Major difficulties in studies monitoring or evaluating general environmental conditions are the intraspecific differences in population descriptors, such as the growth and reproductive period among different localities. The identification of feasible candidates for sentinel species requires basic information on these population parameters. Attention is drawn to the potential of the catfish Genidens genidens as a sentinel species for Brazilian estuarine environments, based on its compliance with a set of population descriptors and characteristics along a latitudinal gradient, and by providing new data from Guanabara Bay, one of the most important estuarine systems on the Brazilian coast. Populations of G. genidens along the southern and southeastern Brazilian coast vary little, showing a similar reproductive period and shape of the length–weight relationship among localities. This catfish is abundant, easy to catch, economically important, tolerant of environmental variations and human impacts, does not migrate, and can be found in reference localities, fulfilling the majority of the requirements for a sentinel species.