5 
civilization as to be unsuitable not only tor 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 ina 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 
