MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



ECOLOGICAL SUBDIVISION OF TUNAS 



On the basis of size and behavior, populations 

 of yellowfin tuna, bigeye tuna, and albacore can 

 be conveniently separated into two groups: (1) 

 the smaller surface fish and (2) the larger deep- 

 swimmers. The two groups (Figure 4) cannot 

 be readily sampled by any one fishing method; 

 most surface fish are caught by trolling and live- 

 bait fishing and most deep-swimmers by long- 

 line. This situation evokes a query as to whether 

 the separation represents a real difference in 

 vertical distribution or whether the two appar- 

 ent groups result simply because one type of fish- 

 ing is more effective for small fish and one for 

 large. The differences in catch rates on longline 

 hooks fished at different relative depths (Table 

 1) strongly suggests that large tunas do, in fact, 

 tend to spend their lives well below the surface. 

 For instance, the catches of large yellowfin tuna 

 on the deep and intermediate hooks were half 

 again as numerous as on the shallow hooks. The 

 differences were even more striking for bigeye 

 tuna and albacore. 



The separation of the population into two com- 

 ponents, surface and deep-swimming, is accom- 

 panied by a horizontal dichotomy, at least in the 

 instance of yellowfin tuna. Sightings and troll 

 catches of small yellowfin tunas (< 36.3 kg) were 

 usually associated with emergent land or shoals 

 (Murphy and Ikehara, 1955). As will be dis- 

 cussed later in the section on surface tunas, the 

 longline when fished in such insular areas cap- 

 tures significantly more small fish (as small as 

 2.3 kg) than when fished in the open sea 

 (>96.5 km from land). When moving away 

 from land, the reduction in numbers of small 

 fish in the longline catch is proportional to the 

 reduction in school sightings which suggests that 

 small yellowfin tuna are associated with land. 



There is another difference between deep- 

 swimming and surface tuna. Generally, surface 

 tuna gather in compact schools, which occasion- 

 ally contain thousands of individuals, whereas 

 Murphy and Elliott (1954) provided evidence 

 from the serial distribution of yellowfin tuna 

 catches along the longline that deep-swimming 

 yellowfin tuna gather in small schools. Little in 

 their data, however, suggests that these groups 



50 

 40 

 30 

 20 

 10 



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 ui 

 a: 



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o 

 a: 



20 

 10 



0- 

 20- 

 10- 



— 

 30- 

 20- 

 10- 



— 

 20- 



10- 







J 



M 



YELLOWFIN TUNA 

 LONGLINE FISHING 

 n = 2,1 14 

 i = 141.12 



ALBACORE | 



LONGLINE FISHING 

 n = 237 

 S = 93.54 



YELLOWFIN TUNA 

 LIVE -BAIT FISHING 

 n = 625 

 X = 88.05 



YELLOWFIN TUNA 

 TROLL FISHING 

 n = 973 

 X = 91.88 



cQ 



BIGEYE TUNA 

 LONGLINE FISHING 

 n = 358 

 151.09 



TkfT 



I1=L. 



\h 



rtt 



40 60 80 100 120 140 160 180 200 



LENGTH (CM ) 



Figure 4. — Length frequencies (fork lengths) of the 

 principal tuna species taken, by type of gear. 



are anything more than loose aggregations of 

 a few large individuals. Thus both the degree 

 of aggregation and the principal depth range 

 differ with size of fish, at least insofar as yel- 

 lowfin tuna are concerned. For these reasons 

 it is convenient, and probably realistic, to treat 

 the two categories separately. 



879 



