Investigating the 'northern Humboldt paradox' from model comparisons of small pelagic fish reproductive strategies in eastern boundary upwelling ecosystems

Eastern boundary upwelling ecosystems are highly productive and sustain the world's largest fisheries, usually dominated by sardine and anchovy species. Stock size is highly variable from year to year due to the impact of the unstable physical environment on fish early stages. Biophysical models of early life-stage dispersal of marine organisms have been built by coupling (i) hydrodynamic models and (ii) life history models (i.e. egg and larva stages), and are therefore useful tools to investigate physical-biological interactions. Here, we review biophysical models of anchovy and sardine ichthyoplankton dispersals developed in the Benguela, Humboldt and Canary Current upwelling ecosystems. We also include a similar study conducted in the California Current upwelling on zooplankton. We then integrate this information into a comparative analysis of sardine and anchovy reproductive strategies in the different systems. We found that the main spawning periods match the season of (i) maximal simulated ichthyoplankton retention over the continental shelf in the northern Benguela, southern Humboldt and Canary (for sardine); (ii) maximal food concentration in the southern Benguela, California and Canary (for anchovy); and (iii) maximal shelf retention of ichthyoplankton and food concentration in the northern Humboldt (for both anchovy and sardine). This specificity of the northern Humboldt ecosystem could explain why it sustains the largest small pelagic fish stock. Finally, the possible effects of climate change on these patterns are discussed.