FISHERY BULLETIN: VOL. 75. NO. 4 



nature of the Transition Zone and its frontal 

 boundaries became evident late in 1973 and per- 

 sisted throughout 1974. 



ALBACORE CATCHES IN 

 RELATION TO OCEANIC FRONTS 



Graphical depictions of the frontal gradients 

 that form the boundaries of the Transition Zone 9 

 and standardized albacore catches for June of each 

 of the three surveys are shown in Figure 9a-c. This 

 figure indicates that the catches were largely 

 made within the Transition Zone in all 3 yr. Dur- 

 ing June 1972 and 1973, productive centers of 

 fishing, indicating high relative abundance of al- 

 bacore, developed in the Transition Zone between 

 lat. 33° and 35°N and west of long. 135°W (Figure 

 9a, b). These centers persisted for several weeks 



^he temperature and salinity fields measured by the research 

 vessel, augmented by the XBT data collected by the charter 

 fishing vessels, were analyzed to delineate the frontal gradients. 



before fishing effort was ended. In these years, the 

 frontal structure was strongly developed and the 

 Transition Zone easily identifiable. During June 

 1974 when the frontal structure was poorly de- 

 veloped and water mass boundaries were less dis- 

 tinct, catches were distributed over a larger range 

 of latitude and longitude (Figure 9c). Overall 

 catches in 1974 were substantial but they were not 

 persistent in any area for more than a few days. 

 Thus, while albacore were still associated with 

 Transition Zone waters, the influence of extensive 

 lateral mixing between water masses and the dif- 

 fuse nature of the boundary frontal structure ap- 

 parently failed to concentrate fish in a given loca- 

 tion for periods of time as had apparently occurred 

 in the previous 2 yr. 



While graphical depictions of frontal structure 

 outline the location of the boundaries associated 

 with Transition Zone water (Figure 9a-c), they do 

 not indicate the intensity of the gradients of the 

 frontal structure. The frontal structure has been 

 shown generally to have weak gradients during 



48 e 



45° 



150° 



145° 



140° 



135° 



130° 



120° 



115° 



40° 



35' 



1 — I — I — T 



-i — r 



X * 



. * • 



30° 



25' 



'. r 



X X o 



48° 



45" 



40° 



35° 



50* 



25° 



150° 



14 5° 



140° 



135° 



.20° 



115° 



FIGURE 9. — Albacore catch per 1 50 line-hours by American Fishermen's Research Foundation charter vessels 

 and locations offronts delineating Transition Zone waters during: a. 1-30 June 1972; b. 1-30 June 1973; c. 1-30 

 June 1974. 



SOS 



