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Introduction
Estuaries have physical and chemical gradients, especially salinity and trace elements, which can be used as natural markers of habitat for aquatic organisms (Albuquerque et al., 2010; Avigliano & Volpedo, 2013; Mohan & Walther, 2015). The exposure of fish to different water masses through ontogenetic development directly influences the incorporation of trace elements into the otolith (Campana, 2014). Otoliths are complex structures of calcium carbonate precipitated mostly as aragonite, and small amounts of other chemical elements immersed within an organic matrix (Campana et al., 2000). The precipitation of calcium carbonate is an extracellular process regulated by fish physiology, environmental factors (such as salinity and temperature), and water composition. Nevertheless, it can also be dependent on genetics, food and even fishing pressure (Morales-Nin, 2000; Elsdon & Gillanders, 2003; Gillanders et al., 2011; Catalán et al., 2018). The otolith is a chemically inert structure, since there is no resorption of the incorporated elements (Gauldie et al., 1998; Elsdon et al., 2008). The precipitation of divalent ions in the otolith is a competitive process, in which these ions substitute Ca2+ (Campana, 1999; Brown & Severin, 2009). For this reason, the natural markers in otoliths such as Ba, Mn, Li and Sr are always studied in relation to the concentration of Ca (element/Ca ratio). Specifically, for several euryhaline species the otolith Sr/Ca and Ba/Ca ratios have respectively positive and negative relationships with salinity (Campana, 2014; Avigliano et al., 2018); while ratios such as Mn/Ca (Limburg et al., 2011) or Li/Ca (Bouchard et al., 2015) may be associated with hypoxia, physiological or environmental factors. Then, the otolith has turned out to be a useful tool to track the life history of teleostean fishes in environments with physicochemical gradients (Shrimpton et al., 2014; Duponchelle et al., 2016; Avigliano et al., 2017a).
The species within the Ariidae family are of high commercial value in the South-western Atlantic (Freire & Pauly, 2005), Madmango sea catfish Cathorops spixii (Spix & Agassiz, 1829) being one of the most important. This is a benthic species widely distributed in estuaries and continental shelf between latitudes 13°N and 25°S (Dantas et al., 2013; Denadai et al., 2013; Santos et al