FRESHWATER FISH EPIDEMICS IN QUEENSLAND RIVERS. 203 



noids — South India] ; Nishikawa, 1901 [Gonyaulax — Japan]. 

 Taylor (1917) rejects these as being a likely cause of certain 

 epidemics in Florida waters, one being previously reported b}- 

 Ingersoll (1882). 



5. OVERSTOCKING. 



Overstocking is commonly regarded as being the cause. 

 Owing to dry weather, the rivers and waterholes shrink con- 

 siderably, and as a consequence there is much less water and 

 food for the fish which come to occupy the restricted areas. 

 It seems likely that overstocking may be a contributing factor 

 to the epidemic. The increased number of fish would use up 

 more food and oxygen and liberate more CO.2, wliich would 

 lead to ill effects unless there were increased plant activity 

 to preserve the balance and so prevent the water from 

 becoming more and more acid. Besides, should the real cause 

 of the epidemic be some protozoan, fungoid, or bacterial 

 organism, then the greater density of the fish population 

 would be favourable to the rapid spread of the disease. 



6. PARASITISM AS A POSSIBLE CAUSE. 



It is well loiown that organisms may cause serious 

 epidemics. Animal parasites likely to be incriminated may be 

 (a) Helminths, and (6) Protozoa such as (i) Sporozoa (Myxo- 

 sporidia and Microsporidia), or (ii) Flagellates, perhaps Cihates. 

 Amongst plant parasites one must consider (c) Fungi and {d) 

 Bacteria. 



(a) Helminth Infection. 



The cyanosed appearance of infected fish suggested the 

 possibility of gill parasites, especially Heterocotylean Trema- 

 todes, being a cause, through their interference with normal 

 gill function. It is known that infection is sometimes extremely 

 heavy, and Ward (1918, p. 374) has stated that Gyrodactylus 

 may be in sufficient numbers to cause death. McCallum (1915, 

 p. 410) has reported that infestation by Diplectanum may be 

 so intense as to bring about the death of its host. Pratt (1919, 

 p. 2) has also referred to gill-infesting trematodes as a cause of 

 considerable mortality, especially in enclosed bodies of water. 

 Magath (1917, p. 59) in a paper dealing with a fluke, Lissorchis, 

 which was regarded as causing heavy losses amongst certain 

 fish in Iowa, U.S.A., mentioned that some parasites so weaken 

 their hosts that the latter may die from some cause which 

 would otherwise not have so affected them. 



