Page:Popular Science Monthly Volume 68.djvu/255

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THOROUGH investigation of the biology of our great lakes is in itself a desideratum worthy the expense. From the purely economic standpoint, however, our Bureau of Fisheries has long recognized the necessity of knowing more of those conditions under which products worth millions annually are produced. Until such an investigation has been made the natural factors which determine the quantity of food fishes these lakes can support must remain unknown.

 Vallisneria spiralis after 7 weeks' growth rooted in lake soil. Plants in figures 1 and 2 originally the same size.

Fish, as all other living forms, reproduce in a geometrical ratio and, other conditions being favorable, will multiply up to the limit of their food supply. Thus it is that the problem of nutrition is very fundamental. To determine the source of nutrition of our fresh-water food fishes is in itself a considerable undertaking, but to ascertain what factors regulate the quantity of this nutrition is a colossal task.

The nutrient relations of aquatic life are perhaps no more complicated than those of terrestrial, but they certainly are more difficult to determine because of the numerous obstacles to observation and