Ecological Effects of Carbon Dioxide 267 



in this manner would accordingly experience difficulty in obtaining 

 oxygen. Fish inhabiting tropical swamps with water heavily charged 

 with carbon dioxide have been found to possess a specially adapted 

 type of blood that is little affected by the environmental carbon diox- 

 ide. Species of fish dwelling in littoral areas where concentrations 

 of carbon dioxide are above normal have similar adaptations en- 

 abling them to extract oxygen from water of wider carbon dioxide 

 range than is possible for pelagic fish. 



Changes in carbon dioxide also tend to modify the pH and the 

 alkalinity of the blood. Alterations of these characteristics can be 

 compensated for by changes in the abundance of hemoglobin and of 

 cations such as Na and Ca. But these modifications of the blood 

 equilibria require time. Fish that can tolerate but little variation in 

 pH and alkalinity of the blood might well be seriously harmed by 

 swimming rapidly across a sharp gradient, as at the thermocline, from 

 water of low carbon dioxide to water of high carbon dioxide or vice 

 versa. 



Another influence of carbon dioxide as an environmental factor is 

 in relation to orientation. The movement of certain aquatic animals 

 is affected by differences in concentrations of this gas. Reactions to 

 increased carbon dioxide may have arisen in relation to the harmful 

 effects of an excessive concentration of this gas mentioned above or 

 in relation to the low oxygen tension commonly associated with it. 

 A search has long been made for the factor or factors that guide 

 anadromous fish migrating upstream to a specific tributary within 

 which their spawning beds are located, and differences in carbon 

 dioxide tension have been suggested as a possible orienting feature 

 for salmon. Experiments were conducted by Collins ( 1952 ) on the 

 alewife (Pomolobus), another anadromous fish, by dividing the 

 stream, in which the fish were migrating, into two channels and add- 

 ing carbon dioxide gas to the water of one channel. These tests 

 demonstrated that the alewife can distinguish between small dif- 

 ferences in free carbon dioxide content (down to 0.3 ppm) and that 

 the majority (72 per cent) "chose" the stream with the lower amount 

 of carbon dioxide. Supplementary tests showed that the fish were 

 oriented primarily by the difference in carbon dioxide itself rather 

 than by the concomitant change in pH. Whether a reaction of this 

 sort to naturally occurring differences in carbon dioxide is a chief 

 factor in directing migrating fish into one branch of a stream rather 

 than into another remains to be investigated. Differences in tem- 

 perature and other factors have been shown to be influential in the 

 foregoing and in other observations, and evidence has been presented 



