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 affected 
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 
