and they will be at the maximum density the 

 environment can support. When a fishery is 

 initiated, fishing will be relatively good. 

 As time passes, the proportion of large, 

 old fish will drop and the catch per amount 

 of fishing per unit of time will also drop. 

 At some point in this trend, added fishing 

 effort will not produce a greater total 

 catch, and biologists generally consider 

 that when this happens, the fish population 

 has been depressed below the maximum sus- 

 tainable yield. 



Since detailed data covering the tuna 

 fisheries of the Eastern Pacific are readily 

 available, I wish to address myself primarily 

 — although briefly — to the evidence available 

 from the far-flung Japanese operations. As 

 we have stated, the Japanese have now cir- 

 cumnavigated the world in search of tuna. 

 Except for their home island fishery, these 

 operations essentially involve the use of 

 long-lines. Long-line gear is rather selec- 

 tive to large, old tuna, particularly yel- 

 lowfin and bigeye and this fact must be kept 

 in mind in evaluating, even in a limited 

 sense, the catch rate of the Japanese fish- 

 eries. The large sub-surface tuna involved 

 in long-line operations are relatively 

 scarce, and fishing of these old fish - while 

 affecting the subsequent harvest of these 

 particular size groups and therefore the 

 future success of long-line operations - will 

 not necessarily have a pronounced impact on 

 the total tuna population being exploited. 



Before proceeding, it is well to consider 

 briefly why the Japanese use long-lines in 

 distant waters. The Japanese have shown 

 themselves to be generally skillful in a 

 variety of fishing operations including 

 the large home island live-bait fisheries. 

 Their use of long-lines in distant fisher- 

 ies would appear to be clearly a function 

 of the logistics of that type of operations 

 as compared with such methods as live-bait 

 fishing. Obviously, the Japanese are well 

 aware that their long-line gear is tapping 

 only a part of the population of yellowfin, 

 bigeye and perhaps other species, but they 

 have thus far not found it feasible to ex- 

 tend significantly their live-bait opera- 

 tions. With this in mind, let us consider 

 what is happening to the catch per 100 hooks 

 in some of the Japanese fisheries. 



In order to closely examine certain 

 catch rates, those for yellowfin in the 

 Indian Ocean were studied by dividing the 

 area into increments of longitude and the 

 average catch rates for each such north- 

 south strip (5 degrees N to 13 degrees S) 

 considered separately. Deviations from 

 these several average catch rates were 

 plotted for each month of the years for 

 which data were available (fig. 22) . Por- 

 tions of the chart above the respective 

 average lines represent above-average 

 catches and portions below these lines 

 indicate below-average catches. In virtu- 

 ally every instance, initial catches were 



CATCH OF ALBACORE, YELLOWFIN, AND SKIPJACK 

 BY PRINCIPAL COUNTRIES, 1957 



WORLD TUNA CATCH BY SPECIES 



IN Mil HON 

 OF POUNDS 



500 1 



300 



200 



100 ISO 



M MIUON 0* »OUNW 



KEY 



RS3 ALBACORE 

 ESO SKIPJACK 

 f^YELLOWFIN 

 iHOTHER TUN A 



■ BONITO 



1948 



1953 



1957 



Figure 20. — Catch by principal countries 

 (1957) of albacore, yellowfin and 

 skipjack. 



Figure 21. — World tuna catch by species 

 (1948, 1953, 1957). 



31 



