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1.
Anchovy biomass and copepod standing stocks and growth rates on the Agulhas Bank were compared during the peak spawning period (November) in 1988 and 1989. In 1988, copepod biomass over the western Agulhas Bank was low (1,0 g dry mass·m?2) relative to anchovy biomass there (14,7 g dry mass·m?2). In November 1989 in the same area, fish biomass was much lower (5,7 g dry mass·m?2), following a recruitment failure, and copepod biomass was higher (2,4 g dry mass·m?2), possibly as a result of lesser predation by anchovy. By contrast, the eastern Agulhas Bank had a larger biomass of copepods (4–6 g dry mass·?2) and a lower biomass of anchovy during both years. Knowing, from laboratory studies, that a prey biomass of 0,78 g·m?2 is required for fish to obtain their daily maintenance ration, it is suggested that spawning on the western Agulhas Bank was food-limited in 1988. Copepods on the western Bank may be replaced by local growth or transport from the eastern Bank. Growth rates of copepods on the western Bank were 10–50 per cent of maximum in 1988, but total production (c. 100 mg dry mass·m?2·day?1) was low, primarily because biomass was low and less than the rate of consumption by anchovy (243 mg copepod dry mass·m?2·day?1). On the eastern Bank, copepod production exceeded anchovy consumption and it is concluded that the flux of copepod biomass onto the western Bank may be as important as local growth in replenishing copepod stocks there. Feeding conditions for anchovy on the western Agulhas Bank are often marginal compared to the situation on the eastern Bank, and it is suggested that the selection of the western Bank as the major spawning area is related more to the success of transport and survival of eggs and larvae on the West Coast recruiting grounds than to feeding conditions per se.  相似文献   

2.
A particle-size spectrum model is used to estimate standing stocks of some pelagic fish from measurements of phytoplankton chlorophyll in two exploited regions in southern Africa (southern Benguela and off South West Africa/Namibia) and in two unexploited regions (the Agulhas Bank and off the eastern Cape Province). The model is based on the assumption that equal biomasses occur in logarithmically equal size classes in the pelagic marine environment. Phytoplankton, with an equivalent spherical diameter ranging from 1 to 128 μm, occupy 21 size classes on the logarithmic scale. Two different size ranges are assumed for some commercial, pelagic fish species, equivalent to exploitation with two different purse-seine mesh sizes. A mesh of 12,7 mm would catch 8 size classes of pilchard, horse mackerel and anchovy whereas a mesh of 32 mm would catch only 3,3 size classes of pilchard and horse mackerel. From the model, the potential biomass of these commercial pelagic fish is estimated, after allowing for the presence of other commercial and non-commercial fish and other taxa in the exploited size range. Total pelagic fish production is estimated by assuming constant turnover rates of 1·y?1 and 1,5·y?1 when exploited with 32 and 12,7 mm mesh nets respectively. Consideration of the maximum and mean reported catches in the exploited areas indicates that only some 25 per cent of pelagic fish production is exploitable by man. On this basis, the unexploited Agulhas Bank region may yield some 400 000 metric tons (wet) of pelagic fish of the species considered, and the East Coast region some 90 000 tons. Exploitation in these regions cannot be recommended, because the Agulhas Bank is an important spawning ground for many pelagic species, and the fish in both regions probably act as a reserve buffer for the heavily exploited pelagic resource of the Western Cape.  相似文献   

3.
Seasonal movements of South African pilchard Sardinops ocellatus in the relatively abundant 1982/83 year-class are inferred by means of length frequency information from predator diet analyses and from commercial and research catches in various localities. Pilchard of approximately one year of age were most abundant in the vicinity of Algoa Bay on the south-eastern Cape coast in the summer of 1983/84. The pilchard length frequency distribution in gannet diet and research midwater trawl samples in this area provided a clear linkage, through the known juvenile growth rate, with fish of an average 6 months of age, which were abundant off the West Coast during May 1983. Pilchard in this year-class were subsequently found to be widely distributed over the Agulhas Bank, with a tendency for them to inhabit deeper water towards the shelf edge as they reached the age of first spawning (about 18 months). During 1985, a noticeable increase in the abundance of large, two-year-old and older pilchard in the diet of West Coast gannets and in West Coast commercial catches indicated a movement of 1982/83 year-class fish into that area. The changes in distribution of pilchard in this year-class are discussed in relation to seasonal differences in the marine environment off South Africa.  相似文献   

4.
Seasonal and short-term variability of environmental parameters influence the spawning strategies of fish species. In this study, the spawning strategies and the transport of early stages of the two Cape hake species off South Africa were investigated. Distribution of eggs and larvae of Merluccius paradoxus and M. capensis was analysed in order to derive more detailed and species-specific information on spawning season, spawning location, and transport of early stages. Samples were collected during three pilot surveys between January and October 2007 and during an extensive survey in September/October 2008 in the southern Benguela upwelling system off South Africa. Eggs and larvae of M. paradoxus were found in greater numbers than those of M. capensis during all surveys. Highest abundances were found from September to October, indicating one spawning peak for M. paradoxus during late austral winter to spring. The western Agulhas Bank was identified as the primary spawning ground, and smaller spawning events occurred on the West Coast. Larvae of both species were mainly distributed in subsurface waters between 25 and 100 m. More than 50% of all larvae caught had a total length between 3 and 4 mm and size increased significantly with decreasing latitude. Merluccius capensis were found closer inshore than M. paradoxus, indicating that early stages of the two species followed separate drift routes. We assume that this distribution pattern most likely evolved from differences in spawning location and phenology. The spawning strategies of M. paradoxus and M. capensis are well adapted to a time-frame of optimal transport conditions favourable for larval survival in the highly variable environment of the southern Benguela upwelling system, but the peak spawning of the two species is separated in time and space.  相似文献   

5.
Data obtained during the Agulhas Retroflection cruise of the Knorr in November/December 1983 suggested the presence of an oxygen-depleted water layer at the boundary of the Agulhas Current/Return Current system. An analysis of historical data shows that such a layer is present off the edge of the Agulhas Bank during much of the year and may be found off the West Coast as far north as 32°S during summer. The origins, temporal and spatial variations and hydrography of this low-oxygen water mass are described, and its influence on the biology of the Western Cape upwelling region is discussed. It is considered that the presence of this low-oxygen water mass can be used to show penetration of Agulhas water into the South-East Atlantic.  相似文献   

6.
Despite much public awareness surrounding the annual migration of sardine Sardinops sagax northward along the east coast of South Africa in winter each year, relatively little research effort has been expended to improve understanding of the ‘sardine run’. For this reason, a dedicated multidisciplinary survey, timed to coincide with the annual sardine run, was conducted off the East Coast in June and July of 2005. The major objective of the survey was to estimate the biomass of sardine off the East Coast during the run, and to compare this with biomass estimates collected during previous surveys conducted in this area during the late 1980s when the South African sardine population was at a considerably smaller size. We also collected data on the distribution of sardine and other small pelagic fish species and their eggs, the biological characteristics of sardine during the run, and data on the hydrography (temperature and currents) and lower trophic levels (phytoplankton and zooplankton) of the region. Results suggest that the biomass of sardine off the East Coast in winter remains relatively small and consistent, regardless of overall sardine population size. The narrow continental shelf to the east of Port Alfred, which is dominated offshore by the fast-flowing warm Agulhas Current, constrains the amount of suitable habitat for sardine and other clupeoids such as anchovy Engraulis encrasicolus, West Coast round herring Etrumeus whiteheadi and East Coast round herring Etrumeus teres, and hence precludes these species from attaining a high biomass in this region. Additionally, primary and secondary productivity levels are much lower than elsewhere on the western and eastern Agulhas Bank off the south coast of South Africa, suggesting that the sardine run is not a feeding migration. A previous hypothesis that the run is mainly a result of an expansion of the distributional range of these fish as conditions become favourable in winter due to sporadic cooling off the East Coast is also not entirely supported by results from the survey. It is suggested that a migration for the purposes of spawning off this coast when conditions become favourable is a more likely incentive for sardine to undertake this arduous journey, despite increased predation and poor feeding conditions.  相似文献   

7.
Direct surveys of Cape anchovy Engraulis capensis spawner and recruit biomass in South African coastal waters were made with a 38-kHz calibrated echo-sounder and custom-built digital echo-integrator. The spawning surveys were conducted each November between 1983 and 1985, predominantly on the Agulhas Bank. Recruit surveys were conducted in autumn 1985 and 1986, mainly on the West Coast. The spawning fish were widely distributed across the continental shelf, whereas recruit concentrations were highest close inshore. Employing a target strength per kilogramme expression for herring Clupea harengus, the spawning biomass in 1984 was estimated at 1,06 million tons and that in 1985 at 0,98 million tons. Similarly, recruit biomass in May/June 1985 and June 1986 was estimated at 310 and 466 thousand tons respectively. From an analysis of error in the latter two estimates, it was considered approximately 80 per cent certain that the recruitment in 1986 was greater than in 1985. The estimates of spawning biomass are 3–4 times higher than earlier estimates made by Virtual Population Analysis (VPA) of commercial catch data, and suggest that the stock is less seriously threatened by present catch levels than was indicated by VPA. It is concluded from the distribution and age structure of the spawning population that the VPA estimates are invalid because of the failure of commercial vessels to sample the major part of the adult population.  相似文献   

8.
A total of 2 077 records of approximately 49 000 small cetaceans, including dedicated and incidental sightings and specimens, was analysed to define distribution patterns of the 28 species found within southern African waters. Distribution analyses reveal distinct component patterns, including cosmopolitan (found in all waters) and pelagic cosmopolitan (found in all pelagic waters) components, tropical, subtropical and warm temperate components of the Agulhas Current system, an Agulhas Bank component, a South and East Coast inshore component, and West Coast neritic and pelagic components. While the offshore distribution appears to be determined by water depth, possibly through distribution of the principal prey, longshore distribution appears to be determined by water temperature. The high diversity of small cetacean species found within the relatively small study region results from the wide range of zoogeographic components present. These components arise from the wide range of water temperature resulting from the warm Agulhas Current and the upwelling Benguela system.  相似文献   

9.
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   

10.
Upwelling from the Benguela Current system on the South African west coast has produced a seaweed flora distinct from that in the region east of Cape Agulhas. The origins of the major red algal component of the West Coast flora appear to be in temperate and polar regions of the southern Pacific Ocean, whereas the South and East Coast floras are more closely related to the floras of the tropical Indian Ocean and of South and West Australia. Although the West Coast flora has a high degree of endemism, recent evidence suggests that events since the initiation of the Benguela system < 12 million years ago have been crucial in its formation. Evidence on the effects of upwelling on growth and physiology suggests that rapid irregular fluctuations in temperature and nutrients on the West Coast have little effect on the strongly seasonal patterns of development, which are probably brought about by aspects of the light regime. Variations in light intensity in the sublittoral caused by upwelling or downwelling conditions appear to have relatively minor effects on seaweed production. Little is known of the effects of high concentrations of available nutrients in upwelled water, although the kelp Ecklonia maxima appears to have evolved both morphologically and physiologically to the West Coast conditions.  相似文献   

11.
Cape anchovy Engraulis encrasicolus adapted its reproductive strategies to the southern Benguela system by spawning over the Agulhas Bank, an area of low productivity that is located upstream of the predominant upwelling system. Frontal jet currents transport eggs and larvae toward the west coast of South Africa, where recruitment takes place. To characterise the recruitment dynamics of Cape anchovy ichthyoplankton, we used an individual-based model forced by a coupled hydrodynamic–biogeochemical model. The results show the importance of food (especially diatoms and copepods) dynamics on the spatial and temporal patterns of recruitment success, and also confirm the importance of the spawning area, timing and water depth on the recruitment success of Cape anchovy larvae.  相似文献   

12.
Cannibalism by adult anchovy on their eggs was investigated within an area of intensive spawning over the Agulhas Bank by comparing estimates of daily per capita egg production and consumption of eggs derived from repeated sampling of plankton by CalVET net dips and offish by midwater trawl. Consumption of eggs over a 15-hour feeding period was estimated to have caused, on average, 70 per cent of the total egg mortality within the study area, although cannibalism was shown to be density-dependent.  相似文献   

13.
The surface and subsurface waters of the Angola and Agulhas Current systems significantly influence the Benguela region and its living resources, and it is probable that the movement of Central Water, which plays a key role in the coastal upwelling process, is controlled by circulation of underlying Antarctic Intermediate Water (AIW) as well as by the dynamics of the overlying subtropical water. The movement of AIW can be inferred from a study of the t-s characteristics, and the data holdings and data base of the South African Data Centre for Oceanography facilitated this investigation. Key findings of the investigation, some confirming earlier theories and hypotheses, are as follows. The mean depth of the AIW core in the South-East Atlantic is 750 m, and in the South-West Indian Ocean, 1 100 m. Agulhas Current AIW, which is modified by Red Sea Water, becomes fresher en route because of entrainment and mixing of water from the south and west. Most of the Agulhas Current AIW per se retroflects east of 18°E. A poleward movement of AIW along the West Coast to around 32°S may be inferred from the salinity and oxygen data, with a freshening en route analogous to the Agulhas Current. Relatively fresh AIW (s < 34,35 × 10?3) is present off the South-Western Cape, the only part of the Benguela where the overlying virgin Central Water upwells. No statistically significant seasonal differences could be resolved.  相似文献   

14.
Once one of the most numerous seabirds of the Benguela upwelling system, the population of Cape cormorants Phalacrocorax capensis has decreased by 60% in the past three decades and the species is listed as Near Threatened. Declines in prey availability and/or abundance brought about by recent changes in the distribution of pelagic fish stocks and industrial purse-seine fishing are hypothesised to be a key driver of seabird population decreases in the southern Benguela. We investigated the foraging behaviour of breeding Cape cormorants by deploying GPS and temperature–depth recorders on 24 breeding adults from three islands off the coast of South Africa, two of them to the north of Cape Point and a third farther south on the western Agulhas Bank. This provided the first measures of foraging dispersal by a cormorant in the Benguela system, and enabled a comparison of foraging behaviour between birds from these islands. Foraging trips of Cape cormorants lasted between 17 min and >7 h, at a maximum distance of between 2 and 58 km away from their colony. Foraging effort was significantly greater for birds from farther north off the West Coast in terms of trip duration, distance travelled, number of dives and time spent flying compared to those from the southernmost island (Dyer), which is probably a response to low prey availability in the north. Coastal reserves that exclude pelagic fishing from inshore feeding grounds around Cape cormorant breeding colonies may result in increased local prey availability, which would benefit Cape cormorant populations.  相似文献   

15.
Fishery independent biomass surveys and commercial linefish catch returns were used to elucidate the spatial patterns of carpenter Argyrozona argyrozona distributed along the South African continental shelf. Two distinct areas of abundan ce ere determined, one on the central and the other on the eastern Agulhas Bank. Tagging studies revealed little exchange between them. Nurseries were identified in Algoa Bay on he eastern Agulhas Bank and on the central Agulhas Bank (CAB). Early juveniles (<100mm total length) on the CAB were found offshore in the vicinity of the Alphard Bank. They were found to move inshore with growth and then back offshore as they approached maturity. Juveniles in Algoa Bay dispersed both eastwards and westwards with growth. Otoith readability and growth rates varied between regions, with fish from the Eastern Cape (Port Elizabeth and Port Alfred combined) having the lowest average percentage error (4.82 vs 5.33 and 7.03) and the slowest growth rates. Size-at-50 % mturity (L50) varied regionally, female fish in the Eastern Cape maturing at a smaller size (L50 = 206mm fork length) than in the Tsitsikamma National Park (L50 = 292mm) or the CAB (L50 = 267mm). Mass-at-length varied between regions, with carpenter in the Park having the highest mass-at-length and those in the Eastern Cape having the lowest. Based on the distribution of carpenter, variability in otolith readability, mass-at-length, variation in growth and size-at-maturity, it is concluded that carpenter exist as two separate stocks and should be managed accordingly.  相似文献   

16.
The distribution of tar balls over the Agulhas Bank during 1977 and 1978, although variable, was consistent with the oceanography of the region. Mesoscale patches of tar-ball concentrations appeared to move onto the Agulhas Bank from the south between 20 and 21°E, under the influence of prevailing winds and currents. Substantial concentrations were observed inshore and offshore in the region east of Cape Agulhas. Data of a hydrological and meteorological nature, on ship's drift and from drift cards have shown the existence of a zone of surface divergence extending southwards from Cape Agulhas. Surface water east of Cape Agulhas (20°E) tends to move slowly eastwards and northwards and water west of the same longitude has a predominantly westerly component. The surface currents east of Cape Agulhas would tend to move the tar balls, which accumulate in eddies in the area, on shore, whereas west of Cape Agulhas on-shore flow is less pronounced. The residence time of tar balls on the Agulhas Bank appears to be substantial, in the range 2—8 weeks. It is concluded that the beaches between Cape Agulhas and Mossel Bay are highly susceptible to pollution by oil and tar balls.  相似文献   

17.
Stomach content analyses and measurements of gillraker morphology were used to assess the diet and feeding ecology of the East Coast redeye round herring Etrumeus wongratanai and provide data for comparisons with other small pelagic fishes off South Africa. Samples were collected by jigging from a kayak off Scottburgh, KwaZulu-Natal (KZN), over the period July 2013–May 2014. In total, 66 stomachs (containing 4 407 prey items) and 66 gill arches were examined. Stomach content analyses indicated that East Coast redeye round herring, at the time of sampling, fed on large (1 500–2 500?µm) particles, with ～2 500?µm particles making the greatest contribution to dietary carbon. The species feeds on larger particles than do anchovy Engraulis encrasicolus and sardine Sardinops sagax, but on particles of a similar size to those consumed by West Coast redeye round herring Etrumeus whiteheadi. There were significant differences in mean gillraker gaps between East Coast redeye round herring and both sardine and anchovy, but not between East Coast and West Coast redeye round herring. The differences in gillraker gaps between East Coast redeye round herring, sardine and anchovy are indicative of resource partitioning through differential feeding, based on zooplankton size. The data suggest that there is no resource partitioning between East and West coast redeye round herring, indicating that competition is likely to occur between them. However, such competition is minimised by limited spatial overlap between these two species.  相似文献   

18.
Changes and fluctuations in sea surface temperature (SST) around the South African coast are analysed at a monthly scale from 1982 to 2009. There is a statistically significant negative trend of up to 0.5 °C per decade in the southern Benguela from January to August, and a cooling trend of lesser magnitude along the South Coast and in the Port Elizabeth/Port Alfred region from May to August. The cooling is due to an increase in upwelling-favourable south-easterly and easterly winds. There is a positive trend in SST of up to 0.55 °C per decade in most parts of the Agulhas Current system during all months of the year, except for KwaZulu-Natal where warming is in summer. The warming was attributed to an intensification of the Agulhas Current in response to a poleward shift of westerly winds and an increase in trade winds in the South Indian Ocean at relevant latitudes. This intensification of the Agulhas Current could also have contributed to the coastal cooling in the Port Alfred dynamic upwelling region. The El Niño Southern Oscillation (ENSO) is significantly positively correlated at a 95% level with the southern Benguela and South Coast from February to May, and negatively correlated with the Agulhas Current system south of 36° S. The correlation with the Antarctic Annular Oscillation is weaker and less coherent. El Niño suppresses upwelling along the coast, whereas La Niña increases it. Although there does not seem to be a linear relationship between the strength of the ENSO and the magnitude of coastal SST perturbation, El Niño and La Niña appear to be linked to major warm and cool events, respectively, at a seasonal scale in summer in the southern Benguela and along the South Coast. However, care must be taken in interpreting low-resolution reanalysed climate data (ERA40 and NCEP) and optimally interpolated Reynolds SST, such as used here.  相似文献   

19.
Information is presented on the distribution of chlorophyll a between the Cunene River (18°S), on the border of South West Africa (Namibia) and Angola, and East London (28°E) on the east coast of South Africa. Spectrophotometric measurements of samples collected during various research cruises and estimates from satellite measurements were used. The coast was divided into a number of oceanographic regions. Spatial and temporal variation of chlorophyll a in the waters off central-northern South West Africa, the Lüderitz region, the South-Western Cape and the Algoa region are discussed in some detail. There was a narrow coastal band of moderate to high chlorophyll a (3 to in excess of 10 mg·m?3) at the surface between Cape Cross (22°S) and Möwe Point (c. 19°S) throughout most of the year, whereas in much of the area between 23 and 33°S concentrations reached maximum values in autumn. Along the South-Western Cape coast, high concentrations of chlorophyll a were observed in the St Helena Bay area up to 90 km off shore throughout the year, evenly distributed in the upper 30 m. A narrower band of high concentrations of chlorophyll a extended southwards to Cape Agulhas during summer when upwelling was most active. During late summer and autumn a subsurface maximum developed on the Agulhas Bank associated with the thermocline. Low to moderate concentrations were widespread over the entire coastal zone during winter, with strong mixing in the upper 50 — 100 m. A fairly consistent feature of the Algoa region was the presence of moderate concentrations of chlorophyll associated with a wedge-shaped zone of coastal and dynamic upwelling. The implications of the distribution of chlorophyll in time and space are discussed with respect to the distribution and migration of pelagic fish species, particularly anchovy.  相似文献   

20.
Pleistocene climatic and oceanographic changes have influenced the distribution of marine biodiversity in southern Africa. Most evidence, however, has been derived from rocky shore or demersal taxa; data on sandy shore species are limited, despite severe threats to many sandy beach ecosystems. To test the effects of past climatic changes on sandy shore species, we sampled 140 smooth plough shell Bullia rhodostoma individuals from eight localities and generated phylogeographic data derived from the mtDNA cytochrome c oxidase subunit 1 (COI) marker. Nuclear markers (ATPSα, ATPSβ, ANT, SRPS4, TBP, LTRS and ZMP) showed no sequence variation. Bullia rhodostoma exhibited shallow genetic differentiation (ΦST = 0.07, p < 0.05) across its range. Isolation-by-distance suggests a stepping-stone model of migration, which is expected given the species’ direct-development life-history strategy. Demographic reconstruction suggests a post-LGM (Last Glacial Maximum) range expansion, concordant with the signal of shallow genetic differentiation. Phylogeographic patterns obtained suggest that, during lower sea levels than current, B. rhodostoma could have been restricted to the South-West Coast, the central Agulhas Bioregion and probably also the East Coast. When climatic conditions changed and temperatures began to rise after the LGM, the species began a rapid westward range expansion from these refugial regions.  相似文献   

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