ORIENT ATIOX OF MIGRATING ANADROMOUS FISHES 



393 



the relative influence of COo and temperature 

 demonstrated conditions under which the response 

 to temperature diiTerences could dominate the re- 

 sponse to CO2 differences when the factors were 

 in o]iposition. Tlie reverse situation was also 

 <l('iuoustrated. Tlie experiments further revealed 

 circumstances under wliich the two factors were 

 equal and the influence of the one balanced the 

 influence of the other. Had an observer at these 

 experiments been considering only temperature, 

 for example, he would have seen the fish choose 

 the warmer water in one group of tests and the 

 colder water in another. During the third group 

 of tests he would have concluded that the fish 

 were indifferent to temperature. It is quite pos- 

 sible that many apparently contradictory observa- 

 tions may be explained in this way, e.g., the 

 observations of Ward (1920), Chamberlain 

 (1907), and Foerster (1929), regarding the in- 

 fluence of temperature on sockeye salmon. 



One of the considerations that had to be taken 

 into account during the experiments at Bourne- 

 <lale was that the fish might be reacting to a 

 I'liange in water conditions. This possibility be- 

 came evident in one of the exploratory tests ex- 

 amining the influence of light intensities. One 

 channel of the experimental trough was shaded 

 and the other was left exposed to full sunlight. 

 When the undivided lower portion of the experi- 

 mental trough was shaded, the fish chose the 

 sliaded channel. When the lower part of the 

 trough was exposed to sunlight, the fish chose the 

 sunlit channel. Tlius the fish showed no prefer- 

 ence for either the sunlit or the shaded channel. 

 They simply avoided any change in light 

 conditions. 



During experiments in which water was modi- 

 fied by tlie introduction of gaseous CO2 the fish 

 avoided the modified water. Although the fish 

 entered tlie trougli in a mixture of water from 

 both channels (fig. 2), and before entering they 

 had been subjected to a mixture of botli waters in 

 the upstream end of the retahiing pool (fig. 4), 

 the possibility that the reaction of the fish was 

 one to change in chemical conditions still had to 

 be considered. However, in one experiment 

 which was made at Bournedale, the CO, content 

 of the water of one channel was reduced by the 

 addition of NaOH. In that experiment (see 

 p. ySy) the fish showed a preference for the modi- 



fied water. It, therefore, seems improbable that 

 the response of the fish in the other CO2 experi- 

 ments was a response to change in chemical 

 conditions. 



Powers and Clark (194:3), disc\issing certain 

 gradient-tank experiments with brook trout, 

 Salvelimis f. fontinaiis, and rainbow trout, Sal mo 

 aalrdnen iridus, presented evidence indicating 

 that the reactions of the trout to CO.., in these 

 experiments might lie characterized as reaction 

 Id cliaiige. They made tlie ol)servation that "The 

 iiiinu'diate response to carbon dioxide tension of 

 the water depends to a large extent upon the car- 

 lion dioxide tension (o which the fish is adjusted." 

 The period of adjustment in their experiments 

 was only 10 to 15 minutes. 



If this observation were true for migrating fish 

 I lien CO2 would proliably have little directive in- 

 lluence. The fish adjusted to the CO2 tension (jf 

 tlu' sea would be restrained from entering fresh 

 water with a different CO2 tension. At a junc- 

 tion between two streams the fish would tend to 

 select the stream with the larger volume in an 

 effort to remain in the CO- tension nearest to the 

 one to which it had become adjusted. 



A possible exjihuiation is suggested by an ear- 

 lier statement of Powers (19.j9) : 



The ova and si>eim heads coutaiu a pruiciu (.pro- 

 tamine) containing large percentages of argiuine. It re- 

 iiuires a vast protein metabolism to obtain tlie necessarj- 

 arginine. I'lotein metabolism and especially fasting — 

 both necessary lor the liberation of argiuine from the 

 muscular tissue of Ihe salmon — tends to produce acidosis 

 of the blood, i. c, lower the alkali reserve of the blood. 

 This is common knowledge. A salmon with a low alkali 

 reserve blood would find low carbon dioxide tension 

 water more advantageous. 



The special physiological state of the migrating 

 fish might prevent it from becoming adjusted to 

 its environment. 



As experimental evidence is acquired indicating 

 physical and chemical differences may have a di- 

 rc'ctional influence upon migrating fish, the 

 method by which the fish is oriented to these dif- 

 ferences becomes an important consideration, par- 

 ticiilarly when one is trying to relate artificial ex- 

 [leriniental conditions to situations found in 

 nature. 



There are two methods by which fish might be- 

 come oriented in a gradient. The fish might make 

 a sinmltaneous comparison of intensities of stinm- 



