Sex-change rules, stock dynamics, and the performance of spawning-per-recruit measures in protogynous stocks

Predicting and under-standing the dynamics of a population requires knowledge of vital rates such as survival, growth, and reproduction. However, these variables are influenced by individual behavior, and when managing exploited populations, it is now generally realized that knowledge of a species’ behavior and life history strategies is required. However, predicting and understanding a response to novel conditions—such as increased fishing-induced mortality, changes in environmental conditions, or specific management strategies—also require knowing the endogenous or exogenous cues that induce phenotypic changes and knowing whether these behaviors and life history patterns are plastic. Although a wide variety of patterns of sex change have been observed in the wild, it is not known how the specific sex-change rule and cues that induce sex change affect stock dynamics. Using an individual based model, we examined the effect of the sex-change rule on the predicted stock dynamics, the effect of mating group size, and the performance of traditional spawning-per-recruit (SPR) measures in a protogynous stock. We considered four different patterns of sex change in which the probability of sex change is determined by 1) the absolute size of the individual, 2) the relative length of individuals at the mating site, 3) the frequency of smaller individuals at the mating site, and 4) expected reproductive success. All four pat-terns of sex change have distinct stock dynamics. Although each sex-change rule leads to the prediction that the stock will be sensitive to the size-selective fishing pattern and may crash if too many reproductive size classes are fished, the performance of traditional spawning-per-recruit measures, the fishing pattern that leads to the greatest yield, and the effect of mating group size all differ distinctly for the four sex-change rules. These results indicate that the management of individual species requires knowledge of whether sex change occurs, as well as an understanding of the endogenous or exogenous cues that induce sex change

A prior for steepness in stock-recruitment relationships, based on an evolutionary persistence principle

Priors are existing information or beliefs that are needed in Bayesian analysis. Informative priors are important in obtaining the Bayesian posterior distributions for estimated parameters in stock assessment. In the case of the steepness parameter (h), the need for an informative prior is particularly important because it determines the stock-recruitment relationships in the model. However, specifications of the priors for the h parameter are often subjective. We used a simple population model to derive h priors based on life history considerations. The model was based on the evolutionary principle that persistence of any species, given its life history (i.e., natural mortality rate) and its exposure to recruitment variability, requires a minimum recruitment compensation that enables the species to rebound consistently from low critical abundances (Nc). Using the model, we derived the prior probability distributions of the h parameter for fish species that have a range of natural mortality, recruitment variabilities, and Nt values

Regulatory Mechanisms and Information Processing in Uncertain Fisheries

We study the effects on fisherman decision processes of periodic (e.g., weekly) individual quotas. In the model, the fisherman must choose at the start of each week which of two grounds to fish on. The catch per week on each ground is a random variable and the fisherman does not know with certainty the parameters of the distribution of that variable. He does have estimates on each parameter and can improve these estimates by Bayesian updating. The choice of a fishing ground takes into account the expected catch on that ground and the expected improvement in information from fishing on that ground. Our study is concerned with the effect of weekly quotas on the joint production of information and fish. Various policy implications are discussed, and the results are compared with the policy analysis of Clark (1980) in the deterministic case. We show that the quota affects the value of Information and that if quotas are transferable, then the quota may limit its own value.Environmental Economics and Policy, Research Methods/ Statistical Methods, Resource /Energy Economics and Policy, Risk and Uncertainty,

Pest science in Pasteur’s Quadrant

More than 25 years ago, Donald Stokes argued that we must move beyond the false dichotomy of basic or applied research and suggested that when considering a program of scientific research it is important to ask whether (i) the work is motivated by use and (ii) if there is a search for fundamental understanding. Giving yes/no answers to these questions allows us to characterize research more fully, replacing the “or” of “basic or applied” by a richer understanding of the process of science. Stokes proposed that research that was motivated by a consideration of use and sought fundamental understanding be called research in Pasteur’s Quadrant. One advantage of such work is that the search for fundamental understanding means that the problem-solving tools are more likely to be transferrable. After reviewing Stokes’s formulation of research, I illustrate it with examples from the control of tephritid flies and the use of insect parasitoids for biological control. Thinking about one’s work within Stokes’s framework has many advantages for individual scientists, including guidance for journal selection, how to organize and conclude papers and seminars, and the “elevator speech.” Furthermore, since research in Pasteur’s Quadrant has the characteristic of simultaneously increasing our understanding of how the world works and improving applications, it will more likely benefit the community of pest scientists.publishedVersio

Operational analysis for COVID-19 testing: Determining the risk from asymptomatic infections

Testing remains a key tool for managing health care and making health policy during the coronavirus pandemic, and it will probably be important in future pandemics. Because of false negative and false positive tests, the observed fraction of positive tests—the surface positivity—is generally different from the fraction of infected individuals (the incidence rate of the disease). In this paper a previous method for translating surface positivity to a point estimate for incidence rate, then to an appropriate range of values for the incidence rate consistent with the model and data (the test range), and finally to the risk (the probability of including one infected individual) associated with groups of different sizes is illustrated. The method is then extended to include asymptomatic infections. To do so, the process of testing is modeled using both analysis and Monte Carlo simulation. Doing so shows that it is possible to determine point estimates for the fraction of infected and symptomatic individuals, the fraction of uninfected and symptomatic individuals, and the ratio of infected asymptomatic individuals to infected symptomatic individuals. Inclusion of symptom status generalizes the test range from an interval to a region in the plane determined by the incidence rate and the ratio of asymptomatic to symptomatic infections; likelihood methods can be used to determine the contour of the rest region. Points on this contour can be used to compute the risk (defined as the probability of including one asymptomatic infected individual) in groups of different sizes. These results have operational implications that include: positivity rate is not incidence rate; symptom status at testing can provide valuable information about asymptomatic infections; collecting information on time since putative virus exposure at testing is valuable for determining point estimates and test ranges; risk is a graded (rather than binary) function of group size; and because the information provided by testing becomes more accurate with more tests but at a decreasing rate, it is possible to over-test fixed spatial regions. The paper concludes with limitations of the method and directions for future work.publishedVersio

Modeling Coupled Nonlinear Multilayered Dynamics: Cyber Attack and Disruption of an Electric Grid

We study the consequences of cyberattack, defense, and recovery in systems for which a physical system is enabled by a cyber system by extending previous applications of models from the population biology of disease to the cyber system and coupling the state of the cyber system to the physical system, using the synchronous model for the electric grid. In analogy to disease models in which individuals are susceptible, infected, or recovered, in the cyber system, components can be uncompromised and vulnerable to attack, uncompromised and temporarily invulnerable to attack, compromised, or reset and thus not able to contribute to the performance of the physical system. We model cyber defensive countermeasures in analogy to the adaptive immune system. We link the physical and cyber systems through a metric of performance of the physical system that depends upon the state of the cyber system using (i) a generic nonlinear relationship between the state of the cyber system and the performance of the physical system and (ii) the synchronous motor model of an electric grid consisting of a utility with many customers whose smart meters can become compromised, in which a steady state in the difference in rotor angles is the metric of performance. We use the coupled models, both of which have emergent properties, to investigate two situations. First, when an attacker that relies on stealth compromise is hidden until it is either detected during routine maintenance or an attack is initiated. The probability that compromise remains undetected declines with time and the level of compromise increases with time. Because of these dynamics, an optimal time of attack emerges, and we explore how it varies with parameters of the cyber system. Second, we illustrate one of the Electric Power Research Institute scenarios for the reverse engineering of Advanced Metering Infrastructure (AMI) by coupling the synchronous motor equations for the generator and utility to the model of compromise. We derive a canonical condition for grid failure that relates the level of compromise at the time of detection of compromise and the dissipation parameter in the synchronous motor model. We conclude by discussing the innovative aspects of our methods, which include (i) a fraction of decoy components in the cyber system, which are not connected to the rest of the cyber system or the physical system and thus do not spread compromise but increase the probability of detection of compromise, (ii) allowing components of the cyber system to return to the un-compromised state either temporarily invulnerable or immediately vulnerable, (iii) adaptive Defensive Counter Measures that respond in a nonlinear fashion to attack and compromise (in analogy to killer T cells of the immune system), (iv) a generic metric of performance of the physical system that depends upon the state of the cyber system, and (v) coupling a model of the electric grid to the model of compromise of the cyber system that leads to a condition for failure of the grid in terms of parameters of both compromise and the synchronous motor model, directions for future investigations, and connections to recent studies on broadly the same topics. We include a pseudocode as an Appendix and indicate how to obtain R script for the models from the first author.publishedVersio

Construction of multidimensional clustered patterns

Journal ArticleEcological processes often depend upon the patterning, as well as the absolute density, of resources. In this paper, we develop methods for describing pattern from the perspective of the organism encountering and exploiting the resources, and for reconstructing pattern from the description

The effects of size-selective fisheries on the stock dynamics of and sperm limitation in sex-changing fish

Fisheries models have traditionally focused on patterns of growth, fecundity, and survival of fish. However, reproductive rates are the outcome of a variety of interconnected factors such as life-history strategies, mating patterns, population sex ratio, social interactions, and individual fecundity and fertility. Behaviorally appropriate models are necessary to understand stock dynamics and predict the success of management strategies. Protogynous sex-changing fish present a challenge for management because size-selective fisheries can drastically reduce reproductive rates. We present a general framework using an individual-based simulation model to determine the effect of life-history pattern, sperm production, mating system, and management strategy on stock dynamics. We apply this general approach to the specific question of how size-selective fisheries that remove mainly males will impact the stock dynamics of a protogynous population with fixed sex change compared to an otherwise identical dioecious population. In this dioecious population, we kept all aspects of the stock constant except for the pattern of sex determination (i.e. whether the species changes sex or is dioecious). Protogynous stocks with fixed sex change are predicted to be very sensitive to the size-selective fishing pattern. If all male size classes are fished, protogynous populations are predicted to crash even at relatively low fishing mortality. When some male size classes escape fishing, we predict that the mean population size of sex-changing stocks will decrease proportionally less than the mean population size of dioecious species experiencing the same fishing mortality. For protogynous species, spawning-per-recruit measures that ignore fertilization rates are not good indicators of the impact of fishing on the population. Decreased mating aggregation size is predicted to lead to an increased effect of sperm limitation at constant fishing mortality and effort. Marine protected areas have the potential to mitigate some effects of fishing on sperm limitation in sex-changing populations