Chapter 11: Growth Estimation: Summarization
Daniel E. Shoup and Paul H. Michaletz
One of the primary reasons for estimating fish age is to describe growth rates. Growth is simply a change in size (length or weight [mass]) over some period of time. This chapter will primarily focus on growth in terms of length, but most of the methods are equally applicable to summarizing growth based on weight. Growth is an important rate function because survival and reproductive success are often related to size. For example, many fishes change habitat or diet and experience changes in predation risk as they grow (Werner and Hall 1988; Olson 1996b; Mittelbach and Persson 1998); growth rates can predict when these changes occur. Growth is important in the recruitment process of age-0 fishes because smaller fish in a cohort are more vulnerable to predation and (or) overwinter starvation (Post and Evans 1989; Cargnelli and Gross 1996; Garvey et al. 1998). Reproduction is often correlated with size (Danylchuk and Fox 1994; Michaletz 1998), so growth is an important factor influencing the timing of maturation and ultimately an individual’s lifetime fecundity. For sport fishes, growth determines how long it takes for a year-class to reach sizes relevant to fisheries management goals (e.g., legal harvestable size, trophy size). Therefore, summarizing growth information is critical for managing fish populations.
Understanding the difference between growth and growth rate is important. Growth is the absolute change in size that was observed. Although growth implies the passage of time, time is not used in the calculation. Growth rate is growth divided by the time interval between size measurements. Growth can be directly compared between individuals or groups of fish if the time elapsed between size measurements is the same. Growth rate should be used if moderate differences in elapsed time exist between the size measurements of the fish or groups of fish. If large differences in elapsed time exist between the size measurements of fish or groups of fish, it may be inappropriate to directly compare growth or growth rate given the size- and age-specific nature of growth rates of most fishes.