Coastal upwelling fronts as a boundary for planktivorous fish distributions

Fronts have long been considered as bio-aggregators across the food web, serving as important foraging grounds for multiple trophic levels. However, the effect of fronts on intermediate trophic levels is not well understood. We hypothesized that for animals whose metabolic rates are strongly temperature dependent, physiological tolerance will have a more significant impact on their distributions than other biotic factors. We examined this hypothesis through assessment of the spatial variability of planktivorous fish and their dominant zooplankton prey associated with the seasonal and latitudinal variability of the upwelling fronts in the Northern California Current System. Acoustically observed fish biomass dominated by planktivorous species was higher offshore of the upwelling front than inshore. In contrast, zooplankton scattering layers dominated by euphauslids were generally associated with the 200 m isobath, regardless of the position of the front. Fish distributions were consistently found offshore of the upwelling front, aggregating them in the regions of warmer temperature. This suggests that the upwelling front acts as a shoreward boundary for planktivorous fish. With the offshore movement of the upwelling front away from the 200 m isobath as the upwelling season progressed, overlap between planktivorous fish and their zooplankton prey would be decreased. The boundary effect of coastal upwelling fronts on the distributions of mid-trophic level organisms indicates their important role in predator-prey interactions and energy transfer through food webs via a radically different mechanism than previously assumed.