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Estuaries have long been recognized as nursery habitats for various marine fish species. As seascapes consisting of a mosaic of habitat patches, estuarine nurseries incorporate ecological processes such as ontogenetic shifts. To improve our understanding of the habitat requirement changes of nursery species within the estuarine seascape, habitat suitability models were built using boosted regression trees for three size classes of European flounder ( Platichthys flesus ) juveniles. The models combined several dynamic and static predictors, from physical and chemical water parameters to biological and physical habitat variables. The relationships between juveniles and predictor variables varied with size, potentially indicating the dynamics of habitat use for the juvenile flounder inside the estuarine nursery. Younger post-settled juveniles tended to be spatially constrained to a shallow upstream area and, with size, there were ontogenetic shifts to the middle estuary. This demonstrates the importance of considering different life stages when developing essential fish habitat maps, which reflect fine scale temporal and spatial distributions. Given the high level of anthropogenic pressures to which estuaries are subjected, the consideration of fish ontogenetic habitat requirements within the estuarine nursery is needed to identify high-value areas, and is valuable for developing effective spatial and temporal management plans. These are essential to fully protect the most sensitive life stages, such as early post-settlement stages occupying transient habitats. Additionally, and as the juvenile-habitat associations can rapidly change, fine-scale connectivity in the estuarine nursery becomes even more relevant in determining recruitment. Connectivity should therefore be central in conservation planning, in order to sustain estuarine and marine adult populations, and ultimately maintaining the fisheries. [ABSTRACT FROM AUTHOR]

Connectivity between coastal spawning grounds and estuarine nurseries is a critical step in the life cycle of many fish species. Larval immigration and transport-associated physical–biological processes are determinants of recruitment success to nursery areas. The recruitment of the European flounder, Platichthys flesus , to estuarine nurseries located at the southern edge of the species distribution range, has been usually investigated during its juvenile stages, while estuarine recruitment during the earlier planktonic life stage remains largely unstudied. The present study investigated the patterns of flounder larval recruitment and the influence of environmental factors on the immigration of the early life stages to the Lima estuary (NW Portugal), integrating data on fish larvae and post-settlement individuals (< 50 mm length), collected over 7 years. Late-stage larvae arrived at the estuary between February and July and peak abundances were observed in April. Post-settlement individuals (< 50 mm) occurred later between April and October, whereas newly-settled ones (< 20 mm) were found only in May and June. Variables associated with the spawning, survival and growth of larvae in the ocean (sea surface temperature, chlorophyll a and inland hydrological variables) were the major drivers of flounder occurrence in the estuarine nursery. Although the adjacent coastal area is characterized by a current system with strong seasonality and mesoscale variability, we did not identify any influence of variables related with physical processes (currents and upwelling) on the occurrence of early life stages in the estuary. A wider knowledge on the influence of the coastal circulation variability and its associated effects upon ocean-estuarine connectivity is required to improve our understanding of the population dynamics of marine spawning fish that use estuarine nurseries. [ABSTRACT FROM AUTHOR]