GEOGRAPHIC DISTRIBUTION OF THE PACIFIC SALMON 
679 
temperature data given in figures 2 and 3 it is obvious that the optimum range for 
growth in the sea must shift to a lower level. The mean annual temperatures of the 
ocean waters at 200 meters throughout almost the entire distribution of the salmon do 
not exceed 8° and in the northern half of the range they fluctuate close to 3° C. Even 
if the salmon frequented only the surface waters during their sojourn in the sea they 
would encounter temperatures of less than 15° C. throughout the greater part of the 
year, for it is only during the summer that the surface waters reach this temperature 
Figure 4. — The geographic distribution of the Pacific salmon and the mean annual surface salinities in parts per 1,000. The bars 
indicate the native distribution of the salmon, the solid black areas indicate the regions in which the salmon have been trans- 
planted successfully, and the dots indicate the regions in which the transplantations were unsuccessful. The isohalines give the 
mean annual surface salinities in parts per 1,000. 
and then only in the southern part of the range. In the northern part of the range 
the temperature of the surface waters at no time exceeds 10° C. Hence, the salmon 
must become acclimated to the colder waters of the ocean during their sojourn in them, 
and are able to grow and survive at lower temperatures than in fresh water. 
Since the salinity of the water forms an important environmental component of 
marine habitats, it was likewise studied in relation to the distribution of the salmon. 
Although salinity data for the subsurface waters would have been more desirable it 
was found that only surface data were available for all of the oceanographic regions. 
The mean annual surface salinities in parts per thousand for the North Pacific are 
shown by the isohalines in figure 4. The data from which these isohalines were 
