In the analysis of the results of fishing, the 
catch rates of yellowfin tuna and bigeye tuna 
(I. obesus), by season and area, were subjected 
to simple statistical tests. Catch rates from 
two or more independent areas or seasons were 
tested to see whether they could have been 
drawn from the same population (Mann-Whitney 
U test or the Kruskal-Wallis one-way analysis 
of variance by ranks--Siegel, 1956). I concluded 
that fish were relatively more abundant in one 
area or season if differences in the catch rates 
were Significant, 
Unless otherwise stated, all the seasonal 
designations in this paper refer to seasons in 
the Southern Hemisphere. 
DISTRIBUTION AND ABUNDANCE 
OF TUNAS 
The data on longline fishing from the three 
cruises already have been published (Austin, 
1957; Wilson and Rinkel, 1957; and Wilson, 
Nakamura, and Yoshida, 1958). The tuna catch 
of these cruises totaled 642 fish, comprising 
438 yellowfin tuna, 102 bigeye tuna, 51 albacore 
(IT. alalunga), and 51 skipjack tuna (Katsuwonus 
pelamis). 
The distribution and abundance of yellowfin 
tuna are considered in greatest detail, since 
this species dominated the catches. 
Yellowfin Tuna 
Previous investigations in the equatorial 
Pacific Ocean disclosed the presence of con- 
centrations of yellowfin tuna within a band of 
latitude several degrees to the north and south 
of the Equator, and also indicated north-south 
and east-west shifts in these concentrations 
(Murphy and Shomura, 1953a, 1953b, 1955; 
Shomura and Murphy, 1955;Iversen and Yoshida, 
1956). 
In the winter of 1956, deep-swimming yellow- 
fin tuna were caught from lat. 1°32’ N. to as far 
south as lat. 13°26’ S., the southernmost fishing 
station, on long. 132° W. (fig. 2). The catch 
rates were highest at the northernmost fishing 
station of the north-south transect and ranged 
from 0 to less than 1 fish per 100 hooks for the 
remainder of the transect. A rather discontin- 
uous distribution is suggested. 
The latitudinal range fished during the sum- 
mer of 1957 on long. 132° W., was from lat. 
4°29’ to 14°02’ S. Yellowfin tuna were found 
throughout this range and were relatively more 
abundant than in the previous winter (fig. 2 and 
5 

4 | OZ 
WINTER 1956 | 
3 - 







0 Sisleose NRE ES 
5 T 7 









Ae 12a | 
SUMMER 1957 | 
é 2 
ise 1 T 
iy! auUo 
eS 5 
Boa 150° W. 
I SUMMER 1957 
& 3 
oO 




om ob A 
5 
4 150° W. 
SUMMER 1958 













15°S. 10° 5° 
LATITUDE 

Figure 2.--Oceanic catch rates (number of fish 
per 100 hooks) for yellowfin tuna on long. 
132° W., winter 1956 and summer 1957, and on 
long. 150° W., summers 1957 and 1958. 
‘table 2), The catch rate was highest at lat. 
3°29’ S. In the same season, but farther to the 
west, on long. 150° W., yellowfin tuna were less 
abundant and were irregularly distributed be- 
tween lat, 3°01’ N. and 16°34’ S, (fig. 2 and table 
2). The highest catch rate, 1.6 fish per 100 
hooks, was at lat. 0°06’ N. Another transect 
was fished on this meridian the following sum- 
mer. On this 1958 transect, fishing was con- 
fined to a narrow band across the Equator, be- 
tween lat. 4°44’ N. and 0°45’ S. Catches of yel- 
lowfin tuna again were rather low and were 
concentrated within a few degrees of latitude 
north of the Equator. 
In summary it is evident that yellowfin tuna 
were relatively unavailable to longline fishing 
on long. 150° W. At long. 132° W., where a 
seasonal comparison can be made, yellowfin 
tuna were relatively more abundant in the sum- 
mer than in winter. They also were more 
abundant to the east, at long. 132° W., than at 
long. 150° W., at least during the summer of 
1957. In addition to these seasonal and longi- 
tudinal differences, relative abundance exhibited 
latitudinal shifts. 
