to gain knowledge of the tuna stocks and to maintain and improve 

 the stability of the f isheryo In the warm seaSj particularly, 

 where the water temperatures are always within the range habit- 

 able by the tunas, water temperatures, especially surface water 

 temperature, has very little significance and value as an 

 indicator of the distribution and fishing potentialities of 

 tuna^ For this reason it is considered necessary that close 

 attention be always given to changes in the position and 

 strength of the currents^ 



iiio Topography and fishing grounds 



^^Yhere there are shoals and sea mounts in the ocean, tuna 

 grounds often develop in their vicinityo Examples in Japanese 

 waters are the Kinan Reefs south of Shionomisakis the KoKo Bank, 

 named for the Kochi Prefecture research vessel Koho" Maru, and 

 many others in the vicinity of the Okinawa, Izu.c and Ogasawara 

 archipelagoes 



In places with this sort of topography one sees the 

 phenomenon of so-called resident schools ( setsuki) p and because 

 the schools remain in the locality j, fishing continues good over 

 long periods of time or at all timeso The following explanation 

 is given for this phenomenono 



In such places the sea water is stirred up and the water 

 from the lower levels, rich in nutrient salts, is pushed up to 

 the surfacec As a result the growth of plankton is stimulated 

 and fish are attractedo Or a sort of vortical current may be 

 formed which will concentrate the small animals upon which fish 

 feed, and therefore food will be always abundanto 



It is thought that the foregoing is quite a reasonable 

 explanation of the phenomenon of resident schools in the vicinity 

 of bankSp but there is some room for doubt as to whether or not 

 it exhausts the subjectc Problems of this sort require further 

 study in the ecology of fishes o IThatever the theoretical basis 

 may be, however,, it is a fact that in seas having this sort of 

 topography there are tuna grounds of a permanent and stable 

 character o 



Until recent years the notion that tuna longline grounds 

 were restricted to waters of this sort was strongly prevalentp 

 and there was a tendency for large numbers of boats to con-" 

 gregate to fish in limited areaso Furthermore, exploration for 

 new fishing grounds was largely a matter of searching for such 

 banks o 



Waters where islands occur in the migratory paths of the 

 tunas are also notable for their tuna fishing groundsc, There 

 is probably no need to cite examples or to explain the reasonso 



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