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WILLIS, MURPHY, AND GUY REVIEWS IN FISHERIES SCIENCE V. STOCK DENSITY INDEX CORRELATIONS A. SINGLE SPECIES CORRELATIONS An appropriate question concerning the use of stock density indices is whether they reflect the density and dynamics of fish populations. In some situations, the answer is certainly yes (Table 3). As the density of a population increases, the PSD tends to decrease -especially after the population reaches carrying capacity. However, low PSD also can occur at low population density because of problems with poor habitat, food supply, or angler overharvest (Gabelhouse, 1984a). As growth in- creases, there is a tendency for PSD to increase. As total annual mortality increases, there is a tendency for PSD to decrease. In situations where recruitment is high enough to result in overpopulation, PSD values will be low. Where recruitment is low enough that populations remain below carrying capacity, PSD can be high. These relationships are most likely to be found in similar environments in single geographic areas. Good examples are provided by the Central States Small Impound- ment Work Group (formerly Central States Pond Management Work Group) studies on largemouth bass and bluegills (Novinger and Legler, 1978; Reynolds and Babb, 1978) and largemouth bass and crappies (Gabelhouse, 1984b). Those studies were completed in similar environments (small impoundments) in a specific region of the U.S. (midwest). Guy and Willis (1990) noted that largemouth bass PSD (r = 0.86, p = 0.0002) and RSD-P (r = 0.70, p = 0.009) were correlated with mean length of largemouth bass in South Dakota ponds. Variations in factors such as productivity and growing season also can preclude establishment of a clear relationship between stock density indices and population parameters. In fact, these factors also may help explain the wide variability found in the relationships reported in Table 3. Coefficients of determination typically are low, and much variability in the relationships is unexplained. Based on modeling results, Carline et al. (1984) suggested that PSD increased linearly with survival, but increased curvilinearly with growth. Willis and Scalet (1989) found that curvilinear regressions provided a better fit for PSD and growth of northern pike; however, the improvement in correlation coefficients was slight. 8. PREDATOR-PREY RELATIONSHIPS Another question concerning the use and interpretation of stock density indices revolves around the possible inverse relationship between size structure of predator and prey populations. We have summarized the correlations between predator and prey size structure that we are aware of in Table 4. All the examples deal with largemouth bass as the predator species. In small impoundments, largemouth bass PSD tends to decline as bass density increases. As largemouth bass density increases, predation on "panfish" tends to increase. Therefore, panfish PSD tends to increase as largemouth bass density increases and an inverse relationship between PSD of predator and prey species results. Carline et al. (1984) suggested that the likelihood of an inverse relationship declined as impoundment size increased and fish community complexity increased. 212