Information on currents in the Cape Peninsula area,South Africa

Statistical information and some interpretative detail, based on currents measured at 28 mooring sites between Cape Point and Yzerfontein since 1981, are presented. The most striking feature is the continual presence of wavelike reversals of current close to the coast and in deeper water offshore. These currents have a periodicity of 3–4 days and exhibit a net poleward transport. The energy at these frequencies is greater than that at tidal frequencies and dominates the flow. The observation of equatorward flow over the 230-m contour confirms the presence of a jet there, identified previously by drogue tracks and current profiling studies.     

The influence of catchment management on salinity,nutrient stochiometry and phytoplankton biomass of Eastern Cape estuaries,South Africa

Three estuaries with differing catchment use and freshwater input were investigated in terms of their nutrient status, phytoplankton biomass, freshwater inflow and salinity between 1993 and 1995. The nutrients analysed include phosphate, nitrate, nitrite, ammonia and total particulate nitrogen. All the parameters were investigated for their relationship with land-use and freshwater abstraction. The Kromme River catchment area is relatively pristine, the river is impounded for ca. 133% of its mean annual runoff, and consequently, freshwater input into the estuary is only episodic. Nutrient and chlorophyll-a concentrations are low, but become elevated when freshwater does reach the estuary. The Geelhoutboom tributary contributes nutrients to the Kromme estuary during high freshwater inflow conditions, but is not a viable nutrient contributor during low flow conditions. Freshwater abstraction from the Swartkops River catchment is limited, and it is characterised by urbanisation and industrial development. The Swartkops River was the main source of phosphate in the estuary, whereas other small tributaries along the estuary were additional point sources for nitrate, ammonia and nitrite. The third system, the Sundays estuary, has no tributaries or other point sources except the Sundays River, where the catchment is extensively used for agriculture and freshwater input relatively high. The phytoplankton biomass (in terms of chlorophyll-a) was highest in the Sundays estuary, although phosphate concentrations were as low as in the Kromme estuary. Trends over time indicated a decrease in phosphate concentrations and showed variations for inorganic dissolved nitrogen concentrations since the previous 15 years in all the three estuaries. Nutrient stochiometry had changed in favour of inorganic dissolved nitrogen.     

Comparison of the abundance of pilchard in Cape gannet diet and commercial catches off the Western Cape,South Africa

A hypothesis is presented that, in the absence of quota restrictions, commercial catches of pilchard Sardinops ocellatus in the western Cape and the abundance of pilchard in the diet of Cape gannet Morus capensis obtained at three South African colonies are directly related to pilchard biomass when the latter is low. There were significant correlations (p < 0,05) between the abundance of pilchard in gannet diet at Lambert's Bay and Malgas Island combined, and the total catch and CPUE at annual, though not monthly, time-scales. Trends in levels of correlation are discussed in relation to short-term variability, sampling intensity, statistical chance and the distribution of pilchard. Gannet diet and commercial catches reflect regional, but not local, changes in biomass. The 1983 pilchard year-class was relatively strong. Gannet diet is suggested to be a reliable monitor of the trend in pilchard biomass at low pilchard biomasses, a time at which direct surveys are least reliable.     

Recolonisation of the Robberg Peninsula (Plettenberg Bay,South Africa) by Cape fur seals

The Cape fur seal Arctocephalus pusillus pusillus colony at Robberg Peninsula, Plettenberg Bay, on the south-east coast of South Africa, was driven to extinction by indiscriminate harvesting by the late 1800s. Seals only began to recolonise this site in the 1990s. This study describes the recolonisation process from 2000 to 2009, exploring both within- and between-year count data of seals using the site. Counts increased over the study period from <300 animals to >3 100. Generalised linear models indicated the importance of year and month in explaining variability in the counts. Withinyear variability in the counts decreased over the study period, which may be related to an increasing proportion of resident (as opposed to transient) seals in the colony. However, the colony is currently still in a transition phase with a low ratio of breeding to non-breeding animals, based on the low numbers of pups born in the colony (currently still <100 per year). The influx of seals to the Robberg area may be associated with shifts in prey availability at the ecosystem level. The colony benefits from the protection afforded by the reserve status of the Robberg Peninsula and the existence of a marine protected area adjacent to it. However, human interference associated with fishing and/or ecotourism on the peninsula may inhibit development into a substantial breeding colony. Potential interventions for the conservation and management of this colony are discussed.     

Regional and temporal changes in bivalve diversity off the south coast of Portugal

From 2000 to 2006, a total of 75 bivalve species were identified, varying from 29 (spring 2001) to 54 species (spring 2005) per year. Seasonal tendencies in diversity varied according the year, thus the interpretation of long-term and regional scales is essential before drawing any conclusions in other studies. Richness and diversity consistently decreased with depth and increased with sediment grain size (from low in very coarse sand to high in coarse silt). Diversity decreased progressively from 3 to 16 m depth, thus the harsher shallower environments (due to waves and tidal air exposure) showed greater diversity than the most stable areas. Communities in finer sediments were more diverse than those in coarser sand. Evenness showed patterns opposite to diversity, overall.Diversity and evenness maps (produced with multivariate universal kriging), showed that most geographic areas with greater diversity were farer from river outflows and wastewater treatment plants. Two types of geographic pattern were observed: areas with persistently greater bivalve diversity through time and areas that changed locally from year to year. This spatial analysis can be used to establish priority conservation areas for management purposes, and to analyse the persistency of regional diversity patterns. The area with most habitat heterogeneity (Sotavento) corresponded to greatest diversity.There was a positive relationship between Spisula solida and Chamelea gallina landings and bivalve diversity 2 years and 1 year later, respectively. Possibly, local fisheries, by selectively withdrawing the commercial numerically dominant species from the ecosystem, increased diversity 1 to 2 years later, as the ecological niches of the dominants are quickly filled by several other species thereby creating a more even community. On regional scales, no significant impact was found on long-term bivalve diversity in local fisheries,     

Aspects of the upwelling regime between Cape Point and Cape Agulhas,South Africa

A progressive increase in mean ovary mass (standardized by the cube offish length) over time was observed in specified length classes of pilchard Sardinops ocellatus sampled from purse-seine catches off the west coast of South Africa. This, in earlier investigations, was considered to have been a density-dependent response following the collapse of the stock in the mid 1960s, and it appeared to result mainly from the increasing frequency of occurrence of pilchard entering the batch-spawning cycle, particularly among fish of 16–20 cm L_c . The possibility that the observed trend was an artifact of changing distribution of fishing or shifting seasonality of spawning relative to the three-month period of sample collection was rejected following analysis of variance on spatially disaggregated data and inspection of seasonal patterns in ovary mass. Other causes of the increase in mean ovary mass could have been density-dependence, declining age structure, changes in the environment and selection for early maturity under prolonged high rates of mortality. The data are critically examined to evaluate the likelihood of each of these explanations.     

Distribution of and predation on saury Scomberesox saurus scombroides in continental shelf waters off the Cape Province,South Africa

Although the size distribution of larvae and early juveniles of the saury Scomberesox saurus scombroides in continental shelf waters off the Cape Province, South Africa, is consistent with a south-north passive dispersal by known currents, the size of late juveniles and adults increases from north-west to south-east. Occurrence of these stages is highly seasonal: they are found in summer off the Western Cape and mainly from late summer to winter in Algoa Bay in the Eastern Cape, possibly reflecting longitudinal migration. Large numbers of late juveniles and adults are often found downstream of upwelling plumes off the Western Cape when warmer waters lie close inshore. They are sometimes taken in purse-seines in association with adult round herring Etrumeus whiteheadi and pilchard Sardinops ocellatus. Important predators of late juvenile and adult saury are fast-moving, surface-feeding species: yellowfin tuna Thunnus albacares, Cape gannet Morus capensis and Cory's shearwater Calonectris diomedea. Cape gannets consume an estimated 2 277–6 044 tons of saury annually in South African waters. Predator diets provide important time-series of occurrence, abundance and length-frequency distributions of late juvenile and adult saury.     

The feeding ecology of skates (Batoidea: Rajidae) off the Cape south coast,South Africa

Six species of skates were trawled commonly on the continental shelf off the Southern Cape during routine surveys of demersal fish between 1986 and 1990. Raja miraletus is a small skate found principally in shallow (<50 m) water that feeds mainly on small crustaceans, such as mysids and macrurans. The large Raja alba occurs over most of the continental shelf, is a piscivore throughout its life and preys heavily on flatfish, especially Cynoglossus zanzibarensis. R. cf. clavata is ubiquitous on the shelf but most abundant between 50 and 150 m deep. Feeding predominantly on crustaceans, this skate switches from carids, penaeids and mysids when small to brachyurans and small benthic fish when larger. R. pullopunctata is found over a similar depth range but attains a much larger size. It eats small mysids and macrurans when small, but brachyurans and fish become more important in the diet with increasing size of the predator. R. wallacei is most commonly taken at depths of 80–>200 m, feeds initially on small crustaceans, but switches to fish, including eels, as it grows. The small Cruriraja parcomaculata is found at the edge of the continental shelf and over the upper slope; it preys on small crustaceans, including mysids, isopods and macrurans. The distribution pattern of the skates and their prey suggest overlap in prey taken, the overlap being greatest in those species which feed on crustaceans. The large biomass of skates in the area suggests that they are an important component of the demersal faunal assemblage of the continental shelf there, and also that they may have a significant impact on the benthic fauna.     

Saldanha Bay,South Africa I: the use of ocean colour remote sensing to assess phytoplankton biomass

The efficacy of ocean colour remote sensing in assessing the variability of phytoplankton biomass within Saldanha Bay is examined. Satellite estimates of chlorophyll a (Chl a) were obtained using the maximum peak-height (MPH) algorithm on full-resolution (300?m) data from the Medium Resolution Imaging Spectrometer (MERIS). Subsurface Chl a maxima often occur within Saldanha Bay below the mean detection depth of the satellite (1.5?m) during periods of thermal stratification. Consequently, the MPH product was poorly correlated to in situ data from 4?m depth (r² and average relative percentage difference [RPD] of 0.094 and 53% respectively); however, the coefficient of determination was much improved if limited to in situ data collected under conditions of mixing (r² and RPD of 0.869 and 89%, respectively). Composites of monthly MPH Chl a data reveal mean concentrations consistent with in situ seasonal trends of phytoplankton biomass, confirming the capability of the MPH algorithm to qualitatively resolve surface Chl a distribution within the bay.     

The spatial and temporal development of the spring phytoplankton bloom in the Celtic Sea,April 1979

The results are presented from three hydrogrpahic surveys in April 1979 of a 40 × 50 km region of the Celtic Sea, centred at 7°W and 51°N, using a towed undulating sensor system. In the $\text{5}\text{1}\text{2}$ days between Surveys 1 and 2, the seasonal thermocline was established, with surface to bottom temperature differences reaching 1.5°C, the average surface chlorophyll a level increased from ～ 1 to ～ 5.5 mg m^?3 due mainly to the growth of diatoms, and the surface nitrate concentration decreased from 6 to 1 μM. The third survey was carried out after a further two days and, although surface properties changed little, there was a general deepening of the mixed layer due to stronger winds, and a further increase in the standing stock of phytoplankton.By applying appropriate techniques of horizontal spatial averaging, which took into account possible advective effects, a quantitative comparison was made of the changes at two positions in the survey area which showed significant differences in the rate of development of the phytoplankton population. A simple model of phytoplankton growth was then developed, based on calculations of eddy diffusivity, on measurements of rates of photosynthetic carbon assimilation, and on observations of carbon to chlorophyll ratios, subsurface light attenuation and inorganic nutrient levels. In the absence of any data on zooplankton populations, the loss of phytoplankton by grazing was left as a free parameter.The model was only partly successful in reproducing the observed changes in chlorophyll concentrations during the first $\text{5}\text{1}\text{2}$ days and showed serious limitations for the subsequent 2 days. However, it emphasised several features of the dynamics of spring phytoplankton populations which require further experimental or observational investigation. These include more precise measurements of carbon to chlorophyll ratios and grazing pressure to which the model is sensitive over a rather narrow range (this has important implications in terms both of control by grazing and nutrient limitation), the potential significance of physiological photoadaptation by the plant cells in determining their vertical distribution, and the role of eddy diffusion across the developing thermocline in relation to the sinking of phytoplankton cells and the upward mixing of inorganic nutrients.     

Socio-economic characteristics of gillnet and beach-seine fishers in the Western Cape,South Africa

Data collected by questionnaire and telephone surveys conducted during 1998 and 1999 are used to describe the socio-economic characteristics of inshore netfishers in the Western Cape. Approximately two-thirds of netfishers work or have worked in other fishing sectors and a further 6–50%, depending on the area surveyed, are retired. Very few (0–11%) permit-holders in most areas classified their occupations as netfishers and the majority claimed to make <5% of their income from netfishing. Estimated costs and returns to net permit-holders suggest that, in most areas, commercial netfishing at current levels of catch and effort is not economically sustainable in the long-term. Only Saldanha-Langebaan gillnetters and beach-seine permit-holders, on average, manage to cover their opportunity costs and make an economic profit. The lack of profits in other areas is compelling evidence that the net fisheries are at or beyond the open access equilibrium point, suggesting that effort reduction in the order of 60% is necessary if maximum economic yield is to be obtained from the fishery. The netfisheries provide part-time employment for approximately 2 000 crew in the Western Cape. Additional economic benefits and employment directly related to the fishery in the form of equipment and fuel purchases made by fishers, maintenance of fishing gear and the sale of fish are estimated to contribute at least R15 million to the regional economy annually. Between 42 and 76% of respondents felt that their catches had declined since they had started netfishing and most felt that no new permits should be issued. Knowledge of catch restrictions among respondents was low (53–73%), indicative of a lack of communication between management and fishers, poorly defined permit conditions and a lack of enforcement. Many fishers interviewed feel it is unfair that they are restricted to catching only low-value target species and do not adhere to the catch restrictions, even if they do know them. The importance of the netfishery for participants varies greatly between and within areas. In order to reduce effort equitably, current and potential new permit-holders should be assessed on an individual merit basis.     

南海北部浮游植物生物量的研究特点及影响因素

介绍了浮游植物生物量变化的研究方法。综述了南海北部浮游植物生物量在营养盐、光照、季风等理化因子影响下出现的变化特点。该海域生态环境复杂,由富营养的珠江口、沿岸带、北部湾和广阔的陆架及贫营养的开阔海区等不同生态区组成,因此浮游植物群落和生物量有其自身复杂的空间和时间变化特点。     

Tectonic evolution of the Cape and Karoo basins of South Africa

The Cape and Karoo basins formed within the continental interior of Gondwana. Subsidence resulted from the vertical motion of rigid basement blocks and intervening crustal faults. Each basin episode records a three-stage evolution consisting of crustal uplift, fault-controlled subsidence, and long periods of regional subsidence largely unaccompanied by faulting or erosional truncation. The large-scale episodes of subsidence were probably the result of lithospheric deflection due to subduction-driven mantle flow. The early Paleozoic Cape basin records the combined effects of a north-dipping intra-crustal décollement (a late Neoproterozoic suture) and a right-stepping offset between thick Rio de la Plata craton and Namaqua basement. Following the Saldanian orogeny, a suite of small rift basins and their post-rift drape formed at this releasing stepover. Great thicknesses of quartz sandstone (Ordovician–Silurian) and mudstone (Devonian) accumulation are attributed to subsidence by rheological weakening and mantle flow. In contrast, the Karoo basin is a cratonic cover that mimics the underlying basement blocks. The Permian Ecca and lower Beaufort groups were deposited in a southward-deepening ramp syncline by extensional decoupling on the intra-crustal décollement. Reflection seismic and deep-burial diagenetic studies indicate that the Cape orogeny started in the Early Triassic. Deformation was partitioned into basement-involved strike-slip faults and thin-skinned thrusting. Uplift of the Namaqua basement resulted in erosion of the Beaufort cover. East of the Cape fold belt, contemporaneous subsidence and tilting of the Natal basement created a late Karoo transtensional foreland basin, the Stormberg depocentre. Early Jurassic tectonic resetting and continental flood basalts terminated the Karoo basin.     

Age and growth of Cape stumpnose Rhabdosargus holubi (Pisces: Sparidae) in the Eastern Cape,South Africa

Rhabdosargus holubi is a small (maximum weight=2.4?kg) yet important fishery species in the estuaries of the south-east coast of South Africa. Little is known of its biology and specifically its growth rate, which is essential for sustainable management of the fishery. We examined and counted the opaque zones in the sectioned otoliths of 134 R. holubi to determine its age and growth parameters. The otoliths from two recaptured fish marked with oxytetracycline confirmed that one opaque zone was deposited annually. The species reached a maximum age of 18 years and growth was adequately described by a von Bertalanffy growth function of the form: L_t = 358.1 (1 – e^{?0.24(t+0.77)}) mm fork length. There were no significant differences between any of the male and female growth parameters (likelihood ratio test: p = 0.3). The growth was slow (omega index: ω = 86.56); however, despite this, the unique life history of R. holubi may provide a degree of resilience to heavy fishing pressure in estuaries.     

Chemotaxonomic investigation of phytoplankton in the shelf ecosystem of the KwaZulu-Natal Bight,South Africa

Phytoplankton community structure was elucidated during summer and winter in the KwaZulu-Natal Bight using pigment and CHEMTAX analyses. The surface pattern in January 2010 indicated that diatoms, haptophytes and prasinophytes tended to be the most prominent groups inshore and in the southern sector of the bight, whereas Prochlorococcus and Synechococcus were more dominant in the north and towards the offshore region. At the deep chlorophyll maximum (DCM), diatoms were dominant in the inner bight and in the south, with prasinophytes being the prominent flagellates, but this changed to Prochlorococcus and pelagophytes being the important groups in the northern sector and outer part of the bight. A different pattern was observed at the surface in July 2010, where diatoms, haptophytes, prasinophytes and cryptophytes in varying proportions comprised most of the community in the inner half of the bight, whereas Synechococcus and haptophytes were the main groups in the outer sector. A similar pattern occurred at the DCM, except that Synechococcus was less prominent and pelagophytes were distributed across the bight in both the inshore and offshore zones. Observations and relationships between phytoplankton groups and environmental parameters indicated that the groups were most closely related to temperature, accounting for 24–64% of the deviance. The influence of nutrients on phytoplankton was less clear but nitrate and silicate seemed to account for some of the patchy distribution patterns.     

Variability in the diet of the Cape gannet at Bird Island,Algoa Bay,South Africa

Results are presented following a long-term study of the diet of the Cape gannet Morus capensis at Bird Island, Algoa Bay, examining 4 178 regurgitations representing 36 351 prey items collected during 12 breeding cycles from April 1979 to March 1991. In all, 32 teleost and two cephalopod species were recorded. The pelagic shoaling pilchard Sardinops ocellatus, anchovy Engraulis capensis and saury Scomberesox saurus scombroides were identified as the main prey species. The diet composition showed marked interannual fluctuations during the sampling period, which appear to be related to changes in the abundance of the main prey species. The relative abundance of pilchard, anchovy and saury was also found to change intra-annually, the first two species occurring more frequently in the diet during the breeding season and saury dominating the diet in the non-breeding season. These changes are thought to originate from temporal variation in the oceanographic regime within the foraging range of the gannets.     

广西白龙半岛邻近海域污损生物群落结构分析

2011年12月-2012年11月,在广西白龙半岛邻近海域进行周年污损生物挂板试验,共记录到污损生物60种,以亚热带暖水种为主,其中优势种类有河蜾蠃蜚Corophium acherusicum、钩虾Gammarus sp.、网纹藤壶Amphibalanus reticulatus、翡翠贻贝Perna viridis、华美盘管虫Hydroides elegans、太平洋侧花海葵Anthopleura nigrescens、冠瘤海鞘Styela canopus、凸壳肌蛤Musculus senhousia和多齿围沙蚕Perinereis nuntia。全年均有生物附着,四季平均附着密度和附着湿质量分别为45 136个/m2和4 246.03g/m2,其中以秋季附着密度最高,达到153 514个/m2,春季附着湿质量最大,为9 418.42g/m2。污损生物群落Shannon-Wiener多样性指数(H′)、Pielou均匀度指数(J)和Margalef物种丰富度指数(d)均以夏季最高,冬季最低。丰度/生物量比较法结果表明,冬季、秋季、下半年和周年板上的污损生物群落结构不够稳定。     

Long-term changes of phytoplankton community in Xiagu waters of Xiamen, China

Long-term changes of phytoplankton community by water sampling method in Xiagu Sea waters of Xiamen, China, were investigated in this study. Species composition of the phytoplankton community in these waters changed greatly since the 1950s. The numbers of Dinophyta species increased significantly, although Bacillariophyta species are generally dominant. The succession of dominant species in phytoplankton community is obvious:large-size dominant species such as Biddulphia sinensis of the 1950s were gradually replaced by small-size ones such as Cyclotella striata and Nitzschia closterium, and species that still maintain dominant such as Skeletonema costatum are also small ones, leading the whole phytoplankton community of smaller size. Cell density of phytoplankton community increased greatly, among which cell density of the most dominant species Skeletonema costatum have been increasing in exponent function. Margalef index of phytoplankton community decreased, indicating decline of biodiversity of the community, and dominant character of Skeletonema costatum increased. Generally, the structure of the entire phytoplankton community is becoming more and more singular and unstable, which makes the occurrence of red tides more frequent. The succession in the phytoplankton community is related to the long-term changes in marine environment, influenced by human activities and global climate changes, especially the increases of nutrient content.     

Distribution and abundance of eggs,larvae and early juveniles of saury Scomberesox saurus scombroides (Richardson) off the south-western Cape,South Africa, 1977/78

The distribution patterns of saury eggs, larvae and early juveniles have been investigated over the continental shelf off the South-Western Cape from August 1977 to August 1978. Young saury tend to occupy water of 18°C or warmer in all seasons and are found offshore of the Benguela oceanic front when inshore upwelling is evident. Their distribution in the region appears to be linked to seasonal hydrography. Spawning in the study area occurs mainly between Cape Columbine and Cape Point, and to a lesser extent east of Cape Point. The ratio of large larvae and early juveniles to small larvae is greater off the West Coast than off the South Coast. Most spawning appears to take place in summer, with a peak abundance of larvae and early juveniles towards the end of this season. Instantaneous natural mortality was calculated: 5 – 30 mm, Z = 0,14·day^?1; 30 – 100 mm, Z = 0,02·day^?1. There is therefore a marked but unexplained decline in mortality shortly after the transition from the larval to the juvenile phase.