surface-schooling species (Shomura and Murphy, 
1955), Around the Marquesas Islands, where 
they are abundant (Wilson, MS.2/), longlines 
catch only small numbers. Catches of this fish 
on long. 132° and 150° W. also were small and 
sporadic, 
Albacore first began to appear in the catches 
at about lat. 7° S. on long. 150° W. and at about 
lat. 11° S. on long. 132° W, The number caught 
at any one station was small. This distribution 
of albacore is interesting in light of the obser- 
vations by Yamanaka (1956) in the western 
South Pacific. Yamanaka noted a conspicuous 
discontinuity, characterized by sharp changes 
in water temperature, salinity, and sigma-t 
values, in the oceanic structure running in an 
east-west direction and centered on lat. 10°S, 
He suggested that this discontinuity probably 
limits distribution of albacore in the south- 
west Pacific. Our observations on the oceanic 
structure in this area indicate that this discon- 
tinuity extends as far east as long. 132° W, 
Only small numbers of albacore were taken 
around the Marquesas Islands in both winter and 
summer; the catch rates ranged from 0 to 2.2 
albacore per 100 hooks, 
VERTICAL DISTRIBUTION OF TUNAS 
Iversen and Yoshida (1957) showed that the 
longlines tend not to fish as deep near the Equa- 
tor as they do several degrees to the north. 
They suggested that this difference was caused 
by the shear between the westerly South Equa- 
torial Current at the surface and the easterly 
Equatorial Undercurrent beneath. In general, 
results were similar on the north-south fishing 
transects across the Equator on long. 132° and 
150° W. (fig. 6). The mean maximum depths 
reached by the longlines within about 2° north 
and south of the Equator were not as great as 
those recorded farther north or south; this 
trend was not strongly evident during Charles 
H. Gilbert cruise 38. 
The Pacific Equatorial Undercurrent first 
was described by Cromwell, Montgomery, and 
Stroup (1954). Knauss and King (1958), who 
measured the velocities of the Undercurrent, 
found that it was symmetrical about the Equator 
and that the core of the current was flowing be- 

eiisone Robert C. MS. The surface tuna re- 
sources of the Marquesas Islands. Division of 
Foreign Fisheries, Bureau of Commercial Fisher- 
ies, Washington, D.C. 20240. 


















2 
B 
Zz 
= 
o 
& | 
5 | 
= 200 
eo al 
= | 
2 40 150° W., 
2 | 
2 80}— 
2 |JRM-34 : 
2 120 \ : x = 
g | ie oe’ 
\\ 
160 | “I 
| CHG-38 | 
| 
2 
200) 15° S: 10° 5¢ 0° 5°N. 

LATITUDE 
Figure 6.--Latitudinal variation of the mean 
maximum depths of longline gear on long. 
132° and 150° W. CHG is Charles H. Gilbert 
and JRM is John R. Manning. 
tween 2 and 3.5 knots at 0°, long. 140° W. They 
also found that the Undercurrent was weaker on 
either side of the Equator; the velocity was 0.6 
knot at lat. 2° N. and 2° S, The depths reached 
by our gear demonstrate this configuration, be- 
cause the longlines usually were shallowest at 
the Equator, where the velocity of the Under- 
current is greatest, and became deeper on 
either side, where the Undercurrent is weaker. 
Longline catches have been used to evaluate 
the vertical distribution of tunas (Murphy and 
Shomura, 1953a, 1953b, 1955; Shomura and 
Murphy, 1955). Chemical sounding tubes at- 
tached to the gear have provided estimates of 
the absolute depths of the longlines, and ac- 
cordingly, the vertical distribution of tunas 
(Shomura and Otsu, 1956; Iversen and Yoshida, 
1957). Catches of tunas usually have been 
greatest on the deeper hooks of the longline. 
For example, Murphy and Shomura (1955: p. 27) 
stated that ‘‘...yellowfin are usually but not 
consistently more abundant at the deeper levels 
in the equatorial Pacific, the best bigeye catches 
are more regularly associated with the deeper 
fishing levels, and albacore are clearly caught 
in greatest abundance on the deepest fishing 
hooks,’’ 
To determine the vertical distribution of 
tunas, the catch rates for all stations were 
combined and averaged by species according to 
