Seagrass meadows shape fish assemblages across estuarine seascapes

Estuarine seascapes are a mosaic of habitat types that are connected, to varying degrees, by the movement of organisms. In these seascapes, the attributes of the habitats themselves, as well as the spatial context in which they are embedded, can shape faunal assemblages. Here, we explicitly test how habitat and connectivity interact to shape fish assemblages across estuaries. Our model system comprised 6 distinct estuarine habitats (log snags, mangroves, rocky outcrops, seagrass, sand/mud and urban structures), which represent the full diversity of estuarine habitats and seascape contexts in our study systems. Fish were sampled with cameras at 318 sites in 13 subtropical estuaries in eastern Australia. Habitat type was the principal predictor of fish assemblage composition, an effect that was shaped by spatial connections to other habitats, particularly seagrass beds. Seagrass structured fish assemblages, not only by providing a complex habitat that contained more species and individuals than all other habitats, but also by having a 'footprint' beyond the edge of meadows. Fish were more diverse and abundant in non-seagrass sites that were closer to seagrass meadows. Two other seascape attributes, proximity of sites to the sea and to mangroves, also influenced the composition of fish assemblages, albeit less consistently than distance to seagrass. Conservation and fisheries management programs that seek to enhance or restore fish populations in subtropical estuaries should prioritise seagrass conservation initiatives, which encompass critical ecological linkages with other habitats in estuarine seascapes.