YELLOWFIN 

BIGEYE ALBACORE 






105-11I9WY 


OCEANIC 


DEPTH (M.) 







WML 


INSHORE 













7 8 
CATCH PER 100 HOOKS 
Figure 7.--Vertical distribution of longline catches of 
oceanic and inshore stations. 
of stations used 
the average maximum fishing depth of the long- 
lines by 15-m. depth intervals. No zero catch 
rates were used in calculating the averages. 
The vertical distribution of tunas at oceanic 
stations is shown in figure 7. Yellowfin tuna 
were caught at all fishing depths; the highest 
catch rate was at the 75- to 89-m. depth. Com- 
plicating factors were latitudinal differences in 
both the catch rates and the depths at which the 
longlines fished. For example the depth at 
which longlines fish is relatively shallow near 
the Equator and here the catch rate of yellowfin 
tuna was greatest. Therefore, the peak catch 
rate at the 75- to 89-m. depth may be merely a 
reflection of the greater abundance of yellowfin 
tuna close to the Equator, or on the other hand, 
it may indicate that the vertical distribution of 
yellowfin tuna is real and that more are caught 
here because the longlines fish the depths in 
which tuna are most abundant, 
Bigeye tuna, like yellowfin tuna, were caught 
at all depths fished. The highest catch rates 
were at 90 to 104m. Here again, it is difficult 
fo assess the relative influence of the areal 
distribution and the vertical distribution of the 
ftunas. 
As has been noted, no albacore were caught 
north of lat. 7° S, It could be argued that the 
tunas at 
Figures in parentheses are number 
in calculating the average catch rate (number 
of fish per 100 hooks). 
longlines did not fish deep enough north of lat. 
7° S,to catch them. This argument is apparent- 
ly not valid, however, for the longlines fished 
as deep between lat. 2° and 5°N. and 4° and 7° S., 
as they did south of 7° S, It can be seen (fig. 7) 
that albacore were distributed rather evenly 
between 105 and 164 m. 
The longlines fished relatively deep around 
the Marquesas--112 to 166 m., average maxi- 
mum depth. The vertical distribution of tunas 
in this area is shown in figure 7. Although 
yellowfin tuna were caught at all depths, greater 
catch rates were within the two deepest strata. 
Bigeye tuna were caught between105 and169 m. 
with more fish taken in the 150- to 164-m. 
range. 
SIZE OF TUNAS 
Longlines typically catch greater numbers 
of the larger tuna than does pole and line or 
troll fishing, either by inherent selectivity of 
larger fish by the longlines or because the 
deep-swimming populations are made up chiefly 
of large fish (Murphy and Shomura, 19538a). 
The length-frequency distributions of oceanic 
and inshore catches of yellowfin tuna (fig. 8) 
indicate essentially no difference. The length 
