排序方式： 共有39条查询结果，搜索用时 359 毫秒
1.
《African Journal of Marine Science》2013,35(1):131-144
An age-structured population model was used to examine the variability of yield created by randomly fluctuating recruitment success in the South African anchovy Engraulis capensis stock. Three different harvesting strategies were examined: (1) constant age-specific pattern of fishing mortality, (2) constant quota with effort limitation and (3) annual quota adjustments by means of an F msy procedure. Variable recruitment was generated by a stochastic stock-recruit relationship, and mean yields, mean spawner biomasses and probability distributions of yield were calculated at each exploitation level after 150 projections from the population structure of anchovy in 1981. Under conditions of constant fishing mortality, the variability of yield increased continuously as the exploitation rate was increased. The maximum average yield (MAY) is the stochastic equivalent of the maximum sustainable yield (MSY) of deterministic models, but an attempt to sustain this MAY by a constant quota resulted in a very high risk of stock collapse. An Fmsy policy based on pre-season adult biomass resulted in more variable yields than were obtained at the equivalent constant fishing mortality at age. Stability of yield therefore demanded the acceptance of lower average yields than could be attained in the long term by quota adjustment. It was considered that the South African purse-seine industry could not cope with the wide fluctuations of yield necessary to attain MAY. Specific management policies sufficiently robust to withstand both fluctuating recruitment success and the uncertainties in the parameters of working population models would be required. 相似文献
2.
《African Journal of Marine Science》2013,35(1):79-92
A simulation model is used to investigate possible ecological effects of up- and downwelling on nitrogen flows through a kelp-bed food web off the Cape Peninsula, South Africa. The model depicts the flow of nitrogen, which is often limiting in marine ecosystems, from kelps, other macrophytes and phytoplankton, through filter-feeders to carnivores, with a feedback loop via faeces and bacteria to detritus and filter-feeders. When modelled as a closed system, bacteria associated with detritus and animal faeces form a large component of the particulate nitrogen available to filter-feeders, and the faeces feedback loop dominates nitrogen flow. When measured rates of water transport are incorporated into the model, bacteria have little opportunity to accumulate before being transported out of the system. Animal faeces and kelp detritus are the dominant filter-feeder food components under upwelling conditions, whereas phytoplankton is the major contributor to particulate organic nitrogen under downwelling conditions. When realistic pulses of upwelling/downwelling derived from wind indices are used as model input, filter-feeders are shown to decline during the summer upwelling season when much potential food is advected out of the system, and they increase during the winter when downwelling conditions are more prevalent, bringing in nitrogen-rich phytoplankton from the blooms developing offshore. 相似文献
3.
《African Journal of Marine Science》2013,35(3-4):493-509
High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones. 相似文献
4.
《African Journal of Marine Science》2013,35(3-4):723-727
Public perception has been that an apparent increase in the nearshore occurrence of white sharks Carcharodon carcharias in False Bay, on the south coast of South Africa, can at least be partly attributed to beach-seine (treknet) operations attracting sharks into this coastal area. To assess the merit of these concerns, all available beach-seine catch-and-effort data from the False Bay fishery over a 32-year period were analysed. A total of 27 cartilaginous species from 15 families was recorded in around 11 400 hauls from 1974 to 2006. Most (98%) of these comprised small benthic invertebrate feeders such as smooth houndshark Mustelus mustelus and lesser guitarfish Rhinobatos annulatus. Large sharks such as C. carcharias and ragged-tooth shark Carcharias taurus were rare, occurring in <0.2% of hauls. The only medium to large sharks that occurred frequently (15% of hauls) in any appreciable numbers (0.3 per haul) were bronze whalers Carcharhinus brachyurus. The relatively high numbers of C. brachyurus compared with C. carcharias, their overlapping size distributions and the difficulty of identifying sharks from a distance, suggests that many of the sharks observed following beach-seine nets are the bronze whalers. Overall, the frequency of occurrence of C. carcharias in the nets is much lower than would be predicted from the high number of observations in the nearshore region. Furthermore, beach-seine fishing rights in False Bay have been reduced from around 170 in the 1970s to five at present. There has been no corresponding decrease in shark incidents. On the contrary, shark incidents have increased from two in the 1970s to six during the period 2000–2005. Overall, there appears to be no strong link between beach-seine activity and human incidents with white sharks in False Bay. 相似文献
5.
《African Journal of Marine Science》2013,35(2):311-336
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
6.
Population structure and growth of polydorid polychaetes that infest cultured abalone Haliotis midae
《African Journal of Marine Science》2013,35(3):499-509
Polydorid polychaetes can infest cultured abalone thereby reducing productivity. In order to effectively control these pests, their reproductive biology must be understood. The population dynamics and reproduction of polydorids infesting abalone Haliotis midae from two farms in South Africa is described using a length-based, age-structured model. Shells were infested mainly by introduced Boccardia proboscidea. Polydora hoplura and Dipolydora capensis were also present but in numbers too few to identify factors influencing infestation. At both farms, B. proboscidea lived for a minimum of 12 months. Growth rate, size at maturity, maximum size, infestation intensity, recruitment, percentage of the population brooding and mortality appear to be affected by abalone feeding regime and water temperature, and these factors need to be considered in controlling infestation. Brooders and recruits were present throughout the year, but increased significantly during mid- to late winter/early spring when water temperature and day length increased. Treatment measures should therefore be implemented throughout the year but with increased effort when water temperature increases. 相似文献
7.
《African Journal of Marine Science》2013,35(3):473-490
Hypotheses regarding temperature, food abundance and food size were tested to explore niche separation between Calanoides carinatus, an abundant copepod in the cool and food-rich southern Benguela upwelling system, and Calanus agulhensis, the dominant copepod on the warmer, relatively food-poor Agulhas Bank off the south coast of South Africa. Under non-limiting food conditions, egg production by both species increased linearly with temperatures between 9°C and 18°C. Egg production by C. carinatus was relatively faster at 21°C, but was offset by greater mortality. Both species showed similar functional responses to food concentration in the field, reaching satiation at ～15mg Chl a m?3, or ～3–4ppm. Food abundance was the most important predictor of egg production, whether measured as Chl a or as particle volume. Both species preferred larger particles that dominated the biomass peak, but particle size appeared more important for C. carinatus, with increasingly faster rates of egg production as the proportion of large cells (>10μm) exceeded 50%, and slower ingestion of small (<10μm) cells. Omnivory may be more important to C. agulhensis. Niche separation between the two species appears unrelated to temperature, food abundance or diel vertical migratory behaviour, and is more likely a function of variability in food availability, although food size may also play a role. 相似文献
8.
《制图学和地理信息科学》2013,40(3):205-224
Over the last three decades analytical cartography has grown from Tobler's concept of "solving cartographic problems" into a broader and deeper scientific specialization that includes the development and expansion of analytical/mathematical spatial theory and model building. In many instances Tobler himself has led the way to these new insights and developments. Fundamental concepts begin with Tobler's cartographic transformations; Nyerges' deep and surface structure and data levels; and Moellering's real and virtual maps; the sampling theorem; and concepts of spatial primitives and objects. This list can be expanded to include additional analytical concepts such as spatial frequencies, spatial surface neighborhood operators, information theory, fractals, Fourier theory, topological network theory, and analytical visualization, to name a few. This base of analytical theory can be employed to analyze and/or develop such things as spatial surfaces, terrain analysis, spatial data schemas, spatial data structures, spatial query languages, spatial overlay and partitioning, shape analysis, surface generalization, cartographic generalization, and analytical visualization. More analytical uses of theory, strategies of analysis, and implementations are being developed and continue to multiply as the field continues to grow and mature. A primary goal is to expand the mathematical/analytical theory of spatial data analysis, and theory building and analytical visualization as analytical cartography takes its place in the geographic information sciences. The research future for this area appears very bright indeed. 相似文献
9.
《制图学和地理信息科学》2013,40(4):237-247
Modeling line or surface phenomena digitally involves two tasks: discretization of the phenomenon, which yields a finite set of data, and subsequent interpolation, which reconstructs the continuum. Many mathematical techniques exist for the latter task, and most methods require a number of parameters to be specified. The shape of digital line or surface models between the data points (that is, the local shape) and the information derived from these models both depend on the selected method and, possibly, on the specification of parameters. The reconstruction of the continuum thus introduces uncertainty. This paper examines the sources and effects of this type of uncertainty. For this purpose, the modeling of lines and surfaces is separated into an abstraction, an implementation, and measurement. The individual factors affecting uncertainty of local shape in each step are identified and discussed. The paper concludes that local shape uncertainty, unlike positional uncertainty of given data, cannot be numerically assessed. Instead, measures of plausibility have to be used to denote the quality of digital models of lines and surfaces. Finally, the concept and potential problems of future empirical investigations are discussed. 相似文献
10.
《制图学和地理信息科学》2013,40(5):332-334
Dramatic changes in the way that spatial data have been collected and processed over that last 20 years is leading to a rethinking and restructuring on the most efficient ways to handle geographical information. These changes are taking place at the federal, state, and local governmental levels with great potential for the private sector as well. The formal adoption of the Spatial Data Transfer Standard (SDTS) as the federal database transfer standard for spatial databases signals a new era in this long chain of developments. It offers more flexible and efficient database transfers than earlier tools, and will become the workhorse for implementing the new National Spatial Data Infrastructure. It offers organizations a standard that will make possible and practical a much wider sharing of databases than is currently being done today. Use of the SDTS presents an opportunity to many organizations to share data more easily and reduce the duplication of expensive spatial database resources. 相似文献