48 Interrelationships of Fishes and Lake Habitats 



indicate that while natural turbidities in ponds seldom if ever cause direct 

 lethal eflFects, over a period of years they may be responsible for poor 

 production of fish and indirectly for the disappearance of certain species. 



OXYGEN AND CARBON DIOXIDE 



Both oxygen and carbon dioxide sometimes occur in w^ater in excessive 

 and subnormal amounts with deleterious effects on fish. In bright sunlight 

 abnormally high oxygen tensions ( supersaturation ) may occur within 

 dense stands of submerged vegetation. On the other hand, unusually high 

 carbon dioxide tensions may occur where rapid decay of organic material 

 is taking place on the bottom of a pond or lake. Although high oxygen 

 tensions are usually associated with low carbon dioxide tensions, this is 

 not always the case. 



Fish living in a medium in which the tensions of oxygen and carbon 

 dioxide change gradually, but markedly, with changes in depth or with 

 time of day, are able to make certain physiological adjustments to com- 

 pensate for changes in the amount of dissolved gases in their habitat. 

 However, these adjustments cannot be made instantly. If forced to make 

 rapid physiological adjustments to compensate for sudden severe changes 

 in dissolved oxygen or carbon dioxide, fish may become deranged and die. 

 Fish have been observed not only avoiding elevated concentrations but 

 also reacting strongly to sudden very small changes in carbon dioxide 

 tension.^- 



Any fisherman who has operated trap nets in ponds or lakes during 

 summer months knows that fish sometimes make trips into the lower 

 waters where dissolved oxygen may be low and carbon dioxide tension 

 high."^ When caught in nets set in deep water, these fish may remain alive 

 for some time at such depths, but if left too long they suffocate. Physiol- 

 ogists have demonstrated the presence of oxygen in the swim bladders of 

 some fishes and have been able to measure the adjustments in alkalinity 

 of the fishes' blood, resulting from changes in tension of carbon dioxide. 

 The length of time a fish may survive low oxygen tension varies inversely 

 with the tension of carbon dioxide.* 



Interest in the effects of rapid change in carbon dioxide tension was 

 stimulated by the death of fish in Norris Reservoir (Tennessee) in De- 

 cember of 1937.-^ Tributary rivers were pouring ice-cold water into the 

 lake at a time when the lake level was being lowered a foot per day. 

 This river water, which was as cold or colder than the lake water, caused 

 a pushing up (because it was heavier) of the bottom water of the 

 lake. This upwelling and mixing of the carbon dioxide-saturated bottom 

 water was indirectly responsible for the death of fish. Small shad moving 

 about in this heterogeneous mixture of waters passed from high carbon 



