OCEANOGRAPHY IN THE UNITED STATES 371 



find their way very quickly into the bays and estuaries where they 

 penetrate well into the marshes, or up the rivers often into fresh water. 

 How these delicate organisms perform such formidable migrations, 

 unless they are carried by currents, is difficult to understand. Yet if 

 they are at the mercy of ocean currents, which are known to vary in 

 strength and direction, they may at times be carried to unfavorable 

 areas and perish. The effect of currents on survival of shrimp, 

 menhaden, and other important resources has an important bearing 

 on the success of fishing at some later date. 



TEMPERATURE 



Our knowledge of the general distribution of surface water tem- 

 peratures in the oceans is reasonably good. The boundary between 

 what we call warm and cold waters falls somewhere near 55° F. In 

 the northern hemisphere in winter, this line runs from the South China 

 Sea through Japan to the northern California coast, and from Cape 

 Hatteras to Cape Finisterre in northern Spain. 



In summer this boundary shifts to a line from the southern islands 

 of the Kurile chain to southeastern Alaska, and from Newfoundland 

 to the northern coast of Norway. It is interesting to see how the line 

 falls much further north on the eastern sides of both oceans, especially 

 in summer, and how it corresponds in general with the paths of 

 Kuroshio and Gulf Stream. 



If we now look at the worldwide distribution of one of our most 

 important tunas, the albacore, we see how closely it matches the tem- 

 perature pattern. The northern and southern limits correspond very 

 closely to the positions of the 55° isotherm, and the blank space in 

 the tropical Pacific probably means that temperatures here are too 

 high for this species. Note how the northern limit in the Pacific 

 runs diagonally across the ocean, conforming very closely to the posi- 

 tions of Kuroshio, North Pacific drift, and Alaska current, and also 

 to the temperature distribution. The information on which this chart 

 was based comes mostly from records of the fishery, lience the irregu- 

 larities are not too significant. We know less about tunas in the 

 Atlantic, and this is why the pattern is less complete. 



Look now at the distribution of another important tuna, the yellow- 

 fin. This species is more tropical than albacore, and its northern and 

 southern limits are bounded by 70° or 75° F. temperatures. You will 

 remember that the current system takes warm water north on the 

 western sides of the oceans, and cold water south on the eastern sides. 

 This corresponds with the broad distribution of yellowfind tuna in 

 the western Pacific, and its absence in the eastern part, except for a 

 narrow coastal band. 



Look also at the known distribution of yellowfin tuna larvae based 

 on the records of oceanographic cruises. This tells us things about 

 this tuna that the commercial fisheries do not. Note particularly how 

 it adds to our knowledge in the Atlantic Ocean. Clues to the effect 

 of oceanographic factors upon tuna abundance must be sought by 

 surveying these vast ocean waters. 



We have a few other clues to the importance of temperature to com- 

 mercial fisheries. The movements of migratory marine animals 

 in and out of our coastal bays and estuaries, and their behavior and 



