9,655 research outputs found
Age and growth of spiny dogfish (Squalus acanthias) in the Gulf of Alaska: analysis of alternative growth models
Ten growth models were fitted to age and growth data for spiny dogfish (Squalus acanthias) in the Gulf of Alaska. Previous studies of spiny dogfish growth have all fitted
the t0 formulation of the von Bertalanffy model without examination of alternative models. Among the alternatives, we present a new two-phase von Bertalanffy growth model
formulation with a logistically scaled k parameter and which estimates L0. A total of 1602 dogfish were aged
from opportunistic collections with longline, rod and reel, set net, and trawling gear in the eastern and central
Gulf of Alaska between 2004 and 2007. Ages were estimated from the median band count of three independent readings of the second dorsal spine plus the estimated number of worn bands for worn spines. Owing to a lack of small dogfish in the samples, lengths at age of small individuals were back-calculated from a subsample of 153 dogfish with unworn spines. The von Bertalanffy, two-parameter von Bertalanffy, two-phase von Bertalanffy, Gompertz, two-parameter Gompertz, and logistic models were fitted to length-at-age data for each sex separately, both with and without back-calculated lengths at age. The two-phase von Bertalanffy growth model produced the statistically best fit for both sexes of Gulf of Alaska spiny dogfish, resulting in L∞ = 87.2 and 102.5 cm and k= 0.106 and 0.058 for males and females, respectively
Demographic Effects on the Swedish Pension System
The present study describes the effect that different demographic developments will have on the Swedish pension system. Projections of expenditures for old age pensions, survivor pensions, and disability pensions were made for the period 1985-2050 on the basis of future developments of the population and its structure (age, sex, and marital status). Six demographic scenarios were formulated: Benchmark, High Fertility, Low Mortality, West European, National 1, and National 2 scenarios. Together they cover a wide range of demographic developments, not to say all probable developments.
A model of the current Swedish pension system is combined with all six demographic scenarios. Projections of expenditures as well as of contributions and benefits in the pension system are made. The pension system will be put under severe strain whatever the demographic development. In all scenarios, expenditures will continue to increase until 2030, in the beginning as a result of the maturing of the system, but after the turn of the century mainly as a result of demographic changes. Expenditures will increase by about 75% in the "most favorable" scenarios (Benchmark/High Fertility, Western European) and by 100% to 130% in the "least favorable" scenarios (Low Mortality, National 1 and 2). After 2030, expenditures decrease in all scenarios except in National 2 where they remain constant. The contribution rates will have to be increased from about 20% in 1985 to between 36% (National 1) and 49.9% (Low Mortality) of the wage sum in 2030.
The impact on contributions and benefits of three selected policy measures are studied: a raising of the retirement age by two years, an extension of the number of years on which benefits are based and an increase in labor force. All three measures will ease the pressure on the system but only to some extent. The main conclusion is that there is a need for a fundamental change in the Swedish pension system
Measuring the Regional Economic Response to Hurricane Katrina
Naturkatastrophe; Sturm; Makroökonomischer Einfluss; USA
Self-organization and Mechanical Properties of Active Filament Bundles
A phenomenological description for active bundles of polar filaments is
presented. The activity of the bundle results from crosslinks, that induce
relative displacements between the aligned filaments. Our generic description
is based on momentum conservation within the bundle. By specifying the internal
forces, a simple minimal model for the bundle dynamics is obtained, capturing
generic dynamic behaviors. In particular, contracted states as well as solitary
and oscillatory waves appear through dynamic instabilities. The introduction of
filament adhesion leads to self-organized persistent filament transport.
Furthermore, calculating the tension, homogeneous bundles are shown to be able
to actively contract and to perform work against external forces. Our
description is motivated by dynamic phenomena in the cytoskeleton and could
apply to stress-fibers and self-organization phenomena during cell-locomotion.Comment: 19 pages, 10 figure
Morphogen Transport in Epithelia
We present a general theoretical framework to discuss mechanisms of morphogen
transport and gradient formation in a cell layer. Trafficking events on the
cellular scale lead to transport on larger scales. We discuss in particular the
case of transcytosis where morphogens undergo repeated rounds of
internalization into cells and recycling. Based on a description on the
cellular scale, we derive effective nonlinear transport equations in one and
two dimensions which are valid on larger scales. We derive analytic expressions
for the concentration dependence of the effective diffusion coefficient and the
effective degradation rate. We discuss the effects of a directional bias on
morphogen transport and those of the coupling of the morphogen and receptor
kinetics. Furthermore, we discuss general properties of cellular transport
processes such as the robustness of gradients and relate our results to recent
experiments on the morphogen Decapentaplegic (Dpp) that acts in the fruit fly
Drosophila
In situ characterization of an optical cavity using atomic light shift
We report the precise characterization of the optical potential obtained by
injecting a distributed-feedback erbium-doped fiber laser (DFB EDFL) at 1560 nm
to the transversal modes of a folded optical cavity. The optical potential was
mapped in situ using cold rubidium atoms, whose potential energy was spectrally
resolved thanks to the strong differential light shift induced by the 1560 nm
laser on the two levels of the probe transition. The optical potential obtained
in the cavity is suitable for trapping rubidium atoms, and eventually to
achieve all-optical Bose-Einstein condensation directly in the resonator.Comment: 3 pages, 4 figure
Requirements for contractility in disordered cytoskeletal bundles
Actomyosin contractility is essential for biological force generation, and is
well understood in highly organized structures such as striated muscle.
Additionally, actomyosin bundles devoid of this organization are known to
contract both in vivo and in vitro, which cannot be described by standard
muscle models. To narrow down the search for possible contraction mechanisms in
these systems, we investigate their microscopic symmetries. We show that
contractile behavior requires non-identical motors that generate large enough
forces to probe the nonlinear elastic behavior of F-actin. This suggests a role
for filament buckling in the contraction of these bundles, consistent with
recent experimental results on reconstituted actomyosin bundles.Comment: 10 pages, 6 figures; text shortene
Robust formation of morphogen gradients
We discuss the formation of graded morphogen profiles in a cell layer by
nonlinear transport phenomena, important for patterning developing organisms.
We focus on a process termed transcytosis, where morphogen transport results
from binding of ligands to receptors on the cell surface, incorporation into
the cell and subsequent externalization. Starting from a microscopic model, we
derive effective transport equations. We show that, in contrast to morphogen
transport by extracellular diffusion, transcytosis leads to robust ligand
profiles which are insensitive to the rate of ligand production
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