Relative Length Frequency: A Simple, Visual Technique to Evaluate Size Structure in Fish Populations

—I developed a way to rapidly evaluate the size structure of fish populations designed to be readily understood by a wide audience. Relative length frequency compares the length frequency of fish from a particular water body with an average developed for a region. This technique is analogous to relative weight, relative condition, or relative growth, which are used to compare the condition or growth of fish in a particular water body with rangewide or regional standards. Relative length frequency can be used in conjunction with relative weight or relative growth to rapidly identify factors potentially limiting fish production. I demonstrate this method with data on largemouth bass Micropterus salmoides from Washington State and roundtail chub Gila robusta from Arizona that were collected during standardized electrofishing surveys. Length-frequency histograms are among the oldest methods used to assess size structure in fish populations (Everhart and Youngs 1981). Although a length-frequency histogram is simple and inexpensive to construct, it often can be difficult to interpret given the few standards by which to assess whether the length frequency is optimal or expected for a given situation. Proportional stock density (PSD), relative stock density (RSD), and young–adult ratio are indices used widely to assess size structure in freshwater sport fish populations (Anderson 1976, 1980; Reynolds and Babb 1978; Wege and Anderson 1978). Comparing the structural index of a stock with a standard developed from ‘‘balanced’’ populations (e.g., PSDs of 40–70 for largemouth bass Micropterus salmoides in central U.S. areas) can help evaluate the status of the particular population. A stock density index compresses the entire length-frequency distribution into a single number, which can result in the loss of much information from an entire length-frequency histogram (Gabelhouse 1984). Therefore, two other types of indices have been developed; based on five-cell length categories, they provide means to evaluate the length distribution in greater detail (Gabelhouse 1984). Traditional RSDs (e.g., RSD-P, RSDQ, RSD-M, RSD-T) are the percentage of stocklength and larger fish exceeding a particular length (e.g., trophy, T; memorable, M; preferred, P; quality, Q; and stock, S). Incremental RSDs (e.g., RSD S-Q, RSD Q-P, RSD P-M, RSD M-T) are the per* E-mail: [email protected] Received January 22, 2001; accepted January 14, 2002 centage of stock-length and larger fish that fall between two length categories. One limitation to stock density indices is the restriction of their application only to sport fishes. Gabelhouse (1984) recommended that his lengthcategorization system be only for fishes regularly caught by anglers because the fish lengths used in his five-cell categories and for PSD are based on angling records. A method that could be used to analyze length frequency data for all fishes, both sport and nongame, would be useful to fisheries