Genetic tags are among the most commonly used markers in these cases, though their usefulness is conditioned by the existence of significant genetic variability among the target populations (references). In this context, natural chemical tags present in certain tissues, such as the inner ear bones of teleost fish (otoliths), are a promising alternative.
The unique properties of these calcified pair structures render them especially useful to unravel the life-history characteristics and movement patterns of fish (Campana & Thorrold, 2001; Walther & Thorrold, 2011). Otoliths grow continuously throughout the life of the fish (from birth to death), are acellular metabolically inert materials and the concentration of certain accreted elements in the successive growth layers is largely determined by their concentration in the ambient water (Campana, 1999; Bath et al., 2000; Walther & Thorrold, 2006; Morais & Daverat, 2016). Therefore, if there are natal habitats with different chemical signatures, the otolith core of the fish hatched in them will record for life these differences, allowing the retrospectively determination of natal origin (Thorrold et al., 2001). Because of these peculiar characteristics, otoliths microchemistry has been used since the end of the 20th century to unravel different features of the life cycle of diadromous fish at fine spatial scales, such as migration ecology, dispersal capacities, patterns of population connectivity and natal origin (Daverat et al., 2005, 2006, 2011; Walther & Thorrold, 2008; Walther et al., 2008; Martin et al., 2013 a, b; Nachon, 2016). The challenge when using the natural marks of the otoliths for the identification of the natal origin in anadromous fish is the appropriate classification of fish of unknown origin to a given river. Because of technological advances, there are sophisticated statistical packages and computation tools that allow calculating with high precision the natal origin of each unknown fish through Bayesian probabilistic analysis (Plumer, 2003; Plummer et al., 2006; P?ugeisen and Calder, 2013; Neubauer et al., 2013).