全文获取类型
| 收费全文 | 102篇 |
| 免费 | 10篇 |
| 国内免费 | 21篇 |
专业分类
| 测绘学 | 11篇 |
| 大气科学 | 11篇 |
| 地球物理 | 12篇 |
| 地质学 | 38篇 |
| 海洋学 | 27篇 |
| 综合类 | 14篇 |
| 自然地理 | 20篇 |
出版年
| 2024年 | 1篇 |
| 2023年 | 2篇 |
| 2022年 | 4篇 |
| 2021年 | 2篇 |
| 2020年 | 8篇 |
| 2019年 | 10篇 |
| 2018年 | 13篇 |
| 2017年 | 5篇 |
| 2016年 | 5篇 |
| 2015年 | 11篇 |
| 2014年 | 12篇 |
| 2013年 | 7篇 |
| 2012年 | 4篇 |
| 2011年 | 9篇 |
| 2010年 | 12篇 |
| 2009年 | 7篇 |
| 2008年 | 5篇 |
| 2007年 | 1篇 |
| 2006年 | 5篇 |
| 2005年 | 1篇 |
| 2004年 | 5篇 |
| 2003年 | 2篇 |
| 1992年 | 1篇 |
| 1987年 | 1篇 |
排序方式： 共有133条查询结果，搜索用时 46 毫秒
51.
刺网选择性对鱼类表型性状的选择作用研究 总被引：2，自引：0，他引：2
本研究从刺网选择性对鱼类表型性状的影响入手,应用数值模拟方法,模拟了一个具有一定体长特征的鱼类种群,利用具有多项选择参数(曲线标准差σ,最适体长l0,选择性因素R0)的刺网曲线对其进行长期捕捞作用,并在去除捕捞的情况下探讨这种影响的稳定性。结果表明,刺网捕捞作业长期作用于鱼类种群,会对种群体长组成产生深远的影响,开发率(E)越大,效果越明显。当刺网最适捕捞体长与初始种群优势体长重合时(l0=15),造成种群体长分布峰值的分化;如果l0>15,则导致种群体长组成向小型化方向偏移;反之则会造成种群体长组成向大型化方向偏移;而且这种影响可能伴随遗传因素,具有不可逆性。 相似文献
52.
利用线粒体细胞色素 c 氧化酶Ⅰ亚基(COI)部分序列研究了南海区鲳属鱼类(Pampus)银鲳(P. ar-genteus)、珍鲳(P. minor)和中国鲳(P. chinensis)的系统进化与种群遗传结构.通过 PCR 扩增与序列测定获得了长度为643 bp 的 COI 基因片段,其 A、T、G、C 碱基的平均含量分别为25.4%、33.6%、18.9%和22.1%, A+T 含量高于 G+C 含量.64条序列共定义了20种单倍型,包含152个变异位点,简约信息位点148个,单变异位点4个,产生169个点突变.结果表明,银鲳与中国鲳的遗传差异最小,银鲳与珍鲳的其次,而珍鲳与中国鲳的遗传差异最大,3种鲳属鱼类的遗传多样性均呈现较低的水平,应采取有效的保护措施,以避免其遗传多样性水平的进一步丧失.本研究结果为鲳属鱼类资源保护和合理开发利用提供必要的参考 相似文献
53.
新疆塔河流域洪水量级、频率及峰现时间变化特征、成因及影响 总被引：2，自引：0，他引：2
采用塔里木河流域（塔河流域）8个水文站及相应气象站数据,全面分析了洪水发生量级、频率和峰现时间等特征,研究洪水发生成因及其影响。结果表明：1980s中后期塔河流域气温与降水持续增加,整个塔河流域年及季节洪峰流量普遍呈上升趋势,大部分在1980s中后期发生突变。1980s中期以后塔河流域年及季节洪峰流量呈持续增加或者显著增加趋势,量级位于整个观测时期均值之上,处于洪水“丰富”期。“丰富”期暴雨型和升温型洪水发生次数及造成的灾害损失均呈显著增加趋势,引起严重洪灾损失的洪水也集中在这一时期,且多由暴雨型洪水引发。大量级洪水（最大三场洪水及重现期大于10年的洪水）多集中发生在1990年之后,并且易引发多个水文站点同时出现。 相似文献
54.
55.
前兆观测干扰信号频谱特征分析 总被引：2，自引：2，他引：0
地震前兆观测数据中干扰信号频繁出现,大大增加了地震前兆异常信息提取难度。本文选用基于傅里叶变换的功率谱、傅里叶谱以及基于希尔伯特-黄变换的边际谱方法,针对黑龙江地区数字化水位和竖直摆倾斜测项中的典型干扰信号进行分析,提取了水位高频干扰、周期干扰以及竖直摆倾斜的风扰、同震应变等常见干扰信号的频谱特征。通过对比3种频谱分析方法的差异性,发现功率谱法和傅里叶谱对提取干扰和噪声等平稳信号更加有效,而边际谱法对提取同震应变波等非平稳信号的效果更加显著。 相似文献
56.
57.
Steroid sex hormones,such as estradiol-17β(E2)and testosterone(T),are important regulators of sex change in fish.In this study,we examined the effects of E2 treatment on the dynamics of E2 and T during gonadal differentiation in the olive flounder Paralichthys olivaceus using histology and radioimmunoassay(RIA).Flounder larvae were divided into five groups(G0–G4),and fed with 0 (control),0.2,2,20 and 100 mg E2/kg feed from 35 to 110 day post hatching(dph).Fish growth in the G1 and G2 groups was not significantly different from that of the control group(P0.05),while fish in the G3 and G4 groups were less active and showed growth depression and high mortality.The gonads of fish in the G3 and G4 groups were smaller and surrounded by hyperplastic connective tissue.The frequency of females in the G0–G4 groups was 54.5%,75.0%,100%,100%and 93.3%,respectively.The RIA analyses of E2 and T showed that T levels decreased during gonadal differentiation,and increased slightly at the onset of ovarian differentiation,while E2 levels increased gradually and peaked at the onset of ovarian differentiation in the control group.In the E2-treated groups,T levels decreased before the onset of ovarian differentiation.E2 levels were high on the 48 dph,but declined to a lower level on the 54 dph, and then increased gradually during gonadal differentiation.And a sharp increase of E2 levels were observed in all E2-treated groups at the onset of ovarian differentiation.The data suggest that T and E2 play important roles during gonadal differentiation,and an E2 dose of 2 mg/kg feed could induce sex reversal in P.olivaceus. 相似文献
58.
The estimation of gear selectivity is a critical issue in fishery stock assessment and management. Several methods have been
developed for estimating gillnet selectivity, but they all have their limitations, such as inappropriate objective function
in data fitting, lack of unique estimates due to the difficulty in finding global minima in minimization, biased estimates
due to outliers, and estimations of selectivity being influenced by the predetermined selectivity functions. In this study,
we develop a new algorithm that can overcome the above-mentioned problems in estimating the gillnet selectivity. The proposed
algorithms include minimizing the sum of squared vertical distances between two adjacent points and minimizing the weighted
sum of squared vertical distances between two adjacent points in the presence of outliers. According to the estimated gillnet
selectivity curve, the selectivity function can also be determined. This study suggests that the proposed algorithm is not
sensitive to outliers in selectivity data and improves on the previous methods in estimating gillnet selectivity and relative
population density of fish when a gillnet is used as a sampling tool. We suggest the proposed approach be used in estimating
gillnet selectivity. 相似文献
59.
Steroid sex hormones, such as estradiol-17β (E2) and testosterone (T), are important regulators of sex change in fish. In
this study, we examined the effects of E2 treatment on the dynamics of E2 and T during gonadal differentiation in the olive
flounder Paralichthys olivaceus using histology and radioimmunoassay (RIA). Flounder larvae were divided into five groups (G0-G4), and fed with 0 (control),
0.2, 2, 20 and 100 mg E2/kg feed from 35 to 110 day post hatching (dph). Fish growth in the G1 and G2 groups was not significantly
different from that of the control group (P>0.05), while fish in the G3 and G4 groups were less active and showed growth depression and high mortality. The gonads of
fish in the G3 and G4 groups were smaller and surrounded by hyperplastic connective tissue. The frequency of females in the
G0-G4 groups was 54.5%, 75.0%, 100%, 100% and 93.3%, respectively. The RIA analyses of E2 and T showed that T levels decreased
during gonadal differentiation, and increased slightly at the onset of ovarian differentiation, while E2 levels increased
gradually and peaked at the onset of ovarian differentiation in the control group. In the E2-treated groups, T levels decreased
before the onset of ovarian differentiation. E2 levels were high on the 48 dph, but declined to a lower level on the 54 dph,
and then increased gradually during gonadal differentiation. And a sharp increase of E2 levels were observed in all E2-treated
groups at the onset of ovarian differentiation. The data suggest that T and E2 play important roles during gonadal differentiation,
and an E2 dose of 2 mg/kg feed could induce sex reversal in P. olivaceus. 相似文献