Analysis and Interpretation of Freshwater Fisheries Data

9: Size Structure

Robert M. Neumann and Micheal S. Allen


Size structure analysis is one of the most commonly used fisheries assessment tools. The size structure of a fish population at any point in time can be considered a snapshot that reflects the interactions of the dynamic rates of recruitment, growth, and mortality. Thus, length-frequency data provide valuable insight into the dynamics of fish populations and help identify problems such as inconsistent year-class strength, slow growth, or excessive mortality (Anderson and Neumann 1996). In most cases, a thorough interpretation of size structure data is complemented by other population assessment tools, such as catch per unit effort (C/f), age-and-growth analysis, recruitment analysis, mortality, and body condition.

Proper analysis and interpretation of size structure data should begin with a clear understanding of how, when, and where data were collected. Specifically, a fisheries scientist should know how size structure data are influenced by the sampling gear, time of the year, and location where fish were sampled. The fisheries scientist should also consider whether an appropriate sample size was obtained to estimate size structure reliably.

Fisheries scientists use several techniques to analyze size structure data. In the simplest case, a length-frequency histogram (see section 9.2) is constructed or a size structure index is calculated. Oftentimes, the primary objective is to compare size structure among samples. In these cases, a fisheries scientist may be interested in answering several questions. For example, does the size structure of white crappie populations differ among water bodies? Did the size structure of a rainbow trout population change over time in response to a management action? Are the size structures obtained from a channel catfish population different between two or more sampling gears? What factors influence the size structure of walleye populations?