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换能器的转换效率是它的一项重要技术指标,效率的高低将直接影响整机的性能指标和使用效果。效率的测量方法通常有直接测量法、阻抗(导纳)园图法、功率曲线法等等。由于这几种方法都要求有一定的专用仪器设备和条件,而且测量计算繁琐。所以,一些生产单位和修理、使用部门,通常只用谐振法和替代法测量出谐振频率及其所对应的阻抗值,来估算换能器的好坏。 相似文献
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斑尾复鰕虎鱼系沿海及河口的习见种类,矛尾复鰕虎鱼为它的同物异名。该鱼属底栖肉食性鱼类,主要以小鱼和小虾蟹为食。在繁殖期间产两次卵,属多次产卵类型,产卵后不久死亡,寿命为壹年。斑尾复鰕虎鱼为鰕虎鱼科中的大型种类,其体长和体重的相关关系式为:W=0.01739~(2.827?),生长方程为:L_t=41.24[1-e~(-0.0?(t+0.0?45?)],W_t=606.15[1?e~(-0.0?(t+0.0645?)]~(2.8?),体重生长拐点 t_r=11.5(月龄)离拐点处体重为 176.5克。本文分析了斑尾复鰕虎鱼在养虾业中的危害情况,并探讨其资源的发展。 相似文献
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基于2012年2~3月南海中部鱼类资源声学调查资料,利用Echoview系统中虚拟变量模块的功能,探讨扣除背景噪声对声学映像、单脉冲回声信号图、海里面积散射系数(NASC)和体积反向散射强度(Sv)的影响。结果显示,背景噪声对120kHz数据的干扰明显强于38kHz;随着水深逐渐增加,扣除背景噪声后,120kHz声学映像变化亦比38kHz更加明显;扣除背景噪声时,换能器表面1m处的Sv值在不断改变过程中,相同水层中不同频率数据的NASC和Sv的变化差异明显,其差异程度因具体水层而异,相同频率数据在不同水层的NASC和Sv的变化差异亦明显。分析认为,扣除背景噪声非常必要;本研究的方法能够较好地排除背景噪声,是提高资源评估准确度的有效手段。 相似文献
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南海北部主要捕捞种类最适开捕规格研究 总被引:15,自引:0,他引:15
根据南海水产研究所使用elefan技术基于体长数据求得的南海北部50种主要捕捞种类的von Bertalanffy生长参数,以Beverton-Holt模型计算最适开捕年龄后用von Bertalanffy生长方程计算其相应开捕体长,再参考实际调查最小性成熟体长、临界体长、拐点体长和Froese-Binohlan经验公式计算的初次性成熟体长范围等数据确定最适开捕规格;部份分海区研究的种类先确定同一种类于各海区的最适开捕规格,再考虑不同海区的适用性确定其在南海北部的最适开捕规格。本文研究了南海北部50种主要捕捞种类的最适开捕规格,结果表明,所求得的最适开捕规格比原广东、广西、福建“水产资源繁殖保护实施细则暂行规定”的开捕规格都大;建议选择19种在以往的生产中和历次调查研究中出现频率较高、产量较高的主要捕捞种类作为南海北部实行开捕规格保护幼鱼幼虾措施的指标种,并提出开捕规格的随机抽样检查方法。 相似文献
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《Fisheries Research》2007,87(2-3):153-158
Gulland's [Gulland, J.A., 1965. Estimation of mortality rates. Annex to Arctic Fisheries Working Group Report (meeting in Hamburg, January 1965). ICES, C.M. 1965, Doc. No. 3 (mimeographed)] virtual population analysis (VPA) is commonly used for studying the dynamics of harvested fish populations. However, it necessitates the solving of a nonlinear equation for the instantaneous rate of fishing mortality of the fish in a population. Pope [Pope, J.G., 1972. An investigation of the accuracy of Virtual Population Analysis using cohort analysis. ICNAF Res. Bull. 9, 65–74. Also available in D.H. Cushing (ed.) (1983), Key Papers on Fish Populations, p. 291–301, IRL Press, Oxford, 405 p.] eliminated this necessity in his cohort analysis by approximating its underlying age- and time-dependent population model. His approximation has since become one of the most commonly used age- and time-dependent fish population models in fisheries science. However, some of its properties are not well understood. For example, many assert that it describes the dynamics of a fish population, from which the catch of fish is taken instantaneously in the middle of the year. Such an assertion has never been proven, nor has its implied instantaneous rate of fishing mortality of the fish of a particular age at a particular time been examined, nor has its implied catch equation been derived from a general catch equation. In this paper, we prove this assertion, examine its implied instantaneous rate of fishing mortality of the fish of a particular age at a particular time, derive its implied catch equation from a general catch equation, and comment on how to structure an age- and time-dependent population model to ensure its internal consistency. This work shows that Gulland's (1965) virtual population analysis and Pope's (1972) cohort analysis lie at the opposite end of a continuous spectrum as a general model for a seasonally occurring fishery; Pope's (1972) approximation implies an infinitely large instantaneous rate of fishing mortality of the fish of a particular age at a particular time in a fishing season of zero length; and its implied catch equation has an undefined instantaneous rate of fishing mortality of the fish in a population, but a well-defined cumulative instantaneous rate of fishing mortality of the fish in the population. This work also highlights a need for a more careful treatment of the times of start and end of a fishing season in fish population models. 相似文献
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本文是根据云南星云湖、杞麓湖中所产大头鲤进行一些生态习性的研究。大头鲤的年龄鉴定是依据鳞片上环片切割现象。依星云湖的标本实测平均体长,Ⅰ龄为135.12毫米,Ⅱ龄鱼为196.93毫米,Ⅲ龄鱼为241.92毫米,Ⅳ龄焦为288毫米。星云湖和杞麓湖大头鲤体长和体重的关系式分别为:W=0.00001283L3.0719,W=0.00002181L2.9895。食性极为单纯,主要是以枝角类和桡足类为食。不同大小个体的食性差异不大。Ⅰ龄鱼部分达到性成熟,Ⅱ龄鱼全部性成熟。其中性成熟最小个体雌鱼体长为140毫米、体重52克。雄焦体长为117毫米,体重32克。生殖季节是4~9月,以5~6月为旺季。分批产卵,卵粘附于水草上。从星云湖五批大头鲤渔获物的长度组成来看,捕捞对象是以Ⅰ、Ⅱ龄鱼占绝对优势,这可以说明捕捞过度已引起资源的衰退。为此我们提出大头鲤在湖内捕捞最低规格是全长200毫米、体重100克。并且建议在每年5~6月生殖旺季时可实行禁捕一个月。 相似文献