THE SALMON AND WATER TEMPERATURE 145 



In the summer of 1906 an experiment was made 

 at my request in the laboratory of the Fishery 

 Board at Bay of Nigg, near Aberdeen, to test the 

 length of time sea lice will remain attached to those 

 summer fish which so rapidly ascend in the compara- 

 tively warm conditions referred to. Two grilse were 

 obtained from a bag-net in the Bay of Nigg, and 

 placed in sea water in a tank of the hatchery. The 

 temperature of the water was 52"9° (11*6° C). Each 

 fish had attached to it a number of sea lice. The 

 density of the water was then reduced by allowing 

 fresh water to enter. This operation was regulated 

 so as to represent approximately in time the period 

 of a single flood tide. The fish showed considerable 

 distress at first, from which it is natural to suppose 

 that the transference to brackish water was too 

 rapid, and that in all probability an interval of some 

 twenty-four hours or so — as had originally been 

 intended — would have been more natural. 



The sea lice may also have been adversely afiected 

 by the rapid transference in eight hours to pure 

 fresh water, but in spite of this some of the sea lice 

 remained attached for six days. Dating from the 

 time when the water was quite fresh, the sea lice 

 remained on one fish for four days and on the other 

 for five days. The fresh water temperature was 55° F. 



The constant action of the current of a river 

 under natural conditions of a fish's ascent may cause 

 the parasites to drop off somewhat sooner, but the ex- 

 periment shows that a fish taken in upper waters with 

 one or two sea lice attached may have occupied at 

 least three or four days in its passage from the tide. 



K 



