HIGHER ANIMALS IN ECONOMY OF LAKES 



141 



what would be expected to account for such 

 a correlation. However, if the experiments 

 are considered as throwing light on a physi- 

 ological mechanism, thev have much more 



meaning. 



It is well known that carbon dioxide has 

 a marked effect on the transport of oxygen 

 by the blood of mammals. In the presence 

 of relatively low coneentrations of carbon 

 dioxide their blood can become saturated 

 Avith oxygen at relatively low tensions of 

 that gas. On the other hand if the carbon 

 dioxide tension is increased the blood can- 

 not take up or hold oxygen to nearly the 

 same extent. Accordingly the moderate 

 tension of carbon dioxide in the alveoli and 

 the higher tension in the tissues facilitate 

 the shift of oxygen from the lungs to the 

 blood and from the blood to the tissues. 

 This effect of carbon dioxide on the ability 

 of blood to take up oxygen is known as the 

 "Bohr effect." Carbon dioxide has a simi- 

 lar effect on the blood of fishes, as has been 

 showai by many workers. Particularly 

 since the discovery of the biological anti- 

 coagulant heparin, it has been shown that 

 the magnitude of the Bohr effect in fish 

 blood differs greatlj^ from one fish to an- 

 other. It is often much greater than that 

 displayed by the blood of mammals or there 

 may be hardly any effect at all as Black 

 (personal communication) has found to be 

 the case in the catfish. 



Blood curves for two fresh-water fish, the 

 sucker and the carp, from Black (1938), 

 are given in the upper left panel of Fig. 3 

 which shows the oxygen tension necessary 

 to half saturate the blood in the presence 

 of a given tension of carbon dioxide. These 

 two blood curves occupy about the center 

 of the range of sensitivity which has been 

 found in fishes, and by way of a landmark 

 it might be pointed out that a similar curve 

 for man lies very closely to that of the carp. 



When the curves for the levels to which 

 oxygen can be removed in the presence of 

 different concentrations of carbon dioxide 

 by these two species, shown in the upper 

 right panel of Fig. 3, are compared with 

 the curves for the drawn blood, it will be 

 apparent that the sensitivity toward carbon 



dioxide displayed by the fisli may be de- 

 pendent upon tlie magnitude of the Bohr 

 effect shown by their bloods. This is what 

 would be expected on a priori grounds, 

 for fish with blood very sensitive to carbon 

 dioxide would be less able to take up oxy- 

 gen in the presence of increasing carbon 

 dioxide than would fish with blood that was 

 more indifferent to the presence of that gas. 

 Thus, and there is further evidence to sup- 

 port this view, the measure of sensitivity 

 towards carbon dioxide as shown in Fig. 3 

 is in all likelihood a measure of difference 

 in the Bohr effect displayed by the bloods 

 of different species. Consequently it seems 

 reasonable to infer that the correlation ob- 

 served between sensitivity and vertical dis- 

 tribution of these species in summer is due 

 to differences iu the oxygen transport 

 capacity of the blood. 



With this brief mention of the part ex- 

 periment plays ill limnology our considera- 

 tion of the position of fish and other higher 

 animals in the economy of lakes is finished. 

 Xo attempt has been made to review the 

 subject exhaustively or to give more than 

 a few references to works which are easily 

 accessible in America. It may be felt that 

 vertebrates other than fish have been 

 shamelessly neglected throughout this dis- 

 cussion. It must be confessed that they 

 have, but in justification it can be pointed 

 out that the principles which have been 

 mentioned here apply to other forms of life 

 as well as fish. Again it may be felt that 

 applied limnology has been overstressed. 

 Ill defense it may be maintained that the 

 only valid elassifieation of limnology is into 

 the practical and the not practical rather 

 than into pure and applied. The differ- 

 ences between pure and applied science lie 

 not in the subject matter but in the attitude 

 taken towards it. 



Eeferences Cited 



Battle, Helen I., Huntsman. A. G., Jeffers, 

 Anne M., Jeffers, G. W., Johnson, W. H. and 

 McNairn, N. a. 1936. Fatness, Digestion and 

 Food of Passamaqnoddy Young Herring. Jour. 

 Biol. Bd. Can., 2 : 401. " 



Black, E. C. 1938. Oxygen Transport by the 



