Non-stationary responses in anchovy (Engraulis encrasicolus) recruitment to coastal upwelling in the Southern Benguela

Anchovy Engraulis encrasicolus is economically important and ecologically critical to the structure and function of the Benguela Current ecosystem, as it transfers energy from plankton to piscivorous fishes, seabirds, and marine mammals. Like other small pelagic fishes, annual recruitment strength varies substantially, but the drivers of these fluctuations are not well understood. To address this issue, we investigated the relationship between 30 yr of anchovy recruitment estimates derived from acoustic survey data and a new coastal upwelling index for the Southern Benguela defined as the monthly sum of offshore Ekman transport over the region 29-36 degrees S. Cumulative December-March upwelling was significantly and positively related to recruitment, and this relationship was improved by integrating the upwelling index over multiple years (i.e. adding autocorrelation). A threshold-generalized additive model further showed that the slope of the linear regression between integrated upwelling and recruitment increased when anchovy spawner biomass on the west coast of South Africa in the preceding year exceeded similar to 0.74 million tonnes. By combining these 2 simple linear regressions into a single model, we were able to account for 82 % of the variability in anchovy recruitment from 1985 to 2014. The biomass threshold in the upwelling-recruitment relationship could relate to the presence of a strong spawner-recruit relationship, or to a shift in the dominant driver of recruitment variability as adult biomass increases, such as effective transport of eggs, larvae retention in coastal habitats, and primary productivity.