Three comparable ecosystems
Three different ecosystems:
The Benguela Current   |  The Canaries Current  |  The Humboldt Current
Modelling, GIS, acoustic observation...

The Benguela Current

Along the west coast of western Africa, the Benguela upwelling distinguishes itself from other upwelling systems by the intrusion of warm waters at its two boundaries: to the north, via the Angola-Benguela front, and to the south, via the Agulhas Current which terminates the western boundary current of the Indian Ocean. To spawn, sardines (Sardinops sagax) and anchovies (Engraulis encrasicolus, ex E. capensis) migrate towards the Agulhas Bank, where the warm waters transported by the Agulhas Current create a highly stratified environment. Eggs and larvae are then rapidly transported northwards by a coastal current. After several days, they reach the upwelling region of the east coast where certain numbers are transported out to open sea by wind-driven currents, a source of mortality. A retention mechanism enables other individuals to aggregate within coastal waters.
Other sources of early life cycle mortality must not be overlooked, in particular those linked to starving larvae and to predation of eggs and larvae. A week after hatching, larvae must feed. As their swimming abilities are limited, they need not only a high density of suitably­sized prey (mainly microzooplankton), but also optimal turbulence conditions. Excessive turbulence disperses plankton aggregations and reduces capture.
Precise modelling of currents and production should shed new light on these issues. Although primary productivity of the Benguela system seems globally in excess, deficits may occur in certain spatio-temporal strata such as that of the Agulhas Bank or the offshore feeding area during the reproductive season. In these conditions, feeding oases may be found within the large offshore eddies caused by the meeting of different currents and water masses. The low productivity of the Agulhas Bank creates intense food competition among the millions of tons of spawners which aggregate there during the spawning season. This encourages parental cannabilism and the predation of eggs and larvae by other species.  Modelling of this phenomenon remains difficult in the absence of monthly spatialised data on fish distribution but efforts at quantification should be made.
The Benguela ecosystem can be divided into two subsystems, the Northern Benguela (southern Angola and Namibia) and the Southern Benguela (South Angola and Namibia), separated by the permanent upwelling cell off Luderitz, the strongest in the world. In South Africa, historical fisheries data suggests a stock collapse of sardines in the late 1960s followed by slow recovery, whereas the first level of collapse was noted in Namibia at the same period, worsening in the late 1970s. The biomass of the Namibian stock has remained at a very low level. Were these collapses due to an environmental process or rather to overfishing? The structure and ecosystem dynamics of the Southern Benguela appear to have been undergoing progressive changes for over 30 years, whereas the Northern Benguela ecosystem has been completely “reorganized” following what is commonly known as a “regime shift”. Today, the Northern Benguela is dominated by jellyfish and fish feeding on detritus (Gobies).  Pelagic fishes have become low in  abundance and fisheries for most commercial species (hake and horse mackerel) are threatened. The reasons for this regime shift with disastrous consequences for future and ongoing fishing activities are still hypothetical, although interactions between the environment and exploitation are strongly suspected.