Page:Popular Science Monthly Volume 87.djvu/98

94 unhatched, and where they have been placed in favored locations along the shores of the streams at low stage. We may then only guess at the destruction which must ensue when the entire condition is suddenly and drastically altered by the untimely arrival of the flood. Clear shallow waters, warmed by the sun, are in a brief space of time replaced by deep and turbid torrents, and the very banks and bottoms are torn away or displaced. To fish life another catastrophe has occurred.

It will not be maintained that any practicable scheme of control, however comprehensive, will prevent altogether the occurrence of high and low stages, but it has been attempted to show that the regulation of the flow of rivers has a very direct relation to the reproduction of fish.

Without successful reproduction we certainly can not have fish; but the abundance of fish, even under natural conditions, does not depend alone upon successful propagation. The young fish must survive and grow, and for these ends their requirements are similar to those of other animals, namely, food and oxygen, principally. Likewise, just as in the case of other animals, the food is derived ultimately from the essential chemical constituents through the intermediation of plants. The rains that wash the soils bring the needed constituents into the streams, but not necessarily in a form available for animal life; for them to become available requires time, sunlight and vegetation.

It is clear that excessive turbidity and extreme conditions of flood have the most direct bearing upon the conditions of food supply for fish. Not only is this the case, but extreme low stages may have a highly deleterious effect. The first result of the decomposition of organic matters brought into the water is the exhaustion of the oxygen supply, and this may proceed to such a point as to make the environment distinctly unfavorable for any form of aquatic animal life. The beginning of mortality among the animals, whether smaller or larger forms, by adding to the amount of decomposing material, only serves to increase the rate of deoxygenation of the water and to accelerate the course of destruction. Such a catastrophe can be checked or restricted as to its duration or territory of action only by the development of sufficient plant-life to effect a restoration of equilibrium, or by the diluting and cleansing effect of an increased flow in the stream. Some of the instances not infrequently reported of enormous mortality of fish in portions of rivers just below cities and in times of low water are most certainly due to this very fact of a disturbance of the established equilibrium between sewage, plants and animals, with a consequent mortality that is self-accelerative to the point of inducing a conspicuous catastrophe.

There has been developing in recent years almost a new science which deals with the gas-content and the chemical analysis of water,