The surface dynamics of the northern Benguela upwelling system and its relationship to patterns of clupeoid production

Abstract

The environmental dynamics of the northern Benguela upwelling system was investigated primarily using a time series of mean, weekly satellite images of sea surface temperature (SST), and observed trends were related to patterns in the reproductive activity and recruitment success of the region's pilchard and anchovy stocks. Three main methods were used to look at environmental trends: overall and seasonal trends in the system's behaviour were derived from maps of mean SST and SST variability, charts of mean weekly SST per 0.5° latitude were constructed to show spatio-temporal variability in coastal upwelling activity, and standardised principal components analysis was employed to identify the major modes of spatial SST variability and to quantify the evolution of the system's spatial structure through time. Important new findings include the observation that interannual variability itself is largely due to interannual differences in conditions during February, March and April, and that longshore intrusions of warmer water from both ends of the system and onshore intrusions of oceanic water account for a large proportion of the system's temporal variability in spatial structure. Comparisons of clupeoid reproductive activity with environmental trends further confirms that spawning activity and anchovy larvae abundance tend to be greatest when upwelling activity is reduced and conditions are suitable for the retention of eggs and larvae. As regards recruitment variability, pilchard recruitment was found to have a generally positive relationship with SST conditions, whilst anchovy recruitment appeared to be favoured by cooler conditions than pilchard. Different 'domains' in the relationship between the system's spatial structure, as revealed by the principal components analysis, and inshore SSTs were found to correspond closely with differing levels of anchovy and pilchard recruitment between 1982 and 1987. Should these relationships hold when tested over more years, they might, in the future, provide a basis for predicting clupeoid recruitment success in the region