676 
BULLETIN OF THE BUREAU OF FISHERIES 
flows southward along the island. The southern limit of occurrence of the Pacific 
salmon on this shore of the island is at Cape Inuboye near the point of confluence of 
these currents. A similar relation exists between the distributional limits of the 
salmon along the shores of the Japan Sea and the confluence of the Japan and Okhotsk 
Sea Current. The Okhotsk Sea Current, which is of arctic nature, flows southward 
along the continent and is dissipated in the waters off the northern shore of Korea. 
The Tumen River, which marks the distributional boundary of the salmon in this 
region, flows into these waters. After entering the Japan Sea the Okhotsk Sea Cur- 
rent influences the Japan Current flowing northward along the west coast of Honshu 
Island. The salmon are found on this shore of the island as far south as the Jogan- 
jigawa River which is near the southern point of confluence of these currents. There 
is a definite correlation between the distribution of the salmon and the influence of 
ocean currents in these regions. 
The Japan Current, after encountering the Bering Sea Current off the east coast 
of Honshu Island, takes a northeasterly course across the Pacific to form a fan-shaped 
divergence commonly known as the West Wind Drift. During this course it is 
greatly tempered. Upon nearing the coast of North America, off Vancouver Island, 
it divides into two branches; the northern branch forming the Alaska Current and 
the southern branch the California Current. According to McEwen (1912), an upwell- 
ing of cold waters along the coasts of Oregon and California influences the California 
Current as it flows southward. The southern distribution of the salmon on the North 
American continent appears to be correlated with the region dominated by this cold 
upwelling current. 
The adult salmon are subjected to the influence of surface temperatures in the 
ocean during their spawning migration to the streams, for it is definitely known that 
they frequent the surface waters at this time. Accordingly a study was made of the 
mean surface temperatures from June through September in relation to the distribution 
of the salmon. The mean temperatures rather than the limits of temperature during 
this period were used owing to the continuous character of the spawning migration 
which in some areas extends over the entire period from June through September. 
The mean surface temperatures in the North Pacific during this period are given 
by the isotherms in figure 2. These isotherms were determined from the seasonal 
and monthly surface temperatures given by Dali (1880), Rosse (1881), McEwen 
(1912), Uda and Okamoto (1930), Uda (1931), Kolcubo (1932), Zeusler (1934), and 
Schott (1935). Along the coasts of Japan and Korea the mean 20° C. isotherm 
touches the shores near the southern limits of the native range of the salmon. Accord- 
ing to Uda and Okamoto (1930), and Uda (1931), the surface temperatures of these 
coastal waters range from 15° C. in June to approximately 24° in September. On the 
North American continent the mean 15° C. isotherm touches the shores of California 
below the southern limits in the distribution of the salmon. According to Schott 
(1935), the surface waters along the coast of California have an annual range of only 
3° C. 
The northern distribution of the salmon on both continents is bounded by the 
mean 5° C. isotherm. However, the salmon migrating to and from the Mackenzie, 
Lena, and other streams tributary to the Arctic Ocean may be subjected to surface 
temperatures only a few degrees above freezing. It is, therefore, possible that the 
