234 THE WATER OF ARTIFICIAL PONDS. 



of ponds constructed in salmon-rivers, or fed by- 

 water directly emanating from them. Both the 

 development of the fish in ovo and ah ovo depends 

 upon the temperature of the water, and we know 

 that a single frosty night will reduce by many 

 degrees the temperature of rills and rivulets, 

 whereas the currents of large rivers are little 

 affected by it. Fry hatched in ponds fed by these 

 hill-streams must be stinted in growth — kept in 

 a status quo during many weeks — and they can 

 never arrive at the smolt state in the same period 

 of time as fry produced and bred in the waters of 

 rivers. These latter fry are in their natural ele- 

 ment — natural in its temperature and in the food, 

 insects and so forth, it produces. On the contrary, 

 fry bred in ponds fed by springs or hill-burns are, 

 as it were, subjected to a different climate, strange 

 and unnatural to them, barren, or nearly so, of 

 insects, and foreign to their innate tastes. Their 

 progress in growth, therefore, cannot equal that 

 of fry bred in favourable localities. 



When the ponds are perfectly formed and con- 

 structed they should be filled with water, and it 

 should be allowed to run freely into and out of 

 them for a few days previously to depositing the 

 spawn in them. This is necessary in order that 

 the newly laid gravel may be washed well, the 



