The shared data are (i) sizes (in cm) and densities (indiv.per1000m²) of juvenile fish and (ii) isotopic data (deltaC and deltaN) for macro- and mega-invertebrates, juvenile fish, and organic matter sources. These data are for 3 habitats (i.e. Sternaspis scutata, Amphiura filiformis/Owenia fusiformis, and Haploops nirae habitats, respectively coded as Code.Habitat 1, 2, and 3) in two coastal bays of South Brittany; the Bay of Concarneau (Code.Bay 1) and the Bay of Vilaine (Code.Bay 2). See the map attached to the dataset for more details.