LAURS and LYNN: SEASONAL MIGRATION OF THUNNUS ALALUNGA 



200 



P 



w 400 

 uj 



u. 



i 600 

 I- 

 a. 

 ill 



Q 



800 

 1000 



31 30 29 28 



1972 



27 26 25 



mm' 



Along I37°30' W. 



tr 



100 £ 



200 



d300 



3I°N 



32°N 



38°N 



39°N 



40°N 



200 - 



400 



600 



a 



800 



1000 



3I°N 



32°N 



33°N 



34°N 



35°N 36°N 



LATITUDE 



37°N 



38°N 



1973 



Along I37°30' W 



39°N 



40°N 



200 <?; 



300 



4I°N 



200 



400 



600 



a. 

 a 



800 



1000 



>4/0/?S? /J7°J(?' J*' 



200 



300 



32°N 



33°N 



34°N 



35°N 36°N 



LATITUDE 



38°N 



39 <> N 



40°N 



4I°N 



FIGURE 8. — Vertical sections of temperature and salinity along long. 137°3G" W during June 1972, 1973, and 1974. Low salinity water 

 (<33.8%o) indicative of Subarctic water is hatched and crosshatched. High salinity water (>34.2%o) indicative of Central water is 

 shaded with a dot pattern. The 58° and 62°F isotherms are shown by heavy dashed lines. 



than 150 m. A temperature gradient on the order 

 of 0.6°C in 13 km was often found to mark these 

 fronts at the sea surface. At other times, however, 

 seasonal heating in the surface layer eroded the 

 horizontal temperature gradient at the surface. 

 Mixing was evident in the Transition Zone in 1972 

 with low-salinity water penetrating southward 

 and some high-salinity water northward at inter- 

 mediate depths. 



Oceanographic conditions were different in the 

 region of the Transition Zone in 1974 from those 

 which were observed in 1972 and 1973. In 1974, 

 boundaries of the Transition Zone were poorly de- 



veloped and broken. Salinity gradients were dif- 

 fuse and changes in depth of the isotherms gradual 

 and variable in the regions of the Subarctic and 

 Subtropic fronts. The Subarctic front was virtu- 

 ally nonexistent and Transition Zone waters 

 graded gradually into Subarctic waters. The Sub- 

 tropic front was weak and spread between lat. 

 31°30' and 33°30'N. Saur 8 found that the diffuse 



8 Saur, J. F. T. 1976. Anomalies of surface salinity and temper- 

 ature on the Honolulu-San Francisco route, June 1966-June 

 1975. NORPAX Highlights 4:2-4. (Unpubl. rep.) 



807 



