293 research outputs found
Survey- and fishery-derived estimates of Pacific cod (Gadus macrocephalus) biomass: implications for strategies to reduce interactions between groundfish fisheries and Steller sea lions (Eumetopias jubatus)
Survey- and fishery-derived biomass estimates have
indicated that the harvest indices for Pacific cod (Gadus macrocephalus) within a portion of Steller sea lion (Eumetopias jubatus) critical habitat in February and March 2001 were five to 16 times greater than the annual rate for the entire Bering Sea-Aleutian Islands stock. A bottom
trawl survey yielded a cod biomass estimate of 49,032 metric tons (t) for the entire area surveyed, of which
less than half (23,329 t) was located within the area used primarily by the commercial fishery, which caught 11,631 t of Pacific cod. Leslie depletion analyses of fishery data yielded biomass estimates of approximately 14,500 t (95% confidence intervals of approximately 9,000–25,000 t), which
are within the 95% confidence interval on the fished area survey estimate (12,846–33,812 t). These data indicate
that Leslie analyses may be useful in estimating local fish biomass and harvest indices for certain marine fisheries that are well constrained spatially and relatively short in duration (weeks). In addition, fishery effects on prey availability within the time and space scales relevant
to foraging sea lions may be much greater than the effects indicated by annual harvest rates estimated from stock assessments averaged across the range of the target spe
Temperature equilibration in a fully ionized plasma: electron-ion mass ratio effects
Brown, Preston, and Singleton (BPS) produced an analytic calculation for
energy exchange processes for a weakly to moderately coupled plasma: the
electron-ion temperature equilibration rate and the charged particle stopping
power. These precise calculations are accurate to leading and next-to-leading
order in the plasma coupling parameter, and to all orders for two-body quantum
scattering within the plasma. Classical molecular dynamics can provide another
approach that can be rigorously implemented. It is therefore useful to compare
the predictions from these two methods, particularly since the former is
theoretically based and the latter numerically. An agreement would provide both
confidence in our theoretical machinery and in the reliability of the computer
simulations. The comparisons can be made cleanly in the purely classical
regime, thereby avoiding the arbitrariness associated with constructing
effective potentials to mock up quantum effects. We present here the classical
limit of the general result for the temperature equilibration rate presented in
BPS. We examine the validity of the m_electron/m_ion --> 0 limit used in BPS to
obtain a very simple analytic evaluation of the long-distance, collective
effects in the background plasma.Comment: 14 pages, 4 figures, small change in titl
Charged Particle Motion in a Highly Ionized Plasma
A recently introduced method utilizing dimensional continuation is employed
to compute the energy loss rate for a non-relativistic particle moving through
a highly ionized plasma. No restriction is made on the charge, mass, or speed
of this particle. It is, however, assumed that the plasma is not strongly
coupled in the sense that the dimensionless plasma coupling parameter
g=e^2\kappa_D/ 4\pi T is small, where \kappa_D is the Debye wave number of the
plasma. To leading and next-to-leading order in this coupling, dE/dx is of the
generic form g^2 \ln[C g^2]. The precise numerical coefficient out in front of
the logarithm is well known. We compute the constant C under the logarithm
exactly for arbitrary particle speeds. Our exact results differ from
approximations given in the literature. The differences are in the range of 20%
for cases relevant to inertial confinement fusion experiments. The same method
is also employed to compute the rate of momentum loss for a projectile moving
in a plasma, and the rate at which two plasmas at different temperatures come
into thermal equilibrium. Again these calculations are done precisely to the
order given above. The loss rates of energy and momentum uniquely define a
Fokker-Planck equation that describes particle motion in the plasma. The
coefficients determined in this way are thus well-defined, contain no arbitrary
parameters or cutoffs, and are accurate to the order described. This
Fokker-Planck equation describes the longitudinal straggling and the transverse
diffusion of a beam of particles. It should be emphasized that our work does
not involve a model, but rather it is a precisely defined evaluation of the
leading terms in a well-defined perturbation theory.Comment: Comments: Published in Phys. Rep. 410/4 (2005) 237; RevTeX, 111
Pages, 17 Figures; Transcription error corrected in temperature equilibration
rate (3.61) and (12.44) which replaces \gamma-2 by \gamma-
Compton Scattering on Black Body Photons
We examine Compton scattering of electrons on black body photons in the case
where the electrons are highly relativistic, but the center of mass energy is
small in comparison with the electron mass. We derive the partial lifetime of
electrons in the LEP accelerator due to this form of scattering in the vacuum
beam pipe and compare it with previous results.Comment: Comments revised, 16 pages, ReVTeX, 2 Postscript figure
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