Features, boundaries and connecting mechanisms of the Mauritanian Province exemplified by oceanic fish larvae

Abstract

The Mauritanian Province, which comprises the area from the NW-African shelf to about 20°W and from Cape Timiris to Cape Verde, is redefined on the basis of fish larvae distribution patterns, direct and indirect current measurements, CTD-O2 hydrography and SST-remote sensing. Its northern and western boundaries coincide with, the permanent, although not stationary, Cape-Verde-Frontal-Zone separating North- and South-Atlantic-Central-Water masses. Its southern boundary is characterized by tropical temperatures throughout the year. The oceanic Mauritanian Province is a myctophid dominated system that lacks genuine endemic species. Many deep-mesopelagic species of Myctophidae as well as other families are absent or rare in its centre, whilst some northern oceanic Myctophidae acquire a pseudoceanic character and prosper. On the other hand, tropical species of both oceanic and neritic origin co-occur alongside such cold-temperate Myctophidae. Distribution patterns of fish larvae were partly directly dependent on currents intruding into the province and partly indirectly on the impact of the circulation system on the local hydrography. Local hydrography could affect the survival capability of either larvae or adults. While an oxygen deficit southeast of the frontal zone is a limiting factor for subsurface-epipelagic and mesopelagic species, southwestward Ekman-drift and the related temperature deficit hampers the intrusion or survival of tropical fish larvae with neustonic or shallow vertical distribution. Southern, deep living or eurybathic species with lower temperature preferences are transported into or even beyond the province by northward currents. A transport model describing the input/output mechanisms of the province and defining its limiting features is presented and discussed in respect to recent literature.