188 DURATION OF THE HATCHING PROCESS. 



As some of the ova are deposited and impreg- 

 nated ten or more days sooner than other portions, 

 we must expect to see the incubating progress 

 completed by degrees. Such, in fact, is the case. 

 The evolving of the fish-foetus is not simultaneous, 

 but gradual, and the infant fry come out from 

 their sand-bed by degrees; at intervals of time 

 corresponding with the intervals that took place 

 during the deposition of the ova. We may, there- 

 fore, have young fish from one and the same nest, 

 differing in age from one to ten or fourteen days. 



The length of time necessary for the comple- 

 tion of the incubating process varies according to 

 the localties of rivers, because locality produces 

 different temperatures. The temperature of river 

 water is also very sensibly affected — heightened 

 or diminished by the mildness or rigour of the 

 season. In the rivers of the north of Scotland 

 salmon ova are hatched in a period varying in du- 

 ration from 100 to 140 days.* In conformity 



* Mr. Young, in his pamphlet, mentioned at page 160, 

 says, — " The ova, after being deposited, progress very 

 slowly, particularly during the winter season, when the 

 temperature of the water is low ; as the length of time be- 

 tween deposition and hatching depends entirely on the heat 

 or cold when the seed is lying amongst the gravel. Ova 

 deposited in September, when the temperature is high, will 

 produce fry in the course of ninety days, whilst those de- 



