The impacts of extreme El Nino events on sea turtle nesting populations

The El Nino Southern Oscillation (ENSO) is the predominant interannual pattern of climate variability in the world and may become extreme approximately once every 20 years. Climate-forced interannual variability in fecundity rates of long-lived species are well-studied, but the effect of extreme events is less clear. Here, we analyzed the effect of the extreme 2015-16 El Nino event on three long-lived sea turtle species in a region highly influenced by ENSO. The effect of this extreme event varied considerably among species. While reproductive success dramatically declined in leatherback turtles (Dermochelys coriacea), the reduction was only marginal in green turtles (Chelonia mydas). Nevertheless, the number of nesting green turtles decreased following the extreme El Nino event, likely due to decreased ocean productivity. We used global climate models to project an increase in the decadal occurrence of extreme events from similar to 0.7 events (beginning of twentieth century) to similar to 2.9 events per decade (end of twenty-first century). This resulted in a projected decline in the reproductive success of leatherback turtles (similar to 19%), a milder decline in olive ridley turtles (Lepidochelys olivacea) (similar to 7%), and no decline in green turtles (similar to 1%). Extreme El Nino events can have a strong detrimental effect on East Pacific leatherback turtles, a population that is already critically endangered due to other anthropogenic impacts. Our results highlight the importance of conducting species-specific and site-specific analyses of climatic impacts on sea turtles.