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1.
A biochemical genetic study of the mussels Perna perna and Choromytilus meridionalis on the west coast of South Africa revealed the presence of an unreported mytilid mussel that had previously been mistaken, because of its shell colour polymorphism, for either P. perna or a hybrid between P. perna and C. meridionalis. The gene products of 19 protein-coding loci in P. perna. C. meridionalis and in the newly recognized mussel were examined by means of horizontal starch-gel electrophoresis. The results showed that there was no allele-frequency overlap between any of these mussel taxa, thus implying little or no genetic relatedness between them. A morphological examination showed that the previously unreported mytilid has a pitted resilial ridge (similar to that of Perna), an anterior adductor muscle (unlike either Perna or Choromytilus), and an undivided posterior foot retractor-muscle scar (unlike Perna). Such features are diagnostic of the genus Mytilus. The South African Mytilus has morphological traits that are more characteristic of M. galloprovincialis of the Mediterranean Sea than of the more cosmopolitan M. edulis. which occurs in the northern and southern Pacific and Atlantic oceans. A large heterozygosity in Mytilus sp. argues against a recent dispersal from the Mediterranean Sea via a small founder population. Rather, the presence of Mytilus sp. in South Africa may represent a relict population of a wider geographic distribution of M. galloprovincialis resulting from Pleistocene cooling. However, the warmer water at lower latitudes could have still prevented dispersal of M. edulis, a species adapted to colder waters.  相似文献   
2.
Assessment of the status of southern bluefin tuna (SBT) by Australia and Japan has used a method (ADAPT) that imposes a number of structural restrictions, and is similar to methods used for a number of stocks world-wide. A flexible method for assessment of the SBT population is presented that is much less restrictive and has potentially wide applicability. The three key features are: (1) all fitting to data is within the context of maximum likelihood, (2) catch-at-age data are not assumed to be without error (as in existing methods), but rather to be random variables, while age-specific selectivity is allowed to change over time within the bounds of specific structure, and (3) autocorrelation in recruitment processes is considered within the likelihood framework of the model. While the results suggest the stock has been depleted considerably from its virgin biomass, and are generally consistent with previous assessments, they also indicate that it is not as much below the biomass that will produce maximum suitable yield as previously estimated and that the extent of stock rebuilding necessary may not be as large as has been argued. The available data are shown to provide little information on the steepness parameter of the stock-recruitment function, and hence on sustainable catch levels for the stock.  相似文献   
3.
Analysis of sea-level data obtained from the Atlantic Global Sea Level Observing System (GLOSS) sea-level station at Takoradi, Ghana, West Africa, clearly reveals a tsunami signal associated with the Mw = 9.3 Sumatra earthquake of 26 December 2004 in the Indian Ocean. The tsunami arrived at this location on 27 December 2004 at approximately 01:38 UTC (which is close to the expected tsunami arrival time at that site), after travelling for more than 24 hours. The first wave was negative (trough), in contrast with the South African stations where the first wave was mainly positive (crest). The dominant observed period at Takoradi was about 42 minutes. The maximum trough-to-crest wave height (41cm) was observed on 28 December at 00:15 UTC. There were two distinct tsunami 'bursts', separated in time by about 14 hours, the larger being the second burst. A small residual lowering of the sea level (～15cm) during the tsunami and for several days afterwards, and a delayed (～4.5 days) lowering of seawater temperature (up to ～4.5°C), was observed, possibly indicating the presence of internal waves through the Gulf of Guinea associated with propagating tsunami waves. The prominent tsunami signal found in the Takoradi record suggests that tsunami waves could also be found at other sites off the West African coast.  相似文献   
4.
This study compares the fish assemblages of the permanently open cool-temperate Olifants and warm-temperate Breede estuaries on the west and south coasts of South Africa respectively, and their responses to altered freshwater flows. Both estuaries have experienced a >35% reduction in mean annual runoff (MAR) from a historical reference condition to the present day with a >60% reduction possible under future flow scenarios. With the exception of species that have both marine and estuarine breeding populations, the Olifants Estuary fish assemblage has experienced an overall 20% decrease in abundance from reference (pristine state) to the present day and will gradually decline to 55% of reference with a predicted future 60% reduction in MAR. Consequently, future reductions in flow are likely to result in the Olifants Estuary progressing towards a low biomass, low diversity, marine-dominated system. In contrast, reduced freshwater flows in the Breede Estuary are likely to experience an overall reduction in the abundance of species that breed only in estuaries, and in freshwater and catadromous species. Collectively, entirely estuarine-dependent fish will increase in abundance, but considered individually some important exploited species such as Argyrosomus japonicus and Pomadasys commersonnii will collapse to 50% of historical numbers once there has been a 64% reduction in MAR. Overall, fish abundance in the estuary has increased by 6% from reference to the present day and is likely to increase to 115% of reference with future reductions in flow. Some species with a preference for fresh and brackish water will be all but lost from the system, but overall diversity is likely to increase with the range expansion of warm-temperate and subtropical marine species westward. In all, the fish assemblage of the Breede Estuary will experience a gradual change from a relatively high-diversity, low-abundance, freshwater rich system under historical flow conditions to a high diversity, high-abundance, marine-dominated system with future reductions in flow.  相似文献   
5.
Cape anchovy Engraulis encrasicolus spawners in the southern Benguela showed an eastward shift in their distribution on the Agulhas Bank that occurred abruptly in 1996 and has since persisted. We assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf of the Agulhas Bank to the east of Cape Agulhas abruptly became 0.5°C colder than in previous years and has since remained that way. In addition, signals, coherent with the 1996 shift recorded in sea surface temperatures, were also found in atmospheric surface pressure and zonal wind data for that region; interannual coastal SST variability is also shown to be correlated with zonal wind-stress forcing. As a result, increased wind-induced coastal upwelling east of Cape Agulhas is proposed as the main driver of the observed cooling in the coastal region. The synchrony between the environmental and biological signals suggests that the eastward shift in anchovy spawner distribution was environmentally mediated and arose from a change in environmental forcing that altered the relative favourability for spawning between regions to the west and east of Cape Agulhas. The results highlight how a relatively minor change in environmental conditions can lead to a drastic spatial reorganisation of the life history of one species in an ecosystem.  相似文献   
6.
Virtual environments and related technology generate interest and excitement. Their power is strengthened with empirical evidence of their utility for scientific inquiry and decision-making. This paper reports on a study to evaluate the effectiveness of virtual environment (VE) presentations about issues typical of those facing decision makers in a rapidly growing urban area. The presentations consisted of the explanation and visualization of two phenomena—groundwater overdraft and the urban heat island. The virtual environment utilized in this study, Arizona State University's Decision Theater, is purported to help policy makers and the larger community visualize complex model output and make decisions about scientific issues. To begin to assess these claims, we carried out a user test during which a group of research participants were given two surveys, one before the presentations, to determine a priori understanding, and a second afterwards, with the same questions. This methodology allowed us to carry out within-subjects tests concerning contrasting phenomena, in order to assess two primary hypotheses: (1) knowledge and perceptions of environmental phenomena will change after the viewing, and (2) understanding will vary based on the phenomena in the presentations. Our analysis shows at least some level of support for the hypotheses, with evidence that the virtual environment positively influenced understanding, and that there may be important differences in insight generation based on characteristics of the phenomena represented. Finally, we outline critical areas of future research to further knowledge about the impact of visual VE settings on understanding and decision making.  相似文献   
7.
The northern Benguela stock of sardine Sardinops sagax used to be considered one of the major clupeoid stocks of the world; it supported an average annual catch of >700 000 tons throughout the 1960s. The stock has been in a depressed state for more than two decades, as demonstrated by annual catches that averaged around 50 000 tons between 1978 and 1989 and only slightly more in the 1990s. It has experienced fluctuations in abundance of several orders of magnitude during the most recent decade. Population size increased until 1992, when the acoustic estimate of biomass was about 750 000 tons. Catches increased accordingly, averaging 100 000 tons between 1992 and 1995, but from 1992 to 1996 the stock was in decline and the lowest annual catch in the history of the fishery was taken in 1996. Although there was a small increase during the last three years of the decade, the stock remains seriously depleted. Survey-based recruitment indices suggest that the changes in the 1990s were initiated by fluctuations in recruitment, but the decline was almost certainly exacerbated by continued fishing. Poor recruitment and decreasing catch rates between 1993 and 1996 in a number of other key resources suggest that system-wide environmental changes were an important factor in the decline of the sardine stock at that time. Anomalous oceanographic conditions, such as extensive hypoxic shelf waters in 1993/94 and a Benguela Niño in 1995, support this conclusion.  相似文献   
8.
There are 21 and 15 species of seabirds that breed in the Humboldt and Benguela upwelling systems respectively. Only two species of gull are common to both systems, one as an endemic subspecies to the Benguela system. Eleven species and two subspecies are endemic (or nearly so) to the Humboldt system; seven species and one subspecies to the Benguela system. Each system has an endemic penguin, sulid, cormorant and tern that feed mainly on anchovy Engraulis spp., sardine Sardinops sagax or both these fish. The Peruvian pelican Pelecanus thagus also feeds primarily on these prey items. A plentiful availability of food has resulted in many of these seabirds attaining high levels of abundance. For the four pairs of species that feed on anchovy and sardine, those in the Humboldt system all have a biology that enables them to increase more rapidly than their Benguela counter-parts. This reflects the higher frequency of environmental perturbations that depress seabird populations in the Humboldt system. In addition, both systems have a small endemic cormorant that feeds near the coast and a small endemic tern that breeds in the adjacent mainland desert and feeds at the sea surface. Several seabirds endemic to a system have no obvious ecological equivalent in the other system: the pelican, a diving-petrel, four storm-petrels and a gull in the Humboldt system; a cormorant and a gull in the Benguela system. Some species with tropical or subantarctic affinities breed at the boundaries of the systems. Others breed also in freshwater systems. The grey gull Larus modestus, which feeds in the Humboldt system, breeds in montane deserts.  相似文献   
9.
Understanding the spatial dimension of fear of crime in the urban environment is important to understanding behaviors in response to this concern. Making this connection between perception and action has long been a goal of scholars in the social and health sciences, though this complex relationship has yet to be fully elucidated. Specifically, in studies on fear of crime and its influence on behavior, a variety of definitions and methods have been employed. This situation has yielded insights, as well as inconsistencies. In the past decade, Geographic Information Systems (GIS) has been added to this methodological mix, though it too has contributed limited understanding of the environmental perception-behavior nexus. During this time, some scholars have integrated a traditional technique for accessing environmental perception, the sketch map, with this newer technology. This article provides a review and critical assessment of the way GIS has been used to understand fear of crime, specifically through the integration of sketch maps. This focus is framed by an overview of substantive and methodological concerns and concludes with a discussion of continued research needs. As behavioral responses to fear of crime are acknowledged to impact physical and mental health and overall well-being, in addition to the viability of neighborhoods, research in this area will continue apace. However, for integration of sketch maps in GIS to be a valuable methodological contributor to this line of inquiry, users of the approach must understand its complexities. This article outlines these issues so that they may be considered in future research and may improve the ability for this approach to yield new understanding of fear of crime.  相似文献   
10.
Saldanha Bay, which lies on the south-west coast of South Africa, has undergone major development over the past 30 years, including breakwater and harbour construction, harbour extension, dredging, mining, fishing, fish processing and mussel culture. This study examines whether the benthic macrofaunal communities in the bay have been altered over this period. Twelve stations that had been sampled in the 1960s, prior to harbour construction, were re-sampled in 2001 (40 years later) with a surface dredge. Species abundances were coded from 1 (present) to 5 (abundant) to make the two sets of samples comparable. The benthic communities before harbour development were significantly different from those in 2001 (ANOSIM, p < 0.001). Increases in the abundances of the whelk Nassarius speciosus and the crab Hymenosoma orbiculare were mainly responsible for these differences. Two main benthic communities were recognised in the 1960s and, at that stage, covered large extents of the bay, whereas in 2001 the communities were divided into distinct 'Small Bay' and 'Big Bay' groups, reflecting the division of the bay into two areas by the development of a harbour wall. Although natural fluctuations may account for some of these changes, it is more likely that the considerable anthropogenic activities within Saldanha Bay, which are known to have altered physical conditions, have led to the changes observed in the benthic communities.  相似文献   
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