37 



civilization as to be unsuitable not only for the delicate ova 

 and fry, but also for mature Salmonidae. ' 



River water, however, if unpolluted and not overcharged 

 with sediment, can generally by an expensive system of 

 settling tanks be rendered safe for hatching purposes, and 

 the alevins from ova incubated in river water in Scotland 

 commencing to feed at a late period in the spring obtain 

 much natural sustenance. Filtered water cannot be used 

 with good results in a hatchery. The best filters deprive 

 water of all animal life, while inferior filters cannot be run 

 for six months continuously without great risk either of 

 their fouling or passing an uncertain supply, especially 

 where they have to contend with leaves and heavy frosts. 

 Spring water is more equitable in temperature, generally 

 free from sediment, and not liable to rapid fluctuation in 

 the supply, but as the ova hatch out earlier the fry are 

 entirely dependent for some months on artificial food. 

 Taking everything into consideration, spring water should 

 be used for the permanent supply to the hatching house, 

 care being taken that it contains nothing in solution dele- 

 terious to the embryo, and of this, experience is the only 

 safe guide ; but it is very important that a supply of river 

 water for summer use be also laid on, otherwise the fry 

 must be removed from the trays four or five weeks after 

 they commence feeding. 



TEMPERATURE. 



The average temperature of the water during the period 

 of incubation fixes the length of that period, which varies 

 in Scotland from 70 to 150 days. I formerly considered 

 41 Fah. as the best average temperature, Salmon eggs 

 hatching in 97 days, while fungus does not grow readily, 

 byssus taking from three to four days to generate to a 



