carabids, or minor modifications in one possible track could diminish its adverse effects. While
many other studies would be needed for a comprehensive assessment of the biotic impact of the highway, we provide an example
on the usefulness of network analysis for land use management.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
120.
Habitat specificity indices reflect richness (α) and/or distinctiveness (β) components of diversity. The latter may be defined by α and γ (landscape) diversity in two alternative ways: multiplicatively () and additively (). We demonstrate that the original habitat specificity concept of Wagner and Edwards (Landscape Ecol 16:121–131, 2001) consists of three independent components: core habitat specificity (uniqueness of the species composition), patch area and
patch species richness. We describe habitat specificity as a family of indices that may include either area or richness components,
or none or both, and open for use of different types of mean in calculation of core habitat specificity. Core habitat specificity
is a beta diversity measure: the effective number of completely distinct communities in the landscape. Habitat specificity
weighted by species number is a gamma diversity measure: the effective number of species that a patch contributes to landscape
richness. We compared 12 habitat specificity indices by theoretical reasoning and by use of field data (vascular plant species
in SE Norwegian agricultural landscapes). Habitat specificity indices are strongly influenced by weights for patch area and
patch species richness, and the relative contribution of rare vs. common species (type of mean). The relevance of properties
emphasized by each habitat specificity index for evaluation of patches in a biodiversity context is discussed. Core habitat
specificity is emphasized as an ecologically interpretable measure that specifically addresses patch uniqueness while habitat
specificity weighted by species number combines species richness and species composition in ways relevant for conservation
biological assessment.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献