Chapter 15: Age and Growth
Michael C. Quist, Mark A. Pegg, and Dennis R. Devries
The phrase “age and growth” is common in fisheries science and captures two related, but different, concepts. Age refers to a quantitative description of how long an organism has been alive. Growth represents a change in size (e.g., length or weight), typically as a function of time. Age and growth can be measured across a variety of temporal scales, from hours and days to months and years. Although age and growth data are often presented and discussed together, each component provides unique and valuable information on individuals and populations.
Growth, recruitment, and mortality are the primary functions that regulate fish population dynamics and thereby influence the ecology and management of fishes (Ricker 1975). Age and growth data can provide direct and indirect information on each of these functions. Consequently, collecting accurate information should be a high priority for all fisheries scientists because age and growth information is critical to almost every aspect of fisheries science.
Estimation of age provides valuable insight regarding the characteristics of individual fish (e.g., age at maturity) as well as information on the age structure of the entire population. Age structure data have a number of important uses, such as estimating mortality rates and providing information on recruitment dynamics (Hsieh et al. 2006; Cassoff et al. 2007). Growth information is important because it provides an integrated evaluation of environmental conditions (e.g., prey availability, thermal conditions, and habitat availability) and genetic factors. Growth also has direct and indirect effects on recruitment dynamics, trophic interactions, and mortality through its effects on age at maturity, size structure of both predator and prey populations, and the susceptibility of fish to environmental alterations and harvest (Birkeland and Dayton 2005; Winemiller 2005; Rowell et al. 2008).
Although age data and growth data each individually provide important information on individuals and populations, their greatest value is obtained when they are combined. When combined, age and growth information may identify potential problems (e.g., recruitment limitations or overfishing; Beamish et al. 2006) or provide feedback on the effectiveness of management practices (e.g., harvest regulations or habitat or prey manipulations). In addition to providing a unique assessment of fish populations, age and growth information can supplement other studies, such as those focusing on movement patterns and habitat use, population densities, or food habits (Buktenica et al. 2007). As such, determining the age and growth of fishes has been and will continue to be one of the most important activities conducted by fisheries professionals (Hilborn and Walters 1992; Jackson 2007).