PACIFIC SALMON 



83 



water gradient is not without effect, as is 

 shown by the delay of the fresh-salt water 

 passage of the salmon to and from the salt 

 water. Other evidence favoring the car- 

 bon dioxide tension gradient is that the 

 response is seasonal with both mature and 

 immature salmon as is the carbon dioxide 

 tension gradient. The physiological evi- 

 dences favoring the carbon dioxide tension 

 gradients are (1) the presence of receptors 

 in the morphological regions of the gills 

 among vertebrates sensitive to carbon diox- 

 ide tension as such and (2) the special type 

 of metabolism necessary for the ripening 

 of the germ cells. Finally, the red salmon 

 seems to be responding to a carbon dioxide 

 tension gradient in its movements when in 

 the fresh water and when selecting a 

 spawning ground. 



Summary 



1. Chemical receptors sensitive to carbon 

 dioxide tension are found in the branchial 

 arches or in structures derived from bran- 

 chial arches. 



2. It has been shown in the laboratory 

 that fishes including salmon respond to gra- 

 dients of physical and chemical character- 

 istics of the water. 



3. Tagging experiments in Alaskan 

 waters have shown that the red salmon mi- 

 grate to the spawning grounds along paths 

 of fresh-salt water gradients. 



4. The red salmon as a rule choose the 

 fork of the river that drains a lake. 



5. Rivers draining lakes, as has been 

 shown by actual observation, have carbon 

 dioxide tensions more nearly in equilibrium 

 with the carbon dioxide partial pressure 

 of the air than do spring-fed rivers. 



6. Streams having the most vigorously 

 aerated (low carbon dioxide tension) water 

 are chosen as spawning grounds by the red 

 salmon as is shown by observations made 

 on the Karluk River system. 



7. These views are further supported by 

 the fact that the greatest number of all 

 species of salmon are moving against the 

 ocean current (drift) during their spawn- 

 ing migration. 



8. It is suggested that the movements 



into the sea are largely drifts and at the 

 same time the salmon are held within the 

 bounds of a fresh-salt water gradient. 



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