MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



yellowfin tuna schools are found near land than 

 offshore. In fact, in the Line and Phoenix groups 

 yellowfin tuna appear to have replaced skipjack 

 tuna near land. Thus the presence of land seems 

 to create favorable circumstances for the occur- 

 rence of tuna schools in general and yellowfin 

 tuna schools in particular. 



This distribution of yellowfin tuna in relation 

 to land is also borne out by longline catches of 

 yellowfin tuna near land and offshore. Figure 

 36 shows that when longline gear is fished near 

 islands it catches more of the smaller yellowfin 

 tuna that compose surface schools than it does 

 when fished oflFshore (compare Figure 36 with 

 Figure 4). This increased abundance of small 

 yellowfin' tuna near islands is also shown in Fig- 

 ure 37, which indicates that about half of the in- 

 crease in longline catch rates near shore stems 

 from the addition of small fish to the catch. Fur- 

 thermore, the increase in small fish in the long- 

 line catches is roughly proportional to the in- 

 crease in the rate of surface sighting of this 

 species near land. A final indication that small 

 yellowfin tuna are more abundant near land is 

 that surface trolling took 10 times as many yel- 

 lowfin tuna per unit of effort within 96.5 km of 

 land as beyond tl^at limit (Murphy and Ikehara, 

 1955) . Since all three sampling methods — long- 

 lining, sighting, and trolling — clearly show small 

 surface yellowfin tuna to be more abundant near 

 land, we conclude with little or no reservation 

 that the occurrence of young yellowfin tuna in 

 the central Pacific is intimately related to the 

 presence of land. 



SMALL YELLOWFIN TUNA«I20 CM ) 

 LARGE YELLOWFIN TUNA (> 120 CM ) 



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Figure 37. — Relative occurrence of schooling yellowfin 

 tuna in surface sightings and longline catches for in- 

 shore and offshore areas. 



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OCEANIC DISTRIBUTION 



Despite the low average abundance of tuna 

 schools in the open ocean (Figure 34) certain 

 areas or zones appear to support populations 

 with a density comparable to that of the island 

 areas. Over most of the central Pacific our sam- 

 pling is not adequate to focus attention on par- 

 ticular areas except in the central portion of the 

 study area (long 140°-170°W) . Here, there are 

 enough days' observations to make meaningful 

 zonal comparisons (Figure 38 A). It is appar- 

 ent that the number of schools sighted in some 



Figure 38. — Latitudinal distribution of surface tuna 

 schools and deep-swimming yellowfin tuna. A. Sightings 

 of surface tuna schools in the open ocean (Murphy and 

 Ikehara, 1955). B. Yellowfin tuna catches from fishing 

 stations more than 96.5 km from land and between long 

 140° and 170°W. 



of these zones is nearly the same as for the Ha- 

 waiian Islands area, e.g., 0.9 school seen per day 

 in the Countercurrent (Figure 38A, lat 5°-10°N) 

 and 1.7 near the Hawaiian Islands (Figure 34). 

 Considering how far removed these zones are 

 from land, we might conjecture that consider- 

 ably more schools would have been sighted if 



905 



