The Canaries Current
There are three major areas in the Canaries Current ecosystem:
– the northern Moroccan coast with seasonal upwelling in summer ;
– the south Moroccan and north Mauritanian coast with permanent upwelling (Sahara Desert) ;
– the south Mauritanian and Senegalese coast with boreal upwelling.
The southern part of this ecosystem is characterized by high seasonal variability, alternating between an ecosystem under tropical influence in summer and a coastal upwelling ecosystem in winter. This alternation is accompanied by a migration of certain tropical species (tuna and tuna-like fishes) up to 20°N during summer, and by a southward extension of the habitat of temperate species such as the sardine S. pilchardus during winter.
The Canaries Current system has a unique proportion of wide continental shelves in the south, whereas the eastern regions are generally characterized by narrow continental shelves, due to their young geological age. Principal spawning areas have been linked to regions with wide shelves. This association would be the result of physical processes developing over wide and shallow continental shelves, which would limit exchanges between the coast and offshore.
WestAfrican fisheries have been studied for several decades by the IRD and its partners. The Canaries Current ecosystems were dominated by large demersal fish, which were rapidly overexploited.. In the southern part of the area, the adaptability of artisanal fisheries took advantage of the seasonal migration of numerous species (a balance between seasonal upwelling and the warm season dominated by tropical influences).
Over a longer period of time, West African fisheries seem to have been spared the sudden collapses observed in other systems. A lower fishing exploitation rate until the 1980s and high seasonal fluctuations could have contributed to this relative resilience. Three countries experienced rapid and unexpected growth of octopus stocks in the 1970s. Octopus fishing has become in commercial value one of the essential components of West African fisheries. This shift in species was interpreted as being the result of the absence of top-down control, following the overexploitation of demersal species and which favoured the development of short-lived prey species, such as the octopus (as well as shrimps and pelagic fishes). Further to the north, the reasons leading to the southern displacement of seiner fleets exploiting sardines remain largely unknown: is this due to stock displacement or better economic profitability in the south?
Offshore of the Sahara Desert, the brutal decline of exploitation in this region in the 1990s following the departure of Soviet block trawling fleets constitutes a unique example of reduction in fishing effort. The Sahara region will be therefore, in addition to the Senegalese upwelling area, a privileged research zone for the study of physical and biological interactions.