Chapter 6: Scales
Michael C. McInerny
This chapter provides a summary of the historic and current use of scales for estimating age and growth of fish, accuracy of scale ages, and basic function, structure, and features of scales. This chapter also describes various processes for estimating age of fish using scales. These processes include collection and preparation of scales, various methods of viewing scales, and guidelines for distinguishing annuli from other scale features. Last, this chapter provides an overview on timing of annulus appearance in scales and measuring scales for estimating growth. Specific details on scale microstructure are found in Chapter 2, and methods for estimating back-calculated lengths at age are in Chapter 11; thus, these topics are not discussed in detail in this chapter.
The potential for scales to provide information on age and growth of fish was first hypothesized in the 1600s, but biologists did not embrace the use of scales for age estimation until the late 1800s and early 1900s (Van Oosten 1929). Antonie van Leeuwenhoek, the Dutch pioneer in microscopy, speculated in one of his letters written in 1684 that the surface of an eel scale could reveal its age. By the late 1800s, biologists found that a unique mark formed once per year on scales, scales remained on fish throughout their lives, and scale length increased proportionately with fish length. Thus, estimates of age and growth could be made. Scale use expanded by the early 1900s when biologists used scales for estimating age and growth of many marine and freshwater fishes throughout Europe and North America (Van Oosten 1929). In general, age and growth estimates via scales have been made for most scaled fish species of commercial or recreational value.