ANADROMY: GENERAL CONSIDERATIONS. 41 



places because the conditions in them and leading to them satisfy the physiological 

 demands and that they arrive at those places by force of similar stimuli. A salmon, then, 

 is led down into an outlet by the same sort of forces that induces another to ascend an 

 inlet for the same purpose, reproduction. 



From observation and experiment made upon various rivers of France, Roule (1920) 

 came to the conclusion that dissolved oxygen is the inducing agent leading salmon into 

 and up the streams to the breeding places. He supports his argument by statements of 

 the results of these experiments and studies of the streams, both those which are and 

 which are not ascended bj' salmon. It is very clear, too, that so far as the conditions of 

 the rivers are concerned his argument is well sustained. Roule's idea is that the migra- 

 tions of salmon appear to be due to some forces of the nature of tropisms, and his theory, 

 briefly stated, is that the fish during Ufe and growth in the sea assimilates intensively, 

 accumulates in its tissues numerous reserves, and reaches that condition of physiological 

 surfeit, when further acquisition of that kind becomes uncomfortable. 



Roule's theory is thoroughly logical and in full accordance with all the known facts 

 concerning the conditions of rivers, both those frequented and avoided by salmon. But 

 like Gurley's theorj' it fails to explain clearly certain questions which arise in connection 

 with it. He states that fresh-water salmonids are affected in a similar manner but to a 

 lesser degree, but he, Uke Gurley, fails to explain why fresh-water salmon sometimes 

 resort to outlets to breed. Indeed he does not refer to the fact at all. 



There still remains the question, pertaining to the sea Ufe of the salmon, as to what is 

 the actuating factor that brings it in contact with the exact proportions of oxygen, cold 

 water, etc., which starts it up river. Chance contact in search for relief from discomforts 

 of satiety is not in accordance with a 'reign of law.' If it were so, many salmon might 

 never migrate and reproduce. The old plethora theory from intensive feeding in the sea, 

 which has been advanced from time to time, appears formerly to have been regarded as a 

 sort of psychological effect. More recently Taylor (1922, p. 121-126) has enlarged upon 

 the idea by connecting it with the specific gravity of the salmon. His theor}^ is based 

 on several demonstrated facts, one of which is that the specific gravity of fish varies with 

 the amount of fat present in the tissues. 



He finally reaches the following conclusions : 



'1. Fish on migrating from water of low salinity to that of high sahnity may adjust 

 specific gravity by reducing the size of air bladder. In the reverse direction there is no 

 apparent means for voluntary adjustment. 



'2. As a fish puts on fat its body specific gravity diminishes, 'pari passu, and in pro- 

 portion to the amount of fat present (a) its navigation in salt water is more difficult; 

 (b) fresher water is better suited as a physical medium. Until a certain amount of fat is 

 accumulated migration from salt to fresh water must be difficult or impossible. 



'3. Reduction of volume of air bladder may possibly be effected by (a) resorption 

 of gas from bladder to blood and expulsion through gills; (b) direct expulsion of gas 

 through pneumatic duct (except in Acanthopteri) ; (c) di\dng, whereby hydrostatic 

 pressure reduces the volume. The effect produced by diving a given depth is proportional 

 to the absolute volume of the air bladder. 



