of the ocean, flows along the coastal area 

 towards the Equator. This current is cool, 

 but not cold, and transports cool water 

 well into the tropics, where it turns off- 

 shore and the cycle repeats itself with the 

 current again crossing the ocean in the 

 equatorial area. 



This circulation gives rise to sub- 

 stantial differences in the general condi- 

 tions on the western and eastern sides of 

 the oceans. The western sides are charac- 

 terized by a very broad belt of tropical 

 water and a very narrow belt of temperate 

 water with rapid transition to the cold 

 waters of higher latitudes. The eastern 

 sides, however, are characterized by a 

 very broad belt of temperate water and a 

 very narrow belt of tropical water. These 

 differences affect the thickness of the 

 upper mixed layer in which there are virtu- 

 ally no vertical temperature differences. 

 Generally in the tropical areas in the 

 eastern portion of the oceans this layer is 

 quite thin, from 50 to 200 feet thick, with 

 a layer of cold water lying immediately 

 below. Conversely, on the western side of 

 the ocean the upper mixed layer is quite 

 thick, frequently over 1*00 or 500 feet. 

 Figure 2 is a cross section along the 

 Equator of the Pacific Ocean from west to 

 east, showing the relatively greater thick- 

 ness of the mixed layer of warm water on 

 the western side. 



Seasonal warming and cooling of the 

 northern and southern portions of the 

 oceans also influences the depth of the 

 upper mixed layer. In the northern summer 

 months the northern waters warm up, forming 

 a shallow warm layer over the surface of 

 the sea (figs. 3 and h, page k) . As winter- 

 approaches and temperatures fall, this 

 mixed isothermal layer becomes deeper but 

 colder (figs. 5 and 6, page 5). The same 

 process takes place six months out of phase 

 in the Southern Hemisphere. In effect the 

 depth of the mixed layer changes seasonally 

 in temperate latitudes, becoming shallow 

 and warm in summer and deep but colder dur- 

 ing the winter, but the seasonal pattern is 

 less pronounced in the tropics (fig. 7, 

 page 6) . The effect of a limited tropical 

 situation on the eastern side and an ex- 

 tended one on the western side in the sum- 

 mer seems to have an important influence 

 on the distribution of the tunas and their 

 fisheries. Attention is drawn to the 

 marked seasonal change in sea surface tem- 



perature in North Western Pacific as con- 

 trasted with those in the North Eastern 

 Pacific (fig. 7). 



The areas which have a shallow warm 

 water layer, (fig. 8, page 6) the eastern 

 margins of the tropical oceans, are the only 

 ones where major live-bait fisheries for 

 yellowfin exist and where purse seining as a 

 method for harvesting tropical tuna seems 

 successful. 



, ' 7 , ' ? ' ■ ' i ) ' 



VERT TEMP. I T I DISTRIBUTION ALONG EQUATOR 

 ( THERMOCLINE DEPTH: 



inr iff 



ntr ire- ire- »«■ 'W w 'M* 



Figure 2. — Cross section of the Pacific 

 along the Equator showing greater 

 thickness of mixed warm water above 

 the ttermocline from vicinity of the 

 Line Island westward (scale of depth 

 in feet at right). 



DISTRIBUTION OF TUNA 



Essentially, what we know of the dis- 

 tribution of tunas comes from the location 

 and volume of tuna catches. The character- 

 istic of the fishery therefore affects the 

 picture we have of the distribution and 

 this should be kept in mind. Nevertheless, 

 it is apparent that in general the distri- 

 bution of the tunas falls within certain 

 temperature limits and that they tend to be 

 more abundant in regions of high biological 

 productivity. 



Defining distribution in terms of 

 these and other environmental features is 

 not yet practical, however, because so 

 little is known of the specific habitat 

 requirements of the fish. Distributions 

 can only be described at present on the 

 basis of catch data. 



