Remotely sensed variability of temperature and chlorophyll in the southern Benguela: upwelling frequency and phytoplankton response

High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones.     

The reproductive biology of Diplodus argenteus (Sparidae) in the coastal upwelling system of Cabo Frio,Rio de Janeiro,Brazil

The reproductive biology of the seabream Diplodus argenteus, a dominant coastal fishery species, was investigated over two consecutive seasons (2001–2002) at Cabo Frio, Brazil, a low-latitude upwelling system. The sex ratio was dominated by females (1.4:1.0) and the length-at-50% sexual maturity (females) was 203mm total length. Females were multiple spawners and the reproductive pattern appeared to be digynous protandrous hermaphroditism. Monthly variations in gonadosomatic index and proportion of ripe females indicated that reproductive activity occurred from late winter to summer (August–February), following coastal upwelling.     

Achievements and lessons learned from the Benguela Environment,Fisheries, Interaction and Training (BENEFIT) research programme

This paper considers the scientific achievements of the Benguela Environment, Fisheries, Interaction and Training (BENEFIT) Programme as reflected in 29 internally evaluated BENEFIT research projects carried out between 1998 and 2007 when the programme ended, and draws a number of important lessons relevant to future regional marine research programmes in the Benguela from the manner in which the projects were initiated and executed. The founding of BENEFIT, its organisational structures, and the way in which the projects were planned, formulated and executed through these structures are outlined. The major findings and other achievements of the research on fish resources, the environment and the linkages between them are discussed under these headings, and in the case of projects on resources, in relation to key questions developed from guidelines in the BENEFIT Science Plan. Based largely on a final evaluation by the programme's International Scientific Advisory Panel, it is concluded that BENEFIT added materially to greater knowledge of the dynamics of key resources in the Benguela, markedly increased the extent of environmental monitoring in the region, and the capacity for future such monitoring, and added significant new insights into key physical, chemical and biological processes of relevance to stock dynamics. BENEFIT's most important achievement in a wider sense was the pioneering development, almost from scratch, of a regional marine research community which, inter alia, was crucial in the development of other regional marine research activities in the Benguela, such as the Benguela Current Large Marine Ecosystem (BCLME) Programme whose primary objective was the creation of a Benguela Current Commission. In this sense, the establishment of the Commission in 2007 to deal with transboundary management of the living resources of the Benguela Current and regional biodiversity issues can be regarded as the greatest single legacy of BENEFIT.     

Metabolic adaptations and reduced respiration of the copepod Calanoides carinatus during diapause at depth in the Angola-Benguela Front and northern Benguela upwelling regions

Stage C5 copepodids and adult females of the herbivorous copepod Calanoides carinatus were sampled in the Angola-Benguela frontal region and northern Benguela upwelling area off Namibia in February–March 2002, using a multiple opening/closing net system. Respiration rates of C5s collected between 400m and 700m were measured onboard at the simulated in situ temperature of 8°C and at sea surface temperature (SST ≥20°C). These data were compared to the oxygen demand of epipelagic individuals of C. carinatus caught in the upper 30m and incubated at ambient SST. Deep-living C5s consumed 0.21 ± 0.08ml O₂ h^?1 (g dry mass)^?1 at 8°C and 0.96ml O₂ h^?1 (g dry mass)^?1 (range 0.84–1.09) at 25.9°C. These results were substantially lower than respiration rates of 5.23 ± 0.55ml O₂ h^?1 (g dry mass)^?1 in epipelagic individuals incubated at SST. The results reveal a reduction by 96% of metabolic rate in deep-living, diapausing C5s relative to surface-dwelling, active individuals. Only 14.4% of this metabolic reduction is explained by the lower ambient temperature at depth and a Q₁₀ value of 2.34. Therefore, the major fraction (81.6%) of the metabolic reduction is attributable to active physiological changes or processes during diapause at depth. The study emphasises the importance for herbivorous copepods, in areas with a highly variable food supply, to adopt a dormant phase in their life cycle in order to survive long periods of starvation.     

An evaluation of artificial shelters in the artisanal spiny lobster fishery in Gazi Bay,Kenya

Two designs of artificial shelters were evaluated for use in the artisanal spiny-lobster fishery in Gazi Bay, Kenya. Both types of shelter were effective in aggregating the spiny lobster Panulirus ornatus in nearshore sea-grass beds. Lobsters aggregated at the shelters were caught by free-diving fishermen, using spearguns and hand nets. Mean lobster catches taken from the shelters ranged from 0,38 to 0,83 kg·trip^?1, around 50% of those taken farther offshore in natural reefs. No significant difference was found in the size or sex composition between shelter and reef-caught lobsters.     

Characterising and comparing the spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem

The spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem were characterised by comparing their egg abundances with environmental variables measured concomitantly during two different survey programmes: the South African Sardine and Anchovy Recruitment Programme (SARP), which comprised monthly surveys conducted during the austral summers of 1993/94 and 1994/95; and annual pelagic spawner biomass surveys conducted in early summer (November/December) from 1984 to 1999. Eggs were collected using a CalVET net. Physical variables measured included sea surface temperature (SST), surface salinity, water depth, mixed-layer depth, and current and wind speeds; biological variables measured included phytoplankton biomass, and zooplankton biomass and production. Spawning habitat was identified by construction of quotient curves derived from egg abundance data and individual environmental variables, and relationships between these variables were determined using multivariate co-inertia analysis. SARP data showed that anchovy spawning was associated with cool water and moderate wind and current speeds, whereas sardine spawning was related to warmer water and more turbulent and unstable conditions (i.e. high wind speeds and strong currents) than for anchovy. SARP data also showed significant differences in selection of spawning habitat of the two species for all environmental variables. The relationship between anchovy egg abundance and salinity was strongly positive, but strongly negative with water depth, phytoplankton biomass and zooplankton production. Sardine egg abundance was strongly positively related to current speed. The spawner biomass survey data demonstrated that the spawning habitat of anchovy was characterised by warm water and high salinity, whereas sardine spawning was associated with cool water and low salinity. The survey data showed significant differences in spawning habitat selection by anchovy and sardine for SST, salinity and zooplankton biomass, but not for the other environmental variables. There was a positive relationship between anchovy egg abundance and SST, salinity and mixed-layer depth, and a negative relationship with water depth, phytoplankton biomass and zooplankton production. For sardine there was a strong positive relationship between egg abundance and current speed and wind speed. Differences in the results between the two survey programmes could be attributable to differences in their spatio-temporal coverage. Spawning habitats of anchovy and sardine appear to be substantially different, with anchovy being more specific than sardine in their preference of various environmental conditions.     

Impact and implications of large-scale environmental anomalies on the spatial distribution of spawning of the Namibian pilchard and anchovy populations

Trends in the spatial pattern of spawning of pilchard Sardinops ocellatus and anchovy Engraulis capensis during the period 1971–1985 were examined in relation to large-scale environmental anomalies outside the spawning area. Shifts in the locality of peak spawning of anchovy often coincided with that of pilchard, although sometimes anchovy were slower to respond to an environmental change. With negative sea surface temperature (SST) anomalies spawning shifted northwards, whereas with positive anomalies spawning took place both in the north and farther south. With a transition from negative SST anomalies early in the spawning season to zero or positive anomalies later in the spawning season, the impact on anchovy and pilchard spawning differed. Anchovy spawning was either reduced or absent during the negative phase of transitional seasons, thus avoiding unfavourable conditions. Pilchard, on the other hand, were spawning mainly in the north during the negative phase of the transitional seasons, but also in the south during the zero to positive phase. The shifts in spawning locality in response to environmental anomalies outside the spawning area imply that explanations for such shifts need not be sought in changes in biomass or age structure or in the fact that stocks may be genetically different, all theories previously advanced.     

Density distribution by depth of Geryon maritae on the northern crab grounds of South West Africa/Namibia determined by photography in 1983, with notes on the portunid crab Bathynectes piperitus

The density of red crab Geryon maritae between 18°00′ and 18°30′S and in depths of 350-1 000 m was quantitatively examined by photography. This area, estimated to be approximately 92 000 ha, is part of the commercial crab ground off South West Africa. The biomass of red crab, up to 55,5 kg·ha^?1, is one of the highest recorded off the African coast for the species. Density of red crab was highest in depths exceeding 600 m, the greatest density (350,2 crabs·ha^?1) being recorded in the 600-699 m depth zone. Another crab, Bathynectes piperitus, was also photographed in the shallower depths of the survey area (300-500 m), and some density estimates for this species at those depths are presented.     

Wintering seabirds in West Africa: foraging hotspots off Western Sahara and Mauritania driven by upwelling and fisheries

Cold-water upwelling supports abundant and diverse faunas. Upwelling off Mauritania has been highlighted as being important for seabirds, but very few systematic offshore surveys have been conducted in that region. Mauritanian waters are increasingly targeted by commercial fisheries along the shelf break and, with the likelihood of future exploitation of mineral oils on the continental shelf, the absence of information on seabirds is worrying. This paper describes the distribution of wintering seabirds in the context of fisheries and hydrography. The avifauna was dominated by surface-feeding and shallow plunge-diving, often planktivorous, seabirds, originating from West Palaearctic breeding grounds (Arctic, subarctic and temperate zones). Many seabirds were associated with fishing trawlers around the shelf break, but the fleets were more evenly distributed than the birds, with certain hydrographical parameters influencing or overruling the attraction of trawlers by seabirds. The results support the belief that West African waters are of prime importance for seabirds.     

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.