By F. E. J. FRY
ONTARIO FISHERIES RESEARCH LABORATORY, DEPARTMENT OF BIOLOGY, UNIVERSITY OF TORONTO, TORONTO, ONT., CANADA
ALL vertebrates which are not strictly marine contribute to the economy of lakes. However, there are great differences in the intimacy of their relations with the lacustrine environment. Many are related to conditions in the water only through their presence in the drainage areas. Others, while completely dependent on air for breathing, may obtain their food from lakes. To this group belong the aquatic mammals and birds. The aquatic reptiles and amphibia are more closely associated with the water than are the other air breathing vertebrates, since they can utilize the dissolved oxygen in the water with varying degrees of efficiency. The group of vertebrates most completely related to the economy of lakes is the fish. In our latitudes fish are confined to the water all their lives, except possibly for a short nocturnal excursion through the grass which eels may undertake. Because of their complete dependence on the aquatic environment, the relations of the fishes will be discussed at greatest length here, in spite of the fact that man, a mammal whose feet are not even webbed, is the species which has the most profound effect on the economy of lakes, and might from that point of view claim the major portion of our attention.
Since the fishes depend for their very existence upon conditions in the water, it might be more appropriate, if the title of this paper were to refer to fishes only, to speak of the position of lakes in the economy of fishes rather than of the position of fishes in the economy of lakes, for the lake is the independent variable in the relation. The fish has to conform to the extent of being physiologically compatible with the conditions of existence in a given body of water, and then within this limited sphere it may contribute its share to the economy of lakes.
The relation between fishes and their aquatic environment is a very intimate one. Fish possess no hair to insulate their body from the ambient temperature. There is no vestibule between the environment and the respiratory surfaces comparable to the lung of the air breather. It falls to the lot of the fish to avoid what unfavorable conditions it can by migration, and to endure to the limits of its adjustment those which it cannot avoid. Thus in certain lakes, conditions are impossible for the existence of a given species of fish. In other lakes the species exists under more or less unfavorable circumstances. In still others the physical and chemical environment may be so completely favorable as to allow the same species full scope for its development, provided its members can obtain sufficient food.
Since the development of an animal depends on nutrition, as well as upon favorable physical and chemical conditions in its surroundings, the presence of a suitable supply of food is also essential. The food organisms are in their turn limited in their own existence to a range of physical and chemical conditions which they can tolerate, or by the necessity for a suitable supply of food, either in the form of other organisms, organic matter, or more primary chemicals. As well as conditions in the immediate environment and the nature of the food supply, factors which affect each individual, there is the factor of predation which affects the welfare of the species as a whole.
It is by compromise, by surviving under a wide range of physical and chemical conditions, and by being able to live with and upon a wide range of associates, that fishes contribute as much as they do to the economy of lakes. Fishes can go far in this compromise. Twenty pound lake trout can turn to Daphnia when no other food comes