Which are otoliths?
Otoliths are concretions composed by calcium carbonate (95.0%, mainly crystallized in the aragonite form), organic matter (3.0-5.0%) and trace elements (Campana, 2004) present in the auditory capsule of the bony fish. They are part of a system related to the balance and hearing mechanisms (Moyle & Cech, 2004). Bony fish have three pairs of symmetrical otoliths: the sagitta, lapillus and asteriscus, and several studies have indicated that the sagittae are the largest and most utilized.
Otoliths grow by the addition of concentric layers of calcium carbonate on a protein base, resulting in an asymmetrical structure in which different rates of organic and inorganic deposits become zones with different optical properties over time. Those depositions result in alterations in growth, temperature fluctuations, diseases, reproductive activity and food resource availability and consumption, among others.
Therefore, otoliths are one of the most important structures to understand the cycle of life of the fish and their populations (Campana, 2004), and several studies (Secor et al., 1995; Volpedo; Echeverrķa, 2003; Assis, 2004; Campana, 2005; Monteiro et al., 2005; Volpedo et al., 2008; Green et al., 2009; Rossi-Wongtschowki et al., 2014) have summarized the utilization of those structures.