Chapter 10: Age Structure
Craig P. Paukert and Jonathan J. Spurgeon
Assessing and understanding age structure of fish populations is a key component of the fisheries management process (McMullin and Pert 2010). Age structure refers to the distribution or frequency (i.e., pattern) of the number of fish at specified ages within a population. Age-structure data are often combined with information regarding growth but can also be informative to the fisheries management process in itself. Age-structure data have a number of important uses in fisheries science and management (Table 10.1), and the analysis of catch-at-age data is considered one of the most important approaches for evaluating fish population dynamics (Quinn and Deriso 1999). Age information can be used to describe longevity, estimate timing of maturity, evaluate sampling bias, identify recruitment dynamics (e.g., strong or weak year-classes within or across water bodies), identify hatch date, estimate dynamic rate functions (e.g., mortality), and assess the resilience of a fish population to factors such as harvest or climate change (Hansen et al. 2015). Although a time series of age information is often most beneficial when assessing population responses to management, a single sample of age data can provide information regarding recruitment variability and year-class strength (Guy and Willis 1995; Maceina 1997; Isermann et al. 2002).
Age structure is often examined at annual time steps (e.g., the number of age-3 or age-4 fish in a population); however, age structure can also be assessed at different time scales with implications for the type of management actions proposed. For instance, daily age has been used to estimate hatch dates and daily growth and to examine abiotic influences on population dynamics (Pine and Allen 2001; Sakaris et al. 2014). Fish response (e.g., somatic growth) to daily flow patterns can be assessed using daily age data and used to recommend differing flow scenarios or enhance nursery habitat during critical life history stages (Humphries et al. 2013). Assessing fish age in years rather than days offers insight into the dynamics of fish populations after recruitment and may help determine when management actions are needed to change population structure. For instance, a fish population composed mainly of young fish may indicate overharvest (Pitlo 1997), gear selectivity (Colombo et al. 2008), or variation in behavior that is related to age (Vokoun and Rabeni 2002). Therefore, analyzing age-structure data might provide important guidance on management strategies (e.g., harvest regulations) or sampling techniques. An understanding of age structure at both daily and annual temporal scales can provide complementary information with respect to different life stages in a fish population (i.e., larval and adult). The scale of age data needed is inherently linked to the scientific question at hand, and assessment of both daily and annual age structure may benefit conservation and management of fish populations.