FACTORS CONTROLLING SALMON MIGRATION 



By HENRY B. WARD 



DEPARTMENT OF ZOOLOGY, UNIVERSITY OF ILLINOIS, URBANA, ILL. 



The following paper represents an effort 

 to summarize the views on the interpreta- 

 tion of the life cycle of the salmon. The 

 main facts in this story were established 

 in North America by the work of Jordan, 

 Evermann and Gilbert, three distinguished 

 American ichthyologists. To their records 

 have been added many observations and sur- 

 mises by a long list of subsequent students 

 of the problem. There still remain gaps 

 in this record and conspicuous differences 

 in interpretation which call for further in- 

 vestigation and discussion before the final 

 account can be written. 



Among the salmon two groups are recog- 

 nized ; the Atlantic and the Pacific, assigned 

 by ichthyologists on the basis of structure 

 to separate genera. Both are anadromous, 

 spending the larger part of life in the sea 

 and when sexual maturity approaches mi- 

 grating to spawning grounds in fresh water. 

 The young, hatched out in fresh water, 

 descend to the sea and seek out feeding 

 grounds where most of their growth is 

 achieved. The life cycle of the Pacific 

 salmon is completed with a single journey 

 from fresh water to the ocean and a return 

 to fresh water for spawning ; at the close of 

 this period the fish dies. On the other hand 

 the Atlantic salmon survives the spawning 

 period, goes back to salt water and later re- 

 turns one or more times to fresh water to 

 repeat the process of spawning. 



My own studies, which have covered the 

 summer migration period during many 

 years, has been devoted chiefly to observa- 

 tions on the activities of Pacific salmon of 

 the species Oncorhynchus nerka, the red or 

 sockeye salmon, in fresh water. Here the 

 case is simpler than with the Atlantic 

 salmon since the entire life span embraces 

 only one cycle; it includes only a single 



journey down stream, a single trip in the 

 sea to the feeding grounds and return to 

 some river, a single vigorous dash up stream, 

 a single reproductive season and then the 

 end of life. 



This journey has often been regarded as 

 similar to the movements of migratory birds 

 but the assumption is a false analogy, based 

 on incomplete data. No Pacific salmon sur- 

 vives to repeat any part of the life cycle. 

 Neither experience of its own nor guidance 

 of companions with knowledge acquired 

 from a previous similar trip can determine 

 or influence its responses to the new en- 

 vironmental conditions which the sockeye 

 encounters at every step of the journey. 

 Add to this the further limitation involved 

 in restricting this study to the part of life 

 spent in fresh water and the problem, com- 

 plex though it still remains, has yet been 

 reduced to lowest terms. One may rightly 

 hope that continued observations on the ac- 

 tive, living salmon will afford some clue to 

 the factors that control salmon migration. 

 To avoid the dangers of narrowly limited 

 investigation of any problem, I have seized 

 the opportunity afforded at times to study 

 the sockeye in different and widely sepa- 

 rated areas from Oregon to Alaska, and to 

 compare this species with the other types of 

 Pacific salmon and with the Atlantic salmon 

 as I have had chances occasionally to study 

 them also. ■ The real problem lies in the 

 interpretation of the facts assembled. 



The Parent Stream Theory 



The migration of the Pacific salmon as 

 ordinarily explained is expressed by David- 

 son and Christey (1938: 644) as due to ''a 

 high degree of homing instinct, i.e., the 

 majority of the adults return to spawn in 

 the streams of their origin." The thesis is 



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