ON THE RELATIONSHIP BETWEEN SCATTERING LAYER, THERMAL STRUCTURE AND TUNA ABUNDANCE IN THE EASTERN ATLANTIC EQUATORIAL CURRENT SYSTEM

A migrating Sound Scattering Layer (SSL) was observed in the Eastern A tlantic equatorial system during warm and cool seasons. Averaged colum n scattering strength was nearly the same during daytime at 350-400 m depth and during night-time close to the surface. The depth of the noc turnal SSL was correlated with the depth of the thermocline. In the wa rm stable season, the maximum column scattering strength was found sou th of the equator in a dome shaped structure. In the cool season, the maximum was found at the northern convergence where the thermocline wa s well-marked, whereas upwelled waters were acoustically poor. For the warm season, this pattern is consistent with the distribution of chlo rophyll, zooplankton and micronekton. For the cool season, it fits als o rather well, except for the net-sampled micronekton : this discrepan cy could be related to the poor efficiency of the net in sampling adeq uately a thin layer. Finally, SSL strength coincides well with tuna ca tch distribution in this area, suggesting a trophic relationship.