FISHERY BULLETIN: VOL. 70, NO. 3 



I 



(£ 

 Ul 



a. 



(A 



8 



X 



u 



v> 



UJ 



o 



z 



2.0 



1.5 



1.0 



0.5- 



0-60 61-180 

 MILES 



0-60 61-180 

 MILES 



0-60 61-180 

 MILES 



HAWAI IAN IS. 



LINE IS. 



1 



PHOENIX IS 



OPEN 

 OCEAN 



Figure 34. — Frequency of sighting fish schools in var- 

 ious zones (distance from land) and areas of the central 

 Pacific (data from Murphy and Ikehara, 1955). 



Systematic trolling is a well-known and estab- 

 lished method of quantitatively surveying the 

 availability of surface fish. The only serious flaw 

 in our data has been the difference in availability 

 to troll lures of the principal surface species, 

 skipjack and yellowfin tunas (Murphy and Ike- 

 hara, 1955). For instance, near the Hawaiian 

 Islands half as many yellowfin tuna were taken 

 by trolling as skipjack tuna, whereas other mea- 

 sures, e.g., the commercial catch (June, 1951), 

 suggest that there are only about one-tenth as 

 many yellowfin tuna as skipjack tuna. 



DISTRIBUTION OF SURFACE TUNAS 

 RELATIVE TO LAND 



The general abundance of surface schools de- 

 picted in Figure 34 shows that schools are seen 

 several times more frequently near islands than 

 in the open ocean. This can be interpreted as 

 merely indicating the abundance of birds, but, 

 as is shown later, it is also possible to regard 

 the islands as creating in their vicinity special 

 conditions that are conducive to the occurrence 

 of surface-schooling tunas. 



In addition to diff"erences in abundance be- 

 tween island waters and the open ocean, species 

 composition appears to shift, in that yellowfin 

 tuna schools tend to favor the vicinity of islands 

 (Figure 35) . About 47% of all schools sighted 



YELLOWFIN TUNA 



SKIPJACK TUNA 



100 



(III) 



(18) 



(73) 



(14) 



(23) 



(6) 



(35) 



80 



(A 

 (9 



Z 

 (- 

 X 

 (9 

 V> 

 O 

 u 60 



Z 

 UJ 



9 



u. 

 o 





40- 



20- 



_Ell 





psi 





0-60 61-180 

 MILES 



0-60 61-180 

 MILES 



0-60 6! 

 MILES 



-180 



HAWAIIAN IS. 



LINE IS. 



PHOENIX IS. 



OPEN 

 OCEAN 



Figure 35. — Relative occurrence of yellowfin and skip- 

 jack tuna schools at various distances from land. Num- 

 ber above bar diagram represents total number of schools 

 sighted. 



15 



o 



z 



UJ 



o 



o 



UJ 

 O. 



10 



5- 



T — I — I — r 



n — I — I — I — I — r 



Ql.,.1 — I — I — 1„ 



40 50 60 



90 100 no 



SIZE (CM) 



Figure 36. — Length frequency of longline-caught yellow- 

 fin tuna at all insular fishing stations (within 96.5 km 

 of land). 



were identified as either yellowfin tuna or skip- 

 jack tuna, a small percentage were identified as 

 other species, e.g., the common dolphin, and the 

 rest were unidentified (Murphy and Ikehara, 

 1955) . The relative percentages of the two im- 

 portant species plotted in Figure 35 show very 

 clearly that (1) skipjack tuna are dominant in 

 the Hawaiian Islands area and in all off'shore 

 areas, and (2) in each instance relatively more 



904 



