Observations on the near-surface behaviour of sardinella schools in Angolan waters

Abstract

The schooling dynamics and swimming behaviour of sardinella Sardinella maderensis and S. aurita schooling near the surface in Angolan waters were recorded visually, using a calibrated echo integration system and a 95kHz high resolution sonar on board RV Dr Fridtjof Nansen during three cruises; July 1996, May 1997 and May 1998. The mean packing density of the sardinella schools was about 3 fish m−3, but this figure varied by two orders of magnitude among the schools. There was a linear relationship between the area and the biomass of the schools, which can be used in future surveys to convert recordings of school area by camera, lidar or sonar to school biomass. The sardinella schools swam at an average horizontal speed of 1.24m s−1, mostly towards or away from the coast. The schools seemed to be feeding near the surface and behaved rather dynamically. Intra-school events such as change of shape, reorganisation, splitting and leaving occurred every 2.4 minutes on average. Inter-school events such as approach and join was every 14.2 minutes on average. The schools were visible on the surface in the morning and in the afternoon, but seemed remarkably undisturbed by predating birds, fish and seals during the day. It is suggested that the low predation activity near the surface enables the sardinella to exploit plankton prey that is concentrated at surface fronts of internal waves and along the convergence lines of coastal and oceanic currents.