Investigation of submicron volcanic aerosol-particles by photoelectron emission

Volcanic aerosol emissions have been studied for the first time by in situ photoelectric charging. Explorative studies on Mt Etna reveal large concentrations of particles below 1000 nm with the spectrum peaking in the size range of 100–1000 nm diameter. Although a large fraction of the particles is already charged upon emission, the net electrical charge carried by the aerosol turned out to be close to zero.Particles with high photoelectric yield vary greatly in their relative abundance and seem to occur mainly at active points of the volcano.     

Earthquakes,catastrophic sediment additions and the response of urban stream communities

Disturbance events can regularly impact stream ecosystems; however, large-scale catastrophic disturbances are rare. From September 2010 to September 2011 Christchurch City experienced over 8500 earthquakes including a magnitude 7.1. One consequence was catastrophic additions of silt and sand into waterways throughout the city. Of 161 km of permanent waterways, 102 km (63%) were affected by earthquake siltation. Benthic invertebrates and fish communities were compared across 16 streams with differing siltation. Invertebrate taxonomic richness decreased significantly (mean 17 taxa reduced to 10 taxa) and EPT taxa (Ephemeroptera, Plecoptera and Trichoptera) were removed entirely from streams receiving heavy siltation. Fish richness and density decreased significantly, with fish absent from some heavily silted streams. Many of these urban streams are sourced from springs and their stable flows and low gradient limit their ability to flush sediment. We predict that without human intervention there will be a long-term sediment legacy and it may take many years for these streams to recover from this catastrophic disturbance.     

Cape cormorants decrease,move east and adapt foraging strategies following eastward displacement of their main prey

Numbers of Cape cormorants Phalacrocorax capensis breeding in South Africa decreased by nearly 50% from approximately 107 000 pairs in 1977–1981 to 57 000 pairs in 2010–2014. Although four colonies had >10 000 pairs in 1977–1981, there was just one such colony in 2010–2014. Almost all the decrease occurred after the early 1990s off north-west South Africa, between the Orange River estuary and Dassen Island. South of this, the number breeding in the two periods was stable, with some colonies being formed or growing rapidly in the 2000s. The proportion of South Africa’s Cape cormorants that bred south of Dassen Island increased from 35% in 1977–1981 to 66% in 2010–2014, with the opposite situation observed in the north-west. This matched a shift to the south and east in the distributions of two of the Cape cormorant’s main prey species, anchovy Engraulis encrasicolus and sardine Sardinops sagax. In 2014, an apparent scarcity of prey in the north-west resulted in Cape cormorants attempting to take bait from hooks of fishing lines over an extended period, a behaviour not previously recorded. The number of Cape cormorants breeding in the south may be constrained by the absence of large islands between Dyer Island in the west and Algoa Bay in the east. If so, it may be possible to bolster the southern population through the provision of appropriate breeding habitat, such as platforms, or restricting human disturbance at suitable mainland cliff breeding sites.     

Interpretation of frequency-dependent transmission-lossinterference patterns

Experimental observations of broad-band acoustic propagation in a known geological region of the Atlantic Generating Station (AGS) site (1994) has prompted a new approach to understanding frequency-dependent behavior in shallow-water regions. A modal-based theory is presented to explain quantitatively the interference patterns of transmission loss versus frequency observed in the experimental data. It is shown that the higher modes are responsible for observed interference patterns and that these can be related to modal group velocities using an ideal waveguide model. This may provide new insights applicable to existing inverse techniques     

Modal analysis of geoacoustic influences on shallow-waterpropagation

Features observed in horizontal wavenumber spectra from low-frequency acoustic measurements taken off the New Jersey Shelf are analyzed through forward modeling using PE models. Environmental models which produce very good agreement with relative intensity data may only match wavenumbers of lower order modes. A prominent feature in contours of modal amplitude versus horizontal wavenumber and depth is the presence of double images of some higher order modes. Two possible causes of this phenomenon are examined. Range dependence in the environment can produce multiple modal images, but the magnitudes of variations required are much larger than those expected at the experiment site. The more likely cause of the double modes is shown to be a duct deep (two acoustic wavelengths) within the sediment. It produces modes with wavenumbers that are close together and with shapes that are very similar in the water column. Comparisons of model predictions with 50-Hz experimental wavenumbers and mode shapes show good agreement, except for amplitudes of some higher order modes. Predictions at 75 Hz typically require more detailed information about the sediment structure in order to accurately match all the features of higher order modes in the data     

Patterns and trends in seabird bycatch in the pelagic longline fishery off South Africa

Both foreign and domestic pelagic longline fishing vessels operate in South Africa’s Exclusive Economic Zone and adjacent international waters where they kill hundreds of seabirds each year as bycatch. To update assessments of the impact of the pelagic longline fishery on seabirds off South Africa, information on necropsied seabirds and national fisheries observer bycatch records were summarised for 2006–2013. Foreign-flagged (Asian) vessels had 100% observer coverage throughout the study period, whereas only 6% of the fishing effort by South African-flagged vessels was observed (with no coverage in 2011–2013). Vessels with observers caught seabirds at a rate of 0.132 birds per 1 000 hooks, resulting in an estimated mortality of 2 851 individuals (356 per year) comprising 14 species. Extrapolation of the observed fishing sets to the unobserved fishing sets by the South African domestic longline fleet suggested that approximately 750 additional birds were likely killed during the study period, therefore a combined 450 birds were killed per year. White-chinned petrel Procellaria aequinoctialis was the most frequently killed species (66%), followed by ‘shy-type’ albatrosses Thalassarche cauta/steadi (21%), black-browed albatross T. melanophris (7%), Indian yellow-nosed albatross T. carteri (3%), and Cape gannet Morus capensis (2%). The seabird bycatch rates were lower than in 1998–2005. Nationality of the vessel, time of line-setting, moon phase, year, season, fishing area, and seabird bycatch mitigation measures all influenced seabird mortality. Concurrent with 100% observer coverage, significant reductions in the seabird bycatch rate occurred in the Asian fleet in the latter years of the study, and these rates now approximate the national target (0.05 birds per 1 000 hooks). However, seabird bycatch rates remained high in the South African fleet, where no observers were deployed during 2011–2013, highlighting the need for independent observer programmes in fisheries—a matter of global interest. Suggestions are made as to how seabird bycatch by pelagic longline fisheries off South Africa may be further reduced.     

Does one size fit all? An evaluation of length–weight relationships for New Zealand's freshwater fish species

Length–weight relationships are a fundamental tool for assessing populations and communities in fisheries science. Many researchers have collected length–weight data throughout New Zealand, yet parameters describing these relationships remain unpublished for many species of freshwater fish. We compiled 285,124 fish records from researchers and institutions across New Zealand to parameterise length–weight equations, using both power and quadratic models, for 53 freshwater species belonging to 13 families. The influence of location and sex on length–weight relationships was also assessed. Location, in particular, generated different length–weight relationships for 65% of the species examined. Length–weight equations were validated by comparing predicted weights against independently measured weights from 25 electrofished sites across New Zealand and the equations were highly accurate (R²>0.99). Recommendations are made about how to robustly apply this new resource which should assist freshwater fisheries researchers throughout New Zealand.     

Effects of dissipation on internal waves in a contained rotating stratified fluid

Abstract

Boundary layer techniques are used to examine the modifications due to dissipation in the normal modes of a uniformly rotating, density stratified, Boussinesq fluid in a rigid container. Arbitrary relative influence of rotation and stratification is considered. The existence of critical regions of the container boundary is discussed. In cylindrical geometry a formula is derived for the decay factor on the homogeneous “spin-up” time scale which reveals how the dominant dissipation varies as a function of several parameters. For the situation where the buoyancy and inertial frequency are exactly equal, all boundaries are everywhere critical. In this case the method of multiple time-scales is employed to investigate the confluence inertial-gravity mode which is shown to persist until the diffusive time-scale is achieved.     

Parabolic Equation Techniques for Seismic Waves

— Parabolic equation techniques for solving seismic wave propagation problems are discussed. These techniques provide an excellent combination of accuracy and efficiency for problems involving laterally varying media. Due to recent improvements, the parabolic equation method is applicable to problems involving wide propagation angles, large variations in the properties of the medium, and all types of waves.