30 FISH CULTUKB. 



DuUiii.1 The best way, therefore, to meet this evil 

 is to boil the gravel for about an hour and then to 

 wash it thoroughly in a sieve before depositing it 

 in the boxes. 



There is also another very fatal cause of destruction 

 to ova in boxes — viz. from the confined nature of the 

 boxes any alluvial deposit which may be held in the 

 water is much more easily precipitated than in the 

 open bed of the stream itself, and a coating of this is 

 often fatal to the embryo in the egg. To avoid this, 

 even in the clearest streams, it is advisable to have a 

 long and soft-haired brush and to gently sweep the 

 gravel and ova in each box over every morning, so 

 that any deposit may be disturbed and carried away 

 by the stream. Even in the clearest streams, which 

 never mud or thicken with rain and which may come 

 from springs at no great distance, there are often 

 matters held in solution which will deposit upon the 

 ova, and from this deposit a conferva grows. Now, 

 when the temperature of the water is low, let us say 

 below 40°, this is not of so much consequence, as it 

 can easily be disturbed and removed; but as the 



' Mr. Brown, in his admirable little work on the " Natural History 

 of the Salmon, "as elicited by the Stormontfield operations, also bears 

 testimony to the great destruction caused amongst the ova by the 

 larva of the may -fly and by certain water-beetles. 



