THE POSITION OF FISH AND OTHER HIGHER 

 ANIMALS IN THE ECONOMY OF LAKES 



By F. E. J. FRY 



ONTARIO FISHERIES RESEARCH LABORATORY, DEPARTMENT OF BIOLOGY, 



UNIVERSITY OF TORONTO, TORONTO, ONT., CANADA 



All vertebrates which are not strictly The relation between fishes and their 

 marine contribute to the economy of lakes, aquatic environment is a very intimate one. 

 However, there are great differences in the Fish possess no hair to insulate their body 

 intimacy of their relations with the lacus- from the ambient temperature. There is no 

 trine environment. Many are related to vestibule between the environment and the 

 conditions in the water only through their respiratory surfaces comparable to the lung 

 presence in the drainage areas. Others, of the air breather. It falls to the lot of 

 while completely dependent on air for the fish to avoid what unfavorable condi- 

 breathing, may obtain their food from lakes, tions it can by migration, and to endure to 

 To this group belong the aquatic mammals the limits of its adjustment those which it 

 and birds. The aquatic reptiles and am- cannot avoid. Thus in certain lakes, condi- 

 phibia are more closely associated with the tions are impossible for the existence of a 

 water than are the other air breathing verte- given species of fish. In other lakes the 

 brates, since they can utilize the dissolved species exists under more or less unfavor- 

 oxygen in the water with varying degrees of able circumstances. In still others the 

 efficiency. The group of vertebrates most physical and chemical environment may be 

 completely related to the economy of lakes so completely favorable as to allow the same 

 is the fish. In our latitudes fish are con- species full scope for its development, pro- 

 fined to the water all their lives, except vided its members can obtain sufficient food, 

 possibly for a short nocturnal excursion Since the development of an animal de- 

 through the grass which eels may under- pends on nutrition, as well as upon favor- 

 take. Because of their complete dependence able physical and chemical conditions in its 

 on the aquatic environment, the relations surroundings, the presence of a suitable 

 of the fishes will be discussed at greatest supjDly of food is also essential. The food 

 length here, in spite of the fact that man, organisms are in their turn limited in their 

 a mammal whose feet are not even webbed, own existence to a range of phj^sical and 

 is the species which has the most profound chemical conditions which they can tolerate, 

 effect on the economy of lakes, and might or by the necessity for a suitable supply of 

 from that point of view claim the major food, either in the form of other organisms, 

 portion of our attention. organic matter, or more primary chemicals. 



Since the fishes depend for their very ex- As well as conditions in the immediate en- 

 istence upon conditions in the water, it vironment and the nature of the food sup- 

 might be more appropriate, if the title of ply, factors which affect each individual, 

 this paper were to refer to fishes only, to there is the factor of predation which affects 

 speak of the position of lakes in the economy the welfare of the species as a whole, 

 of fishes rather than of the position of fishes It is by compromise, by surviving under 

 in the economy of lakes, for the lake is the a wide range of physical and chemical con- 

 independent variable in the relation. The ditions, and by being able to live with and 

 fish has to conform to the extent of being upon a wide range of associates, that fishes 

 physiologically compatible with the condi- contribute as much as they do to the econ- 

 tions of existence in a given body of water, omy of lakes. Fishes can go far in this 

 and then within this limited sphere it may compromise. Twenty pound lake trout can 

 contribute its share to the economy of lakes, turn to Daphnia when no other food comes 



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