梁子湖乌鳢种群动态及最大持续渔获量的研究

根据1997年3月—1998年2月在梁子湖取得的672尾乌鳢标本研究了其种群动态及最大持续渔获量。用鳞片鉴定年龄,鳞径与体长的关系为L=2.7683 9.0184S;体长与体重的关系为W=0.0126L~(3.0197),体长与体重的von Bertalanffy生长方程为:L_t=81.03×[1-e~(-0.1755(t 0.5690))];W_t=7310×[1-e~(-0.1755(t 0.5690))]~(3.0197)。种群的年总死亡率为0.5232,自然死亡率为0.3049,捕捞死亡率为0.2182。通过比较各种捕捞强度和最小捕捞规格的不同的总渔获量和群体的相对数量,提出获得持续渔获量的捕捞强度F=0.40和最小捕捞年龄t_c=4。     

基于持续高产的近海渔业双寡头捕捞策略

海洋鱼类是人类一种重要的生活物质资料,当代人在进行渔业捕捞满足自身生活需求时,应合理确定捕捞努力量以实现渔业资源的可持续利用,不危及后代人的需求,假定存在两个捕捞主体,分别就Cournot模型和Stackelbeerg模型分析了两个主体为了自身获得最大持续产量而投入的捕捞努力量,研究表明,与只有一个捕捞主体相比,当存在两个捕捞主体时,每个捕捞主体都将投入更多的捕捞努力量,但最大持续产量不随之增加,甚至还会减少,产生这种后果的原因在于每个主体只考虑自己投入的捕捞 努力量对自己产出量的影响,而不考虑对对方或社会产生的负面影响,对渔业捕捞进行全面规划,综合管理是消除这种后果所必要的。     

On the potential for illogic with logically defined outcomes

Logically defined outcomes are commonly used in medical diagnoses and epidemiological research. When missing values in the original outcomes exist, the method of handling the missingness can have unintended consequences, even if the original outcomes are missing completely at random. In this note, we consider 2 binary original outcomes, which are missing completely at random. For estimating the prevalence of a logically defined "or" outcome, we discuss the properties of 4 estimators: the complete-case estimator, the available-case estimator, the maximum likelihood estimator (MLE), and a moment-based estimator. With the exception of the available-case case estimator, all the estimators are consistent. The MLE exhibits superior performance and should be generally adopted.     

The dynamics of bacteria-plasmid systems

We introduce a general model for the dynamics of a single plasmid type and a single bacterial cell type, following Stewart and Levin (1977) in subdividing the population into plasmid-bearing and plasmid-free cells. For the particular case of mortality being a linear function of population sizes, we demonstrate the existence of multiple stable states and threshold values, thus illustrating that, in some cases, the establishment of a population of plasmids may depend on introduction of plasmids at sufficiently high levels. We also analyze in detail, for a general monotonically increasing mortality function, the epidemiological case in which plasmids confer a net cost on their hosts, and demonstrate that it is possible for such plasmids to become established. Stewart and Levin previously demonstrated this effect in a more restricted model.     

An adaptive management approach to an octocoral fishery based on the Beverton-Holt model

Coral reef species are frequently the focus of bio-prospecting, and when promising bioactive compounds are identified there is often a need for the development of responsible harvesting based on relatively limited data. The Caribbean gorgonian Pseudopterogorgia elisabethae has been harvested in the Bahamas for over a decade. Data on population age structure and growth rates in conjunction with harvest data provide an opportunity to compare fishery practices and outcomes to those suggested by a Beverton-Holt fishery model. The model suggests a minimum colony size limit of 7–9 years of age (21–28 cm height), which would allow each colony 2–4 years of reproduction prior to harvesting. The Beverton-Holt model assumes that colonies at or above the minimum size limit are completely removed. In the P. elisabethae fishery, colonies are partially clipped and can be repeatedly harvested. Linear growth of surviving colonies was up to 3 times that predicted for colonies that were not harvested and biomass increase was up to 9 times greater than that predicted for undisturbed colonies. The survival of harvested colonies and compensatory growth increases yield, and yields at sites that had previously been harvested were generally greater than predicted by the Beverton-Holt model. The model also assumes recruitment is independent of fishing intensity, but lower numbers of young colonies in the fished populations, compared to unfished populations, suggest possible negative effects of the harvest on reproduction. This suggests the need for longer intervals between harvests. Because it can be developed from data that can be collected at a single time, the Beverton-Holt model provides a rational starting point for regulating new fisheries where long-term characterizations of population dynamics are rarely available. However, an adaptive approach to the fishery requires the incorporation of reproductive data.     

An epidemic model in a patchy environment

An epidemic model is proposed to describe the dynamics of disease spread among patches due to population dispersal. We establish a threshold above which the disease is uniformly persistent and below which disease-free equilibrium is locally attractive, and globally attractive when both susceptible and infective individuals in each patch have the same dispersal rate. Two examples are given to illustrate that the population dispersal plays an important role for the disease spread. The first one shows that the population dispersal can intensify the disease spread if the reproduction number for one patch is large, and can reduce the disease spread if the reproduction numbers for all patches are suitable and the population dispersal rate is strong. The second example indicates that a population dispersal results in the spread of the disease in all patches, even though the disease can not spread in each isolated patch.     

A stochastic modeling of early HIV-1 population dynamics

In a recent paper, Tuckwell and Le Corfec [J. Theor. Biol. 195 (1998) 450-463] applied the multi-dimensional diffusion process to model early human immunodeficiency virus type-1 (HIV-1) population dynamics. The purpose of this paper is to assess certain features and consequences of their model in the context of Tan and Wu's stochastic approach [Math. Biosci. 147 (1998) 173-205].     

A dynamic model of water gain and loss in larval Mexican bean beetle

Abstract. Quantitative effects of temperature, vapour pressure deficit, host, and larval body size were experimentally determined. A simulation model for dynamic water balance in the Mexican bean beetle, Epilachna varivestis Mulsant, is presented and parameters are estimated from laboratory data for water gain/loss equations. The model is based on water loss through the cuticle, spiracles and frass, and water gain through ingestion.