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A migrating Sound Scattering Layer (SSL) was observed in the Eastern Atlantic equatorial system during warm and cool seasons. Averaged column 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 nocturnal SSL was correlated with the depth of the thermocline. In the warm stable season, the maximum column scattering strength was found south of the equator in a dome shaped structure. In the cool season, the maximum was found at the northern convergence where the thermocline was well-marked, whereas upwelled waters were acoustically poor. For the warm season, this pattern is consistent with the distribution of chlorophyll, zooplankton and micronekton. For the cool season, it fits also rather well, except for the net-sampled micronekton : this discrepancy could be related to the poor efficiency of the net in sampling adequately a thin layer. Finally, SSL strength coincides well with tuna catch distribution in this area, suggesting a trophic relationship.