ANADROMY: GENERAL CONSIDERATIONS. 43 



other factors that primarily induces salmon to ascend, Nordquist (1924, p. 50) thus 

 refers to Roule's conclusions: 'Roule (1920, p. 41-53) says that the salmon in entering 

 rivers are guided by the amount of oxygen in the water, but not by any instinct or other 

 psychical faculty. The salmon thus enter the rivers with a lower amount of this gas.' 

 Also: 'Roule (p. 45) pretends also that the interruption of the ascent of salmon, which 

 takes place in French rivers in summer is caused by a lower amount of oxygen.' 



The unfavorable effects of turbidity of the water and other conditions are then dis- 

 cussed, all of which lead to his final conclusion, viz.: 'notwithstanding that we yet want 

 more details about the connection between the ascent of salmon and other factors, the 

 facts brought forth above, seem to show that high water in the rivers, when not con- 

 nected with an excessive turbidity, promotes the ascent, while skreed and great turbidity 

 of the water prevent it.' 



In another connection on page 48 Nordquist says: 'I have here tried to show that the 

 time of ascent of the salmon stands in direct relation to the water level in the respective 

 rivers. Of course it is not the water level itself but accompanying circumstances that affect 

 the ascent, viz.: how far out in the sea the river water extends; the swiftness of the water, 

 and the height of the water-falls. But there may still be other factors that affect the time of 

 ascent.' (ItaUcs the present writer's.) 



The following quotation from page 52 should be compared with Gurley's theory: 

 'The influence of the height of water upon the ascent of salmon manifests itself in two 

 different ways, viz.: the high water-level directly promoting, the low water checking 

 the ascent and further so that the favourable height of the water recurring every year 

 about a certain time, has brought about a concentration of the chief entering of the 

 salmon to this time. This latter effect has probably been caused indirectly by selection and 

 heredity [italics the present writer's], so that individuals that ascend during a period with 

 a favourable height of the water are more Ukely to attain the spawning places than 

 individuals that ascend later, when the water has sunk so much that the surmounting 

 of high waterfalls becomes impossible.' 



Shelford and Powers (1915, p. 331-332), concerning the results of their experiments, 

 say that in connection with the entrance of salmon into fresh water, the orientation of 

 the specimens concerned in the experiment was with reference to acidity and alkalinity 

 rather than salinity. Sea water is less acid than fresh and the reactions of the salmon 

 accord with their recent entrance into salt water. They also found that the fish selected 

 aerated water in preference to water directly from the tap and further that when oxygen was 

 added to the water in opposition to that drawn directly from the tank the preference for the 

 higher oxygen content was decided. 



The authors concluded that the extreme sensitiveness of the fish enabling them to 

 detect shght deviations from neutraUty, makes it clear that there is no difficulty in 

 fishes determining the direction to large rivers from hundreds of miles out at sea or of 

 finding their way into a bay or harbor or river or other arm of the sea which their par- 

 ticular physiological condition at a given time demands ; and that it is not necessary to 

 appeal to 'instinct' to explain the return of certain salmon to certain rivers. 



