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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]

Habitat structure and complexity influence the structuring and functioning of fish communities. Habitat changes are one of the main pressures affecting estuarine systems worldwide, yet the degree and rate of change and its impact on fish communities is still poorly understood. In order to quantify historical modifications in habitat structure, an ecohydrological classification system using physiotopes, i.e. units with homogenous abiotic characteristics, was developed for the lower Lima estuary (NW Portugal). Field data, aerial imagery, historical maps and interpolation methods were used to map input variables, including bathymetry, substratum (hard/soft), sediment composition, hydrodynamics (current velocity) and vegetation coverage. Physiotopes were then mapped for the years of 1933 and 2013 and the areas lost and gained over the 80 years were quantified. The implications of changes for the benthic and demersal fish communities using the lower estuary were estimated using the attractiveness to those communities of each physiotope, while considering the main estuarine habitat functions for fish, namely spawning, nursery, feeding and refuge areas and migratory routes. The lower estuary was highly affected due to urbanisation and development and, following a port/harbour expansion, its boundary moved seaward causing an increase in total area. Modifications led to the loss of most of its sandy and saltmarsh intertidal physiotopes, which were replaced by deeper subtidal physiotopes. The most attractive physiotopes for fish (defined as the way in which they supported the fish ecological features) decreased in area while less attractive ones increased, producing an overall lower attractiveness of the studied area in 2013 compared to 1933. The implications of habitat alterations for the fish using the estuary include potential changes in the nursery carrying capacity and the functioning of the fish community. The study also highlighted the poor knowledge of the impacts of habitat changes on fish due to coastal development and urbanisation and emphasises that ecosystem management and conservation will benefit from a wider understanding of habitat functional roles and habitat changes influencing the functioning and structure of the fish communities. [ABSTRACT FROM AUTHOR]