Biobío River freshwater discharge effect on Engraulis ringens early stages transport in the Gulf of Arauco (Chile) using biophysical modeling

Resumen: The Gulf of Arauco, a highly productive area, sustains some of the primary epipelagic, mesopelagic, and benthic resources in the Southern Humboldt Current System. It is also an important nursery and spawning area. High productivity is driven by seasonal and synoptic wind variability, coastal upwelling, topographic upwelling from the Biobio Submarine Canyon, and freshwater flow, mostly from the Biobío River. The effect of freshwater discharge on early- stage fisheries resources has been poorly studied in central Chile, making our research all the more significant. From 2014 to 2018, a high-resolution interannual biophysical simulation incorporating the Biobío River freshwater discharge was conducted. It allowed us to thoroughly examine the impact of the Biobío River on the abundance and transport of anchovy early-life stages in the Gulf of Arauco. To ensure the accuracy of our findings, we compared model results to in-situ biological/hydrographic data in the Gulf of Arauco from 2007 to 2019. The biophysical model was run with release pulses every five days from September 1 to October 31. In the research area, the months of September and October have traditionally been identified as anchovy spawning months. The simulation was evaluated using General Linear Models, which suggested that larval survival would be influenced by the spawning’s temporal fluctuation (year and day of release). The extension of the Biobo River plume, identified using the potential energy anomaly, was found to be similar to the observations. The model results showed that larvae were largely transported northward, and spawning zones from the research period showed strong retention in the Gulf of Arauco. The highest abundances of anchovy early-life stages were found south of the Biobío River mouth, in highly stratified waters associated with the Biobío River plume in the Gulf of Arauco. Significant correlations between Biobío River’s monthly mean discharge, river plume extension, and anchovy early-life stages abundance were observed from 2014 to 2018, both for simulated and in situ data. However, when considering a larger egg in situ abundance time series data in situ (from 2007 to 2019), significant correlations were observed between egg abundance and the river plume extension area (R²= 0.7 p = 0.03) only up to a threshold value, which ranged from 338 to 357 km²); below and above this threshold, abundance dropped drastically. This research concludes that the Biobío River is an essential factor regulating the early stages of the life of anchovy in the Gulf of Arauco.