Mesoscale distribution of clupeoid larvae in an upwelling filament trapped by a quasi-permanent cyclonic eddy off Northwest Africa

Abstract

The distribution of fish larvae in relation to a filament shed from the Northwest African coastal upwelling was studied in February 2001. During the cruise, the filament was located between 27°N and 28°N, extending from the African coastal upwelling zone to the south of Fuerteventura Island (Canary Islands). This filament was trapped and remained over the quasi-permanent cyclonic eddy observed in previous studies. Almost all Sardina pilchardus, Engraulis encrasicolus and Sardinella aurita larvae caught during the cruise were associated with upwelled waters and filament structures. The sampled larval fish assemblage was composed by 12.6% of clupeoid larvae. These were distributed as follows: 73.9% were S. pilchardus, 20.7% were E. encrasicolus and 5.4% were S. aurita. Their distribution suggested that the coastal upwelling filament is a mechanism of transport from the upwelling area to oceanic waters, but its junction with the generated cyclonic eddy may not always work as a retention structure for those transported larvae, as described in previous studies. Physiological studies based on gut fluorescence and ETS activity of clupeoid larvae, as proxies for grazing and respiration, respectively, denoted a switch from pigmented food near the upwelling zone to unpigmented food toward the ocean. This pattern agrees with observed mesozooplankton feeding along an upwelling filament in previous studies. Therefore, this work confirms the close relationship between clupeoids distribution and mesoscale circulation, as well as constitutes the first assessment of the metabolic activity of those larvae in the region.