全文获取类型
收费全文 | 920篇 |
免费 | 111篇 |
国内免费 | 274篇 |
出版年
2024年 | 10篇 |
2023年 | 28篇 |
2022年 | 51篇 |
2021年 | 56篇 |
2020年 | 44篇 |
2019年 | 48篇 |
2018年 | 48篇 |
2017年 | 33篇 |
2016年 | 50篇 |
2015年 | 31篇 |
2014年 | 67篇 |
2013年 | 55篇 |
2012年 | 55篇 |
2011年 | 65篇 |
2010年 | 62篇 |
2009年 | 52篇 |
2008年 | 50篇 |
2007年 | 62篇 |
2006年 | 73篇 |
2005年 | 51篇 |
2004年 | 50篇 |
2003年 | 42篇 |
2002年 | 27篇 |
2001年 | 27篇 |
2000年 | 19篇 |
1999年 | 23篇 |
1998年 | 7篇 |
1997年 | 18篇 |
1996年 | 6篇 |
1995年 | 10篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1991年 | 11篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有1305条查询结果,搜索用时 0 毫秒
1.
Current issues in fish welfare 总被引:11,自引:0,他引:11
F. A. Huntingford† C. Adams V. A. Braithwaite‡ S. Kadri T. G. Pottinger§ P. Sandøe¶ J. F. Turnbull 《Journal of fish biology》2006,68(2):332-372
Human beings may affect the welfare of fish through fisheries, aquaculture and a number of other activities. There is no agreement on just how to weigh the concern for welfare of fish against the human interests involved, but ethical frameworks exist that suggest how this might be approached. Different definitions of animal welfare focus on an animal's condition, on its subjective experience of that condition and/or on whether it can lead a natural life. These provide different, legitimate, perspectives, but the approach taken in this paper is to focus on welfare as the absence of suffering. An unresolved and controversial issue in discussions about animal welfare is whether non‐human animals exposed to adverse experiences such as physical injury or confinement experience what humans would call suffering. The neocortex, which in humans is an important part of the neural mechanism that generates the subjective experience of suffering, is lacking in fish and non‐mammalian animals, and it has been argued that its absence in fish indicates that fish cannot suffer. A strong alternative view, however, is that complex animals with sophisticated behaviour, such as fish, probably have the capacity for suffering, though this may be different in degree and kind from the human experience of this state. Recent empirical studies support this view and show that painful stimuli are, at least, strongly aversive to fish. Consequently, injury or experience of other harmful conditions is a cause for concern in terms of welfare of individual fish. There is also growing evidence that fish can experience fear‐like states and that they avoid situations in which they have experienced adverse conditions. Human activities that potentially compromise fish welfare include anthropogenic changes to the environment, commercial fisheries, recreational angling, aquaculture, ornamental fish keeping and scientific research. The resulting harm to fish welfare is a cost that must be minimized and weighed against the benefits of the activity concerned. Wild fish naturally experience a variety of adverse conditions, from attack by predators or conspecifics to starvation or exposure to poor environmental conditions. This does not make it acceptable for humans to impose such conditions on fish, but it does suggest that fish will have mechanisms to cope with these conditions and reminds us that pain responses are in some cases adaptive (for example, suppressing feeding when injured). In common with all vertebrates, fish respond to environmental challenges with a series of adaptive neuro‐endocrine adjustments that are collectively termed the stress response. These in turn induce reversible metabolic and behavioural changes that make the fish better able to overcome or avoid the challenge and are undoubtedly beneficial, in the short‐term at least. In contrast, prolonged activation of the stress response is damaging and leads to immuno‐suppression, reduced growth and reproductive dysfunction. Indicators associated with the response to chronic stress (physiological endpoints, disease status and behaviour) provide a potential source of information on the welfare status of a fish. The most reliable assessment of well‐being will be obtained by examining a range of informative measures and statistical techniques are available that enable several such measures to be combined objectively. A growing body of evidence tells us that many human activities can harm fish welfare, but that the effects depend on the species and life‐history stage concerned and are also context‐dependent. For example, in aquaculture, adverse effects related to stocking density may be eliminated if good water quality is maintained. At low densities, bad water quality may be less likely to arise whereas social interactions may cause greater welfare problems. A number of key differences between fish and birds and mammals have important implications for their welfare. Fish do not need to fuel a high body temperature, so the effects of food deprivation on welfare are not so marked. For species that live naturally in large shoals, low rather than high densities may be harmful. On the other hand, fish are in intimate contact with their environment through the huge surface area of their gills, so they are vulnerable to poor water quality and water borne pollutants. Extrapolation between taxa is dangerous and general frameworks for ensuring welfare in other vertebrate animals need to be modified before they can be usefully applied to fish. The scientific study of fish welfare is at an early stage compared with work on other vertebrates and a great deal of what we need to know is yet to be discovered. It is clearly the case that fish, though different from birds and mammals, however, are sophisticated animals, far removed from unfeeling creatures with a 15 s memory of popular misconception. A heightened appreciation of these points in those who exploit fish and in those who seek to protect them would go a long way towards improving fish welfare. 相似文献
2.
Testing for symmetry 总被引:1,自引:0,他引:1
3.
4.
人分裂细胞核抗原基因片段的筛选、测序及对启动子的分析 总被引:1,自引:0,他引:1
经6.6×105个克隆筛选,从装在λ噬菌体载体Charon30中的人基因库中筛选到了一个含人分裂细胞核抗原(PCNA)基因的克隆。经Southern杂交分析插入基因长约14kb,有较长的5'上游区,但3'端缺少一部分。经亚克隆和测序已确定从5'上游1263bp到3'端与λ载体接点共4969bpPCNA基因片段的核苷酸序列。将PCNA基因启动子核苷酸序列与DNA聚合酶α,拓扑异构酶Ⅱα,胸苷酸激酶基因的启动子进行比较有30%以上同源性,具有“看家基因”特征。在转录起始点的5'上游几百bp的范围内都有与CAT,SP1,E2F,NFHB,Oct1和ATF等转录因子的结合位点相似的核苷酸序列。 相似文献
5.
温度对麦长管蚜种群存活率的特征参数的影响 总被引:2,自引:0,他引:2
在测定麦长管蚜种群存活率基础上,本提出反S型曲线模型来描述种群存活率,探讨了麦长管蚜种群存活率过程变化的特征参数,并得到不同的特征值. 相似文献
6.
In this paper, the author studies the quartic bioecology differential system:Here n and q are positive constants, l、m and p are constants. The author also proves some theorems of existence and uniqueness of limit cycles of this differential system. 相似文献
7.
8.
Simon M. Munthali 《Biodiversity and Conservation》1997,6(2):253-261
Most of Malawi's popular food-fish species are in decline. The fishers attribute this decline to overfishing, changes in climate, inadequate fishing technology and supernatural powers. Sedimentation of the lake and rivers due to poor agricultural practices, which can also be implicated in the demise of Malawi's fisheries, was not recognized by the fishers. This study tested and confirmed that the fishers would readily switch to exploiting ornamental fish (Mbuna) for subsistence and cash if the more favoured food-fish species continued to dwindle. However, as most of the Mbuna species are localized to specific rocky habitats and are endemic to certain parts of the lake, they may be vulnerable to overfishing. To safeguard Lake Malawi's rich fish species diversity: (i) alternative economic opportunities for the lakeshore people should be identified, evaluated and promoted to alleviate the fishing pressure, (ii) the Department of Fisheries should collaborate with local fishermen in promoting sustainable fish utilization, e.g., through public campaigns aimed at sensitizing fishermen on the status of the fisheries and the vulnerability of some fish communities to overfishing and environmental degradation, and (iii) the degree of Mbuna exploitation, now that the popular food fishes are in decline, should be monitored and, where necessary, regulated to promote sustainability. 相似文献
9.
外域杂草的产生、传播及生物与生态学特性的分析 总被引:32,自引:4,他引:28
本文分析了浙江省境内较近期传入的外域杂草区系成分,介绍了我国一些常见外域杂草的产生及传播特点,并通过对北美车前(Plantagovirginica)繁殖机制,种群生态、分布及危害特点等方面的研究.阐述了外域杂草所具的生物与生态学特性. 相似文献
10.