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WILLIS, MURPHY, AND GUY REVIEWS IN FISHERIES SCIENCE For largemouth bass, minimum stock and quality lengths are 20 and 30 cm (total length). The PSD for a largemouth bass sample is the percentage of 20-cm and longer fish that also are longer than 30 cm. Anderson (1980) summarized suggested stock and quality lengths for 26 species of fish. Both PSD and RSD are unitless measures (Anderson, 1980; Anderson and Gutreuter, 1983) Biologists need to be careful when applying stock density indices to data collected in English or metric units. Anderson and Gutreuter (1983) listed minimum lengths for metric units. However, confusion can occur during "rounding" when conversion between metric and English units occurs. For example, minimum stock length for white crappie (Pomoxis annularis) is 5 in. (Gabelhouse, 1984a). Stock length in metric units is standardized as 130 mm (Anderson and Gutreuter, 1983; Gabelhouse, 1984a), not 125 mm. Thus, we encourage biologists to note and use the already established values in both English and metric units, rather than convert- ing from English to metric units by various methods of rounding. 2. Relative Stock Density RSD was first used for largemouth bass; the "specified" length was 15 in. (38 cm), and the percentage of stock-length fish that also were 15 in. long came to be known as RSD-15. Gabelhouse (1984a) noted the need for more than atwo-cell (stock and quality lengths) model for size structure analysis. His example involved discussion of two bluegill populations. Both populations had a PSD of 60, meaning that 60% of stock-length (8 cm) bluegills also were longer than quality length (15 cm). However, one population contained no bluegills over 18 cm, whereas the other contained numerous bluegills over 20 cm and a few that even exceeded 25 cm. Thus, Gabelhouse (1984a) developed afive-cell length-categorization system, again based on percentages of world-record length using Weithman's (1978) fish quality index (Figure 1). Gabelhouse (1984a) defined the length ranges from which stock (S), quality (Q), "preferred" (P), "memorable" (M), and "trophy" (T) lengths should be chosen for 70 species of fish (the 70 include 2 hybrids and 1 family). Quality length had been defined as the size of fish most anglers like to catch (Anderson, 1980). Gabelhouse (1984a) suggested that although anglers may like to catch a fish of Q length, they would prefer to catch a larger fish (P). "Memorable" was defined as a size of fish most anglers remember catching, and T was a size considered worthy of acknowl- edgment. He then recommended minimum S, Q, P, M, and T lengths for 27 species (including 1 hybrid) of warm- and coolwater fish. Adoption of these five standard- length categories would facilitate communication within the fisheries profession. Gabelhouse (1984a) did not propose five-cell length categories for coldwater fishes, although he did provide the length ranges from which appropriate categories should be chosen. Anderson and Gutreuter (1983) also did not include length categories for coldwater fishes. However, Anderson (1980) included stock and quality lengths for landlocked Atlantic salmon (Salmo salary, lake trout (Salvelinus namaycush), rainbow trout (Oncorhynchus mykiss), and brown trout (Salmo trutta); plus separate lengths for brook trout (Salvelinus fontinalis) in lakes and streams. We have summarized the standard length categories proposed by Gabelhouse (1984a), as well as recently proposed categories, in Table 2. All lengths recommended in this table are based on the technique originally proposed by Gabelhouse (1984a). Again, 206