El Niño, surface circulation and coral larval dispersal across the world's greatest marine barrier

More than 5000 km separates the frequently disturbed coral reefs of the Eastern Tropical Pacific (ETP) from western sources of population replenishment. It has been hypothesised that El Niño events facilitate eastward dispersal across this ‘East Pacific Barrier’ (EPB). Here we present a biophysical coral larval dispersal model driven by 14.5 years of high-resolution surface ocean current data including the extreme 1997-98 El Niño. We find no eastward cross-EPB connections over this period, which implies that ETP coral populations decimated by the 1998 bleaching event can only have recovered from eastern Pacific sources, in congruence with genetic data. Instead, rare connections between eastern and central Pacific reefs were simulated in a westward direction. Significant complexity and variability in the surface flows transporting larvae mean that generalised upper-ocean circulation patterns are poor descriptors of inter-regional connectivity, complicating the assessment of how climate change will impact coral gene flow Pacific-wide.