FRESHWATER FISH EPIDEMICS IN QUEENSLAND RI\ERS. 11)3 



hours, piovidcd they weic f:lcAvly thawed out. Bumpus (1898) 

 suggested that the heavy mortahty of tile-fifh off Florida in 

 1878 might be due to a sudden diminution of temperature as a 

 result of an alteration of the Gulf Stream. Hofer, in his valuable 

 work on fish diseases (1906, pp. 87-93), gave an account of 

 the effects of cold on the skin of freshwater fish in Europe. 



Wells, in one of his many papers dealing with the relation 

 of fish to their environment, stated that many species can 

 detect and react to temperature differences as small as 1 to 2 

 degrees Centigrade (1913, p. 339). Next year he published a 

 paper giving an account of his investigations regarding the 

 resistance and reactions to temperature (1914). He found that, 

 in the case of freshwater fish, the degree of resistance varied 

 with the species and with the size of the individual, large 

 specimens being more resistant to high temperatures than small 

 fish of the same species, ^^ilile small individuals were able to 

 adapt themselves more successfully to sudden changes from 

 warm to cold. He also reported that in no case did death result 

 from sudden change from a higher to a lower temperature, 

 though the widest range — viz., from the maximum for the 

 species do^^'n to freezing point — was tried. He admitted that 

 it was possible that a sudden and great lowering of tempera- 

 ture might cause death in the case of certain species. His 

 experiments showed that fish can detect and react to variations 

 of temperature amounting to only 01 degree Centigrade. 



Shelford and Powers (1915, p. 325) ascertained that marine 

 fish were capable of detecting differences of -5° to -6° C, and 

 probabh" as low as -2° C. 



3. STATE OF THE WATER. 



This may be considered under various headings^ — viz., 

 alterations in regard to amount of suspended matter or A^eeds, 

 the amount of gases (oxygen and carbon dioxide) present, in 

 the degree of alkalinity or acidity, and in the amount of salts 

 in solution. 



(a) Suspended Matter. 

 We laio"«' that the amount of suspended matter in stagnant 

 pools dej^ends mainly on the chemical composition of the 

 Avater, since the presence of certain substances leads to the 

 precipitation of finely divided and colloidal material. One 

 might draw attention to the muddy water of a stream and the 

 clear water of adjacent " kopai "' holes rich in sulphate of lime. 



