SALMON AND TROUT CULTURE. 437 



hatching apparatus itself. Storage tanks or reservoirs will still be 

 necessary in most cases, as the water should not be taken from 

 a very great pressure. Fish are as frequently killed by water 

 that is too highly charged with air as from a scarcity of water. 



It is perfectly possible to hatch a limited number of eggs 

 in a few oyster shells, a soup plate, or a frying-pan I was 

 almost saying an old hat by which I would convey that the 

 material of which hatching troughs are constructed is not of so 

 much importance as the treatment of the ova placed in them. 

 The most important thing is the water-supply ; if this deposits 

 much sediment, filters must be used. 



There are different opinions as to the structure of hatching 

 troughs. Whilst some prefer the 'glass grille' system, others 

 find slate or metal troughs, wire gauze, or perforated zinc trays 

 answer equally well. The glass grille system appears to me 

 extravagant only as far as space is concerned. Lord Lauder- 

 dale says that in his water this system produces stronger fish. 

 The first cost of the glass is certainly greater, but it entails no 

 annual expense, and has the advantage of being an almost perfect 

 non-conductor of heat or electricity, the latter being an impor- 

 tant factor in hatching, and as yet insufficiently studied. With 

 the water I employ, perfectly healthy young fish are produced 

 by the other methods. 



The ' slate trough ' plan also loses much space, as it is not 

 advisable to place the ova in more than one layer during the 

 period of incubation. 



The perforated tray system, with the 'underflow' supply, 

 will bring on very many more eggs in the same area. To 

 make a comparison upon a superficial area of one inch, twenty 

 to twenty-five ordinary sized trout ova will lie ; therefore to 

 hatch, say ten millions of eggs on grilles, or on the bottom of 

 troughs, an area of over 300 square yards is required (in reality 

 500 square yards would be necessary). In the perforated tray 

 . system, eggs may safely be placed ten or fifteen deep ; the 

 space required to bring on ten millions of ova by this method 

 would thus be but a tenth or a fifteenth part ; but as there is 



