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Volume 1 (Issue #3), 1993 STOCK DENSITY INDICES populations, only an average of ten states and provinces applied PSD for coolwater assessments. For Coldwater species, the average number of states or provinces using PSD declines even further to six. At present, we seem to encounter two schools of thought on the use of stock density indices. At one extreme, some biologists are firmly convinced that stock der}siry indices reflect both the fish population structure (form) and the dynamics or function (recruitment, growth, and mortality). ~n some cases, those biologists believe that stock density indices alone can be used to reliably assess populations and communities. At the other extreme are biologists who are just as firmly convinced thaC stock density indices have no utility whatsoever. We hope to demonstrate that a "nhiddle-of--the-road" approach to use and interpretation of stock density indices would be in the best interest of our profession. Thus, we have chosen to discuss factors that contribute to the proper use of stock density indices as a fisheries assessment tool. ~~ lV.'; SAMPLifNG CONSIDERATIONS Unbiased length-frequency samples are an assumption of many standard analysis techniques used by fisheries biologists. ideally, such tools as age structure, mortality. rates, size (length) structure, and stock density indices would best be calchiated from a random sample of the entire population. In reality, many factors affect sampling data. Three of the more important size-related biases result from seasonal patterns in sampling data,- gear-related length selectivity, and selection of sampling sites. A. ~! SEASONAL INFLUENCES Seasonal patterns in size structure indices for warmwater fishes in impoundments follow a bimodal pattern, with "peaks" in the spring and fall and a "trough" in summer. Carline et al. (1984) in Ohio, Gilliland (1985) in Oklahoma, and Bettross and ~ Willis (1988) in South Dakota reported this pattern for largemouth bass electrofishing PSD from spring to fall, as did Jakes (1987) from spring to late summer in Georgia. Bettross and Willis (1988) also observed a bimodal pattern in PSD for bluegills collected by electrofishing and trap netting. Boxrucker and Ploskey (1988) repa~-ted that size structure of white crappie was appropriately represented by spring and fall fyke net samples in Oklahoma. However, Nlclnerny (1988) reported that RSD of black crappie caught in trap nets was higher in spring than fail. An obvious question is whether the peaks or troughs depict the true size structure of fish popti~lations. Van Den Avyle (1976), Reynolds and Simpson (1978), Simpson (1978), Brenner and Noble (1982), Carline et al. (1984), Gilliland (19853, and Mesa et al. (199b) all inferred that spring or fail peaks reflect the true size structure of populations. hems (1985) noted that the PSD of spawning walleye (Stizostedion vitreum) populations collected by Tyke nets would be low early and late in the spawning period because of the abundance of small males. During the height of the spawn, 209