258 SOME MINUTE ANIMAL PARASITES 



nated water in which the animal is living. Inbreeding 

 tends to intensify any weakness, and thus allows of 

 fiercer attack by the parasite. Again, among artifi- 

 cially hatched fish there is rarely the competition 

 or struggle for existence such as naturally hatched 

 fish undergo, with the result that weaklings survive 

 and even reach maturity, which would never occur 

 under natural conditions. Hence when outbreaks 

 of disease occur in artificially stocked waters, the 

 rate of mortality is much higher than among " wild " 

 fish. At the same time, should the epizootic occur 

 in the hatchery, there is the possibility of adopting 

 rapid disinfection measures and of spore destruction, 

 such as is hardly possible in the open. 



Every year the newspapers report cases of mys- 

 terious deaths among fish. Pollution from factories, 

 road tar, sheep dips, etc., are blamed. Disease is 

 not often suspected, and still less are any active 

 measures taken to suppress the outbreak. As before 

 mentioned, dead fish should be collected and burned 

 wherever possible. The drastic remedy of complet- 

 ing the destruction of the fish by draining off the 

 water and then exposing the bed to light and air 

 for some time, and also lime-dressing it, is most 

 effective, but is rarely practicable, on account of the 

 cost and labour involved. The netting of streams 

 and ponds and destruction of all suspects are far from 

 being so difficult as drainage, nor are they so efficient, 

 but they are very useful in combating any outbreak. 

 Chemical treatment, such as spraying with petrol 

 or paraffin, may help in destroying myxosporidian 

 spores ; but the amount of damage done to the larvae 



