from 5 to 4 mg/1. If the opercular rate is taken as 

 the criterion by which the adequacy of an oxygen 

 concentration is to be judged, then such evidence 

 as we have indicates 6 mg/I as the required dis- 

 solved oxygen concentration. Several field studies 

 have shown, however, that good and diversified 

 fish populations can occur in waters in which the 

 dissolved oxygen concentration is between 6 and 

 5 mg/1 in the summer, suggesting that a minimum 

 of 6 mg/1 is probably more stringent than neces- 

 sary for warm-water fishes. Because the oxygen 

 content of a body of water does not remain con- 

 stant, it follows that if the dissolved oxygen is 

 never less than 5 mg/1 it must be higher part of 

 the time. In some cases, good populations of 

 warm-water fish, including game and pan fishes, 

 occur in waters in which the dissolved oxygen may 

 be as low as 4 mg/1 for short periods. Three mg/1 

 is much too low, however, if normal growth and 

 activity are to be maintained. It has been reported 

 that the growth of young fish is slowed markedly if 

 the oxygen concentration falls to 3 mg/1 for part 

 of the day, even if it rises as high as 18 mg/1 at 

 other times. It is for such reasons as this that oxy- 

 gen criteria cannot be based on averages. Five and 

 4 mg/1 are close to the borderline of oxygen con- 

 centrations that are tolerable for extended periods. 

 For a good population of game and pan fishes, 

 the concentration should be considerably more 

 than this. 



The requirements of the different stages in the 

 life cycles of aquatic organisms must be taken into 

 account. An oxygen concentration that can be 

 tolerated by an adult animal, with fuUy developed 

 respiratory apparatus, less intense metabolic re- 

 quirements, and the ability to move away from 

 adverse conditions, could easUy be too low for eggs 

 and larval stages. The eggs are especially vulner- 

 able to oxygen lack because they have to depend 

 upon oxygen diffusing into them at a rate sufficient 

 to maintain the developing embryos. Hatching, 

 too, is a critical time; recently hatched young need 

 relatively more oxygen than adults, but until they 

 become able to swim for themselves (unless they 

 are in flowing water) they must depend upon the 

 oxygen supply in the limited zone around them. 

 These problems are not as great among species 

 that tend their eggs and young, suspend their eggs 

 from plants, or have pelagic eggs, as they are for 

 salmonids. Salmonids bury their eggs in the gravel 

 of the stream away from the main flow of the water 

 thereby requiring a relatively high oxygen concen- 

 tration in the water that does reach them. 



Recommendation: In view of the above considerations 

 and with the proviso that future research may make 

 revision necessary, the following environmental con- 



ditions are considered essential for maintaining na- 

 tive populations of fish and other aquatic life : 



( 1 ) For a diversified warm-water biota, including 

 game fish, daily DO concentration should be above 

 5 mg/1, assuming that there are normal seasonal and 

 daily variations above this concentration. Under ex- 

 treme conditions, however, and with the same stipula- 

 tion for seasonal and daily fluctuations, the DO may 

 range between 5 mg/1 and 4 mg/1 for short periods of 

 time, provided that the water quality is favorable in 

 all other respects. In stratified eutrophic and dystrophic 

 lakes, the DO requirements may not apply to the 

 hypolimnion. In shallow unstratified lakes, they should 

 apply to the entire circulating water mass. 



These requirements should apply to all waters ex- 

 cept administratively established mixing zones. In lakes, 

 such mixing zones must be restricted so as to limit the 

 effect on the biota. In streams, there must be no blocks 

 to migration and there must be adequate and safe 

 passageways for migrating forms. These zones of pas- 

 sage must be extensive enough so that the majority of 

 plankton and other drifting organisms are protected 

 (see section on zones of passage). 



(2) For the cold water biota, it is desirable that DO 

 concentrations be at or near saturation. This is espe- 

 cially important in spawning areas where DO levels 

 must not be below 7 mg/1 at any time. For good growth 

 and the general well-being of trout, salmon, and other 

 species of the biota, DO concentrations should not be 

 below 6 mg/1. Under extreme conditions they may 

 range between 6 and 5 mg/1 for short periods provided 

 that the water quality is favorable and normal daily 

 and seasonal fluctuations occur. In large streams that 

 have some stratification or that serve principally as mi- 

 gratory routes, DO levels may be as low as 5 mg/1 for 

 periods up to 6 hours, but should never be below 4 

 mg/ 1 at any time or place. 



(3) DO levels in the hypolimnion of oligotrophic 

 small inland lakes and in large lakes should not be 

 lowered below 6 mg/1 at any time due to the addition 

 of oxygen-demanding wastes or other materials. 



Carbon dioxide 



An excess of "free" carbon dioxide (as distin- 

 guished from that present as carbonate and bicar- 

 bonate) may have adverse effects on aquatic ani- 

 mals. These effects range from avoidance reactions 

 and changes in respiratory movements at low con- 

 centrations, through interference with gas ex- 

 change at higher concentrations, to narcosis and 

 death if the concentration is increased further. The 

 respiratory effects seem' the most likely to be of 

 concern in the present connection. 



Since the carbon dioxide resulting from meta- 

 bolic processes leaves the organisms by diffusion, 

 an increase in external CO, concentration will 

 make it more diflicult for it to diffuse out of the 

 organism. Thus, it begins to accumulate internally. 

 The consequences of this internal accumulation 

 are best known for fish, but presumably the princi- 

 ples are the same for other organisms. As the CO2 

 accumulates, it depresses the blood pH, and this 



44 



