chiefly at Misaki in Kanagawa Prefecture^ and vessels based in the 

 outlying areas were husily engaged in fishing in these groundso 

 Elsewhere vessels based in Formosa were active in the South China 

 Sea, the Sulu Sea, the Celebes Sea, and the Bast Philippines Seao 

 It gradually became known that, in addition to tha grounds under 

 exploitation, yellowfin, big-eyed tuna, and the spearfishes ware 

 densely distributed in various other southern sea areaso 



In order that a given sea area may be considered a tuna 

 fishing ground, it is not enough that there merely be biological 

 evidence of the presence of tuna there„ It need hardly be said that 

 in order to have significance as a fishing ground actual operations 

 must be carried on in the area and the results must be a catch at 

 least sufficient to pay expensesc The expenses of operating a 

 fishing enterprise may in general be thought to be proportional go 

 the number of days spent per voyageo The more distant the fishing 

 ground, the higher the expenses mount j as the number of days required 

 to go and come between the base and the grounds increases, the 

 number of days on which actual fishing is possible decreasesc It 

 follows that the farther the fishing ground, the higher the catch 

 rate must be in order to pay expenseso Since the capabilities of 

 fishing boats are limited, unlimited extension of the fishing 

 grounds is naturally impossible under the present system of single- 

 vessel operationso 



The fishing season, needless to say, is the season or period 

 during which fishing is carried on within a given sea areao Since 

 fishing seasons are mainly controlled by seasonal changes in 

 oceanographic conditions, they are generally the same from year 

 to yearo 



The tuna longline fishery in Japan is operated as a complement 

 to the skipjack fishery, so, in general, the fishing season is almost 

 limited to the wintero Actually in most cases it is a matter of 

 vessels which engage in the skipjack fishery from spring to autumn 

 turning to the tuna longline fishery during the winter. This does 

 not, however, indicate that the tuna are most densely distributed 

 during the winterj it should be considered chiefly as a phenomenon 

 controlled by the distribution and migration of the skipjacko 

 Except for albacore and the big-eyed tuna, the distribution of the 

 tunas and the spearfishes in the North Pacific, in general, is 

 denser in the summer than in winter„ As a consequence^ one can see 

 a tendency in recent years toward a gradual increase in the number 

 of vessels operating the year round in the tuna longline fishery. 



Both the fishing grounds and the fishing seasons are determined 

 by the distributional and migrational patterns of the tunaso There- 

 fore, if it were possible to know accurately in advance the times, 

 positions, and routes of these migrations, it should be possible for 

 the tuna longline fishery to become stabilized to a high degreeo 

 However, the factors which control these migrations are chiefly 

 changes in oceanographic conditions„ 



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