Larval fish assemblage dynamics in response to ENSO events in the Eastern Tropical Pacific: evidence from Gorgona island, Colombia

The El Niño–Southern Oscillation (ENSO) is a major driver of hydroclimatic variability in the Eastern Tropical Pacific (ETP), with cascading effects on marine ecosystems and biodiversity. This study examines larval fish assemblages around Isla Gorgona, Colombia, a strategically important Marine Protected Area within the Eastern Tropical Pacific Marine Corridor (CMAR), during a strong El Niño (October 2015) and a short-lived La Niña (March 2018). Contrasting thermocline depths (45 m vs. 10 m), sea surface temperatures, and salinity profiles were likely associated with differences in larval abundance, richness, and assemblage composition. A total of 573 fish larvae representing 59 taxonomic units were collected. Assemblages during El Niño were dominated by coastal taxa such as Cetengraulis mysticetus and Coryphopterus urospilus , whereas La Niña assemblages included mesopelagic taxa such as Bregmaceros bathymaster and Vinciguerria lucetia . Diversity metrics indicated higher Shannon diversity and lower dominance during La Niña, while beta diversity analyses revealed pronounced compositional turnover between phases. These patterns are consistent with previous studies reporting ENSO-linked larval dynamics across the ETP and may reflect ichthyoplankton sensitivity to hydroclimatic anomalies. However, interpretation remains constrained by seasonal differences between sampling periods, reliance on single-event comparisons, and the absence of direct measurements of upwelling, productivity, and nutrient dynamics. Despite these limitations, the study provides case-based evidence from an insular tropical system, establishing a valuable baseline for biodiversity monitoring in the Colombian Pacific. The findings underscore the importance of sustained ichthyoplankton surveys for adaptive management and for anticipating ecosystem responses to future ENSO scenarios and broader climate change projections in the ETP.