Shifts in plankton size spectra modulate growth and coexistence of anchovy and sardine in upwelling systems

Fluctuations in the abundance of anchovy (Engraulis spp.) and sardine (Sardinops scigax) are widespread in marine ecosystems, but the causes still remain uncertain. Differences between the planktonic prey availability, selectivity, and predation between anchovy and sardine have been suggested as factors influencing their dynamics. Using a dynamical multispecies size -spectrum model, we explore the consequences of changes in plankton size composition, together with intraguild predation and cannibalism, on the coexistence of these species. The shift towards smaller plankton has led to a reduction in the growth rate of both species. The effect was more deleterious on anchovy growth because it is unable to filter small particles. In model scenarios that included the effects of cannibalism and predation, anchovy typically collapsed under conditions favouring smaller sized plankton. The two species coexisted under conditions of larger sized plankton, although strong predation in conjunction with weak cannibalism led to the loss of sardine. The model provides new testable predictions for the consequences of plankton size structure on anchovy and sardine fluctuations. Further empirical work is needed to test these predictions in the context of climate change.