Chapter 2: Morphology, Composition, and Growth of Structures Used for Age Estimation
Gregory W. Whitledge
A variety of calcified structures are commonly used for estimating the age of fishes, including otoliths, scales, and skeletal bones (Maceina et al. 2007). Estimating the age of individual fish using any of these structures is based on visual examination for periodic, concentric marks formed as a consequence of environmental and endogenous factors, and distances between these incremental marks can be used to estimate growth rate. Although the fundamental approaches to age and growth estimation of fishes are similar among different types of calcified structures, there are substantial differences in morphology, composition, development, growth, and other processes that are relevant to choosing a structure for age estimation (Chapter 4) and to the techniques used to prepare and interpret structures (Chapters 6–9). To fully understand the potential applications and limitations of using calcified structures for estimating age and growth, it is important to understand how and when these structures are formed and the mechanisms underlying their growth, particularly processes that influence the establishment, retention, and spacing of incremental marks. Additionally, knowledge of factors influencing the production and appearance of irregular marks that do not correspond with particular time periods or life history events is also important, as failure to recognize and distinguish such marks is a frequent source of error in estimating age and growth (Chapter 3).
The purpose of this chapter is to provide an overview of morphological features, composition, development, and growth of calcified structures routinely used for estimating the age of fish, with emphasis on the processes regulating formation, visual appearance, retention, and positioning of growth marks.