排序方式： 共有12条查询结果，搜索用时 31 毫秒
1.
A roving creel survey of the recreational shore fishery along the 16.4-km coastline in the Goukamma Marine Protected Area on the south coast of South Africa was conducted from 2009 to 2011. Some 838 patrols were stratified equally among months, areas and years, but intentionally biased towards weekends. Angler densities at Buffalo Bay and Groenvlei were 0.59 and 0.28 anglers km?1, respectively. Weekend densities were double to quadruple weekday densities and fishing during winter was more popular than during summer. Area, habitat and distance to access points explained variation in angler densities. Shannon–Wiener diversity in catches declined from 2.18 in an earlier (1993–2002) survey to 1.79. Although the order of species abundance in the catches remained largely unchanged, blacktail Diplodus capensis dominance increased to 57.3% by number, at the expense of galjoen Dichistius capensis. Habitat explained 27% of the variance in catch composition. The catch per unit effort (CPUE) for the top nine species ranged from 0.19 to 6.35 fish 100-h?1. The CPUE of all species, except spotted grunter Pomadasys commersonnii, declined. Blacktail and galjoen CPUE declined by 17% and 77%, respectively. The total catch estimate was 2 986 fish y?1. Transgressions of size limits were common. The results suggest that the fishery is overexploited and that catch rates are declining. 相似文献
2.
3.
贝塞尔曲线在虚拟森林景观单树几何模型构建中的应用 总被引：10，自引：0，他引：10
虚拟景观模型的生成,需要对实际景观进行抽象和简化。树几何模型和数字高程模型(DEM)是构成虚拟森林景观的两大要素。而树几何模型的构建又是林业GIS三维可视化的基础。在树几何建模中,树干及树枝形状的可视化表达具有突出意义。由于实际树种的复杂性,在计算机中建树模型,通常采用基于过程建模的方式,而这将面临用户如何对模型进行交互式控制的问题：参数太少,不足以描述树形态特征,而参数太多,则模型难以生成和理解。为了解决这个问题,我们引进贝塞尔(Bezier)曲线这种基于控制点的自由曲线来描述树木中的树干以及树枝形状,利用对曲线控制点的变动来控制曲线的形状,进而生成形态各异的树形状,对此,我们基于ActiveX技术生成Bezier曲线控件。结果表明,该方法能够表示多种形状的树干与树枝,具有操作简捷、计算速度快,可视化效果良好的优点。 相似文献
4.
RH Bennett PD Cowley A-R Childs CG Attwood L Swart TF Næsje 《African Journal of Marine Science》2017,39(4):475-489
Understanding movement behaviour is essential for effective management of fishery species. Dart tags were used to study coastal movement patterns of white steenbras Lithognathus lithognathus, an overexploited seabream (Sparidae) in South Africa. In total, 6 962 fish (190–1 080 mm fork length) were tagged throughout the species’ distributional range, in four long-term fish-tagging programmes. The predominant behaviour recorded was residency, with relatively short-ranging movements. More than 60% of the 351 recaptured fish were recaptured within 1 km of their tagging site, some of which were at liberty in excess of three years. Most of the juveniles (93.9%), subadults (71.7%) and adults (64.0%) were recaptured within 10 km. Occasional long-distance movements of up to 800 km were recorded. Recapture distances were positively, but weakly, correlated with fish fork length (n = 257 fish measured at recapture; r2 = 0.166, p < 0.001). Low levels of connectivity among coastal areas suggest that large-scale annual spawning migrations, as previously hypothesised for this species, are unlikely, which raises the possibility of multiple spawning sites. Seventy-seven percent of L. lithognathus tagged within three marine protected areas (MPAs) were recaptured within the same MPA, suggesting that area closures provide protection for L. lithognathus through the post-estuarine juvenile, subadult and adult life stages. We confirm that the country’s current network of coastal MPAs plays a vital role in sustaining this species, and suggest that additional closures, or otherwise substantial reductions in catch rates, are necessary for the species’ recovery. 相似文献
5.
The South African midwater trawl fishery targets adult horse mackerel Trachurus capensis. The bulk of the catch is taken by a single freezer-trawler, the biggest fishing vessel operating in South African waters. As fishing takes place off the south coast in ecologically sensitive areas, there are concerns about the potential impacts of this fishing operation on non-target species. Fishing behaviour and bycatch of this fishery from 2004 to 2014 were investigated by analysing observer records with regard to catch composition, volume and temporal and spatial patterns. The midwater trawl fishery was estimated to have caught 25 415 tonnes annually, with a bycatch of 6.9% of the total catch, by weight. There are species overlaps with various fisheries, namely the demersal trawl, small-pelagic, line, shark longline and squid fisheries, yet the total bycatch estimates from this fishery are generally small relative to catches taken in the target fisheries. Bycatch species with the highest average annual catches were chub mackerel Scomber japonicus, redeye roundherring Etrumeus whiteheadi, ribbonfish Lepidopus caudatus and hake Merluccius spp. Large-fauna bycatch species included sunfish Mola mola as well as a number of CITES II- and IUCN-listed species, such as Cape fur seal Arctocephalus pusillus, dusky shark Carcharhinus obscurus, smooth hammerhead shark Sphyrna zygaena and thresher sharks Alopias spp. The 97.9% observer coverage is high and the 6.9% bycatch rate low compared to other South African fisheries; however, due to the large size of the individual hauls (average of 46.3 t), the average sampling rate of 1.56% is low. Our analyses suggest that bycatch in the South African midwater trawl fishery has been lower than in other South African fisheries and similar fisheries elsewhere, but due to the combination of high catch volumes and low sampling rates, estimation errors for rare species are high and there is a substantial risk of incidental unmonitored bycatch of rare large fauna and aggregations of small fauna. This could be mitigated by spatio-temporal management of this fishery, to avoid fishing in high-risk areas, and the introduction of an electronic monitoring programme. 相似文献
6.
A real-time colour display of the Culgoora acousto-optical radio spectrograph is described and illustrated. 相似文献
7.
The declaration of a state of emergency in the South African linefishery in 2000 has had a positive impact on a few overexploited endemic seabreams (Family Sparidae). However, the population of the reef-dwelling dageraad Chrysoblephus cristiceps has been unresponsive to progressively stricter management regulations. After decades of unsustainable fishing mortality the species is in a critical state. In this paper, a method based on standardised probability of capture is used to develop an index of relative abundance for rare species, such as dageraad, which is more robust to changes in output control regulations than conventional methods. The results show a severe decline in dageraad capture probability in the south-west region of South Africa, from 8% in 1985 to <0.1% in 2011. The east region experienced a more rapid decline, from 24% in 1999 to <0.1% in 2011. A spatial comparison of the distribution of historical and recent dageraad probability of capture along the South African coastline indicates a range contraction commonly associated with the collapse of a population. The once-widespread species is now largely limited to a few locations in the Eastern Cape province and dageraad is mostly absent across its former western distribution. Although the historical decline has commonly been attributed to commercial overexploitation, the recreational sector had an estimated 3–5 times greater impact on the dageraad population than the commercial sector in recent years. Given that previous management regulations have been unsuccessful in arresting the decline of dageraad populations, assertive management is necessary to ensure the species’ sustainability. Rehabilitation of dageraad stocks will require a further decrease in fishing mortality, in conjunction with maintaining an appropriate marine protected area (MPA) network. 相似文献
8.
R.D. Hedger T.F. Næsje P.D. Cowley E.B. Thorstad C. Attwood F. Økland C.G. Wilke S. Kerwath 《Estuarine, Coastal and Shelf Science》2010
Acoustic telemetry was used to study patterns of habitat use and movements of Pomatomus saltatrix L. (common name elf/shad/bluefish/tailor) within the Saldanha Bay with Langebaan Lagoon coastal embayment on the west coast of South Africa. Thirty six mature P. saltatrix were tagged with acoustic transmitters and released within the lagoon in May 2006 and November–December 2007, and their positions were monitored until late-November 2008 using 28 hydrophones positioned throughout the embayment. The detection pattern of P. saltatrix suggested a tendency to residence within the embayment throughout the thirty month long study period, with nearly 60% of released individuals only being detected within the lagoon in the inner part of the embayment. However, there was a long-term trend of movement from the lagoon into the bay. One individual was recaptured off the east coast of South Africa 21 months after being tagged, 1760 km away, suggesting that P. saltatrix are capable of undertaking long along-shore migrations. Over finer scales within the inner lagoon, P. saltatrix ground speed increased (1) with an increase in tidal current speed, (2) with an increase in photoperiod, and (3) during day. Pomatomus saltatrix tended to move seaward during ebb tides, and to occupy greater depths during day. 相似文献
9.
10.