Analysis and Interpretation of Freshwater Fisheries Data

5: Age and Growth

J. Jeffery Isely and Timothy B. Grabowski


The ability to determine ages of fishes without bias is critical to effective management and research. Accurate age information can provide valuable insights into critical life history events. Often, migrations related to spawning or ontogenetic changes in environmental requirements are also age dependent. Age data can be coupled with numbers of individuals to produce an age-frequency distribution, from which patterns in mortality can be determined. Similarly, deviations in expected numbers at age can provide insights into year-class strength variability and the effects of environment on survival.

When age and size information are combined, we can evaluate growth. Growth provides us with some indication of resource utilization and the effectiveness of our management strategies. Our ability to model growth and to understand variables that affect growth both within and among populations is critical to our ability to manage fisheries effectively. When we evaluate age, growth, and mortality (see Chapter 6) in combination, we begin to understand the complex relationship between population size and biomass (see Chapter 8). This understanding is the basis of modern fisheries resource allocation and management.

Primary methods employed by fisheries scientists to estimate ages of fishes are recovery of known-age fish, evaluation of length-frequency distributions, and interpretation of calcified structures. Under unique circumstances, additional methods employed by researchers include evaluations of isotope decay rates and chemical microanalysis.