Response of rock-lobster populations in the Benguela ecosystem to environmental change — a hypothesis

Studies some years ago suggested that yields of Jasus lalandii in the northern Cape and Namibia had declined as a result of severe overfishing and progressive reductions of minimum size limits, especially between 1960 and 1970. Although catches were temporarily boosted to artificially high levels by reductions in minimum size, evidence is presented which suggests that sustainable yields may have declined largely as a result of environmental changes during and after the 1960s. It is postulated that a progressive expansion of oxygen-deficient shelf water may have forced lobsters to occupy a much-reduced habitat in shallow waters, where overcrowding has resulted in reduced growth and survival, and production and yields have declined accordingly. It is suggested that increased competition for food and space in the better-oxygenated shallows has led to a diminution in the size at sexual maturity of female lobsters and reduced adult growth rates. In addition, low levels of dissolved oxygen may have direct physiological effects on rates of feeding, growth and mortality. Possible reasons for the likely change in oxygen concentrations after the mid 1960s are discussed. Reduced grazing of phytoplankton by planktonic herbivores (zooplankton and clupeoid fish) as well as increased phytoplankton production per se are considered to be possible causative factors leading to the carbon-overloading prevalent in the central and northern Benguela system during the past two decades.     

A demonstration of the hydrographic partition of the Benguela upwelling ecosystem at 26°40'S

Continuous CTD data from a series of recent cruises show that the distribution of the water mass characteristics in the central Benguela region from the Orange River mouth (28°38'S) to alvis Bay (22°57'S) is discontinuous in the central and intermediate waters at about the latitude of Lüderitz (26°40'S), Namibia. The central and intermediate water masses at the shelf edge and shelf break north of the Lüderitz upwelling cell have a high salinity relative to the potential temperature compared to similar waters south of the upwelling cell. It is shown that the feed waters for the wind-induced upwelling on the shelf to the north and south of the Lüderitz discontinuity are different in character and source. The distribution of the water masses shows that the shelf-edge poleward undercurrent provides low-oxygen water from different regions in the Atlantic Ocean to be upwelled onto the shelf. North of th Lüderitz upwelling cell, the central and intermediate waters come from the oxygen-depleted Angola Basin, whereas south of the discontinuity those waters are from the interior of the adjacent Cape Basin, which is less oxygen-deficient. This has implications for the dispersion of low-oxygen water and the triggering of anoxic events, and consequences for the biota on the shelf, including commercially important fish species.     

Predatory fish and their prey — an overview of trophic interactions in the fish communities of the west and south coasts of South Africa

Fish communities over continental shelves are of three types: pelagic, rocky reef and soft substrata. The pelagic community of the South African west coast is dominated by clupeiform fish, which are the principal prey of snoek, the dominant piscivorous teleost in the area, and other fish, including tuna and pelagic sharks. Keystone prey species are broadly similar on the South African south and west coasts, but predator composition differs. Studies of rocky reef fish suggest that the fauna of the South Coast is more diverse than that of the West, the West Coast having fewer species and only two reef fish species being taken in linefisheries. Feeding interactions of some of the dominant species have been established, but the status of commercially less important fish such as elasmobranchs and small species is poorly known. The community and feeding interactions of fish living over soft substrata are better known and part of this fauna is taken by demersal trawlers. The two species of Cape hake dominate the communities on both coasts, but the fauna is more diverse on the Cape south coast. Generally, much of the present ichthyological knowledge is derived from the ecologically narrow perspective of commercial fisheries. Nevertheless, the patterns of diversity appear to conform to previous work, which has indicated a lower species diversity in the Atlantic than in the Indian Ocean. Knowledge of commercially unimportant fish is fragmentary, and understanding of the ecological interactions on the South Coast generally lags that of the West.     

Seasonal variability of δ15N in sinking particles in the Benguela upwelling region

Temporal changes in δ¹⁵N values of sinking particles collected with sediment traps in the Benguela upwelling regime off southwest Africa mirrored variations in the input of inorganic nitrogen to the surface water. Reductions in δ¹⁵N (to as low as 2.5‰) corresponded to low sea surface temperatures during austral spring and late austral autumn/early winter, indicating increased nitrate availability due to the presence of recently upwelled water. High particulate fluxes accompanied the low δ¹⁵N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High δ¹⁵N values (up to 13.1‰) coincided with high sea surface temperatures and low particle fluxes. In this area, the seaward extension of upwelling filaments, which usually occurs twice yearly, brings nutrient-rich water to the euphotic zone and leads to elevated productivity and relatively lower δ¹⁵N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with δ¹⁵N changes. The observed isotopic pattern does not appear to have been caused by variations in the species composition of the phytoplankton assemblage. Calculations based on δ¹⁵N of the sinking particulate nitrogen show that the surface nitrate pool was more depleted during late austral summer/early fall and mid-winter and that supply exceeded demand during the intense spring bloom and in late austral fall. The main uncertainty associated with these estimates is the effect of diagenesis on δ¹⁵N and possible variability in preservation of the isotope signal between periods of high and low particle flux.     

Studies on the role of marine bacteria in the upwelling ecosystem——Ⅰ.Bacterial biomass in the upwelling regions

-Six cruises were carried out off the south bank of Fujian - Taiwan during the period of December 21, 1987 to November 15, 1988 to estimate the contribution of bacterial biomass carbon (BBC) to the totai particulate organic pools using epifluorescent microscopic technique. The results show that the standing crop of bacteria fluctuated from 0. 95 to 66. 60 mg /m3 (dry weight). Upwelling phenomena appeared in the region around Nanpeng Island in summer while in the region of Waixie in all seasons. The average value of BBC was 27. 60(±6. 08)mg/m3and 21. 32 (±2. 34) mg/m3 respectively. The seasonal and spatial distribution is discussed in relation to environmental factors as well as upwelling phenomena. The role of bacteria in the flow of material and energy in the upwelling ecosystem is emphasized.     

Morphological characteristics in the area off eastern Taiwan Island and their tectonic implications

On the basis of bathymetric data and other geological and geophysical data obtained during the first survey conducted by Chinese Mainland in the area off eastern Taiwan Island from May to June in 2000, the morphological features of the region, the tectonic control to the seafloor topography and their tectonic implication are studied and discussed. The results have revealed that both the slope zone of the Ryukyu arc and the Ryukyu Trench present a typical morphotectonic characteristics controlled by the trench-arc system in the West Pacific Ocean. At the slope of eastern Taiwan Island the isobathic lines parallel to the coastline and distribute densely in nearly N-S direction and the slope gradient of topography is large. Such a unique feature is attributed to the collision of the Luzon arc and Eurasia continent. In the Huatung Basin, turbidity fans and submarine canyons are well developed, the formations of which are mainly related to the steep topography of the slope of the Luzon arc and the abundant s     

Geochemical changes of the terrigenous sediments in the southern South China Sea and their paleoenvironmental implications during the last 31 ky

Fifty-six subsamples of the 86GC core were collected from the southern South China Sea continental slope. Major and trace elements, including rare earth elements (REEs), of detrital matters in these sediments were measured. The patterns of chondrite normalized REEs and the discrimination plots based on major and trace elements, respectively, indicate that the terrigenous sediments around this area had been supplied with one dominating provenience during the last 31 ky. The co-operation of sea level changes with the East Asian winter monsoon-related rainfall and sea water circulations should be the main mechanism causing the geochemical characteristics variations. Higher ratios of Al/Ti and some other elements to Ti, except P/Ti and Mn/Ti, are present during the last glacial sea-level low stand and the winter monsoon periods owing to increased fluvial input of material with low Ti content from the Mekong River, followed by strengthened chemical weathering intensity, causing higher Al₂O₃ and most other elements contents. The elemental patterns imply general warm and very humid climate in the tropical areas during the last glacial times; however, the climate since the last deglaciation was very changeable, especially during the Holocene. Even so, it can still be certain that the climate during the early part (29–24 ky B.P.) of the last glacial maximum was much worse than during other stages. Additionally, it is very likely that the 8 k cold event and the Younger Dryas event have been impressed in the geochemical record of this core.     

The effects of heavy metals on the primary productivity in the marine ecosystem enclosures

In April, 1985, scientists from China and Canada made a marine enclosed ecosystem experiment cooperatively in Xiamen. The experiment was carried out in the plastic bags in a shorebased tank. To the bags, heavy metals (Cu, Hg, Zn, Pb and Cd) were added quantitatively and their effects on the primary productivity were studied. The results show that the effects of low level metal mixture (Cu 3.5-10.0, Hg 0.2-1.0, Pb 0.3-1.5, Zn 3.5-17.5,     

Interannual variations of the planktonic ecosystem in the slope water and Kuroshio south of Japan in February in the years 1990–2002

To determine recent interannual variations in the planktonic ecosystem of the slope water south of Japan, an area between 1000 m isobath on the continental slope and assumable Kuroshio front, in reference to preceding reports on the Kuroshio, we examined interannual variations of physical factors, sea surface chlorophyll concentration (SSChl), size-fractioned copepod biomass and the abundance of Calanus sinicus, one of the dominant copepods in the region, in February from 1990 to 2002. In the slope water, SSChl concentration had generally shown a higher value than in the Kuroshio and increase of the SSChl in both areas started in February but lasted longer in the slope water. The regional distribution of copepod biomass and C. sinicus abundance was similar to that of phytoplankton in that they were distributed more densely in the slope water than in the Kuroshio, reflecting assumed higher nutrient supply. The interannual variation in sea surface temperature in the slope water was explained by the rise of air temperature in 1998, a strong El Niño year, and subsequent change in the Kuroshio path (1999–2001). The interannual variation of the planktonic community, i.e. sea surface chlorophyll concentration and copepod biomass, was attributed to the effects of those physical events through the direct effect of local temperature and/or variation in surface irradiance, except for a prominent increase of copepods in the slope water in 2000. The effect of interannual change in the vertical mixing, suggested in previous studies, was not apparent in February, when the primary production is not limited by nutrient concentration which would be more important in regulating biological production in later months.     

Effects of diesel oil on the polychaete Annelida in model benthic ecosystem

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Monitoring of seabirds in the Benguela upwelling system: can seabirds be used as indicators and predictors of change in the marine environment?

Historical and contemporary data on Benguela seabirds assumed to be responsive to changes in food availability are reviewed as potential indicators and predictors of change in the ecosystem. Cyclic variations in the annual yields of seabird products evident at decadal long time-scales may reflect fluctuations in prey populations at similar periods. However, no such relationship has been demonstrated and consequently the value of such annual yields as indicators is not proved. More recently, interannual variation in diets, foraging effort, breeding success and chick growth of two of the three most abundant species of seabird in the ecosystem have been examined in relation to changes in prey abundance. Although the relative abundance of important forage species in the diets of both species of seabird have reflected regional changes in the absolute abundance of these prey, changes in the values of other parameters thought to reflect food availability were not consistently correlated between and within species. This indicates poor potential for their use as indicators of change at least over the range of abundances experienced in this study. Under present management protocol, data from seabirds do not nor are likely to provide abundance indices that may be used directly in stock assessment of commercially important species. To date, insights into distribution and migration of pelagic fish provided by seabird-derived data probably represent the most valuable contribution of birds to fisheries scientists studying the Benguela system.     

The effects of nutrients and heavy metals on the plankton in marine enclosed ecosystem

Applying marine enclosed ecosystem extensively to study the nutrient variation and its relation to biochemical process by marine ecologists and biogeologists in North America, Europe, Japan and Australia has been lasting for many years. Santschi (1982) reviewed the geochemical recirculation of nutrients systematically. Kremling et al. (1978) described the changes in nutrient in marine enclosed e-     

Calcareous nannoplankton in the surface sediments of the East China Sea and its environmental implications

Calcareous nannoplankton flora in the surface sediments of the East China Sea has been investigated many times based on isolated sample collections since 1980. In the present study, 73 surface sediment samples recovered from the East China Sea, extending from the estuaries to the Okinawa Trough (about 10 -2000 m in depth of water), were chosen to investigate the floral distributional pattern and to discuss in greater detail the controlling factors. Thirty-nine species were identified under polarized microscope and SEM. In general, nannoplankton abundance and diversity increase progressively with the increase of depth. In estuaries and the inner shelf nannoplankton are extremely rare. The 50 m isobath is a sharp boundary for the development of calcareous nannoplankton. Another low floral concentration is encountered at the edge of the outer shelf where Pleistocene relict sediments outcrop. The nannoflora displays a scattered distributional pattern in the Okinawa Trough. Nannoplankton of larger sizes is m     

Changes in abundance of the northern Benguela sardine stock during the decade 1990–2000, with comments on the relative importance of fishing and the environment

The northern Benguela stock of sardine Sardinops sagax used to be considered one of the major clupeoid stocks of the world; it supported an average annual catch of >700 000 tons throughout the 1960s. The stock has been in a depressed state for more than two decades, as demonstrated by annual catches that averaged around 50 000 tons between 1978 and 1989 and only slightly more in the 1990s. It has experienced fluctuations in abundance of several orders of magnitude during the most recent decade. Population size increased until 1992, when the acoustic estimate of biomass was about 750 000 tons. Catches increased accordingly, averaging 100 000 tons between 1992 and 1995, but from 1992 to 1996 the stock was in decline and the lowest annual catch in the history of the fishery was taken in 1996. Although there was a small increase during the last three years of the decade, the stock remains seriously depleted. Survey-based recruitment indices suggest that the changes in the 1990s were initiated by fluctuations in recruitment, but the decline was almost certainly exacerbated by continued fishing. Poor recruitment and decreasing catch rates between 1993 and 1996 in a number of other key resources suggest that system-wide environmental changes were an important factor in the decline of the sardine stock at that time. Anomalous oceanographic conditions, such as extensive hypoxic shelf waters in 1993/94 and a Benguela Niño in 1995, support this conclusion.     

The dynamics of the nanophytoplankton community in the coastal ecosystem of the Southern Bight (North Sea) during the winter–spring period

The seasonal dynamics of the nanophytoplankton community (pelagic algae 2–20 μm) was studied at a location in the Southern Bight (North Sea) from October 1993 through May 1994. During the study period the cell number and biomass of the nanoalgae varied between 10⁵–10⁶ cells dm⁻³ and 16–300 μg wet weight dm⁻³. Several ecological events seem common to the boreal coastal nanophytoplankton communities in the winter–spring period. These are: (1) abrupt fall in abundance at the beginning of winter (late October–early November); (2) winter minimum (December–March); (3) steady increase in cell number and biomass (April–May); followed by (4) the spring maximum occurring usually in May.     

Killer whales in South African waters—a review of their biology

The distribution, seasonality and schooling behaviour of killer whales Orcinus orca in South African waters have been investigated from 785 records compiled between 1963 and 2009, and their size, morphometrics, growth, reproduction, food and feeding behaviour described from the examination of 54 individuals, 36 of which were landed at the Durban whaling station between 1971 and 1975. Qualitatively, the species appears to be more frequently encountered offshore, where it forms small schools of generally less than six animals. Seasonality of occurrence is not marked, although circumstantial evidence indicates that some individuals migrate from higher latitudes. Males reach 8.81 m and females 7.9 m, with 16.2% of males exceeding the size of the largest female. Stomach content and observational data suggest that the species can be characterised locally as an opportunistic predator of megavertebrates, rather than as the fish-feeding ecotype previously described. A stranded adult male with extreme tooth wear that was 1.5–2 m shorter than other males of equivalent age may be representative of a separate ‘offshore’ ecotype. Apparent differences between features of the popu lation's life history and those of resident killer whales in the north-eastern Pacific might be attributed to either uncertainties in age determination using dentinal growth layer groups or sampling bias. The basis for the suggestion that killer whales in South African waters should be reclassified as Vulnerable (rather than Data Deficient) is challenged.