Habitat use and food sources of European flounder larvae (Platichthys flesus, L. 1758) across the Minho River estuary salinity gradient (NW Iberian Peninsula)

The European flounder (Platichthys flesus Linnaeus, 1758) exhibits plasticity for several life traits throughout its distribution range, including ontogenetic habitat shifts during early life, as well as the timing and duration of spawning. Estuaries are preferred as nursery habitat; however, the importance of specific salinity zones for larval development is not well-understood. Therefore, we aimed to identify the significance of distinct estuarine salinity habitats (i.e., tidal freshwater, brackish) along the Minho River estuary (NW-Iberian Peninsula, Europe) for larval development by combining field observations with carbon (C) and nitrogen (N) stable isotope analysis. Sampling occurred between January 2015 and January 2016 in six sampling stations across the estuarine salinity gradient. A total of 29 larvae were collected in the Minho River estuary from March till September 2015. Spawning likely occurred near the river mouth because the highest abundance of larvae occurred in the brackish estuary. Timing for migration towards freshwater was variable with metamorphosis likely occurring in both brackish and freshwater habitats. European flounder larvae obtained their diet from the benthic food web, indicating that benthic habitat is fundamental for larval development, including prior to settlement. This study provides further evidence on the behavioral plasticity of European flounder during early life regarding both habitat use and timing of migration towards freshwater habitats. Additionally, this study demonstrates the importance of preserving estuarine connectivity for this migratory species.