This dataset is used to support the paper submitted to Environmental Pollution titled "Elements and essential fatty acids in fishes along a large, dammed river". The dataset includes levels of Hg, 30 other trace elements, stable isotopes of carbon and nitrogen, and the fatty acids EPA and DHA in four fish species collected from four sites on the Wolastoq | Saint John River (New Brunswick, Canada) that were selected in relation to the Mactaquac Generating Station. It also includes physical characteristics of the fish including length, weight, age, sex and percent moisture.