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1.
A semi-quantitative assessment is made of the animals observed in archived videotapes taken from the research submersible Jago, during diamond mining and exploratory surveys off the mouth of the Orange River on the west coast of southern Africa (28°15′S, 29°11′S) in November 1996. The seabed environment is described and nekton associations with substratum features are identified. The area is characterized by heterogeneity to its physical and biological struture. The variety of observed nekton is low, and communities are dominated by goby Sufflogobius bibarbatus, juvenile hake Merluccius spp. and cuttlefish Sepia spp. (on soft substrata), as well as false jacopever Sebastes capensis and kingklip Genypterus capensis (on rocky substrata).  相似文献   
2.
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   
3.
Fishery-dependent and fishery-independent distribution analyses together reveal four discrete areas of white stumpnose Rhabdosargus globiceps abundance between Port Nolloth and the Kei River off the Cape Province of South Africa: the Western Cape (Saldanha Bay), the South-Western Cape, the Southern Cape and the South-Eastern Cape. On the basis of migratory patterns determined from tagging and catch data, and on differences in growth rate and size-at-maturity, it is concluded that these areas of abundance represent four separate stocks. Each stock apparently disperses offshore in winter (to c. 130 m depth) and concentrates inshore (<60 m depth) in response to ocean ographic patterns during summer. Growth rate and size-at-50% maturity (L 50) increased clinally from the South-Eastern Cape through to the South-Western Cape, and in all three regions males matured at larger size than females. Sizes at maturity for male and female R. globiceps were respectively 18.6 and 15.3 cm (fork length, FL) in the South-Eastern Cape, 22.1 and 18.1 cm in the Southern Cape and 24.3 and 23.6 cm in the South-Western Cape. The fitted Von Bertalanffy growth equations for the three regions were: Lt = 349 (1?e?0.114(t+3.60)) mm for the South-Eastern Cape; Lt = 337 (1?e?0.207(t+1.05)) mm for the Southern Cape; and Lt = 379 (1?e?0.290(t+0.16)) mm for the South-Western Cape. Maximum ages recorded in each region were 21 years for the South-Western Cape, 20 years for the Southern Cape and 10 years for the South-Eastern Cape. Lack of older fish in the South-Eastern Cape sample, attributed to inadequate sample size, has probably resulted in overestimates of both L ∞ and K in this region. Spawning is from August to February, with a peak in spring (September–November). Early juvenile R. globiceps recruit into estuarine and surf-zone marine nursery areas at around 2–5cm (±3 months), but move progressively farther offshore with growth; those trawled deeper than 50 m east of Cape Agulhas were predominantly adults (20–35 cm FL). Because of cooler water temperatures west of Cape Agulhas, adults there are found from the surf zone to depths of only 20 m in summer.  相似文献   
4.
Growth rates of dusky kob Argyrosomus japonicus calculated from mark-recapture and otolith-reading methods were compared. Mark-recapture data showed that A. japonicus are resident in an area between the Breede River Estuary and Cape Agulhas on the south-east coast of South Africa. Maximum recapture length was 1 150mm. A von Bertalanffy growth curve, fitted to length-at-age data derived from otolith sections of 168 specimens (1–42 years) from the Breede Estuary and adjacent marine environment, was differentiated to provide instantaneous length-based growth rates. Comparison of mark-recapture and otolith-based growth rates revealed that external dart tags suppress growth of A. japonicus <750mm, but do not affect the growth of larger fish.  相似文献   
5.
The reproductive success of African penguins Spheniscus demersus at Dassen Island from 1994 to 2000 was variable, but much higher than previously reported figures for the species. Breeding success was positively related to the abundance of anchovy Engraulis encrasicolus and sardine Sardinops sagax, and the high reproductive output during the study was attributed to the large biomass and high availability of these two species for much of the study period. De-oiled penguins from the Apollo Sea spill had a slightly lower hatching success but a significantly lower overall breeding success than that of un-oiled birds, driven mainly by the reduced number of chicks which fledged. Nests with two de-oiled Apollo Sea parents were less successful than nests with one de-oiled bird. There was increased mortality of chicks 40 days and older in nests with de-oiled birds. Chicks from nests with one de-oiled Apollo Sea parent grew at a similar rate to chicks from nests with no de-oiled parents. However, chicks from nests that comprised two de-oiled Apollo Sea birds had significantly slower growth rates than these other two groups. Breeding success and chick growth at nests with de-oiled birds were more negatively impacted when feeding conditions were less favourable. These results suggest that one of the main reasons for lower breeding success in de-oiled birds was their reduced ability to provision chicks, especially during the period in which the energy demands of the chicks is greatest. The rate of mate fidelity was lower in de-oiled birds than un-oiled birds, and there was a positive association between mate fidelity and breeding success.  相似文献   
6.
Insufficient research has been done on integrating artificial-neural-network-based cellular automata (CA) models and constrained CA models, even though both types have been studied for several years. In this paper, a constrained CA model based on an artificial neural network (ANN) was developed to simulate and forecast urban growth. Neural networks can learn from available urban land-use geospatial data and thus deal with redundancy, inaccuracy, and noise during the CA parameter calibration. In the ANN-Urban-CA model we used, a two-layer Back-Propagation (BP) neural network has been integrated into a CA model to seek suitable parameter values that match the historical data. Each cell's probability of urban transformation is determined by the neural network during simulation. A macro-scale socio-economic model was run together with the CA model to estimate demand for urban space in each period in the future. The total number of new urban cells generated by the CA model was constrained, taking such exogenous demands as population forecasts into account. Beijing urban growth between 1980 and 2000 was simulated using this model, and long-term (2001–2015) growth was forecast based on multiple socio-economic scenarios. The ANN-Urban-CA model was found capable of simulating and forecasting the complex and non-linear spatial-temporal process of urban growth in a reasonably short time, with less subjective uncertainty.  相似文献   
7.
A method is developed to establish clumps of mussels Perna perna in denuded areas on high-energy rocky shores on the south-east coast of South Africa. A total of 20 small (20–30 mm total length) mussels is placed under a 30 cm half-section of perforated PVC drainage pipe bolted to the rock surface. The pipe is left in position for one month before removal to allow the mussels to attach firmly to the substratum. A pilot study in a marine reserve indicated 76% survival of clumps and individuals over a period of a year. Mussels grew at a mean annual rate of 30.1 mm from an initial size of 31.8 mm. A subsequent study of survivorship and growth of three size-classes: 25–35, 50–60 and >70 mm, showed that greatest survivorship and growth were exhibited by the smallest size-class. Apart from the initial drilling of holes, the procedure is simple and inexpensive. Mussels could be harvested after about 12 months and the holes and PVC pipe re-used for repeats of the procedure. The method may be used as a research tool and as a way of rehabilitating areas denuded by other natural or anthropogenic agencies.  相似文献   
8.
Culture of pearl oysters is rapidly increasing worldwide, including the western Indian Ocean. The oyster Pinctada margaritifera L., which produces the most highly valued black pearls, occurs in East Africa, a nd as been exploited there for the shell for many decades. The growth patterns of P. margaritifera from a natural population in the sheltered back-reef, and fro oysters translocated to a tidal current-swept site, both sites within Gazi Bay, Kenya, are described. The growth rate in the natural population ranged from 31.3mm year?1 (60–65mm size-class) to 7.6mm year?1 (105–110mm size-class). The von Bertalanffy growth coefficient (K), calculated with a fixed L∞ of 127.2mm, was 0.30 for the natural population and 0.38 for the translocated oysters. The mean growth rate during the north-east monsoon season was approximately double that for during the south-east monsoon season. The dailyrate of nacre deposition ranged from 1.3μm to 5.9μm (mean 3.45μm); it declined with the size of oysters and was marginally higher at the high-energy current site. At that rate, it would take approximately two years to produce a marketable cultured half pearl with a 2.5mm layer of nacre. The results of the study are relevant to the understanding of the influence of the environment on growth, and are applicable to the optimisation of growth rate of pearl oysters in the inshore region along the east coast of Africa.  相似文献   
9.
Age and growth of West Coast steenbras Lithognathus aureti, sampled from two separate populations (northern and southern) along Namibia's coast, was determined using sectioned otoliths, and the results were validated using mark-recapture data. For both populations, the special three-parameter Von Bertalanffy growth model described growth adequately. Growth of the southern population was described by the equation Lt = 73.556(1?e?0.065(t+3.92)) cm and for the northern population by Lt = 84.601(1?e?0.088(t+2.756) cm. Environmental conditions, such as difference in sea surface temperature, density-dependent competition for food, or biochemical genetic variations between the two populations, are possible reasons for the geographic differences found in the growth rates and length-at-age. Slow growth and longevity are characteristics of West Coast steenbras that make it extremely susceptible to overfishing; careful management of the resource is therefore essential.  相似文献   
10.
A total of 2 728 spinner sharks Carcharhinus brevipinna was caught in nets that protect the swimming beaches of KwaZulu-Natal between 1978 and 1997. The species constituted 10.3% of the total shark catch during that period. An average of 136 spinner sharks was caught annually, with no trend in catch rate over the study period. The species was caught throughout the year, predominantly in the south and mainly from February to July. Males matured at approximately 150 cm precaudal length and females at 154 cm. Gonad indices in mature males were highest during December and January and in females during February and March. Mating takes place between January and March. The hepatosomic index of pregnant females was highest at the onset of pregnancy and lowest at parturition. Evidence suggests a two-year reproductive cycle in females, with a gestation period of 13–18 months. The average litter size was nine, with an estimated length at birth of between 50 and 60 cm. Near-term pregnant females were caught mainly in the north, indicating the possibility of a nursery there. Both large and small pregnant females produce pups of the same length, but larger females general1y have larger litters. Regional, seasonal and size variations were evident in the diet, with teleosts being the most frequently eaten prey.  相似文献   
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