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Show selves in a few days, and thereby can shift with the waters far more readily than large slow- growing perennial plants of higher evolutionary types like the cattails, pondweeds, and ditch grasses. In general, a drawdown of over 18 inches or two feet would have a deleterious effect on many food plants for ducks, even though cattails, giant bullrush and some others of little or no value would still do all right. On the other hand, a rather slow drawdown of ten or twelve feet, provided it did not have too much effect on the area of the reservoir . . . might be good for fish by minimizing weed growth. 7 Weed growth is deleterious to fish in that the fish- food organisms, to which such growth gives shelter, becomes inaccessible to the fish. As a result, production of fish is less in a pond with weedy vegetation. The effects of fluctuation in a given reservoir obviously are dependent upon many variables. For example, " a short period ( two to six weeks) of flooding is usually good for ducks because it promotes growth of lake- margin food plants without injuring those adjusted to normal level. That is if the flooding occurs before the nesting season, or if after the nesting season only lasts a week or ten days. In the case of draw- down, both the rate and the time of year would be important." 8 Effect on animals. •- There will be no large aquatic plants along the margins of reservoirs having water- level fluctuations of more than 10 feet during the growing season. Since these plants comprise the principal food supply and habitat of waterfowl, the absence of these plants seriously curtails the wildlife and recreational values of reservoirs. The role of such relatively barren bodies of water in the conservation of waterfowl will be relatively minor because nesting opportunities will be few. Therefore, such areas will be of little use except as temporary stopping places along the routes of migration. Unfortunately for recreation and wildlife conservation, most of the existing reservoirs in the Colorado River Basin are of the excessively fluctuating type. Stable reservoirs with ample food and cover are all the more critically needed in this region because of the prevailing scarcity of suitable natural areas for waterfowl. Reservoirs that 7 Bond, 1945, letter. 8 Bond, lit. cit. control floods also eliminate many of the ponds and marshes that normally are formed along river bottoms during periods of overflow. Thus, former breeding grounds of waterfowl, muskrats, and other marsh- dwellers are destroyed, and the need of additional stable reservoirs with food and cover becomes all the greater. Lack of an extensive growth of large aquatic plants does not in itself mean that a reservoir fluctuates so much as to be unsuitable for fish. If the fluctuations are not too violent, and if other growing conditions are favorable, microscopic and filamentous algae may be produced in such enormous quantities that they blanket all the shallow, sunlit depths. This microscopic aquatic forest teems with such " game" as the larvae of midges, mayflies, stone- flies, caddisflies, dragonflies, tiny Crustacea, aquatic beetles, snails, worms, rotifers and other fish foods. Fluctuation, especially if it is at all rapid, may have an extremely deleterious effect on the bottom fauna, such as the Tub if ex worms, and larvae of Chironomus and other midges. These are of very great importance as fish food . . . The busfs that burrow in the surface of the mud ( and these are probably of greatest importance to the fish) have very little motility . . . The fish are concentrated, since most of them stay in the reservoir, while the bottom fauna dies off in considerable part, and the plankton, which has little control over its general movements, drains out with the water or else readjusts its total mass to the new size of the lake within a few days. This leaves the fish in a pretty pickle. 9 The other major source of food in lakes and reservoirs is furnished by the billions of microscopic free- floating and swimming plants and animals known as plankton. Though usually invisible to the casual observer, the plant forms sometimes become so abundant under proper conditions of temperature, light, and fertility as to impart a greenish tinge to the water. Smaller animal plankton like the one- celled protozoas, as well as the plant forms of plankton, are eaten by larger plankton forms such as rotifers, or the barely visible Crustacea known as " water fleas." These in turn are eaten by the aquatic insect larvae and by small fish ( which 9 Bond, 1945, letter. 106 |