Chapter 9: Otoliths
James M. Long and Timothy B. Grabowski
Otoliths are commonly used to estimate ages of bony fishes, particularly teleosts, but otolith use and methods vary among species (Maceina et al. 2007). Otoliths have been less useful for estimating the age of nonteleost bony fishes that have a more basal evolutionary history (Adams 1942; Koch et al. 2009). In this chapter, we discuss the aspects of otolith form and function that are relevant to age estimation and provide an overview of the methods used to extract, prepare, and view otoliths when estimating the age of fishes.
Otoliths are calcium-carbonate-based structures associated with the inner ear of bony fishes, which is situated toward the posterior end of the cranium (Popper et al. 2005; also see Chapter 2). The inner ear is used for both hearing and orientation and consists of three semicircular canals and end organs (saccule, lagena, and utriculus) that contain the otoliths (Popper and Lu 2000; Popper et al. 2005; Figure 9.1). The otolith associated with the utriculus is the lapillus (utricular otolith) and is thus located anterior and superior to the other otoliths in the head of a fish. The otolith of the saccule (saccular otolith) is the sagitta and that of the lagena (lagenar otolith) is the asteriscus. Otoliths occur in pairs (left and right) and can be obtained through dissection for age and growth purposes. In bilaterally symmetrical fishes, otolith pairs are symmetrical and several previous studies have reported no appreciable differences between otoliths in a pair (Boxrucker 1986; Harvey et al. 2000; Cardinale et al. 2004). Conversely, paired otoliths may not be similar in flatfishes (Nolf 1985), affecting the location of the core but not necessarily age estimation (Fischer and Thompson 2004). In juvenile Winter Flounder, otolith dissymmetry increased during metamorphosis as the eye migrated to one side, adding secondary centers of growth and resulting in differing relationships between otolith length and fish length (Sogard 1991).