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344 CHAPTER t 4 Vi a: 1-5 c:x: W > 6-10 - o o 11 -15 a: ~ 16 - 20 W :E 21 - 30 ~ :::::~~::~:~:~::~~~~~::~~~~:~:~~~:~:~:~: :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:. : ~ ~ ~ ~ ~ ~ t t ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~~t t ~ ~ ~ ~ ~~ ~ ~~~~ ~~~~ ~~~~~ ~~~ ~~~~~~~~~ ~ ~~ ~ ~: .................... . .. . .. .. . . . . . . ... .. '" . . '" . .. ... .. . . . ... . . '" . .. . .... .. . . .. . .. . . . .................... .................... .................... .................... .................... o 5 10 15 20 25 30 35 PERCENT Figure 14.4 Length of time that chemical treatment has been found to be effective. Effective time depends on the physical and chemical factors of the treated waters as well as the target species. should be obvious that programs not considered to be beneficial would be discontinued. Cost alone cannot serve as the measure of the value of a treatment program. Common sense must playa large role. Trimberger (1975) reported that increases in the number of game fish available to anglers and increases in fishing pressure followed most chemical reclamation projects in Michigan. Fishing pressure on formerly marginal trout waters after chemical treatment has equalled the pressure on the best trout waters. Trimberger also reported that chemical treatments of warmwater lakes consistently resulted in more angler days per acre than were attributable to other management strategies. Similar statements have been made by fisheries managers throughout North America. Periods of effectiveness range from less than I to 30 years (Figure 14.4). When benefits are substantial, treatment programs can be scheduled at regular intervals. Annual treatments of particular waters are usually considered too costly for benefits realized. 14.5 BIOLOGICAL METHODS FOR MANAGING UNDESIRABLE FISH SPECIES Biological control methods can be grouped into three categories: (1) grazing and predation by protozoa, zooplankton, fish, birds, insects, snails, crayfish, turtles, and mammals; (2) use of pathogens (viruses, bacteria, and fungi); and (3) biomanipulation, in which interrelationships among plants, animals, and their environment are adjusted to achieve the desired control and ecological balance (Schuytema 1977). Biomanipulation is the most promising biological control technique for managing fish populations in that it may minimize competition and establish balanced predator-prey populations (Box 14.4). The use of pathogens for fish control is risky; therefore, pathogens are not generally used in aquatic environments. Mechanical and chemical control of nuisance aquatic plants is generally temporary and expensive. Biological control of such vegetation by using grazers