4 EIGHTH PACIFIC SCIENCE CONGRESS 



conditions of fertility occur for production of such food, and where 

 the cHmate of the sea is most suitable. In other words, the oceanog- 

 rapher must provide information as to the nature and extent of the 

 best pastures and the most desirable sea climate. With such oceano- 

 graphic information, expressed in the form of charts, the commercial 

 fisherman can function economically and efficiently. This oceano- 

 graphic method is in sharp contrast to the so-called historical or hunting 

 method. 



As an example of the oceanographic method, it is deemed desirable 

 to outline its application to recent tuna investigations. The tuna prefer 

 ocean waters where the temperatures are higher than 15° C. and the 

 salinity is greater than 32°/oo- The tuna also require a plentiful supply 

 of smaller fish on which to feed. These small fish are plankton feeders 

 and are found in regions of upwelling near the continental coasts, or 

 around islands, or in some of the ocean currents. These regions of 

 upwelling supply the necessary nutrient material which is essential to 

 plankton growth. Tuna are found in the broad tropical belt of the 

 ocean and can be expected to migrate into some parts of the temperate 

 regions during the summer months. The search for tuna is narrowed 

 to salubrious climates which are well stocked with plankton feeders. 

 Thus the assumption may be made that if the conditions suitable for 

 plankton are known in the warmer seas, there tuna will be found. 

 Hence the oceanographic conditions of the plankton are studied in order 

 to find the tuna. 



In this oceanographic approach some progress has been made. It 

 is known that the basic foods are the respiratory elements, oxygen and 

 carbon dioxide and the various nutrient salts such as nitrates, phos- 

 phates, silicates, etc. 



The respiratory elements are plentiful everywhere in the upper 

 100 meters of the ocean, but the nutrients are often depleted, except 

 in the regions of upwelling, where they are replenished from the cool 

 depths. Physical oceanographic stvidies indicate that such regions occur 

 along the coast lines, and around the islands. Here the phyto-plankton 

 is plentiful, and provides food for the zooplankton, which in turn feeds 

 the small fishes on which the tuna feed. Evidently tuna should occur 

 in the warm waters on the lee side of coasts and in the vicinity of up- 

 welling in the equatorial currents. This has been found to be generally 

 true. However, they also occur in many parts of the open ocean where 

 plankton and the plankton feeders are sparse, and so the simple theory 

 of the food chain is not the only explanation of the fishery. 



Recently it has been learned that great concentrations of plankton 

 exist in the cold water below the thermocline, over all the known seas. 

 This plankton layer is deepest in the tropics and shallowest in the arctic 



