Estimating exploitable stock biomass for the Maine green sea urchin (Strongylocentrotus droebachiensis) fishery using a spatial statistics approach

The objective of this study was to investigate the spatial patterns in green sea urchin (Strongylocentrotus droebachiensis) density off the coast of Maine, using data from a fishery-independent survey program, to estimate the exploitable biomass of this species. The dependence of sea urchin variables on the environment, the lack of stationarity, and the presence of discontinuities in the study area made intrinsic geostatistics inappropriate for the study; therefore, we used triangulated irregular networks (TINs) to characterize the large-scale patterns in sea urchin density. The resulting density surfaces were modified to include only areas of the appropriate substrate type and depth zone, and were used to calculate total biomass. Exploitable biomass was estimated by using two different sea urchin density threshold values, which made different assumptions about the fishing industry. We observed considerable spatial variability on both small and large scales, including large-scale patterns in sea urchin density related to depth and fishing pressure. We conclude that the TIN method provides a reasonable spatial approach for generating biomass estimates for a fishery unsuited to geostatistics, but we suggest further studies into uncertainty estimation and the selection of threshold density values

Theory for Superconducting Properties of the Cuprates: Doping Dependence of the Electronic Excitations and Shadow States

The superconducting phase of the 2D one-band Hubbard model is studied within the FLEX approximation and by using an Eliashberg theory. We investigate the doping dependence of $T_c$, of the gap function $\Delta ({\bf k},\omega)$ and of the effective pairing interaction. Thus we find that $T_c$ becomes maximal for $13 \; \%$ doping. In {\it overdoped} systems $T_c$ decreases due to the weakening of the antiferromagnetic correlations, while in the {\it underdoped} systems due to the decreasing quasi particle lifetimes. Furthermore, we find {\it shadow states} below $T_c$ which affect the electronic excitation spectrum and lead to fine structure in photoemission experiments.Comment: 10 pages (REVTeX) with 5 figures (Postscript

Nursing Home Quality as a Public Good

There has been much debate among economists about whether nursing home quality is a public good across Medicaid and private-pay patients within a common facility. However, there has been only limited empirical work addressing this issue. Using a unique individual level panel of residents of nursing homes from seven states, we exploit both within-facility and within-patient variation in payer source and quality to examine this issue. We also test the robustness of these results across states with different Medicaid and private-pay rate differentials. Across our various identification strategies, the results generally support the idea that quality is a public good within nursing homes. That is, within a common nursing home, there is very little evidence to suggest that Medicaid-funded residents receive consistently lower quality care relative to their private-paying counterparts.

Dirac--Lie systems and Schwarzian equations

A Lie system is a system of differential equations admitting a superposition rule, i.e., a function describing its general solution in terms of any generic set of particular solutions and some constants. Following ideas going back to the Dirac's description of constrained systems, we introduce and analyse a particular class of Lie systems on Dirac manifolds, called Dirac--Lie systems, which are associated with `Dirac--Lie Hamiltonians'. Our results enable us to investigate constants of the motion, superposition rules, and other general properties of such systems in a more effective way. Several concepts of the theory of Lie systems are adapted to this `Dirac setting' and new applications of Dirac geometry in differential equations are presented. As an application, we analyze traveling wave solutions of Schwarzian equations, but our methods can be applied also to other classes of differential equations important for Physics.Comment: 41 page

Factor Substitution and Unobserved Factor Quality in Nursing Homes

This paper studies factor substitution in one important sector: the nursing home industry. Specifically, we measure the extent to which nursing homes substitute materials for labor when labor becomes relatively more expensive. From a policy perspective, factor substitution in this market is important because materials-intensive methods of care are associated with greater risks of morbidity and mortality among nursing home residents. Studying longitudinal data from 1991-1998 on nearly every nursing home in the United States, we use the method of instrumental variables (IV) to address the potential endogeneity of nursing home wages. The results from the IV models are consistent with the theory of factor substitution: higher nursing home wages are associated with lower staffing, greater use of materials (specifically, physical restraints), and a higher proportion of residents with pressure ulcers. A comparison of OLS and IV results suggests that empirical studies of factor substitution should take into account unobserved heterogeneity in factor quality.

Population Dynamics and Spatial Analysis of the Maine Green Sea Urchin (Strongylocentrotus droebachiensis) fishery

Fisheries research on the green sea urchin in Maine has been limited despite its importance to the state\u27s fishing industry. The objective of this thesis was to generate critical information for the management and monitoring of the Maine green sea urchin fishery. In particular there are three main areas of interest: (1) an investigation of biological reference points; (2) spatial analysis and biomass estimation, and (3) the development of a simulation framework approach to determine an optimal sampling strategy for the fishery-independent survey program. Biological reference points are markers conlrnonly used to monitor and manage fisheries. For the Maine sea urchin fishery, no biological reference point had been estimated as a management target, which made it difficult to determine the status of the stock and develop appropriate management plans. The purpose of this study was to investigate if Fo., and Fmax are appropriate management targets for the Maine sea urchin fishery and how uncertainties associated with them affect their suitability as management targets. A Monte Carlo simulation approach was used with fishery-dependent data to estimate uncertainties in the biological reference points FOJ and Fmx. FO.~ was considered a more suitable as a management target than Fmx because it is precautionary, more robust to estimation uncertainty and usually well defined. Current fishing mortality was greater than Fo,, for all tested variations; in other words, the stock is overfished. Estimates of exploitable biomass and current exploitation rate are essential for determining the current status of the sea urchin stock. With the onset of a fisherindependent survey program, it became possible to conduct a stock assessment that incorporates spatial variability. The objective of this study was to investigate the largescale spatial patterns in sea urchin abundance to estimate the fishery\u27s exploitable biomass. Triangulated irregular networks (TINS) were used to characterize the largescale patterns in the fishery-independent density data by size category and depth. Exploitable biomass estimates were almost identical to estimates calculated using a length-structured fisheries population dynamics model on fisheries-dependent data, providing independent validation of the estimates. The 2001 pilot study for the fishery-independent survey program was extensive, time-consuming and costly, and needed to be optimized to ensure its feasibility as a longterm scientific survey. The high degree of spatial variability in sea urchin abundance, however, prevented us fiom using standard optimization techniques, such as traditional statistics or even geostatistics. Kernel estimation and computer simulations were combined to create a framework for survey optimization. Optimization must decrease sampling intensity, yet produce accurate realizations of the large-scale spatial structure and be compatible with the planned statistical analysis. Considering that the sea urchin data will continue to be analyzed by traditional and spatial statistics, we chose the original fishery-independent survey with a reduction to 10 locations per strata as the optimal strategy. The research presented in this thesis provides the DMR with essential information on the sea urchin stock, suggests new analysis techniques, and recommends a cost and time effective plan for collecting quality long-term fishery-independent data