THE SPEED OF MIGRATION OF SALMON 
591 
fish when it passes from the sea-water to the fresh water at the 
mouth of the river. 
Rutter^ has advanced the theory that the sahiion makes the 
journey through tide water by running up stream during the ebb, 
and down stream during the flood tide, that is, the fish stems the 
current during the flow of each tide. Rutter arrived at this con- 
clusion by following the variations in the catch of the fisheries 
at the different towns along the San Francisco Bay and Lower 
Sacramento River. The theories of Rutter explain in part the 
movement of salmon in tide water, but tidal currents are not 
sufficient alone to account for the movements. A much more 
important factor is the condition of water as regards the content 
of salt. Salmon are delicately responsive to stimulation by the 
variation in the degree of admixture of sea-water and fresh water 
in the tidal area. 
The present observations show a veiy much longer time spent 
by salmon in the tidal waters of the Columbia River than that 
deduced by Rutter for the Sacramento River. It is safe to con- 
clude that the salmon spend not less than thirty to forty days in 
passing the tidal area of the lower Columbia River. When once 
through the tidal area they make the journey up the river at an 
average speed of not less than seven and one-half miles a day. 
* Rutter: Bull. U. S. Bureau of Fisheries. 22, 122, 1902. 
