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Volume 1 (Issue #3), 1993 STOCK DENSITY INDICES Vl. INTERPRETATION OF STOCK DENSITY INDICES Fishery managers often wish to evaluate fish populations by comparison of stock density indices to generally accepted objective ranges for balanced populations. Anderson and Weithman (1978) indicated that "balanced populations have a structure that is intermediate between the extremes of a large number of small fish and a small number of large fish. For the structure of a fish population to be balanced, the rates of reproduction, growth, and mortality must be satisfactory." We have summarized objective ranges for balanced fish populations in Table 5. In most cases, these objective ranges were obtained from simple models that used characteristic rates of recruitment, growth, and mortality to predict population size structure. We wish to stress that not all populations are managed for "balance." In some situations, managers may determine that stock density index values outside the balanced range are appropriate. For example, Gabelhouse et al. (1982) suggested that largemouth bass and bluegills in small impoundments may be managed for balance, or for the "panfish option" or "big bass option" (Table 6). In the panfish option, the quality (size) of largemouth bass is sacrificed to produce a higher quality bluegill population. Ahigh-density, slow-growing largemouth bass population preys upon bluegill reproduction, and surviving bluegills have faster growth rates and reach sizes of interest to anglers. In such communities, largemouth bass PSD should be 20 to 40 and bass RSD-P should be 0 to 10; bluegill PSD should be 50 to 80 and RSD-P should be 10 to 30. In the big bass option, the management strategy is to produce fewer, larger largemouth bass, resulting in less predation on bluegills. Bluegill size structure then shifts toward smaller fish. Largemouth bass PSD should be 50 to 80 (RSD-P = 30 to 60) and bluegill PSD 10 to 50 (RSD-P = 0 to 10) in impoundments managed under this option. Stock density indices provide more interpretative information when populations are relatively "steady-state," i.e., where recruitment, growth, and mortality remain somewhat constant. If density is high, recruitment is moderate to high, growth is relatively slow, and mortality is high (at least at a certain age), then PSD will be low. If density is low, growth is moderate to fast, recruitment is low, and mortality is low to moderate, then PSD will be high. These scenarios obviously assume no angler overharvest and a desirable habitat. In other situations, the indices provide less information. As one example, alow- density, fast-growing population may consistently have a low PSD if angler harvest is excessive. To discern whether glow-PSD population is overharvested or over- populated, other assessment tools are necessary. Again, such tools could include CPUE, condition, or growth assessment. As another example, PSD will provide little interpretive information on population rate functions when recruitment is inconsis- tent. Populations with inconsistent recruitment are common in the shallow glacial lakes of eastern South Dakota. Guy and Willis (1991b) sampled black crappies monthly during 1990 in Lake Madison, South Dakota, using trap (modified-fyke) nets. They expected to see peak CPUE and PSD during spring and fall samples, as noted by Boxrucker and Ploskey (1988). The expected seasonal pattern in CPUE was present; catches were highest in spring and fall, and lower in mid-summer. However, 215