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336 CHAPTER 14 Box 14.1 Criteria to Determine if Fish are Undesirable I. The species does not contribute to the sport fishery (may not be available or acceptable to anglers or commercial fishers) or forage base. 2. The species inhibits development or maintenance of desirable fish through predation or direct competition with sport or commercial fishes. 3. The species is detrimental to the biological balance of the aquatic system (e.g., large gizzard shad sometimes constitute most of the biomass in reservoirs). 4. The species may serve as a potential reservoir for pathogenic organ- isms in a hatchery water supply. 5. The species may interfere with other wildlife management practices (e.g., common carp cause turbidity inhibiting the growth of aquatic plants that are used as food by waterfowl). weeds and gardeners (Box 14.\). Early fisheries administrators in the United States (e.g., Spencer F. Baird) were instrumental in the introduction of common carp into North America because it was considered to be a good food fish. Within 20 years (1877-1896) after its introduction, the common carp was disdained because it failed to provide sport for anglers and was not used for food (Cumming 1975). In addition to control of nongame species, stunted stocks of game fish are sometimes controlled because they are not of a size that is acceptable to anglers. 14.2.1 Families of Fish Targeted for Control Control programs have targeted virtually all species of freshwater fishes (Lennon et al. 1970; Dunst et al. 1974). Most (54%) of the current control efforts are directed at three families of fishes: herrings (primarily gizzard shad), minnows (almost exclusively common carp), and sunfish (Table 14.\). An additional 22% of the control efforts involved suckers (11%) and bullhead catfishes (11%). Although the need was recognized as early as 1900, conservation agencies did not routinely implement control measures until after 1930 (Figure 14.\). The management of undesirable fish species in North America appears to be evolving from control of a single species toward an approach that considers relationships with companion species of fish. Modified stocking practices, changes in regulations, and development of strains of stocked fish are being used as strategies to manage undesirable species. 14.3 F ACILIT A TING MANAGEMENT OF UNDESIRABLE FISH STOCKS Three methods are used to control undesirable stocks of fish: chemical, biological, and mechanical. Chemical and biological means are favored over mechanical controls for most families of fishes (Table 14.1). Chemica I control of fish stocks is summarized in Johnson and Finley (1980) and Schnick et al. (1986). Biological control of fish stocks appears to be increasing, perhaps as a result of