13,449 research outputs found
Stock assessment of the gulf menhaden, Brevoortia patronus, fishery
A stock assessment of the gulf menhaden. Brevoortia patronus, fishery was conducted with data on purse-seine landings from 1946 to 1985 and port sampling data from 1964 to 1985. These data were analyzed to determine growth rates,
yield-per-recruit, spawner-recruit relationships, and maximum sustainable yield (MSY). Virtual population analysis was used to estimate stock size, year-class
size, and fishing mortality rates. During the period studied, an average of 27% of age-l fish and 55% of age-2 and age-3 fish were taken by the fishery, and 54% for age-I and 38% for age-2 and -3 fish were lost annually to natural causes.
Annual yield-per-recruit estimates ranged from 6.9 to 19.3 g, with recent mean conditions averaging 12.2 g since 1978. Surplus production models produced estimates of MSY from 620 to 700 kilometric tons. Recruits to age-I ranged from 8.3 to 41.8 billion fish for 1964-82. Although there was substantial scatter about the fitted curves, Ricker·type spawner-recruit relationships were found suitable for use in a population simulation model. Estimates of MSY from
population simulation model runs ranged from 705 to 825 kilometric tons with F -multiples of the mean rate of fishing ranging from 1.0 to 1.5.
Recent harvests in excess of the historical MSY may not be detrimental to the gulf menhaden stock. However, one should not expect long-term harvesting above the historical MSY because of the short life span of gulf menhaden and
possible changes from currently favorable environmental conditions supporting high recruitment.(PDF file contains 24 pages.
X-ray reflection in the nearby Seyfert 2 galaxy NGC 1068
We use the full broad-band XMM-Newton EPIC data to examine the X-ray spectrum
of the nearby Seyfert 2 galaxy NGC 1068, previously shown to be complex with
the X-ray continuum being a sum of components reflected/scattered from cold
(neutral) and warm (ionised) matter, together with associated emission line
spectra. We quantify the neutral and ionised reflectors in terms of the
luminosity of the hidden nucleus. Both are relatively weak, a result we
interpret on the Unified Seyfert Model by a near side-on view to the putative
torus, reducing the visibility of the illuminated inner surface of the torus
(the cold reflector), and part of the ionised outflow. A high inclination in
NGC 1068 also provides a natural explanation for the large (Compton-thick)
absorbing column in the line-of-sight to the nucleus. The emission line fluxes
are consistent with the strength of the neutral and ionised continuum
components, supporting the robustness of the spectral model.Comment: 8 pages. 7 figures. Accepted for publication in MNRA
Properties and Analysis of Thermally Aged Poly(ethylene oxide)
Recent studies have been performed into the use of polyethylene oxide (PEO) as a model system for observing the fundamental effects of adding micro and nano sized fillers to create polymeric composite systems. Many factors contribute to the successful creation of such a composite system, including dispersion of the filler and treatment of the material during creation. For example, while producing thin films of the materials for testing, high temperatures were used for short periods of time in open air to press the samples into small discs. It is well known that prolonged high temperature exposure can alter the chemistry and structure of polymeric materials and that small variations in the original chemistry, such as longer molecular weights or introduction of fillers, can reduce or possibly accelerate this 'ageing' effect. From these previous investigations many property changes were observed during addition of filler or variation of molecular weight, therefore to accurately attribute these changes to a cause the thermal ageing of the material should be observed. This investigation looks at the same 3 molecular weight PEO systems as those used in the previous investigations and analyses them for their vulnerability to thermal ageing. One thermally aged sample is then taken and tested alongside an unaged sample to observe the effects that the process has on the properties. This includes rheology in solution, differential scanning calorimetry (DSC), AC electrical breakdown, dielectric spectroscopy and Fourier transform infra-red (FTIR). By observing the property changes of aged samples it is possible to better understand the thermal ageing process occurring and possibly a way to reduce the effect, along with considering the effect with regard to the behaviour of the previously tested composite samples
The linear rms-flux relation in an Ultraluminous X-ray Source
We report the first detection of a linear correlation between rms variability
amplitude and flux in the Ultraluminous X-ray source NGC 5408 X-1. The rms-flux
relation has previously been observed in several Galactic black hole X-ray
binaries (BHBs), several Active Galactic Nuclei (AGN) and at least one neutron
star X-ray binary. This result supports the hypothesis that a linear rms-flux
relation is common to all luminous black hole accretion and perhaps even a
fundamental property of accretion flows about compact objects. We also show for
the first time the cross-spectral properties of the variability of this ULX,
comparing variations below and above 1 keV. The coherence and time delays are
poorly constrained but consistent with high coherence between the two bands,
over most of the observable frequency range, and a significant time delay (with
hard leading soft variations). The magnitude and frequency dependence of the
lags are broadly consistent with those commonly observed in BHBs, but the
direction of the lag is reversed. These results indicate that ULX variability
studies, using long X-ray observations, hold great promise for constraining the
processes driving ULXs behaviour, and the position of ULXs in the scheme of
black hole accretion from BHBs to AGN.Comment: 4 Pages, 3 figures, accepted for publication by MNRAS
An extended XMM-Newton observation of the Seyfert galaxy NGC 4051. I. Evidence for a shocked outflow
An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 has
revealed a rich absorption line spectrum indicating the presence of a
photoionized outflow with a wide range of velocities and ionization parameter.
At low continuum fluxes an emission line spectrum is well defined with both
narrow and broad components of several abundant metal ions. The absorption line
velocity structure and a broad correlation of velocity and ionization parameter
are consistent with an outflow scenario where a highly ionized, high velocity
wind, perhaps launched during intermittent super-Eddington accretion, runs into
the interstellar medium or previous ejecta, losing much of its kinetic energy
in the resultant strong shock. We explore the possibility that a quasi-constant
soft X-ray emission component may be evidence of this post-shock cooling. This
revised view of AGN outflows is consistent with multiple minor Eddington
accretion episodes creating a momentum-driven feedback linking black hole and
host galaxy growth.Comment: accepted for publication in Monthly Notices of the Royal Astronomical
Societ
On characterising the variability properties of X-ray light curves from active galaxies
We review some practical aspects of measuring the amplitude of variability in
`red noise' light curves typical of those from Active Galactic Nuclei (AGN).
The quantities commonly used to estimate the variability amplitude in AGN light
curves, such as the fractional rms variability amplitude, F_var, and excess
variance, sigma_XS^2, are examined. Their statistical properties, relationship
to the power spectrum, and uses for investigating the nature of the variability
processes are discussed. We demonstrate that sigma_XS^2 (or similarly F_var)
shows large changes from one part of the light curve to the next, even when the
variability is produced by a stationary process. This limits the usefulness of
these estimators for quantifying differences in variability amplitude between
different sources or from epoch to epoch in one source. Some examples of the
expected scatter in the variance are tabulated for various typical power
spectral shapes, based on Monte Carlo simulations. The excess variance can be
useful for comparing the variability amplitudes of light curves in different
energy bands from the same observation. Monte Carlo simulations are used to
derive a description of the uncertainty in the amplitude expected between
different energy bands (due to measurement errors). Finally, these estimators
are used to demonstrate some variability properties of the bright Seyfert 1
galaxy Markarian 766. The source is found to show a strong, linear correlation
between rms amplitude and flux, and to show significant spectral variability.Comment: 14 pages. 12 figures. Accepted for publication in MNRA
Towards Recyclable Insulation Materials for High Voltage Cables
The preferred material for modern extruded high voltage transmission cables is cross-linked polyethylene (XLPE). This material has excellent thermo-mechanical and dielectric properties, however it is not easily recycled at end of use, raising questions as to its long term sustainability [1]. Therefore research work at Southampton has sought to identify suitable recyclable alternatives to XLPE. Such candidate materials need to have low temperature flexibility and high temperature mechanical stability combined with a sufficiently high electrical breakdown strength
Assessment of Atlantic Red Drum for 1999: Northern and southern regions
An assessment of the status of the Atlantic stock of red drum is conducted using recreational and commercial data from 1986 through 1998. This assessment updates data and
analyses from the 1989, 1991, 1992 and 1995 stock assessments on Atlantic coast red drum (Vaughan and Helser, 1990; Vaughan 1992; 1993; 1996). Since 1981, coastwide recreational catches ranged between 762,300 pounds in 1980 and 2,623,900 pounds in 1984, while commercial landings ranged between 60,900 pounds in 1997 and 422,500 pounds in 1984. In weight of fish caught, Atlantic red drum constitute predominantly a recreational fishery (ranging
between 85 and 95% during the 1990s). Commercially, red drum continue to be harvested as part of mixed species fisheries.
Using available length-frequency distributions and age-length keys, recreational and commercial catches are converted to catch in numbers at age. Separable and tuned virtual population analyses are conducted on the catch in numbers at age to obtain estimates of fishing mortality rates and population size (including recruitment to age 1). In tum, these estimates of fishing mortality rates combined with estimates of growth (length and weight), sex ratios, sexual maturity and fecundity are used to estimate yield per recruit, escapement to age 4, and static (or equilibrium) spawning potential ratio (static SPR, based on both female biomass and egg production).
Three virtual analysis approaches (separable, spreadsheet, and FADAPT) were applied to catch matrices for two time periods (early: 1986-1991, and late: 1992-1998) and two regions (Northern: North Carolina and north, and Southern: South Carolina through east coast of Florida). Additional catch matrices were developed based on different treatments for the catch-and-release recreationally-caught red drum (B2-type). These approaches included assuming 0% mortality (BASEO) versus 10% mortality for B2 fish. For the 10% mortality on B2 fish, sizes were assumed the same as caught fish (BASEl), or positive difference in size distribution
between the early period and the later period (DELTA), or intermediate (PROP). Hence, a total of 8 catch matrices were developed (2 regions, and 4 B2 assumptions for 1986-1998) to which the three VPA approaches were applied. The question of when offshore emigration or reduced
availability begins (during or after age 3) continues to be a source of bias that tends to result in overestimates of fishing mortality. Additionally, the continued assumption (Vaughan and Helser, 1990; Vaughan 1992; 1993; 1996) of no fishing mortality on adults (ages 6 and older), causes a
bias that results in underestimates of fishing mortality for adult ages (0 versus some positive value). Because of emigration and the effect of the slot limit for the later period, a range in relative exploitations of age 3 to age 2 red drum was considered. Tuning indices were developed
from the MRFSS, and state indices for use in the spreadsheet and FADAPT VPAs.
The SAFMC Red Drum Assessment Group (Appendix A) favored the FADAPT approach with catch matrix based on DELTA and a selectivity for age 3 relative to age 2 of 0.70 for the northern region and 0.87 for the southern region. In the northern region, estimates of static SPR increased from about 1.3% for the period 1987-1991 to approximately 18% (15% and 20%) for the period 1992-1998. For the southern region, estimates of static SPR increased from about 0.5% for the period 1988-1991 to approximately 15% for the period 1992-1998.
Population models used in this assessment (specifically yield per recruit and static spawning potential ratio) are based on equilibrium assumptions: because no direct estimates are available as to the current status of the adult stock, model results imply potential longer term,
equilibrium effects. Because current status of the adult stock is unknown, a specific rebuilding schedule cannot be determined. However, the duration of a rebuilding schedule should reflect, in part, a measure of the generation time of the fish species under consideration. For a long-lived,
but relatively early spawning, species as red drum, mean generation time would be on the order of 15 to 20 years based on age-specific egg production. Maximum age is 50 to 60 years for the northern region, and about 40 years for the southern region. The ASMFC Red Drum Board's first
phase recovery goal of increasing %SPR to at least 10% appears to have been met. (PDF contains 79 pages
Stock assessment of the Atlantic menhaden, Brevoortia tyrannus, fishery
A stock assessment of the Atlantic menhaden, Brevoortia tyrannus, fishery was conducted with purse-seine landings data from 1940 to 1984 and port sampling data from 1955 to 1984. These data were analyzed to determine growth rates,
maximum sustainable yield (MSY), spawner-recruit relationships, and yield per recruit. Virtual population analysis was used to estimate stock size, year class
size, and fishing mortality rates.
Surplus production models produced estimates of MSY from 450 to 490 kmt compared with yields of 416to 436 kmt based roughly on maximum recruitment from a weak spawner-recruit relationship. Recruitment to age-I ranged from 1.2 to 14.8 billion fish for year classes 1955-81. Recent mean recruitment to age-I for the 1975-81 year classes averaged 5.7 billion fish and compared favorably with the mean of 7.7 billion age-I fish recruited during the late 1950's. Mean recruitment from recent years suggests possible coastwide yields of 416 to 481 kmt. Continued dominance of late age-2 spawners among the spawning stock is of concern, since the stock is at greater risk through poor recruitment if recent favorable environmental conditions change.
Yield-per-recruit estimates ranged from 46 g to 59 g since 1970. The high dependency of the modern fishery on prespawners has increased concerns about fluctuations in year-to-year availability and catches. To increase yield and enhance the stability of the resource, the number of age classes contributing significantly to the fishery should be increased, creating a butTer against future poor
recruitment years and lessening the year-to-year fluctuations in landings. (PDF file contains 24 pages.
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