388 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



of free CO2 or to differences in CO., tension. Tlie 

 difference in CO2 tension associated ^Yitll a differ- 

 ence of 0.3 p. p. m. free CO, under tlie conditions 

 of these experiments would be approximately 0.1 

 mm. Hg. 



The response of tlie fisli to differences in CO2 

 (whether it was to the amount of free CO,, to tlie 

 CO2 tension, or to associated HCO.t) "as similar 

 to the response of the fish to temperature differ- 

 ences in that it appeared to be a uniform response 

 to all differences above the threshold difference. 



RELATIVE ORIENTATIVE INFLUENCE OF CO. 

 AND TEMPERATURE 



As the evidence was acquired indicating that 

 differences in temperature and differences in CO2 

 could influence the orientation of the migi-ating 

 fish, the need for some information on the relative 

 influence of the two factors became increasingly 

 obvious. Under the controlled conditions of the 

 experiments, where all factors other than the one 

 used for testing were maintained equal in both 

 channels, the fish might show as great a response 

 to a relatively minor influence as they would to 

 an important or dominant factor. To examine 

 the relative orientative influence of CO2 and tem- 

 perature, two groups of experiments were under- 

 taken. In one set of experiments the directional 

 influences of the two factors were arranged so as 

 to be in conflict. The differences in CO2 favored 

 the choice of one channel, while the differences in 

 temperature favored the selection of the other. 

 In a second group of experiments the differences 

 both in CO2 and in temperature favored the selec- 

 tion of the same channel. 



The data (table 8) collected indicate that the 

 relative importance of the two orienting factors 

 depends upon their quantitative relationships. 

 By altering the relative amounts of heat and of 

 CO2 added to the water, it was possible to demon- 



strate circumstances under which either factor 

 could balance or even dominate the other when 

 the factors were in opposition. The data also 

 suggest that when the two factors are not in op- 

 position they may actually augment each other 

 and together provoke a response in a greater num- 

 ber of fish than either factor could produce alone. 



ORIENTATIVE INFLUENCE OF pH 



In the preceding experiments involving the ad- 

 dition of gaseous CO, to the water, the differences 

 in the amount of free CO2 between the two chan- 

 nels were always accompanied by differences in 

 pH. (see table 4). The question arises as to 

 whether the response shown by the migi-ating 

 fish is to the differences in CO2 or to the accom- 

 panying differences in pH. 



An attempt was made to answer this question 

 experimentally with the migrating fish. It was 

 necessary to use a substance which would, when 

 added to the water, modify the pH of the water 

 to the same degree that it was modified during the 

 CO2 experiments (see table 9) without, at the 

 same time, materially affecting the amount of free 

 CO2 in the water. It was also necessary that the 

 substance be one to which the fish would not 

 respond by means such as taste or smell. To 

 avoid the difficulties of determining whether the 

 response of a fish was to the taste or to the smell 

 of a chemical, or whether its response was to pH 

 differences created by that chemical, it was neces- 

 sary to select a substance to which the fish did 

 not respond at all. 



In one of a series of exploratory tests (table 10) 

 NaOH was used to modify the water. There was 

 no response to a difference in pH of 0.1 (compare 

 with response to pH difference 0.1 created by the 

 addition of gaseous CO2, table 9). However, it 

 will be noted that when the difference in pH was 

 greater tlian 1.0 pB. unit, the fish favored the 



Table 8. — Relative orientative influence of CO2 and temperature as shown in tests made June 3-11, 1950 



Note.— Stream temperatures, 16.9° to 22.2° C. Unmodified stream water, 0.7 p. p. m. to 2.1 p. p. m. free COi; pH 7.0 to pH 6.5. 



