TEMPERATURE (°C.) 



SALINITY (V..) 



North Lotitude ► 55* 



Oi • — * — * 



DISSOLVED OXYGEN (mg.ot/l.) 



WATER MASSES 



PACIFIC CENTRAL 



Figure 3,- Vertical sections of temperature, salinity, dissolved oxygen, and a schematic diagram of water masses 

 along 175° E. during summer 1958 (from Favorite and Hebard, 1961). 



vertical 4° c. isotherm which occurs at ap- 

 proximately lat, 46° to 48° N. denoted the 

 southern limit of salmon (as determined by 

 surface gill net catches) during summer. After 

 comparing salmon distribution during spring 

 with relatively permanent oceanographic fea- 

 tures, they proposed that the maximum 

 southern limit of salmon is determined by the 

 salinity front at lat. 41° N. (Indicated by 

 the almost vertical 34 °/oo isohaline in the 

 surface layer). The boundaries of these fronts 

 and the associated water masses are shown in 

 figure 2. Further subdivision of these water 

 masses, and a more detailed picture of the 

 water structure along long. 175° E. during 



1958 are presented in figure 3. The position 

 of the 34 °/oo isohaline in the mid- Pacific 

 during summer has been relatively constant 

 since at least 1955. However, because the 

 axis of the North Equatorial Current is known 

 to oscillate from lat. 9° to 11° N. in summer, 

 to lat. 6° to 7° N. in winter (Defant, 1961, 

 p. 569), winter observations were necessary 

 to determine if the proposed boundaries shifted 

 southward during winter. 



A significant feature of the water structure 

 southward of the Aleutian Islands is the 

 vertical intrusion of Deep Water, indicated by 

 the ridging of the isohalines(seefig. 3). During 



