24The whitemouth croaker, Micropogonias furnieri, is a long-lived fish of high commercial 25 importance in the western Atlantic Ocean. Here, we used stable isotope ratios of carbon, sulfur, 26 and nitrogen and isotopic niche metrics (SIBER) to study feeding habits and track habitat use 27 by whitemouth croakers in Guanabara Bay, an estuary in Rio de Janeiro state, Brazil. Our 28 results highlighted size-related habitat segregation, with small juvenile fishes (<30 cm) residing 29 mostly inside estuaries and large adult fishes (>60 cm) feeding mainly in Continental Shelf 30 waters. Medium adult fishes (30-60 cm) appear to feed in multiple coastal and Continental 31 Shelf habitats. Moreover, their feeding ecology showed strong temporal differences, linked 32 with seasonal and, to a lesser extent, interannual variation in oceanographic features of the 33 ecosystem in which they live. Overall, these differences in ecological features suggest that (1) 34 adult and juvenile whitemouth croakers should be treated as different components of the food 35 web and (2) the conservation of these habitats should be prioritized to better manage and sustain 36 the coastal fisheries in Guanabara Bay. 37 38 39 Keywords: whitemouth croaker, Guanabara Bay, Brazilian estuary, SIBER, isotopic niche. 40 41 42 45 muscles associated with the swim bladder, are a widely distributed fish family throughout the 46 world (Nelson 2006). Croakers constitute commercially important resources in the western 47 Atlantic Ocean and are the major fishery resource in Brazil, as they represent 22% of marine 48 and 9% of freshwater fish landings (Chao et al. 2015). The whitemouth croaker, 49 Micropogonias furnieri (Desmarest 1823) (Perciformes, Sciaenidae), is widely distributed in 50 the coastal waters of the western Atlantic Ocean, occurring from the Yucatán Peninsula 51 (Mexico, 28º N) to the Gulf of San Matías (Argentina, 41º S) (Isaac 1988; Vazzoler 1991). 52 However, despite its wide latitudinal distribution, this species becomes abundant from Rio de 53 Janeiro state (RJ) at 23º S latitude southwards to the shelf of Uruguay (Vazzoler 1991). In 54 Brazil, it constitutes one of the main demersal resources with an average annual catch of 42 55 000 metric tons between 2000 and 2015 (Chao et al. 2015; FAO 2017). In addition, the species 56 also represents one of the most important fishery resources in coastal demersal fisheries in 57 Uruguay and Argentina (Vasconcellos and Haimovici 2006; FAO 2017). 58 The whitemouth croaker occurs in coastal waters over large range of salinities (0.1-35) and 59 temperature (11-31.6˚C) (Vazzoler 1991; Carneiro et al. 2005), inhabiting over sandy and 60 muddy bottoms, mainly at depths of up to 50 m and occasionally to 100 m (Carneiro et al. 61 2005). The population distribution of whitemouth croaker in the southeastern and southern 62 regions of Brazil has been investigated since the 1970s. Studies on morphometric and 63 biological features (Vazzoler 1991; Puchnick-Legat and Levy 2006) and population genetics 64 usi...
