I70°E 



180° 



I70°W 



160° 



LONGITUDE 



140° 



Figure 3. --Seasonal variation in the northern limits of the Transition Zone (dashed 

 line) as defined by the abundance of Sagitta elegans or Eukrohnia hamata . 

 The symbols indicate the abundance of the species in that locality. 



In general, the chaetognaths and pteropods 

 characteristic of the Subtropic Zone centered 

 about water types with temperatures of about 

 76°F. (24*C.) and salinities of about 35%o cor- 

 responding to the Pacific Central Water mass, 

 those of the Transition Zone at temperatures of 

 about 71 °F. (22 "C.) and salinities of about 34%o 

 corresponding to the North Pacific Current, and 

 those of the Subarctic Zone at temperatures of 

 about 58*F. (14°C.) and salinities of about 33%o 

 corresponding to the Subarctic Water mass. 

 Figure 4, for the two meridians, shows that the 

 change in temperature-salinity values between 

 the Subtropic and Transition Zones was a grad- 

 ual one while the change between the Transition 

 and the Subarctic Zones was quite abrupt for the 

 180" meridian and less so for the 157°30'W. 

 meridian. These same features were generally 

 evident in the chaetognath distribution (fig. 5), 



thus demonstrating that the differences between 

 regions defined on the basis of fauna were also 

 evident in the characteristics of the water. On 

 the basis of the T-S diagrams and the chaetog- 

 nath data, the southern boundary of the Sub- 

 arctic Zone is defined more sharply on the west- 

 ernmost longitudes (180°) than to the east 

 (157°30'W.), agreeing with Sverdrup et al. 

 (1942: 143). 



Relation to Surface Albacore Distribution 



Initial study of troll and gill-net catches of 

 albacore in the North Pacific in relation to the 

 zones established on the basis of the chaetognath 

 and pteropod distribution indicated that these 

 fish were found in all three zones during the 

 periods covered by our data. In the summer of 

 1955, albacore were caught in the Subarctic and 



