On physical mechanisms controlling inshore aggregations of small pelagic fish in a tropical upwelling system

Acoustically determined fish distribution pattern from bi-seasonal pelagic fish surveys conducted off Angola indicate for the seasonal contrasts in the spatial organization of the sardinella (Sardinella spp.) aggregations. During austral winter, the distributions are dominated by contiguous aggregations, aligned with the inshore areas shallower then 50 m while during austral summer the aggregations are more scattered and spread across the entire continental shelf length. Bi-seasonal acoustic surveys on this stock were conducted during 1994-1998 and 2011-2014. We have compared oceanographic data collected during these surveys with cross-shelf multi-frequency acoustic images constructed from the collected echogram segments in an attempt to asses an impact of the seasonally varying physical conditions on the observed changes in fish distributions patterns. The oceanographic regimes off Angola are remotely controlled by coastally trapped Kelvin waves. The main upwelling season takes place from June to August. The hydrographic and meteorological observations evidence that this upwelling it is characterized by the elevated thermocline and the presence of the cold water pool close to the sea surace despite of the absence of the upwelling favorable coastal wind. The acoustic imagery reveals the omnipresence of the first mode internal solitary waves in the water column. Under these conditions, the observed continuous pelagic fish aggregations are generally located inshore of a shallow front induced by the internal wave breaking, which is located in the depth range of 40-60 m. The concomitant drop in the surface temperature and the rise in the fluorescence suggests that this front is the hot-spot of the planktonic food concentrations. On the basis of these observations, we conclude that in the windless upwelling region of the southeastern tropical Atlantic (6°30´-13° S), it is the internal waves and their breaking which are likely to be a major factor controlling the ocean triad: enrichment, concentration and retention - of the physical mechanisms that control small pelagic fish populations in these waters.