Defining and estimating global marine fisheries bycatch     

R.W.D. Davies  S.J. Cripps  A. Nickson  G. Porter 《Marine Policy》2009

Unselective fishing catches non-target organisms as ‘bycatch’—an issue of critical ocean conservation and resource management concern. However, the situation is confused because perceptions of target and non-target catch vary widely, impeding efforts to estimate bycatch globally. To remedy this, the term needs to be redefined as a consistent definition that establishes what should be considered bycatch. A new definition is put forward as: ‘bycatch is catch that is either unused or unmanaged’. Applying this definition to global marine fisheries data conservatively indicates that bycatch represents 40.4 percent of global marine catches, exposing systemic gaps in fisheries policy and management.  相似文献   

Examination of a physically based,high‐resolution,distributed Arctic temperature‐index melt model,on Midtre Lovénbreen,Svalbard          下载免费PDF全文

T. D. L. Irvine‐Fynn  E. Hanna  N. E. Barrand  P. R. Porter  J. Kohler  A. J. Hodson 《水文研究》2014,28(1):134-149

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Thermal evolution of Kuiper belt objects, with implications for cryovolcanism     

Steven J. Desch  Jason C. Cook  Simon B. Porter 《Icarus》2009,202(2):694-714

We investigate the internal thermal evolution of Kuiper belt objects (KBOs), small (radii <1000 km), icy (mean densities ) bodies orbiting beyond Neptune, focusing on Pluto's moon Charon in particular. Our calculations are time-dependent and account for differentiation. We review evidence for ammonia hydrates in the ices of KBOs, and include their effects on the thermal evolution. A key finding is that the production of the first melt, at the melting point of ammonia dihydrate, ≈176 K, triggers differentiation of rock and ice. The resulting structure comprises a rocky core surrounded by liquids and ice, enclosed within a >100-km thick undifferentiated crust of rock and ice. This structure is especially conducive to the retention of subsurface liquid, and bodies the size of Charon or larger (radii >600 km) are predicted to retain some subsurface liquid to the present day. We discuss the possibility that this liquid can be brought to the surface rapidly via self-propagating cracks. We conclude that cryovolcanism is a viable process expected to affect the surfaces of large KBOs including Charon.  相似文献   

Moving young lives: Mobility, immobility and inter-generational tensions in urban Africa     

Gina Porter  Kate Hampshire 《Geoforum》2010,41(5):796-331

This paper explores young people’s experiences and perceptions of mobility and mobility constraints in poorer urban areas of Ghana, Malawi and South Africa within the specific context of inter-generational relations. Drawing principally on qualitative research findings from a study involving both adult and child researchers, our aim is to chart the diversities and commonalities of urban young people’s mobility experiences in the everyday - how they use and experience the city - developing a comparative perspective across three urban study sites which links young people’s mobility with the power relations that operate to shape their movements in individual locations. In particular, we consider how positive and negative images of young people’s mobility play out in terms of the inter-generational frictions and negotiations generated by their mobility performances. We also reflect on the developmental implications, in terms of young people’s access to services and income and their participation in the social networks and peer culture which may shape their life trajectories. Three themes are explored in detail: mobility as challenge, mobility as temptation and mobility control.  相似文献   

The influence of variable depth on steady zonal barotropic flow   总被引：1，自引：0，他引：1  

Gene H. Porter  Maurice Rattray Jr. 《Ocean Dynamics》1964,17(4):164-174

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Volcanic activity and human ecology: Edited by P. D. Sheets and D. K. Grayson. Academic Press,New York, 1979. 644 pp., $49.50     

Stephen C. Porter 《Quaternary Research》1981,15(1):107-108

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Hydrodynamic effects in dynamic response of simple arch dams     

Craig S. Porter  Anil K. Chopra 《地震工程与结构动力学》1982,10(3):417-431

The dynamic responses of simple arch dams, with different radius to height ratios are analysed for three conditions: the dam alone without water, and the dam with full reservoir, considering water to be compressible in one case and neglecting water compressibility in the other case. The complex frequency response functions for accelerations at the dam crest due to the three components of ground motion—upstream-downstream component, cross-stream component and vertical component–are presented. Based on these results, the effects of dam-water interaction, of water compressibility, and of bank motions on dam response are investigated.  相似文献   

Episodic gullying and paleomonsoon cycles on the Chinese Loess Plateau     

Stephen C. Porter  Zhisheng An 《Quaternary Research》2005,64(2):234-241

Buried gullies exposed in road excavations along the margin of a loess tableland on the Loess Plateau of central China lie within a thick loess-paleosol succession that spans ≥780,000 years. Constraining ages for gully cutting and filling are provided by dates of loess and soil units cut by and capping the paleogullies. An episode of gully cutting begins at the onset of an interglaciation and ceases as the gullies begin to fill with colluvium and airborne dust during the transition to full-glacial conditions. The episodic cutting and filling of gullies implies a basic astronomical (orbital) control of gully evolution involving cyclic changes in dominant summer and winter monsoon climates, surface hydrology, and vegetation cover.  相似文献   

Real-time measurement of volcanic SO2 emissions: validation of a new UV correlation spectrometer (FLYSPEC)     

Keith A. Horton  Glyn Williams-Jones  Harold Garbeil  Tamar Elias  A. Jeff Sutton  Peter Mouginis-Mark  John N. Porter  Steven Clegg 《Bulletin of Volcanology》2006,68(4):323-327

A miniaturized, lightweight and low-cost UV correlation spectrometer, the FLYSPEC, has been developed as an alternative for the COSPEC, which has long been the mainstay for monitoring volcanic sulfur dioxide fluxes. Field experiments have been conducted with the FLYSPEC at diverse volcanic systems, including Masaya (Nicaragua), Poás (Costa Rica), Stromboli, Etna and Vulcano (Italy), Villarica (Chile) and Kilauea (USA). We present here those validation measurements that were made simultaneously with COSPEC at Kilauea between March 2002 and February 2003. These experiments, with source emission rates that ranged from 95 to 1,560 t d⁻¹, showed statistically identical results from both instruments. SO₂ path-concentrations ranged from 0 to >1,000 ppm-m with average correlation coefficients greater than r ²=0.946. The small size and low cost create the opportunity for FLYSPEC to be used in novel deployment modes that have the potential to revolutionize the manner in which volcanic and industrial monitoring is performed.  相似文献   

On the asphericity of nova remnants caused by rotating white dwarf envelopes     

John M. Porter  O'Brien  & Mike F. Bode 《Monthly notices of the Royal Astronomical Society》1998,296(4):943-948

The effect of rotating white dwarf envelopes in determining the structure of nova shells is examined. This is achieved by numerical hydrodynamic simulations of the flows around a binary star system. In previous studies of remnant formation, this rotation has not been included.
It is found that the structures formed in the flow are more consistent with observations of nova shells than the previous theoretical studies. The shells produced by the nova become more prolate with increasing white dwarf envelope rotation. Hence the rotation of white dwarf envelopes must be included in any future discussion of remnant formation.
A possible method of identifying the dominant process by which mixing of accreted and white dwarf matter takes place is suggested.  相似文献