11)4 PROCEEDINGS OF THE ROi'AL SOCIETY OF QUEENSLAND. 



Under this heading one may refer to the presence of 

 organisms, wliether plant or animal, which could act mechani- 

 cally by interfering with the passage of water through the 

 fish -gills. 



Reference was made to the suggestion that overgrowth 

 of water-weeds, including '"' umbrella grass," might cause 

 trouble. It is to be pointed out that weeds grow only under 

 certain conditions of light, depth, etc., and in the case of rooted 

 plants form only a fringe around deep lagoons. Sorhe of the 

 waterholes in which the epidemic occurred were many miles 

 long and up to 30 or more feet in maximum depth, the greater 

 part of the lagoons being too deep for rooted plant growth. 

 Abundance of green water-vegetation improves aeration, 

 though one must admit that organic decomposition results in 

 the using up of oxygen and the liberation of carbon dioxide 

 and other gases. Moreover, the presence of abundant decom- 

 posing matter is associated Avith abundance of saprophytic 

 and sajDrozoic organisms — e.g. fungi, bacteria, and certain 

 protozoa — all of which are using up oxygen instead of 

 liberating it. 



Mr. Colbert referred to the deep-green colour of the water 

 in his locality, Lake Nash, during the periods in wliich the fLsh 

 epidemic occurred. The colour suggests that phytoflagellates 

 A^ere present in enormous numbers. It is not impossible 

 that they might set up some irritation of the gills and become 

 entangled in the mucus produced, and thus lead to partial or 

 complete suffocation. But the presence of this intense coloura- 

 tion does not seem to have been a constant feature of the 

 outbreaks, and suggests to us that the stagnant conditions 

 allowed the organisms to grow at a much more rapid rate than 

 they were being devoured by the various other organisms 

 present. As a rapid decomposition of these might cause the 

 liberation of toxic substances, the matter will be referred to 

 later, under the heading of poisons as possible causes. 



(6) Effect of Alterations in Chemical Composition of 



Water. 



An analy.sis of samples of Longreach water, one from the 

 Thomson River and the other from an adjacent billabong, was 

 made by the Government Analyst, Mr. J. B. Henderson, 



