Remote and Local Environmental Factors Drive Long-Term Trends of an Estuarine-Dependent Marine Fish in a Subtropical Coastal Lagoon

An important component of global climate influencing ecological systems is the El Niño Southern Oscillation (ENSO), which affects hydrological conditions in coastal environments around the world. Based on long-term time series of monthly sampling (1997–2019), we investigated the influence of remote (ENSO) and local environmental factors on the juvenile’s abundance of the Argentine menhaden Brevoortia pectinata in a subtropical estuary. We also evaluated if the abundance over the years is decreasing due to the negative effects of higher intense El Niño events. Despite the apparent increase in abundance over the years, time series decomposition and mixed models did not reveal an increasing trend along the studied period (Chisq = 2.62; p = 0.289). The overall interannual trend varied across sampling sites, and a trend of increasing abundance over the years was observed only at the estuary site closest to the sea (Chisq = 5.59; p = 0.018). The most parsimonious GAM model revealed that salinity, temperature, and ENSO (based on the Oceanic Niño Index, ONI) explained 23.8% of the abundance variation of B. pectinata juveniles. There was a negative effect on the abundance during El Niño years, especially during more intense events (ONI > 1.5), but no apparent effects were observed for La Niña events. Our findings revealed that the variations in the abundance of this species depend not only on local factors such as temperature and salinity, but also on global climatic phenomena that influence estuarine hydrology.