Tuna are at best four or five steps 

 above phytoplankton in the food chain and 

 are, therefore, likely to constitute less 

 than a few percent of the total mass of 

 animal life in the sea. As tuna grow they 

 shift their position along the food chain, 

 feeding generally on larger predaceous 

 fishes and squid, which in turn implies that 

 the total weight of large tuna which the sea 

 can sustain is less, and it may be much less, 

 than the total weight of the smaller sizes. 



This aspect of the relation of tuna 

 sizes in food chains is mentioned since it 

 may be of interest in the consideration of 

 yields from longline fisheries, which seem 

 to fish the larger tunas rather selectively. 



QCEANOGRAPHIC FEATURES 

 AND TUNA HABITATS 



Let us consider the major oceanogra- 

 phic features of the oceans in relation to 

 tuna habitats. First let us review the 

 major current systems of the oceans of the 

 world (fig. 1). In the equatorial regions 

 of the Atlantic and Pacific there is a 

 westward transport of water north and south 

 of the equator by the North and South Equa- 



torial Currents. As these currents approach 

 the western boundaries of the oceans they 

 turn poleward, skirting the eastern shores 

 of the continents, thereby transporting large 

 large volumes of relatively warm water into 

 temperate latitudes. In the Northern Hemi- 

 sphere between 30° and 1jO°N. these currents 

 leave the continental shores to flow in a 

 northeasterly direction. Southerly-flowing 

 cold currents, the Oyashio in the Pacific 

 and the Labrador Current in the Atlantic, 

 join and mingle with the Kuroshio and Qulf 

 Stream as they carry the water back again 

 to the eastern sides of the oceans. Proc- 

 esses of mixing and cooling change the 

 characteristics of these currents, so that 

 when they reach the western shores of the 

 continents they are no longer tropical, but 

 cool and temperate. Here again the currents 

 divide, in the Pacific forming an eddy in 

 the Gulf of Alaska and the southward flowing 

 California Current, in the Atlantic penetrat- 

 ing as far north as Spitzbergen and the 

 Norwegian coast and also forming the Canary 

 Current off Spain and the west coast of 

 Africa. These currents are cool, but not 

 cold, and transport cool water well into 

 the tropics, where they swing offshore 

 once 'again in the equatorial region, com- 

 pleting the cycle. 



Figure 1. — Ocean current systems. 



