Relating sardine Sardinops sagax abundance to environmental indices in northern Benguela

Most attempts to define relationships between the environment and Sardinops sagax recruitment success in the northern Benguela have focused on regional indices. This study used a large-scale perspective and proposed two new indices: one based on the sea surface temperature in the tropical Atlantic, and the other on coastal wind stress. The proposed indices were not only consistent with empirical evidence but were also based on current understanding of the processes underlying environmental variability and pelagic fish recruitment dynamics. Prior to the mid-1980s, sardine recruitment strength was positively correlated with SST and negatively with wind. Subsequently, these relationships reversed, recruitment being negatively correlated with SST and positively correlated with wind. GAM models incorporating recruitment, spawning stock biomass and environmental indices were built and compared, and two hypotheses were formulated to explain the reversal of the environment-fish relationships. The first addressed a switch between two environmental regimes: the first (prior to the mid-1980s) characterised by weak stratification and strong enrichment, and the second (late 1980s and the 1990s) characterised by frequent warm events, stronger stratification, and reduced enrichment and productivity. The second hypothesis suggested that the reversal can be attributed to changed population structure, distribution and migration behaviour of the severely overfished sardine stock resulting in a shift from the optimal spawning area in the vicinity of Walvis Bay to the a less favourable spawning habitat in the north. The present depressed state of the stock does not allow firm conclusions to be drawn about the effect of the environment on its population dynamics, nor to consider the implications for management. However, it is argued that an appreciation of how such relationships may change through time is important in further understanding and modelling oceanic and fisheries systems in the region.