28 American Fisheries Society 



cases for adult fishes where these decomposition products 

 are present. 



Turning to the fishes themselves we note that their 

 presence or absence is controlled by (a), their ability to 

 recognize the presence of strange or deleterious sub- 

 stances and to turn back when they are encountered, and 

 (b), by their survival or death in situations where they 

 cannot escape the deleterious conditions. Their ability 

 to recognize strange or deleterious substances has been 

 shown ( Shelf ord and Allee '11; Shelf ord and Powers 

 '14; Wells, unpublished) to be very elaborate and effect- 

 ive. Fishes recognize exceedingly minute quantities of 

 numerous substances and not only turn back upon en- 

 countering them, but are able to recognize and orient 

 their bodies with reference to increases and decreases of 

 such substances often present in water. Various workers 

 have shown that the products of decomposition are in the 

 main very poisonous to fishes, especially to eggs and 

 young fry (Wells '13 and citations). It must further be 

 noted that all fishes young and adult require oxygen and 

 eggs require a large quantity of it for development. 

 Aside from the ill effects of the decomposition products 

 themselves, there is usually little oxygen in their presence, 

 especially in fresh water. From these considerations it 

 is readily seen that it is useless to expect the presence of 

 food fishes where the bottom of the waters generally con- 

 tain any quantity of decomposition products. Thus in 

 seeking indices of the suitability of bodies of water for 

 fishes we are concerned primarily with decomposition 

 products. The peculiar physiological constitution of the 

 fishes in question is, of course, a matter for consideration, 

 for the character of different species differs in this re- 

 spect. However, the difference between different species 

 is one of degree and special habits. The effects of the 

 various decomposition products are the same in a wide 

 range of species with only slight differences in degree. 

 Furthermore when we consider the best food fishes (we 

 believe both fresh water and marine fishes though our 

 experience with the latter has been limited), we find that, 



