A catch-free stock assessment model with application to goliath grouper (Epinephelus itajara) off southern Florida

Many modern stock assessment methods provide the machinery for determining the status of a stock in relation to certain reference points and for estimating how quickly a stock can be rebuilt. However, these methods typically require catch data, which are not always available. We introduce a model-based framework for estimating reference points, stock status, and recovery times in situations where catch data and other measures of absolute abundance are unavailable. The specif ic estimator developed is essentially an age-structured production model recast in terms relative to pre-exploitation levels. A Bayesian estimation scheme is adopted to allow the incorporation of pertinent auxiliary information such as might be obtained from meta-analyses of similar stocks or anecdotal observations. The approach is applied to the population of goliath grouper (Epinephelus itajara) off southern Florida, for which there are three indices of relative abundance but no reliable catch data. The results confirm anecdotal accounts of a marked decline in abundance during the 1980s followed by a substantial increase after the harvest of goliath grouper was banned in 1990. The ban appears to have reduced fishing pressure to between 10% and 50% of the levels observed during the 1980s. Nevertheless, the predicted fishing mortality rate under the ban appears to remain substantial, perhaps owing to illegal harvest and depth-related release mortality. As a result, the base model predicts that there is less than a 40% chance that the spawning biomass will recover to a level that would produce a 50% spawning potential ratio

Analyzing dropout rates of B.Sc students

Survival analysis is a useful statistical technique for analyzing failure time data. It overcomes the limitations of cross-sectional analysis and convention regression analysis. This study proposes three statistical models that analyze the failure time when university students withdraw from B.Sc courses. The sample comprises all the 91 students who commenced their four-year studies with the Faculty of Science in 2002. The first approach uses the Kaplan-Meier product limit method for estimating the survival functions under non-informative censoring. The second approach uses the Cox regression model, which involves the assumption of proportional hazard functions. The third approach uses a parametric model to estimate the hazard function using an appropriate distribution. The aim of the study is to fit survival models that predict the probabilities of retention and dropout of B.Sc students within each study area using the facilities of SPSS and STATA.peer-reviewe

Properties of sunspot umbrae observed in Cycle 24

We analyzed the size, intensity, and magnetic field strength of sunspot umbrae to compare the present cycle 24 with the previous one. We used data of the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory and selected all sunspots between May 2010 and October 2012, using one image per day. We created two subsets of this data with a manual tracking algorithm, both without duplication. One is containing each sunspot (910 umbrae within 488 spots) and was used to analyze the distribution of umbral areas, selected with an automated thresholding method. The other one contains 205 fully evolved sunspots. We find nonlinear relations between umbral minimum intensity and size and between maximum magnetic field strength and size. The field strength scales linear with the intensity and the umbral size scales roughly linear with the total magnetic flux, while the size and field strength level off with stronger flux. When separated in hemisphere and averaged temporally, the southern umbrae show a temporal increase in size and the northern umbrae stay constant. There is no temporal variation in the umbral mean intensity detectable. The probability density function of the umbral area in the ascending phase of the current solar cycle is similar to that of the last solar cycle. From our investigation of umbral area, magnetic field, magnetic flux and umbral intensity of the sunspots of the rising phase of cycle 24, we do not find a significant difference to the previous cycle, and hence no indication for a long-term decline of solar activity.Comment: 11 pages, 9 figures, to be published in Astronomy & Astrophysic

Imputing Risk Tolerance from Survey Responses

Economic theory assigns a central role to risk preferences. This paper develops a measure of relative risk tolerance using responses to hypothetical income gambles in the Health and Retirement Study. In contrast to most survey measures that produce an ordinal metric, this paper shows how to construct a cardinal proxy for the risk tolerance of each survey respondent. The paper also shows how to account for measurement error in estimating this proxy and how to obtain consistent regression estimates despite the measurement error. The risk tolerance proxy is shown to explain differences in asset allocation across households.