Many commercially important marine fishes are found in estuaries, particularly as juveniles. The estuaries may provide a thermal resource, refuge from predators and a source of abundant prey. Here, carbon (d13C) and sulphur (d34S) stable isotope data from coastally caught common sole (Solea solea) and whiting (Merlangius merlangus) were used in independent mixing models to determine relative contributions of estuarine prey to white muscle composition. The influence of assumed trophic fractionation was also investigated with sensitivity analysis. At the population level, there was evidence for estuarine contributions to muscle tissue in both species (means from 16.37% to 61.28%), though among-individual variability was considerable. d13C and d34S model outputs implied different estuarine contributions for the same individual, likely reflecting the slower turnover of d34S than d13C. Sole exhibited population level plasticity in their feeding as juveniles, separating into two distinct juvenile sub-populations; but they are less plastic as older fish when they adopt increasingly marine diets. Whiting show individual plasticity as both juveniles and adults, feeding on prey of estuarine and coastal origin.