El Niño events and chlorophyll levels affect the reproductive frequency but not the seasonal reproductive output of East Pacific green turtles

Abstract

The El Niño-Southern Oscillation (ENSO), which includes El Niño and La Niña phases, affects oceanographic conditions and primary productivity of marine ecosystems at a global scale. During El Niño events, warm temperatures in the central and eastern tropical Pacific decrease primary productivity, affecting the biology of many species including sea turtles. We aimed to determine the effect of ENSO (as a global index) and primary productivity in the foraging areas (as a local index) on remigration intervals (number of years between nesting seasons) and seasonal reproductive output (clutch size and number of clutches in a season) of green turtles Chelonia mydas in the North Pacific off Costa Rica, an area highly influenced by ENSO. We used the Multivariate ENSO Index (MEI) and mean values of chlorophyll a from previously identified feeding areas for this population over different time periods (up to 3 yr before the nesting season). Chlorophyll a levels in foraging areas and MEI strongly influenced the reproductive frequency of green turtles. Variability in the remigration intervals was better explained by models that included MEI conditions 2 and 3 yr before the onset of the nesting season and chlorophyll levels in the previous 3 yr. Models using global and local indexes were equally good at predicting changes in the remigration intervals. Positive values of MEI (El Niño phase) and lower levels of chlorophyll a corresponded to longer remigration intervals. No statistically significant relationships were found between seasonal reproductive output and MEI or chlorophyll a levels. As El Niño events may become more frequent and last longer due to climate change, the lifetime reproductive output of green turtles in this area may be compromised in the future.

Inter-annual variability in breeding census data across species and regions

“…In some years, for example, oceanographic conditions may drive marine heatwaves and associated sea grass die-off along the west coast of Australia (Arias-Ortiz et al 2018) and so have a major negative impact of green turtle foraging conditions (Nowicki et al 2019). ENSO may also impact green turtle foraging conditions in the eastern Pacific and help drive inter-annual variability in reproductive output in that region (Bruno et al 2020;Ramírez et al 2021). It has been well established that, across their adult lifespans, sea turtles tend to have tight fidelity to individual foraging sites (Shimada et al 2020).…”