82 



THE MIGRATION AND CONSERVATION OF SALMON 



the author that salmon do not respond to 

 factors other than gradients of carbon diox- 

 ide tension and fresh-salt water. There are 

 many other factors, food, etc. These many 

 responses are the factors that keep the 

 salmon distributed throughout their habi- 

 tats. If gradients of factors to which 

 salmon respond are not sufficiently steep to 

 direct the movements of the salmon they 

 might move about seemingly aimlessly and 

 thus be carried by the ocean drift (Hunts- 

 man, 1938b). To illustrate, if fish re- 

 sponded to current only they would all be 

 at the heads of streams and not distributed 

 throughout the streams. 



The movements of fishes, of necessity 

 more than of any other vertebrate, are 

 dominated by responses to the environ- 

 ment. The first development of a cortex 

 from the pallium appears in the amphibia. 

 The cerebra of the fish are mainly corpora 

 striata and olfactory bulbs. The cortex 

 reaches its highest development in mam- 

 mals (man) and the highest level of the 

 corpora striata is reached in birds. We 

 will not discuss the relative exactness of the 

 migratory movements of these two classes 

 of vertebrates. 



Greene (1915, 1918 and 1919) has shown 

 that in the king salmon the muscle protein 

 is reduced about 30 per cent and the fat 

 from 18 per cent to as low as 1.6 per cent 

 during its spawning migratory movements 

 from the brackish water to the spawning 

 grounds. The weights of the ovaries are 

 (Greene, 1921) in brackish water 501 to 

 749 gm, in spawning beds 775 to 2,243 gm 

 and when ripe 2,596 gm. The average de- 

 pression of the freezing point of the blood 

 (Greene, 1905) is -0.762° C for sea salmon, 

 -0.737° C for brackish water and -0.628° 

 C for spawning ground salmon. 



Questions that can legitimately be asked 

 are: Does the electrolyte content of the 

 blood change with the salt content of the 

 environment and is the salmon held back 

 until this change has taken place in tlie 

 blood before ascending the river? Or is the 

 electrolyte content of the blood lowered in 

 coordination with the special metabolism 

 in the ripening of the germ cells and is the 



salmon held back until this change has 

 taken place in the blood before ascending 

 the river? Observations indicate that the 

 second series of events is the more probable. 

 The more nearly ripe the germ cells the 

 shorter the time the salmon lingers in the 

 brackish water before ascending the river. 

 When the germ cells are ripe or nearly ripe 

 the salmon loses no time in passing from 

 the brackish water to fresh water or ascend- 

 ing the river to the spawning ground. 



Huntsman (1936, 1938a, 1938b, and 

 1938c) presents evidence that the Atlantic 

 salmon never gets beyond the influence of 

 the fresh water. That is, a salmon never 

 passes into pure oceanic water and still 

 finds its way back to fresh water. A delay 

 in the salmon's passage from fresh water 

 to salt water and the second delay in its 

 passage from brackish water to fresh water 

 both favor the view here presented. That 

 is, the state of the metabolism of the salmon 

 determines the extent of its movements 

 into waters of greater salt contents. The 

 fact that salmon do not ordinarily leave the 

 salt water and ascend the rivers except 

 when the germ cells are ripening (that is, 

 there are one year, two year, three year, 

 etc., periods in the sea) lends weight to the 

 view that the state of metabolism deter- 

 mines the vigor or intensity of the response 

 of the salmon to the environment. The 

 mature respond more vigorously than the 

 immature to fresh-salt water and carbon 

 dioxide tension (one or the other or both) 

 gradients. 



To summarize, in order that there be a 

 directive response there must of neces- 

 sity be a gradient of the stimulating fac- 

 tor or factors. Evidence from the most 

 exact observations indicates that the salmon 

 which do return never move beyond the in- 

 fluence of fresh water. There are two pos- 

 sible gradients, a fresh-salt water gradient 

 and a carbon dioxide tension gradient. 

 Time will tell which of the two gradients 

 dominates the spawning migratory move- 

 ments of the salmon from the sea to the 

 fresh water. At the present time the evi- 

 dence seems to favor the carbon dioxide 

 tension gradient. However, the fresh-salt 



