P^HESIIWATER FISH El'IDEMICS IN QUEENSTiAND RIVERS. 197 



amount of decaying organic matter, rainfall, and exposure of 

 the surface to winds. Hence great variation may occur within 

 the same body of water at different times. He stated (p. 344) 

 that small variations (e.g. 5 to 10 cc. COg per litre) from the 

 normal in regard to the amount of COg present apparently 

 produce ultimately effects similar to those caused by greater 

 variations (25 cc. COg per litre) in relatively short periods. 

 Certain species are more sensitive than others in this respect 

 and would therefore react first — i.e. they would endeavour to 

 move away from the adverse conditions. Resistance comes 

 into play when organisms cannot move away from unsuitable 

 surroundings but must adapt themselves to the unfavourable 

 environment. We know that fish are able to withstand stagnant 

 water during dry seasons by gulping air at the surface. We 

 also know that some fish are less affected than others. Wells 

 found — (1) that the presence of oxygen in large amounts (10 cc. 

 per litre) counteracted the detrimental effect of liigh CO2 con- 

 tent (50 cc. per litre) ; (2) that Ioav oxygen content (0-1 cc. per 

 litre) in alkaline water caused death sooner than when it 

 occurred in slightly acid water ; (3) that the resistance of fishes 

 to fatal concentrations and combinations of oxygen and carbon 

 dioxide varied A\ith the individual, with the species, and with 

 the weight, small fish being more resistant per unit weight 

 than were large ones. 



Wells (1916) investigated the seasonal resistance of fishes 

 in the United States, and stated that as a result of several 

 years' observations i had been noted that in nature their 

 resistance to detrimental factors in general was lowest in 

 late summer (July to October) and highest in spring (February 

 to May or June). They were found to be least resistant just 

 after the breeding season. 



Shelford and Allee had previously (1913) pointed out that 

 young fish were more sensitive to changes in regard to the 

 amounts of these two gases than older fish were, and that some 

 species reacted to a concentration of CO 2 as low as 5 to 7 cc. 

 per litre, and of oxj^gen as high as -7 to 1 cc. per litre. 



Powers (1914) found that freshwater crayfishes reacted to 

 very weak concentrations of CO.^. 



Wells (1918) stated that at a concentration of 10 cc. per 

 litre CO2 soon proved fatal to more sensitive species, and that 

 it ^^•as doubtful if there were any freshwater fish wliich could 



