FRESHWATER FISH EPIDEMICS IN QUEENSLAND RIVERS. 207 



of bacterial origin, and that various local and climatic con- 

 ditions favour it. We then offer the suggestion that the 

 epidemic may be due to a high acidity on account of excess 

 of CO2 in stagnant water, favouring the spread of a virulent 

 bacterial disease amongst the weakened fish. The presence 

 of Saprolegnia aggravates the condition, as probably also does 

 hydrogen sulphide, which is itself a decomposition product 

 resulting from the effects of the disease. 



SUMMARY. 



A very destructive epidemic makes its appearance amongst 

 freshwater fish in Queensland and Northern Territory at very 

 irregular intervals, usually during dry and cold conditions, 

 ceasing when the rivers run freely. The affected streams belong 

 to various watersheds — e.g., Lake Eyre basin (Cooper's Creek, 

 Diamantina, Georgina, Thomson, and Wilson Rivers) ; Bulloo 

 River ; Warrego River ; Darling system (Mclntyre and 

 Severn Rivers) ; Flinders system (McKinley River) ; Burnett. 

 It is apparently not due to the follo^^^ng : — Use of explosives ; 

 dry weather ; low temperature ; overstocking ; animal para- 

 sites (helminth or protozoon). 



It is suggested that the prime cause will be found to be 

 a bacterial organism, whose spread is favoured by a high 

 acidity of the water due to excess of CO2. The presence of the 

 fungus Saprolegnia aggravates the disease, as no doubt does 

 hydrogen sulphide also. 



ADDENDUM. 



Dr. J. Shirley informed us that a series of epidemics 

 causing considerable mortality occurred in the Logan River, 

 in South-eastern Queensland, during the drought years just 

 prior to 1902, but further information regarding these out- 

 breaks is not yet available. 



Mr W. B. Alexander supplied us with the followmg state- 

 ment which is of interest. Early in July 1921 a steamer 

 arrived at Fremantle with its hold on fire, and large quan- 

 tities of water were pumped into it. Among the cargo was 

 a consignment of cyanide for the Kalgoorlie mines, which 

 partly dissolved and entered the harbour water, causing the 

 death of a large number of fish belonging to many different 

 species. The occurience was reported in the Perth daily 

 Press, 



