Chapter 11: Growth
Greg P. Busacker, Ira R. Adelman, and Edward M. Goolish
Growth may be viewed as an ongoing process or as something that has occurred in the past history of animals. The concept of growth implies many things and is too frequently presented in a misleading way. This imprecision can lead to misinterpretation of observations or experimental results.
Growth may be positive or negative, temporary or long-lasting. It may occur at all levels of biological organization: cells, tissues, organs, whole organisms, populations, communities. Depending on the level of organization, growth can be measured in terms of number, linear dimension, weight, volume, energy content, or the amount of a specific component such as protein. Furthermore, growth can be measured as an incremental change or as a rate of change. Finally, a variety of indices have been developed, such as glycine uptake by scales or RNA:DNA ratios, that are related to growth but in themselves are not direct measurements of growth. Thus, a precise universal definition of growth is precluded by the variety of processes that can be considered as growth and the variety of measurements that can be applied. However, it is imperative that growth be clearly defined within the context of specific investigations and that the method of measurement be appropriate for that definition.
For the purposes of this chapter, growth is defined as any change in size or amount of body material, regardless of whether that change is positive or negative, temporary or long-lasting. In the following sections, we examine techniques that are used in the study of growth and provide guidance to the appropriate uses of those methods.
Numerical expressions of whole-body growth of fish may be based on absolute changes in length or weight (absolute growth) or changes in length or weight relative to the size of the fish being considered (relative growth). Measurements of growth expressed in terms of some interval of time (day, month, year) constitute a growth rate (Ricker 1979). For example, if t1, is the time at the beginning of an interval and t2 the time at the end, and if Y1, and Y2 are the respective fish sizes at those times.